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Newly bought used 1989 300SEL hard start

7K views 39 replies 8 participants last post by  alexstorm 
#1 ·
Hello!

Nice looking forum!

I just bought a 1989 300SEL yesterday
WDBCA25D2KA446759


Had seemingly no issues yesterday morning... so I bought it from granny down the block .... took it for smog, and had a bit of trouble starting it up afterwards... and since. tried this morning again and got it going, held the ignition till it stopped making tone, repeated... then full turn and fired with gas pedal.

when it does start coughing i need to give it some pedal. jumps right up.
needs pedal nearly every time.

there is a whining-hissing- noise somewhat like an arcing-light-switch sound that is coming from very near the fuel pump..... its prominent- if it were your lightswitch you'd call the electrician.
im nearly sure its fuel delivery related for my start up issues.

once running its really nice.... though this morning I just noticed that it seems to be searching for RPM..... when it starts at 1000, and even after it drops to 650... seems like varying between 75 and 100 rpms. yesterday morning while watching the engine run I noticed that cylinder 3 seems to be arcing from the plug to the cyl head.... so im going to be changing plugs and wires and will see how it goes.

main thing is the issue starting- i don't want to be taxing my battery and starter every time I need to go somewhere.


Any input or advice helps,

Thanks for having me.
 

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#2 ·
I should mention.... im leaning towards pressure accumulator.... or check valve as even if the fuelpump is noisy.... it seems to do just fine once running.

also, these cars aren't known for their grunt are they? it seems a bit underwhelming for 3liters.

if anyone in reno is a good judge of 300sel's feel free to PM and set up a drive to give me feedback.... ill grab you some beers... smokes...
 
#3 ·
Welcome to the forum. We have nearly identical cars except that your interior is black and mine's cream. Your color looks to be Desert Taupe? In any event, congrats! These are wonderful cars, but you may find yourself initially dumping some serious coin into getting the ride and everything working like it did twenty years ago—depending of course on the car's mileage.

I had hard-starting problems for a while a couple of years ago, and wound up, through a series of repairs over about a year's time, doing the following—in this order and as trial-and-error dictated:

- Spark plugs
- Plug cables
- Fuel pump relay
- Overload relay
- Fuel pump and filter
- Distributor cap (points) and rotor

My M103 engine still won't instantly turn over, cold or hot, but it generally fires up with a few seconds when cold cranking and slightly less when hot.

Depending on your mileage and storage condition of the car, I'd recommend inspection and potential replacement of the following if you want maximum ride quality:

- Tires. If older than six or so years (see side date stamp XXXX), replace them.
- Front guide rod mounts (they loosen significantly with age as the rubber hardens)
- Lower ball joints
- Shock absorbers (get Bilstein, either standard "comfort" or sport (slightly stiffer) depending on how you like to drive and what load you have in the rear.) I like my car completely factory, so I went with comfort.
- Steering drag link
- Springs (optional; this is a pain in the butt to do, so only as a last resort if yours are weak)
- Motor and transmission mounts (do not buy cheap aftermarket; they fail quickly and aren't worth the hassle)
- Rear sway-bar links (these aren't expensive, but the ball joints rot and and clunk)

I've done all of the above between 165-175K miles, and the car rides significantly different—and way better—than it has in at least a decade. It's a fabulous road car.

Enjoy your ride; these cars are lovely, and they're infinitely fixable because they're mostly mechanical. They are a labor of love, but properly TLC'd and they'll reward you with a wonderful ride and solid comfort for a long time. Just don't neglect the interval maintenance, either.

One other thing: The M103 engine used in the 300SE/SEL is notorious for oil leaks at the front and rear seals as well as the valve guides. If your engine burns a little oil (maybe a quart every 600-800 miles), don't be alarmed, but do keep an eye out for the presence of oil in your coolant reservoir. Once that shows up you need to have the seals replaced or you could ruin the engine. Other than that, the engine is bulletproof, and to answer your other comment: Yes, it's slightly underpowered for the car's weight and size (particularly the SEL since it's longer and weighs a few hundred pounds more) but if you're not Ricky Racer it's totally acceptable. I've averaged 18.1 mpg over about 15,000 miles, so the Mulroney sticker's 17/19 EPA estimate is spot-on.
 

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#4 ·
Thank you! So far I do love the car.
everything works.... except the radio, the gas pedal... and the whatever the hell is causing the hard starting and rpm searching.
Honestly, the radio being a bit wonky, and the fact the gas pedal broke off at the hinge... almost has me more offset than the hard starting... within a day i've found I can jimmy it on basically first try so thats OK.... for now.

Currently the tires are near 60% tread, just rotated a few months ago (havnt checked date though), rides nicer than my 2001, steers better too -no play at all... gets into turns as well as my 325i e30 coupe, I'm pretty impressed overall.
honestly it has ok get up and go for a sedan of its size.


I do have leaks on my seals, engine is a bit 'wet' in apperance on the exhaust side of the engine. but nothing that really concerns me. some oil in the pan below, but again not enough to leave a slick. I havnt checked the coolant- but I will do.
a few more questions though:

-Reputable online source for parts?
- Good online, or book source of mechanical diagrams?
-Vin identifier because I cannot locate the tags on the trans, engine or diff... and the few I've found online don't list those.
-Does the differential have a seal? or is it made with the sealing black gasket goop stuff? i have a bottle from my ford 8.8 rebuild sitting around the garage.
-Is significant noise from the fuel pump normal?
-Does the car have 1 or 2 fuel pumps? -I haven't crawled around yet, ill admit.

Finally.... did they always have this issue starting? I have to imagine good ol' Hanz at Benz engineering doesn't want to stand there spraying starter fluid in the winter.... or turning his key 3 times to start the car once. My reasoning behind that statement is that: at $55,000 *1989 dollars* this thing was worth quite a deal.
 
#8 ·
Currently the tires are near 60% tread, just rotated a few months ago (havnt checked date though), rides nicer than my 2001, steers better too -no play at all... gets into turns as well as my 325i e30 coupe
You're welcome. If the tire date stamp indicates more than about eight years, I'd replace all of them regardless of mileage. It's a function of age, heat and oxidation that weakens the tire compound. These cars have insanely good brakes, so the last thing you want is a blowout if you have to brake really hard.

-Reputable online source for parts?
- Good online, or book source of mechanical diagrams?
See later in this thread; @John350 answered that for you.
-Vin identifier because I cannot locate the tags on the trans, engine or diff... and the few I've found online don't list those.
Not sure about this; most of the major components have VIN ID stickers on them, but I'm not sure where. Are you concerned that your car has parts from different 126s?

-Does the differential have a seal? or is it made with the sealing black gasket goop stuff? i have a bottle from my ford 8.8 rebuild sitting around the garage.
I can't answer this, but I do recall a conversation with my mechanic indicating that the rear differential is rarely an issue; it's the components that work with it that age and fail.

-Is significant noise from the fuel pump normal?
My last pump was very noisy. The replacement I put in last year is quiet as a church mouse. The only thing I hear from the back is when I switch on the ignition as the pump kicks on.

-Does the car have 1 or 2 fuel pumps? -I haven't crawled around yet, ill admit.
One. And fortunately, it's above the rear axle and easy to get to—along with the filter.

Finally.... did they always have this issue starting? I have to imagine good ol' Hanz at Benz engineering doesn't want to stand there spraying starter fluid in the winter.... or turning his key 3 times to start the car once. My reasoning behind that statement is that: at $55,000 *1989 dollars* this thing was worth quite a deal.
No. They should start up pretty quickly. There are days when my M103 turns over instantly...like zero cranking...but it's rare. Usually it takes 2-3 seconds of cranking hot or cold. But yes: Given that the in today's dollars the car would cost $110K+, it's reasonable to expect that Daimler-Benz would not stand for an engine that is a cranky cranker. The M103 is a very good engine—almost as good as its replacement the M104 except that the latter had more HP and an additional valve per cylinder. The bottom half of these engines is, as my mechanic puts it, "buuuuuletproof." Just change your oil regularly and watch your levels. When my oil light comes on I know it's time to add a full quart, and thankfully DB put an oil pressure gauge in these cars, and I watch it all the time, and I am kinda OCD about driving as much as possible with the manifold pressure indicator (the Economy gauge) in the black. I had a friend with a turbodiesel who chose not to watch his gauge and didn't realize it was at zero until it was too late: Seized his engine. I use Kendall synthetic blend. Keep a quart in the trunk; you'll find that you need it.
 
#6 ·
Performance: remember, the car scales at almost 2 tons empty.

Starting: no, they should start right up, but, you're looking at a fuel and ignition system that's over 25 years old. Don't be surprised if you need to replace a few things. I think the 300 has one fuel pump. Cycling the key helps the system build pressure, so you should look for ways the system pressure can bleed off while the car is sitting. The accumulator is a good first place to look.

Service manuals: the factory manuals are available free. See the sticky thread at the top of this forum.
 
#7 ·
help

Hello I Recently Got a Mercedes 1993 300se It has a coolant leak coming from the reservoir not from cap (i replaced cap) its the pillar the cap goes on where it attaches to the tank i keep sealing but it keeps blowing seal.

Also My heater kicks on an off going over bumps works if i park with back end facing down hill. cuts on and off over bumps or if i hit the dash.

Power locks / Speakers not Working aswell- they were but every time it rains they mess up.

Troubleshooting done.
-Fuses- perfect
-wires and sensors -perfect.(from under engine.)
-no water leaks
-Cap replaced

Also What type oil and coolant should I use in a Straight 6.
maybe someone could help.
 
#10 ·
Hello I Recently Got a Mercedes 1993 300se It has a coolant leak coming from the reservoir not from cap (i replaced cap) its the pillar the cap goes on where it attaches to the tank i keep sealing but it keeps blowing seal.
I'd replace the reservoir.

Also My heater kicks on an off going over bumps works if i park with back end facing down hill. cuts on and off over bumps or if i hit the dash.
This one's hard to diagnose without a process of elimination. When you say "heater", do you mean the blower fan? Seems to me whether AC or heat, the result should be the same, and I suspect the blower motor is the culprit. In my 1989 300SE Fuse #5 is the blower motor and HVAC unit control. Check your fuses again (I know you've said you already did this); it might be loose. I think it's an 8 amp. I had a problem with mine until I replaced it. You may also have vent air distribution problems, which is a more complicated problem because the system is actuated by vacuum; pods (six of them) open/close various air flaps. See below under power locks and speakers.

Power locks / Speakers not Working aswell- they were but every time it rains they mess up.
Power locks and speakers are completely different animals. The power locks are operated by a vacuum unit in the trunk. If your system isn't locking, then you likely have a faulty vacuum pump and/or a loose hose connection. With these cars, having a Mighty Vac or similar vacuum line tester is super handy. Consider getting one. I bought one from my favorite online how-to video store: https://goo.gl/BPt4Px

Also What type oil and coolant should I use in a Straight 6.
maybe someone could help.
I use Kendall synthetic blend, usually 5W-30. As for coolant—whatever DB specifies. I think the newer stuff is blue and is designed to last ten years.
 
#9 ·
Further to my last comment, I was thinking about your fuel pump noise and my own history. Before my car left me high and dry on the freeway—complete with a $300, 60-mile tow) in 2015, I did notice that the pump was really noisy—a steady "wheeeeeee" sound from the rear. Because my pump wound up failing (we tested it beyond the fuel pump- and overload-relays), I wonder now if that loud noise is indicative of the pump on its way out, including but not limited to insufficient pumping pressure.

So you might, as a higher priority, think about having it thoroughly checked out. Or, if you have repair records and it's never been replaced, just do it as a matter of course. After replacing mine, and the filter, and the relays, I've had zero issues.
 
#12 ·
so.... I was checking the plug wires.... the one that was arcing was noticeably loose. Pushed it back on... so far so good, I cannot see a spark bridging to the head.

added some premium gas- i started to question how long the lady let the gas sit, and if it was premium to begin with..... surprisingly seems to help the idle quality, and how much juice I get. I guess I underestimated the cars ability to change timing. I think it got back closer to where it should be.


car still does some searching at start up, and less when warm. when I come to a stop though it can seem a little rough- not every time, and not very rough.... but some vibration from the engine.



Main issue is still.... I have to crank it for 3 or so seconds- and even then I hold the starter on after the inital firing.... the engine shoots up to about 1200 RPM, then drops back to 500 or below..... if I don't hold the starter for that longer duration sometimes it will die and be difficult to start for a moment.


I heard a loud slap the first time It started up yesterday afternoon... Was worried the starter didn't disengage.
i'm fairly confident i'm hurting the starter doing this..... im buying a fuel pump, and i'm probably going to get a new pressure accumulator just for the Fk of it.
 
#15 ·
...added some premium gas- i started to question how long the lady let the gas sit, and if it was premium to begin with..... surprisingly seems to help the idle quality, and how much juice I get. I guess I underestimated the cars ability to change timing. I think it got back closer to where it should be.
Never run anything but premium fuel in your car; they're not designed to run on anything else, and you'll have performance issues using lower octane fuel.


car still does some searching at start up, and less when warm. when I come to a stop though it can seem a little rough- not every time, and not very rough.... but some vibration from the engine.
I have a similar issue; at stoplights (idle) it'll misfire occasionally. Usually just once. I suspect it's either an iffy fuel injector, a fouled plug or something similar. I haven't yet fully diagnosed it because it's not often a problem.



Main issue is still.... I have to crank it for 3 or so seconds- and even then I hold the starter on after the inital firing.... the engine shoots up to about 1200 RPM, then drops back to 500 or below..... if I don't hold the starter for that longer duration sometimes it will die and be difficult to start for a moment.

I heard a loud slap the first time It started up yesterday afternoon... Was worried the starter didn't disengage.
i'm fairly confident i'm hurting the starter doing this..... im buying a fuel pump, and i'm probably going to get a new pressure accumulator just for the Fk of it.
I don't consider having to crank the engine for three seconds to be a problem. I had an '06 Honda Civic si, and I routinely had to crank that engine for almost that long every time I started it cold. I also have found on the M103 engine we have that the cold-start choke doesn't rev the engine at 1,000+ rpm like most cars do. When it fires up, it hits the usual 600-700 or so rpm and just hangs there. It's kind of a lumbering engine that performs best when it's at about 3,500 rpm doing around 70. It has to work hard all the time because of the car's mass; you'll discover—if you haven't already—that letting off the gas while on the highway makes for an immediate drop in the car's speed, and you can feel the engine let up—unlike, say, a big V-8. I suppose that's the reason why the car's EPA rating is only 17/19; there's not a big difference in the work it's doing at lower vs. higher speeds. If I fill up and hit the road and don't have a heavy head-wind I can get nearly 20 mpg over a few hundred miles.

As for your fuel pump and accumulator: Good idea. As I've mentioned earlier in this thread, replacing the fuel pump and filter, as well as the fuel pump relay and overload relay fixed my fuel delivery problems. I probably should also do the accumulator, as that has IMO the greatest chance of being the culprit for longer cranking times, particularly when the engine's warmer and the likelihood of fuel evaporation is greater.
 
#13 ·
Now with that said... I don't just lean on the starter so to speak... after it goes to 1200, back to 500 and above 500... its good. i can leave it alone.

all of this happens in 3 seconds or so.... really dont like holding the starter on like that though.

never owned a carbureted car..... is this still sounding abnormal, or is it getting closer to how it is?
 
#14 ·
I have an 88 SE. Yes mine is a bit funky to start cold. First thing I did was remove the idle air control valve and rinse it internally with BRAKELEEN. They get carboned up over the years and the air slide sticks. That will account for the idle search cold. Next, replace the alternator voltage regulator. Very simple on this 6 cyl. In my experience distributor caps/rotors wear out early and will affect cold start. The voltage regulator replacement seemed odd at first but it affects the available voltage stored in the battery. Be aware that there is another fuel filter in the fuel tank. Called a strainer, it can cause a noisy fuel pump if restricted .
 
#16 ·
Very good advice, @carnut62. I strongly suspect that a noisy fuel pump indicates lack of lubrication (by the fuel) and ultimately contributes to failure. I'm almost sure that's what happened to mine. After replacing it, there was no more whining noise coming from the rear when I engage the ignition. I also did the distributor cap and rotor; I found cylinder 5 was corroded and contributing to a misfire. I think doing my voltage regulator is probably a good idea.
 
#21 ·
Ok, I have a list and am now tackling it.

first thing- signature :p

I should also elaborate. the car idles at 1000 rpm for a minute or so before hitting 650 and staying there.

it just drops to 500 very briefly after starting..... so.. prime, fire.... it goes to 1000-1100 rpm, and drops to 520-480 (all within the 3 seconds) before returning to 1000. once it starts climbing past that dip is when I let off the starter, when its gotten back to 1k.... then it will search a little and be a little rouggh.... but overall OK.

also found a video that sounds incredibly similar to my pump noise.


how difficult is it to change the strainer? Please tell me it can be done from the bottom of the car, lol. because I know what through the trunk means.... a damned headache of a fuel tank project.


as for the starter.... is it in a sense like a bike down a hill.... i.e. the wheels(engine) may be going quickly... but its not engaging so your legs(starter) arent getting thrashed?
basically: will it hurt my starter to leav it engaged for the second or two until the engine is under its own power?
 
#24 ·
Ok, I have a list and am now tackling it.

first thing- signature :p

I should also elaborate. the car idles at 1000 rpm for a minute or so before hitting 650 and staying there.

it just drops to 500 very briefly after starting..... so.. prime, fire.... it goes to 1000-1100 rpm, and drops to 520-480 (all within the 3 seconds) before returning to 1000. once it starts climbing past that dip is when I let off the starter, when its gotten back to 1k.... then it will search a little and be a little rouggh.... but overall OK.
Your engine does what it's evidently supposed to as relates to cold-start idle, but all the cranking you have to do is the harder part. I think I have an idle air control or accumulator issue.

also found a video that sounds incredibly similar to my pump noise.
https://www.youtube.com/watch?v=lZ68wBN4sNA.

how difficult is it to change the strainer? Please tell me it can be done from the bottom of the car, lol. because I know what through the trunk means.... a damned headache of a fuel tank project.
Yep: That's the sound. Like a somewhat high-pitched "wheeeeeeeeee" sound. After replacing mine, the sound stopped. Can't help you on the strainer as I don't know where it is. My mechanic did the work, and I didn't ask him about a strainer.


as for the starter.... is it in a sense like a bike down a hill.... i.e. the wheels(engine) may be going quickly... but its not engaging so your legs(starter) arent getting thrashed?
basically: will it hurt my starter to leav it engaged for the second or two until the engine is under its own power?
I don't have a definitive answer to this. My assumption is that after the engine starts firing, the starter motor's RPMs necessarily instantly climb to match the speed of the now-spinning flywheel—meaning that if still engaged, you're effectively making the starter motor spin a lot faster than it's designed to when cranking the engine. What this does to its effective life I can't answer.
 
#25 ·
I would imagine that it does damage..... bearings are specced for a reason lol.....

im just wondering if its like a ratchet.... yeah you can get leverage but if you spin the bolt the same way you're applying force, just faster.... it just clicks
 
#27 ·
@SammyG, you might consider using the site's search engine. Often you can find posts related to the topic. In the search drop-down, click Advanced Search. Then, enter data as shown in the screen capture. I think you can use Boolean operators in the search field. For example, "I used 126 "fuel filter"" (without beginning/end quotation marks) and returned about twenty articles that contained "fuel filter". Give it a whirl.
 

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#30 ·
I only run the starter until its running, not constantly.

it wants to initially surge to 1200, then stumble and fall below 500 rpm... then back up to 1000, where it idles and will go to 650 eventually and surge 75 or so rpm each way



Seriously though, since adding premium techron gas it has gotten much better. NOT ONE long start more than 3 seconds.... first day I bought it sometimes It would take at least 10 seconds of cranking... also, no need for me to give it any pedal anymore.... ignition is enough.


Im going to clean the Idle Control Valve like mentioned when I get home. (bought some of the good red bottle chlorinated BRAKLEEN) as for the idle. I swear it idles smooth as butter if I give the throttle even a hair of gas..... only enough to raise the idle by 50 or 100 rpm.... but I think it takes the ICV out of the picture during that time.
 
#31 ·
I know it was mentioned before, but the M103's are very picky regarding their ignition system. Replace the rotor/cap/wires/plugs if their is no history of recent replacement. Also don't hesitate to hop into the w124 forum, the M103 was the go to engine for the 300e.

My friend has a 300e with the m103, and yes they are a bulletproof fantastic engine (the secret Mercedes J2Z), but can get grumpy at times. While I do suggest like others have, to clean out your IAC with a oil dissolving cleaner like brake kleen, one tip I do with my m116/117 iac (although completely different design then yours), use a 9v battery to actuate it many times it as you flood it with cleaner. Your IAC has a normal voltage range of 0-10v so you wont hurt it.. Just make sure your cleaner is not flammable. :grin
 
#32 ·
This sounds like a good idea. Under certain conditions I've not been able to narrow down for descriptive purpose, my M103 idle is sometimes uneven...wandering back and forth between 500 and 600 or so. I suspect the IAC is struggling a bit, and cleaning it would make good sense. However, I'm a newbie to DIY repairs: Where is this thing, and how hard is it to remove?
 
#33 ·
Took off air filter box and cleaned ICV...

ICV valve is only partially open, ( dont know its resting position normally)
was damned clean. I mean damned clean, as in the cleanest part ive ever taken off of any car ive ever touched even cleaner than some interior stuff.. still sprayed some brakleen through.


seems like the gasket at the bottom of the box is going bad. good amount of grit around it underneath the air filter (somewhat significant) inside of the box past the filter as well, as not major.... but dirty.
cannot find a replacement..... 1020940060 i found on FCP euro but claims it doesnt fit my car. 0010945180 just cant find at all.


the throttlebody butterfly is also gritty. ....


ill add photos once its all reassembled.
 
#34 ·
icv and air filter changed.
fired right up.. - seemed even easier than before....waited to come down to 650... turned off and fired right up again..
still changing pump due to noise. and still want to replace the gasket. likely the valve cover vent hoses too -loose and likely allowing crap to come in.




still a little bit rough idle. but better, and faster crank time.
 
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