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91 Mercedes 300E with rough idle, stumble on acceleration! HELP!

14K views 38 replies 11 participants last post by  LeftCoastGeek 
#1 Ā·
Hello Everyone!

I have a 1991 Mercedes 300E and need some help!

About 3 months ago I was driving and noticed it seems to chug or stumble on acceleration. At first I thought it was the transmission not going back down into the starting gear but then realized it is a gas issue.

So.. what is happening now is that when you start it up.. it's like it stumbles at idle. Even when in Park and I step on the gas you can hear it rev but hesitating/stumbling.

When I drive it now.. it does the same thing. Stumbles on takeoff pretty bad.. Once i kick it down and we get going it seems to be fine. It's just more on the initial takeoff or on lighter throttle.

I have done many things to the car! ha ha

Here is the list:

New Fuel Filter
Accumulator
EHA Valve
Fuel Pump Check Valves
New Injectors
New Plugs

I HAVE adjusted the mixture using the Duty Cycle. I haven't checked it for a few months. I could do that again I suppose.

Your help or input would be awesome!!
 
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#3 Ā·
my 300E started stalling when coming off idle, the distributor I'd installed 5 years and 50000 miles ago, was pretty much toast inside. new cap, no rotor, new plugs, and its humming like new again. the Beru spark plug wires I installed 5 years ago are still in fine shape.
 
#8 Ā· (Edited)
Ok.. so I did a new distributor cap and rotor. Also did a new O2 Sensor.. still having the issue and worse than it was...

Just can't understand for the life of me. Had the EGR valve checked and it was working when they applied vacuum.

I'm at a loss.. I would take it to the Benz dealer but they told me that they don't really have anyone who knows these older cars anymore.

Anymore suggestions?

There is no way that when I changed out my Fuel Pressure Regulator that somehow there could get air in the line or something? Like an air bubble or something.. ha ha Is that even possible?!

Oh.. and no lights on the dash! :)

And they also put some smoke through the vacuum system to see if anything leaked and didn't see anything other than a bit coming out of the air filter.
 
#9 Ā· (Edited)
I'd suggest checking the duty cycle again. By chance, does the car acclerate fair well went it's stone cold and progressively get worst as it warms. Runs fine once it's nearing 2000 rpm? Remember, do the checking when the car is fully warmed.



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#10 Ā·
I'd suggest checking the duty cycle again. By chance, does the car acclerate fair well went it's stone cold and progressively get worst as it warms. Runs fine once it's nearing 2000 rpm? Remember, do the checking when the car is fully warmed.
I just did that! Still 45% - 50%.

It does seem to do it worse when it's warmer out. But it's been around the -1C to +7c lately.

Today it was doing it first think on startup... Yeah.. it stumbles on idle.. Never stalls but just like it catches itself. It's worse if I just do a light acceleration.. It's just putters almost.. like it's trying to get gas.. then once it hits 2000 - 2500 or I push down harder so that it either drops a gear and revs up or gets above that 2000- 2500 rpm, it is fine. No problems once I'm at normal driving speed... Just seems to be on the initial takeoff.
 
#12 Ā·
Ok... went out this morning. Started up good and no stumbles but duty cycle was staying constant at 49.9% until it warmed up.

As soon as it started to fluctuate with I assume means the ECU started working or something.. started to stumble. I shut the car off and unplugged the EHA Valve. No stumble...

Plugged it back in.. Stumble. The EHA Valve was new last spring so I know it can't be bad. However... could be set wrong. Would that cause issues?
 
#13 Ā·
Your cold behavior is normal. It is operating in open loop lambda control when cold. It is almost like a cheap carburetor at this point, the AFM plunger just mechanically controls fuel pressure.

When it warms up (~75C-80C) it switches over to closed loop in which the ECU starts monitoring O2 and various other inputs to best optimize mixture.

When you unplug the EHA your are breaking the loop, so you are back to open loop operation.

I believe you have proven that:

(1) Your closed loop system is busted somehow. Could be a myriad of reasons.
(2) Your open loop system is probably well adjusted because the car runs fine when you break the loop. So I would not mess with any adjustments at this time.

Remember in closed loop operation, your adjustment has to be way off for it to give you any big trouble.
Even if it is not adjusted to be best possible point the closed loop will take care of it. It is designed that way.

I'm still suspecting your AFM potentiometer, or at least the connections around there.

CAVEAT: I'm not an expert, just someone with opinions. There are plenty of members with a whole lot more knowledge and experience than me.
 
#15 Ā·
I hear you. I have two late model BMW's (535-XD and convertible 428i) and still find myself driving my old Mercedes quite often.

Since you are in Canada, (potentially a wet climate), I would start with the low hanging fruit. The easiest things to do that cost you nothing.
I would disconnect all electrical sensors/actuators and clean them with an electric contact cleaner (using q-tips for female side and wiper for the male side).

This should take you and hour of two of labor and no parts swapping/diagnostics needed.

If you are inclined to check out your AFM, please read my very recent "idle stumble" thread. And follow the instructions given to me in it pertaining to the AFM potentiometer.

BTW, my idle stumble appears to have gone away without me swapping anything just yet. I believe what did it was me plugging and unplugging AFM and ICV and EHA sensors/actuators. So I suspect one of them had a poor connection (most likely the EHA in my opinion). And someone has suggested to me what I'm suggesting to you about cleaning all contacts.

I have not done this yet, but if I ever get that very infrequent "stumble/stall" back that is the first thing I will try.
I do not want to touch anything now as the car is now running like brand new again. Even the idle seems more stable (no hunting).
 
#16 Ā·
Ok, still working on this! I AM going to clean all the plugs and stuff! However, it's getting worse and worse with time! It's pretty bad now right from startup.

Any chance my car has an ignition coil? Because I just had similar issues on my BMW motorcycle and had to replace the ignition coils.

It ran a bit rough at idle.. However, with my BMW bike AND my BMW 535xi.. it was more under load that there would be an issue and this is idle and right off the line.

Sorry.. still trying to make sense of this..

Also.. COULD it be possible that I have a bad fuel pump? How could I check that?
 
#17 Ā·
I doubt it would be your fuel pump but it is difficult to rule out. Generally, the pump(s) go slowly and you will have signs like car not starting, or just coasting on the road at 30MPH it will cough out of the blue when not even accelerating, etc.

Also your earlier symptoms appear to be that when the engine was cold it was fine (this is when the most fuel is used as the mixture is artificially rich) and it started stumbling after the engine warmed up when the fuel usage is lower and the closed system kicked in.

But if you are handy with electronic multimeters you can measure the pump current when the car is running as the easiest check. Do you have a single pump or two under the car near the tank? Bad pumps either draw too much current or not enough.

Other proper way is to measure fuel pressure but that can get messy if you know what I mean.
 
#20 Ā·
Cleaned anything with "plugs" today. All the sensors, etc.

Still nothing has changed... Ugh! So frustrated.. I feel like just taking it to the junk yard.

It's just in such great shape everywhere else.

It's also got the hard start or have to start it twice thing again now that it's warmer. I just can't understand. I've done almost every fuel component. Other than pumps.. Can an engine temperature sensor cause anything like this?

I would just think that the pumps if they weren't working would also just cut out and the engine would die... Unless perhaps 1 is just bad and it's just not quite getting enough fuel.

Could an exhaust blockage cause something like this? I mean.. the exhaust seems to be working fine. Exhaust comes out the pipes.. ha ha

Here is a youtube video.. showing the gauges and sound a bit.

 
#21 Ā·
When you have temp related idle issues it may have something to do with temp sensors or voltage resistance. Check which parts that measure temperature and provide voltage are old and replace the cheapest ones until problem is solved.


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#22 Ā·
You have some serious problem with the oil pressure gauge. I would fix that first. Most likely your pressure sensor though. That is next to your oil filter.
If it is really that low, you've got other issues.

So at idle, let's open that oil filler cap and look inside with a flash light. Make sure there is a nice flow of oil flowing into your number one valve cam lobes.

You engine does not sound good either. I thought I heard some high pitch grinding, it could be warn bearings in pulleys up front or maybe something else.

Unfortunately, I did not spot any sign of idle stumble in that video. That slight roaming is somewhat common. I got the impression you had some major stumble. 

I do not know how to test the coil. Never had a bad coil in any of my cars (fortunately).

As for your fuel pump, seriously, the best way is to measure the current thru each one. I believe they are supposed to be 6A-10A each I believe. But do not quote me on that. I know someone reported 4A on one and 10A on the other and the one with 4A was busted. After replacing just that one the car was good as new. Common wisdom is to repair them in pairs.
 
#23 Ā·
Ok.. opened up the cover at idle and there is oil flowing! YAY!

In terms of the "grinding" noise.. must have just been the camera because there is nothing much that I hear like that going on.

Yeah... hard to hear but it IS stumbling and rough idle!

I'll do the test on the pumps next!

Anyone heard of the Henko or other aftermarket brand fuel pumps being any good? $60 for a pair!! ha ha I could buy 4 for the price of 1 Bosch! ha ha

Fuel Pump SET 2 FITS Mercedes Benz 190E 260E 300E C220 500SEL 600SL E320 CL500 | eBay
 
#24 Ā·
Your idle roughness look more like ignition related not fuel related. Your RPM is too low as well.

I would get that oil pressure sender/gauge fixed ASAP. There is a good reason Mercedes put that in your cluster.
It should read ~1.5 when oil is hot and maybe higher when cold. It is a bit oil viscosity related as well but at no time should it red near zero when the engine is idling. It is good you have a decent flow at the valve train.

If I were you I would make sure that you are not running on 5 cylinders to be honest or maybe some other ignition problem.

You have already replaced just about everything fuel related, just make sure your pumps are drawing the right amount of current.
I doubt it is your pumps though.
 
#25 Ā·
I'd check;

1)Plug wires are snapped back on fully on spark plugs.

2)Run the engine in a complete dark area and look to see if plug wires at spark plugs are arcing to head.

3)Test the resistance of coolant temperature sensor on back of head in an x pattern. This tells the EZL and Jetronic how to 'react'.

When you changed the cap, was the suppression disc in good shape and was there any pooling of oil at the bottom? If you aren't careful, you can break the center sprung pin off inside of distributor cap when installing.

Sorry if I missed it, but are the plug wires stiff at all? Couldn't hurt to replace if you haven't already.
 
#26 Ā·
Yeah... I wonder if it's ignition related too! I did not see any oil when I changed the rotor, cap. It was clean. I am quite sure no damage was done as I was careful as I could see how the tip could break if not careful.

It didn't run any different after I put it on anyway..

I HAVE not done the spark plug wires... I should have I guess a long time ago. They SEEMED fine. However, I guess they are a bit cracked so probably a sign they need changed? Also.. I was pulling one off the other day and with my vice grips tore a bit of the rubber on the plug on the 6th cylinder and it was arcing to the head. I had to electrical tape it up but it's running a bit rougher. I have plug wires on the way! We'll do that and see and I did do a test of the Coolant Temp Sensor last year and from what I read it seemed to be right. But I will do it again just to make sure!

I will also change that oil pressure sender! If it IS ok and it still does what it's doing... what does that mean? New oil pump?

Thanks for all your help everyone! REALLY appreciate it!
 
#33 Ā·
In full agreement with sbaert. Having just replaced my Camshaft/rockers/etc, I would not want to repeat that job again. You will never know if that oil stops flowing with that pressure sender not working. It should be a very easy and cheap fix now and would avoid junking the car.
 
#36 Ā·
HI new here... I used Seafoam for a rough idle.. and the oil pressure gauge which was intermittent before now works perfectly now,,,, lol only I get black smoke when accelerating.. any ideas???

and the vacuum hose from the hose that starts on the hose between the air cleaner and valve cover is missing and I can't find where the other end goes... any idea where it goes... 1991 300e base///

thanks
 
#37 Ā·
That vacuum line goes to the back of the fuel pressure regulator... its just a safety thing, if the fuel pressure regulator diaphragm ruptures, this vacuum line would divert the fuel that would otherwise spray on the engine into the intake where it can be burned off.

Black smoke is generally an indication the engine is running way too rich, getting too much fuel. I'd inspect the spark plugs to confirm all 4 are fouled with black deposits more or less evenly, then it would be time to do a thorough ignition and fuel injection diagnosis.

I've never had injector cleaners like Seafoam do any good, rough idle problems typically are caused by air leaks in the intake path, or gummed up idle air control valves, or similar. or worn out ignition systems.... when was the distibutor cap and rotor last replaced? does the engine have the correct spark plugs (use only non-resistor 'copper' (not platinum or iridium or whatever) spark plugs of the correct type. The original /correct/ non-resistor Bosch plugs for the M103 engines are hard to find, there's an NGK equivalent I've found to work quite well.
 
#38 Ā·
Greetings, found this chat today, I have an 88 300E I bought 2 months ago, same issue as yours....the car is solid other than when it stumbles/stalls upon acceleration from complete stops after driving it for a few miles...the fuel pump sys was replaced 4 yrs ago from the previous owner. Do you still own your 91?
 
#39 Ā·
your symptoms match what my 1990 did when I got it.... after a year of tinkering and testing and trying to fix stuff, the actual problem turned out dried out hardened injector seals allowing air to leak in around the injectors. 3 of the injectors were pretty bad in a 'pop test' (this is something a diesel injector shop can do if you bring them the injectors, or a old mercedes specialist who has the gear because they work on lots of CIS cars).

there's a pretty siimple 'smoke test' where with the engine running, you flood various areas of the intake path with a gas like CO2 or propane. if the idle changes when you spray this not-air gas on a specific spot, you have an air leak. obviously, be careful with propane around the running engine.
 
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