[tongzilla]

Guys, I have a 98' S420 and the idle on my car is rough as he**. I took it to the mechanic I always use and he replaced:
1. Spark plugs (he said they were not the right kind)
2. Fuel injector
3. 4 Coils (due to overheated catalytic converter)
4. Catalytic Converter (passenger side) this side was reading 100F higher than the other side.

He then told me that the problem is now probably due to Cylinder 4 (one closest to passenger catalytic converter) that was overheated. He said that it would cost THOUSANDS to repair the engine block.

I then decided not to go with it.

I already spent over $2,000 and idle is still rough. I decided to take it to Mercedes Dealership and the dealership hoping they would be able to fix it. They said that spark plugs were not the right kind (AGAIN!?!) and had to change a few coils. My bill at the dealer is already over $900. It still does not fix the problem. The next procedure is to pull the head cover to see what needed to be done (another $180 to pull head cover). Service consultant think that it might need the camshaft work on (would be around $1,500).

I love the car but it is starting to cost me a fortune -- what should I do? What do you guys think the problem is coming from? Anybody has had the same problem?? Please HELP!

 

[Brett San Diego]

Wow, all that work, and nobody thought to do a compression test and leakdown test? What kind of incompetents are working these shops?

But before that, I would start with the basics. Air/fuel mixture: Check for intake vacuum leaks, clean or replace the MAF sensor (air mass meter). When was the last time you replaced your air filter? Fuel pressure OK? Fuel pump OK? Fuel pressure regulator OK? Basically, check everything involved with fuel delivery.

spark: spark plugs, spark plug wires, ignition coils, camshaft position sensor (but usually this results in non-running),

General: check the condition of all the wiring in the engine compartment. The degrading wiring problem was supposedly fixed for model year 1996, but you never know.

Electronic throttle actuator: The electronic throttles can go south and need fixing or replacement. There are a couple of companies that rebuild them. I can't help you on diagnosing an electronically bad throttle actuator. But, simply cleaning a gunked up throttle body can work wonders. How dirty is yours?

If all the parts outside the engine check out OK, then it's time to go looking for a problem inside the engine, which means compression and leakdown test.

I almost forgot to ask. Any check engine lights/fault codes?

Brett

 

[tongzilla]

I forgot to mention they did a compression test and found that the cylinder #4 is not running the same as the rest. I already had the oil services and fuel filter replaced along with the air filter. I don't think they check the wiring harness though. Also there is no light on. Check engine light is not on. However, once a great while, the ASR/Brake light would come on. I would turn the engine off and then turn it back on and the lights would go away.
I think I'm going to mention about the wiring harness and the elec actuator. What else should I have them looked at?

 

[vee12]

Sounds a lot like the ETA (electronic throttle actuator). They can cause the idle to be rough, even though the car will run fine under full or steady throttle.

 

[deanyel]

I forgot to mention they did a compression test and found that the cylinder #4 is not running the same as the rest.

What does this mean? What are the numbers? Depending on the variation in the numbers this could be very important, or not. By the way, this is not the right car for you. Only if you were dying to do the work yourself could this be the right car for you.

 

[Brett San Diego]

I forgot to mention they did a compression test and found that the cylinder #4 is not running the same as the rest.

I agree with deanyel. This statement is ambiguous without seeing the actual compression test results. If a cylinder is found to have poor compression, and you intend to actually try to fix it, a leakdown test should be performed to help pinpoint the problem area (intake valves, exhaust valves, head gasket, or piston rings).

I would not authorize any work until you find a competent mechanic that will perform a leakdown test and give you a true idea of what is wrong with cylinder 4. If compression and leakdown test are both poor, then you have found your major problem and can stop searching for now... until you get it fixed. Then if it's still running rough, you can start searching again.

Brett

 

[tongzilla]

Check

Ok I just talked to the mechanic at Elite Mercedes (Springfield, MO).They ran the compression test, leak down test, and also checked the wiring harness. The result on cylinder number 4 is about 80%. My service consultant said that this result is as good as a new car. Yesterday, they also pulled the valve cover and check camshaft and valve, eveything is fine. They could not find anything wrong upto this point. They are very determined to find out what causes it. They have NOT checked the electronic throttle actuator. I told them that you guys on benzworld.oeg suggested this to be done. They will do that today. I will let you guys know as soon as I find out on the ETA. If that is the case, who should I contact to get it rebuilt. I rather not spend the money on a new ETA through MB Dealership. Thanx!

 

[Brett San Diego]

Again, I'm not entirely sure what you mean by 80%. If you mean 20% leakdown on cylinder 4, that is a very bad number. Leakdown on a good cylinder is around 2-3%.

Brett

 

[starscream]

check if the throttle house is sooty

 

[deanyel]

To be "as good as a new car" it must be 80 percent of the compression of the other cylinders, which isn't a very good new car but not likely the source of the problem either. The ETA does deserve a good cleaning before you bail out on it. There's at least a couple of good rebuilders:

Welcome to Beckmann Technologies!

Mercedes Benz Accessories

 

[fabbrisd1]

You have a good independent about 1 1/2hrs south

I highly recommend Shuler Autohaus in Fayetteville AR, about 1 1/2hr south of you straight down 540.

(479) 521-8383, Roger is the owner.

If it was me, I would call and schedule to go down for a diagnosis, with all your paperwork in hand...

Tell Roger Terry Fabbri referred you.

It's not uncommon to get the run around, even from the dealers, in South Missouri and Northwest Arkansas... no experienced mechanics is the real problem.

Roger's shop has some long term clients, and I mean long term, that drive from over 250 miles away.

With what you have had done already, and getting your car into one of their bays, I am sure Roger's people will give you the straight poop...

Roger himself has over 20yrs MB experience - and experience, and not more shop hrs, is what you need !

 

[mbnz230]

I already spent over $2,000 and idle is still rough. I decided to take it to Mercedes Dealership and the dealership hoping they would be able to fix it. They said that spark plugs were not the right kind (AGAIN!?!) and had to change a few coils. My bill at the dealer is already over $900.

i've been through this once before. we took it to an independent and they replace the spark plugs and the problem is still there, then we took it to the mercedes dealership and the told us its the wrong spark plugs, we took the bill as proof and brought it to the independent shop and got our money back, he works on mostly european cars, he didnt want us to ruin his rep. but even the dealer replace the plugs the problem is still there, well the problem comes and goes, my best bet its the distributor but we never got to replacing that before we traded in for another car. so take the bill from the dealer ship and bring it to the inddpendent guy to get a refund...

 

[NZ Benz]

i've been through this once before. we took it to an independent and they replace the spark plugs and the problem is still there, then we took it to the mercedes dealership and the told us its the wrong spark plugs, we took the bill as proof and brought it to the independent shop and got our money back, he works on mostly european cars, he didnt want us to ruin his rep. but even the dealer replace the plugs the problem is still there, well the problem comes and goes, my best bet its the distributor but we never got to replacing that before we traded in for another car. so take the bill from the dealer ship and bring it to the inddpendent guy to get a refund...

I don't think the 1998 has a cap and rotor.... But if it does thats a good place to start, I changed the cap, rotor and plugs on my car and man what a difference.

 

[Brett San Diego]

I don't think the 1998 has a cap and rotor.... But if it does thats a good place to start, I changed the cap, rotor and plugs on my car and man what a difference.

Nope, no cap and rotors. Ignition is fully electronic on the W140 from 1996 and on.

Brett

 

[tongzilla]

Stopped by at Elite Mercedes this morning and talked to my service consultant. He explained that parts he put in and did not help with rough idle will be taken off and put my old parts back on. He checked the ETA and it seems to be fine. Now what he wanted to do is to replace the timing chain!! This car is 89K miles and do you think it needs timing chain job already?? It is going to cost me $1200 on this timing chain job. Should I go on and do it?

I called Roger (referred by Fabbrisd1) and I made an appointment with him for Monday next week. I thought between now and then I would give Mercedes-Benz dealer a try. The dealer is giving me a break on parts and also labor. Right now my bill is about $400 which is not too bad considering the works they have done to the car. When I talked to the Service Manager, I asked him if he had seen any other car like this, he said he has but not this engine. I guess they are using my S420 as a test. Well as long as they continue to be reasonable with me on repair bills, I would let them work on it until this Friday, then I will take it to Roger. I will keep you guys posted.

 

[Brett San Diego]

Stopped by at Elite Mercedes this morning and talked to my service consultant. He explained that parts he put in and did not help with rough idle will be taken off and put my old parts back on. He checked the ETA and it seems to be fine. Now what he wanted to do is to replace the timing chain!! This car is 89K miles and do you think it needs timing chain job already?? It is going to cost me $1200 on this timing chain job. Should I go on and do it?

I called Roger (referred by Fabbrisd1) and I made an appointment with him for Monday next week. I thought between now and then I would give Mercedes-Benz dealer a try. The dealer is giving me a break on parts and also labor. Right now my bill is about $400 which is not too bad considering the works they have done to the car. When I talked to the Service Manager, I asked him if he had seen any other car like this, he said he has but not this engine. I guess they are using my S420 as a test. Well as long as they continue to be reasonable with me on repair bills, I would let them work on it until this Friday, then I will take it to Roger. I will keep you guys posted.

Please, keep us informed. The solution to this one could be very informative, especially if it turns out to be something simple.

Brett

 

[scotta]

My '95 has had the same problems. I replaced the harness (no problem for you) the car ran smooth for a 1000mi or so rough idle came back....

Replaced the plugs....same result 500mi or so......rough idle returned.....
(cleaned caps and rotors as well)

Problem persisted as I made a list of replacement items for a major tune up.(same mi as you on mine)

I had an oil change 4/2 and had the fuel filter put on that I was going to do when I was going to do the above....

The car runs great now!!!! Still a stumble ever so often, but most of this is most likely related to the caps/rotors/plug wires. In a non electronic car this can be expected regardless.

I was thinking the worse, MAF, O2 sensor, ETA etc.. I did not see any mention of you replacing your fuel filter?.

My idle was as rough to the point of thinking it would stall at stop lights etc...

If the car is driven sparringly, as mine is, they collect condensate in the tank as they sit. I do not believe the fuel filter will separate water. If this is the case I can see how the service interval would be greatly reduced.

Good luck

 

[tongzilla]

Got It Back - I'm So Happy!!

Guys, Elite Mercedes called me yesterday and told me they were going to look into timing chain. I was where I was had no other option but to do it. They were strongly believed that timing chain was what causing the problem. THEY WERE RIGHT. They found out that engine was running out of time due to timing chain stretch. Replaced timing chain and chain tensioners. Now my car run as smooth as silk. Best of all, they did an extensive checking including but not limited to some of the things I mentioned that was done to the car. They did not charge me for it. AND most parts they replaced and did not help, they replaced it back with my old parts. My total bill from Elite Mercedes was $1,350 and they FIXED the problem.

*Who said MB dealers were trying to screw you up* ;-)

While my S420 was in the shop, I was driving anything from a Camry, Caddy DTS, Audi TT, and E430 - nothing is comparable to my S420. LOVE MY 140!!

 

[tongzilla]

BTW, I've never imagined in my wildest dream that a timing chain job would be needed on a car with less than 100K miles. Who knows what could cause this??

 

[Brett San Diego]

That's good news. Thanks for keeping us informed.

Brett