I have a 1996 SL500 that has a random misfire on cylinders 5,6,7, and 8. I have done the manifold lower gaskets and the donut gaskets, all injector seals, replaced the MAF, ATS, and all plugs. I was able to see the random misfire on my scan tool. When the car is first started cold, the idle is fine. After it runs for a few minutes it starts to stumble at idle. When I bring the idle above about 1500 the random misfire goes away. I can visually see this on the scan tool. When I let the car come back to idle the stumble returns. The stumble appears to be getting worse with time but has yet to make the car stall.

Open to any suggestions. Thanks.

Does this car have 2 distributor caps? If so swap them left to right then see if the problem follows. Then swap rotor , coil wire, coil.

Baring that, the cam timing unit on that side might be having a problem but I can't say for sure how it would act.

Thanks for the reply. The car is coil on plug so no distributor caps. I understand what you are saying though. I've tried switching coils, plugs, and injectors around but still have the same result.

Is the engine numbered 1 2 3 4 on one side and 5 6 7 8 on the other?

Does this engine have 2 throttle bodies / air flow sensors?

So the misfire is on the drivers side bank. Any more info? fuel trim long term and short term. Ever do valve cover gaskets? if yes are the grounds nice and clean and tight on the valve covers? Are the valve covers tight? I assume your using F8DC4 spark plugs and NOT Bosch Platinum's? I assume you have swapped the coils over from left to right banks? I doubt there is a timing issue as that usually shows up on the right bank (if the timing chain is stretched it throws the right bank off more)
Find out the fuel trim on the bank 1 & 2 and let us know. That would help diagnose it.

By the way, stop throwing parts at it until you diagnose it :-)

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The engine is numbered 1234 on the right (pass) and 5678 on the left (driver)side. I did not do the valve cover gaskets. The only part that I "threw" at the car was the MAF and I only did that because I took the TB out cleaned it. I did the intake Gaskets because I found a small leak at the donut gasket in the runner to #7. I have switched coils left to right and there was no difference. I used the F8DC4 plugs. I was able to get into the ECM with a Solus Ultra with the latest software. The o2 voltage range before the cats was 86-826 on the left and 126-626 on the right. The o2 (Lambda) control before three way cat converter was -2.1--1.3 on the left and 0.2-0.3 on the right. I realize this might be all greek because its coming off of a Snap On tool and not from a Mercedes scan tool. I'm not well versed on (Lambda) control. I found the differences in the lambda voltage readings interesting. Right now I'm leaning towards doing a pressure test in each side of the exhaust system as I am suspecting a restricted cat.

I really appreciate all your help guys.

Some things to check

Fuel pressure regulators leak on these and M104 engines, could be a rich misfire as the vacuum source is the left bank (5-8)

Do a compression test?

What are the actual misfire counts? The HFM-SFI stops monitoring misfire counts once the throttle is moved from the idle position. Then you need to read "smooth running" values.

I would say the cat could be an issue as the W210's with the M119's had a similar issue, but they also had low power when hot. I assume powers good? Easy enough to loosen the 4 bolts to see if there is a difference.

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I did a compression test early on and all cylinders were within 10 pounds of each other. The misfire counts on 5678 were 0-.05. On 1234 they were 0. The best way I could see them was when I was graphing. 1234 were steady and did not waver at all. Flat straight line all the way. 5678 were all over until I brought the rpm's up then they smoothed out too. When I first changed the plugs No. 8 was sooty black and the others were clean. I put the new plugs in and ran the engine for about 10 minutes. I pulled the plugs and No. 8 was covered in soot again. I mention this because the FP regulator sits right next to 8.

I will Check the fuel pressure regulator too.

Thanks very much for your help Ben.

Pulled the vacuum line off of the fuel pressure regulator, started the car and waited a couple minutes. Suddenly fuel came pouring out of the regulator nipple. Coincidentally the fuel pressure regulator picks up its vacuum from intake runner No. 8 which was why I kept getting the black plug. I have yet to replace the regulator and see How it idles.

Good to hear, easy to change

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I had a bad stumble on my 96 SL500 R129 with no cel. I cheaped out and did not buy the Bosch MAF ,replaced it and all is well! dont ever buy the cheap $90.00 MAF's on ebay...

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

A note Just to close this thread out. Changed the Fuel Pressure Regulator last weekend and now the idle is nice and smooth. Thanks for everyone's help. Now on to the climate control! Lol