Occasional Severe Power Loss SL500

[gatvol]

My 94 SL500 is below 30k miles and is just a weekend runabout. In the past two weeks I have experienced two sudden losses of power having driven for about 4 miles from a cold start.

Upon starting everything is 100% and there is no indication that there is anything wrong. But then suddenly it feels like it is running on about three cylinders and has virtually no power, the engine lurches (like its about to backfire) until you slowly accelerate to 60 mph then it is reasonably normal again.

The second time this happened I just carried on driving and after about 15 miles the car reverted to being 100% normal again. I stopped and started it from warm and all was well on the 20 mile ride home

It feels to me as though it may be starved of petrol, or choked. The tickover is very lumpy when this power outage occurs. Stopping it and restarting the car does not clear the fault. The car just seems to need time !

This may be entirely unconnected, but a merc garage replaced the power steering pump recently as it had a leak. I can't think that this is connected to my sudden engine fault, but it seems remarkably coincidental

My guess is a fault with the choke or fault with an air intake sensor. Perhaps a fault in the distributor caps ?? but before dismantling and incurring big bills I was after any ideas that more experienced owners may have

Thanks for any help you can give

 

[jeff500sl]

I have exactly that problem after not using the car regularly, my own conclusion is that it is moisture in the Distributor cap causing the problem as otherwise the car starts and runs well.

In my case it is after the car hasn't been run for two/three weeks it starts and runs fine and then after about 2/3 miles misses like mad. My solution so far has been to drop to 2nd gear and do an italian tune up seems to go after another couple of miles and never comes back unless I leave the car not run for a period.

On the basis that at all other times it never misses a beat I am sure enough that it is not anything serious hope yours is the same issue.

 

[Doodle]

+1

First port of call with any poor running on the 119 is distributor caps. They need checking periodically anyway as they get a hard life on this motor.

 

[gatvol]

Dolphin and Jeff. Thank you so much for your posts. I replaced the caps and rotors when i bought the car 1 year ago as I had heard they gave trouble. Regardless both sets were corroded even though the car has only done about 2,000 miles on them.

Scraping this corrosion away has cured the problem, and there have been no more issues. Very quick job and massively more convenient and cheaper than taking it to a garage for repair

 

[Herm]

Hello to Everyone,
The problem described by Gatvol and Jeff500sl in September is identical to the issue with my 1993 500SL (40,000 mi). I have owned the car for 5 years with this problem. As long as I use the car on a reqular basis all is fine, but if I let it sit for a week +, it will start fine, but within 10 min I start losing power. At that point I look for a high speed road and accelerate to 70+ mph and within 5 mi the problem goes away. At this point I am a little tired of the issue and would like to address the issue without fail. Have you solved your problem? Other threads suggest that the problem may be the OVP relay, the MAF or the EGR. Is any one of these the culprit given the current perspective on anyone?
I look forward to your assistance.
Herm

 

[grober]

Try drying out the distributor body-- not just the cap alone ---- with a hair drier. Its moisture deep down inside the distributor body that gets volatilised when the engine gets hot and rises to the HT cap and rotor arm section that does it.

 

[Herm]

Many thanks for your input and quick reply! I will try as you suggest.

 

[donubenz]

My W140 420 was the same and had a couple of caps on it. The issue was that I only used if for a couple of miles drive to work each day and the engine really never got hot enough to drive off the moisture from the crankcase. The distributor is sat in the cold and collects condensation like a cold window in a house.
The answer is to always let the motor warm up thoroughly, say a 40 mile trip once at week to drive off any moisture in the oil or crankcase. Or keep an eye on the caps!

 

[Herm]

Many thanks for the confirmation of the solution.

 

[bing]

This is a very common issue with the M119 engines and lot of owners are forever chasing this type of poor running. All what is described above is a common occurance and yes a lot do live with it thinking it's moisture and that it's normal after the car has sat for a period of time. It should not be the case!!

You have to rule out all the ignition components - plugs when were they last changed, HT leads are they OEM silicon and caps/rotors OEM ? One part of the distributor which is commonly neglected is the "ceramic seal" that sit under the rotor and rubber seal around it. Pricey but worth replacing these if they're still originals. This is what usually causes intermittent misfire.

How's your engine harsness condition? Has this been replaced?

 

[Herm]

To this point in time, MB has done all the work which includes the 30K tune-up. Although the car (1993) only has 40K miles, I am sure that age related deterioration, as opposed to use related, is a factor for many components. I will consider your suggestions as well. I realize that the harness will ultimately need replacement, but in this instances the loss of power problem has been with the car during my experience (5 yr and 28K-40K) which suggests that the harness has not yet been subjected to enough heat for breakdown. My search for information is strongly related to following a logical path based on your knowledge and that of the other responders and limiting the financial damage as I progress down the path to resolution.
Many thanks for your input.

 

[bing]

Herm, below link explains the part of the distributor that's commonly overlooked:

http://www.w124performance.com/docs/mb/M119/ignition_misfiring_STAR.pdf

Also below is the same subject that's often discussed on 500E forum. See post #7 parts diagram, part#17 and 14 is what I'm referring to.

Misfire/rough after sitting

 

[bing]

Herm, below link explains the part of the distributor that's commonly overlooked:

http://www.w124performance.com/docs/mb/M119/ignition_misfiring_STAR.pdf

Also below is the same subject that's often discussed on 500E forum. See post #7 parts diagram, part#17 and 14 is what I'm referring to.

Misfire/rough after sitting

 

[Herm]

Many thanks to all for the continued help.