560 SL Missing/Hesitation/Rough Idle

[JFawcett]

I know there are about 10,000 posts on this and I have reviewed many, but they don't seem to exactly address what is going on with my 87 560SL. I have about 100,000 miles. Within the last 5,000 miles - new wires, cap, rotor, EHA, plugs, chain tensioner, brakes, etc, etc. Now I am getting an intermittent hesitation and very rough idle. Car starts everytime. Idles very roughly - almost feels like it is going to stall - kind of embarrassing because the entire car shakes.....totally not the Mercedes-Benz way. But sometimes, it is just fine .
Based on the posts it sounds like it could be injectors, fuel distributor, fuel accumulator, fuel pump, fuel filter or a big vacuum leak. I would prefer not to throw parts at this problem....as we all know Mercedes isin't exactly giving them away! Any input on how to test to atleast narrow down would be very helpful. I am leaning towards injectors, as they seem to deterioate around 100,000 miles. If I step on the gas, engine revs and seems to be fine - which is why I am leaning away from fuel distributor/pump/filter. Any help is appreciated.

[RickM]

I've seen this happen before when Non-OEM (non shielded) ignition wires were used as a replacement.

If you have the originals and didn't experience these symptoms perhaps a swap will verify. Just a thought.....

[dpetryk]

You need a fuel distributor. Crack the fuel lines on the top of the distributor one at a time and compare the fuel leakage rate. If they are not the same the FD is shot.

[dugald]

I had to replace the fuel distributor on my 560. The idle was rough and, unlike yours, it couldn't rev past 3000 rpm. I wouldn't jump to the conclusion that yours is bad.....

[JFawcett]

If it is the fuel distributor, wouldn't the problem be consistent? Mine is intermittent.......
Any other ideas?
Thanks!

[dpetryk]

Well let put it this way. Long story short - The symptoms you describe exactly match ones I had including inconsistency.. I tried everything saving the FD for last. Well guess what - the FD fixed the problem. Nobody can say for sure what the problem is without proper diagnostic work. But given everything you said and the age of the vehicle and the trend for the FD things to begin failing at this mileage and age etc. My best guess is a FD. I have seen it a number of times.

Nobody wants to accept that the FD might be at fault. 3 of my 5 have had failed FD's and all were within 2 years of each other. Is there any significance to this? Only you can judge.

For years I had my 87 420sel would start right up but for 30 seconds or so it would run rough. I always told myself it was the injectors. Well is slowly got worse, and then the hesitation set in, then rough idle, poor acceleration, bad mileage etc. and so on. After checking and changing all sorts of stuff and no real improvement I replaced the FD and the problem was solved. It was like a new car.

When I read a post like yours I can related to how I went down the road of rejecting the idea the FD is bad mostly because the cost of these things only to find the problem wads the FD after all.

I try to express my opinion which is based on experience to arrive at a conclusion that in my opinion is the most likely culprit. I don't like to type so I tend not to go into lengthy explanations as to why I arrived at my conclusion. Just the conclusion itself. But then the questions come. Also, in many posts where I have made recommendations, the person asking for advise almost never reports back as to that the cure was. It is important for everyone to know what worked and what did not. SO if you would please report what the fix is when you get it repaired and running properly. It helps everybody here. To ask a question, get a dozen suggestions, and not report the outcome is not very helpful. Knowing the cure is.

dugald - what fixed your problem? - Sounds like a fuel delivery problem to me.

So anyway, do as many suggest, swap out stuff for EOM, non-OEM, resistor, non-resistor plugs. Do the "low cost" items first "because it does not cost much" -- ??? -- WTF. I hear all sort of logic as to how to approach finding the problems. Many are expensive if you add up all the time and "low cost items" replaced that really did not need to be changed. It is a waste of time and money in my opinion. I tend to diagnose the problem and be sure of defective parts before replacing things. I don't believe in the "lowest cost first - shotgun approach" method. I don't believe in the "lowest cost first shotgun approach" method. Unless you have unlimited funds.


Let us know what you find. Everybody will be grateful.

Jfawcett - you need to come by my shop sometime - 290 & BW8 area. I would love to see your car.

[RickM]

If the FD is faulty what is the usual route for repair? Replacement with new, rebuilt or have a specialist repair the existing unit?

[dpetryk]

About $500 for a rebuild. No need to get a factory unit. Several good rebuilders exist.

[RickM]

Originally Posted by dpetryk

About $500 for a rebuild. No need to get a factory unit. Several good rebuilders exist.

OK...I know that rebuilding/tweaking is popular with the Porsche 911SC crowd, especially when installing a turbo system and fuel delivery must be optimized.

[JFawcett]

Well, got in car today - actually had an appointment to get diagnosed and car was PERFECT!
No hesitation and no rough idle!
Is there anyway to test FD?

[ctaylor738]

Before I popped for a FD, I would check very carefully for vacuum leaks especially from cracks at the rubber connectors. I had a similar intermittent problem and it turned out to be a small crack in the rubber fitting that connects the little pipe on the back of the intake manifold to the cabin vacuum line.

So everybody has a story.

You might also have a problem with the electro/hydraulic actuator (EHA) not delivering enrichment on acceleration. This could be the EHA itself or an input to the control module.

[JFawcett]

I will check for leaks....is there a good way to check for vacuum leaks?
The wierd thing is that the car accelerates (even when acting up). If I step on the gas, engine revs up and away we go. Maybe there was something in the lines and it has moved through and out?
I have replaced the EHA - but that was about 6 months ago...

[rugby2]

I had a vacume problem, which when fixed solved it. any noise you can hear?