[ensignthomas]

I am totally new to Mercedes-Benz's. But, I just bought an 88 560SEL. . .very, very clean inside/out. . .with 99k on it for $5200. A/c great, all electrical stuff works, never been wrecked, etc. The car will start and idle usually just fine. . .no weird noises or anything else out of the ordinary. However, under acceleration, it will fall flat on its face and stall. I can get maybe up to 30mph before it "misses". It will stall. I put it in neutral and it will start up just fine. . .then the same thing happens again. If this was a car that I was familiar with, I would say either clogged fuel filter and/or bad fuel pump as I have run into this with other cars I've owned. I got a CarFax report on it and the report says that at times over the last 17 years it just sat. . .the use of the vehicle by the prior owner did not seem to be ordinary or daily. Could it sitting around have caused some rusting to occur in the tank, and the rust has clogged the fuel filter? Or, could it be something more complex? It is not the transmission because there are times when it will have a hard time starting after one of these stalling episodes. And, the missing/stalling seems to also occur at low speeds rounding turns. All opinions respected and appreciated.

 

[Photoman]

another thing to check is the injectors. they wear out. i'm getting mine replaced tomorrow as my SEC is missing at low rpm and surges under acceleration

 

[vanir]

Yeah, short of a loose distributor retaining bolt, sounds like a fuel delivery problem to me.
Could mean anything from clogged injectors to a vacuum leak (which matches the description). Has the idle gone up a little in neutral?

Check all your vacuum hoses and fittings, run injector cleaner in the tank, replace your fuel filter, get someone to check your fuel pumps (there's two in-line I heard someone say). Get your injectors professionally cleaned if need be.

All the usual. When you're sure all that's running fine then start at the electricals (anything from burned out plug leads to a faulty timing curve).

Hope it helps.

 

[jdc1244]

Does this happen at high speed or low speed or both? As you’re loosing power and slowing down (although you’ve got the pedal to the floor) does it jerk and surge like the brake pedal is being pumped hard before dying? How often does it happen? In other words, can you go several miles before it happens again or right after you restart and go? Is the fuel pump loud?

It could be a clogged filter but that tends to be consistent, to the point where the engine will not start. The same with a bad fuel pump – its failure will be more consistent. A failing fuel pump relay may result in the good/bad performance.

Change the filter, it more than likely has not been done in a while.
If that does not work then check the pump relay – the pump is activated directly with current bypassing the relay (it’s a pump test as well); the procedure is here, do a search.

My guess is a bad pump or relay.

 

[TheRazz]

Check the Cat also, that's what it sounds like to me. A clogged cat causes that same problem. If your in a area the requires you to have one. Replace it, if your in an area like me, SMASH IT TO DEATH with a REALY big HAMMER, an let the exaust blow out the chunks, an never let it bother you again. Best way to check if it's the cat. get your self a baking thormometer ( a good high temp one ) and tuch the exaust pipe. If it hotter before the cat, it's bad. If it's hotter after the cat, it's good. A clogged catalytic converter can cause bogging on exceleration. It can also cause an engine fire, I've seen it before...

 

[TheRazz]

Duhh, could be clogged fuel filter also. That happened to me as well, forgot. SOmtimes it's the simple things that realy make us think the hardest...

 

[vanir]

under acceleration, it will fall flat on its face and stall.

Either the timing curve is stuffed (could be caused by deteriorated plug leads, but also a cracked distributor gear, clogged advance mechanism, carbonised or oiled up spark plugs, etc.),
Or there's a fuel delivery problem under pressurization (bad pump, clogged fuel filter, a pinhole in the alloy intake manifolding somewhere, a tear in a vacuum hose, or a vacuum line has simply come off somewhere, which happens, clogged or worn fuel injectors, a deteriorated injector mechanism). K-jetronic injection is vacuum operated so that gives vacuum problems a huge likelihood.

I can get maybe up to 30mph before it "misses". It will stall. I put it in neutral and it will start up just fine. . .then the same thing happens again.

That makes it sound like a vacuum leak to me. But could still be mechanical fuel delivery failing to operate at higher pressures.

 

[danwatt]

Does it cut out if you rev it in neutral?

 

[ensignthomas]

UPDATE: I now have the car at a supposedly reliable Mercedes-Jaguar-BMW garage here in Las Vegas. It came recommended to me from a friend of mine. So far, they replaced an O2 sensor that was supposedly bad. . .the Check Engine light had been on and they said replacing it would fix it. Also, they checked out the fuel distribution system and some other things under the hood. I asked them if they thought it was a fuel filter or fuel pump problem. They said no, saying that the filter had been checked and the fuel pumps had given regular voltage readings. So, they concentrated on the above mentioned things. Now, today. . .Wednesday. . .they tell me that the other things they fixed did not change the cars stalling/missing. They are now looking at the fuel pumps and telling me that the pumps appear to be "used". . .as if somebody else had replaced the originals with used parts, saying they look as if they had been "painted." Who knows. . .but has anyone ever heard of this? As a side note, on Monday night before I dropped off the car at the garage, I noticed that at the back of the car a very audible whirring noise that I had not heard before. How loud are fuel pumps on this car supposed to be? I'm beginning to think this is something really simple that these guys are missing. . .like a vacuum line or a corroded ground wire dealing with the pumps or something in the fuel system. Also, the car revs fine in neutral. . .only when it is pulling a load does it miss. What's the life for a fuel pump on this car? The car also has rotted motor mounts that need fixed. . .this car must have sat quite a bit.

 

[jdc1244]

Missing, surging, or loss of power at high speed or at load is a typical failing pump symptom – I wish my pump would ‘misbehave’ so I can justify replacing it. They should not be so loud that you notice – you can hear it but nothing abnormal as with my 300 SE. (I should record the sound…). Bosch does not recommend using remanufactured pumps or pump components…

…and you should only run a Bosch pump, imho…

 

[ensignthomas]

FINALE: It was BOTH pumps. The tech showed me the pumps and one was stock for the 560SEL. The other. . .was something off of a Mercedes but not made for the 560SEL. . .looks at though somebody tried to do a cheap fix and it didn't work. I knew what I was getting into before I bought the car. Now that the car is running right, it is blowing just a wee bit of smoke. Obviously, it is the valve stems leaking. But, it passed emissions. So, looks like I'll start saving for a valve and timing chain job. . .big bucks, I know. . .but with only 100k on the car and it looks like new, I'm still ahead of new car buyers. Thanks everyone.