560sl runs weird after hot start

[bhatt]

Hello all,

My wife's 560SL starts fine after it has been left to sit for a few hours (or overnight) but does require full throttle while turning the key to start it reliably. If you don't give it full throttle, it will start to catch, then die. Not sure if this is normal or not..

The big problem -- when the car is "warm started", it can start to do something weird. While sitting at a traffic light, it will begin to "miss" like a spark plug wire is disconnected. Then when you accelerate, it runs like there is almost no power and it wants to stall. If you give it very light throttle, it will move a bit, but the more you push the gas pedal, the more it chokes and loses all power.

I've found that if you keep the gas pedal floored, it will eventually start running fine (10-15 seconds later or so) and will then act like nothing was ever wrong.

My initial guess is that the car is running really rich when started warm, and that is causing it to stumble and choke.

I've recently replaced the spark plugs, air filter, and fuel filter if that helps. Ignition wires didn't have any visible cracks or broken ends. Car runs fine 99% of the time with full power, except for this problem.

Help! It's tough to enjoy this car in traffic when you know it's gonna die!

Thanks..
Neal

[joe p]

O2 sensors will cause these to run wierd. There are also potential issues with sticking a/f meter disc's. Pop the air cleaner off and see if the big disc is dirty, if so, clean it up with some carb cleaner, start the car and clear it out. (the carb cleaner will load it up badly.)

If the problem persists, check the o2 when its in the mood to mess up. You might find it readin full lean (.2v or so) I've taken and unplugged them while the car was acting up and had the car clear out instantly and run ok. From there the list gets progressivly longer and more complex. 1 car that comes to mind when you talk about what yours is doing was a 85 380 that had a warm up regulator that would flip out on hot soak and runs like azz for 5 -10 minites then clear out and runs fine.




Joe

bhatt - just had the same thing last year on my 88 560sel - it's your fuel accumulator mounted by the fuel pumps in the back of the car - change it. For such a little part it sure is expensive - paid $287 CDN for mine but it cured the problem. If you do a search on fuel accumulator on this forum you will get much info to assure you that that is the problem. Please read the following thread in this tech help section " Hard starting on 1989 420 SEL " - you will understand
Morris

[jhodg5ck]

It also sounds like your fuel injectors are on their way based on the start issues. How many miles are on this car? Once the fuel injectors get old they don't hold the 50 lbs of pressure they should to give you near instantaneous starting, instead they leak the fuel down into the cylinder giving you an Exteremly rich situation under start up. Depressing the accelerator introduces a enough air to balence the situation out and poof..you start.

MB rates their injectors with a life of about 100K.

Hope that Helps!
Jonathan

[RandyA]

This does not sound at all like an accumulator problem. That only serves up high pressure fuel on cold start and is passive once running, unless the fuel pickup sucks air for a moment.

I recently changed plugs on my 560sl, and it ran fine for a few hours, then it started missing. Mine got so bad that the exhaust melted the hangers.

Even though I was very careful removing the plug wires and they showed no visible sign of problem, I discovered that 4 of the wires had failed. Two were delivering very little juice and two were total failures.

Either pull all the plugs and reconnect to wires or use the old ones and crank the engine while looking for spark at all the plugs, or check each wire from end to end with a multimeter.

I still believe that the platimun plugs that I installed on the old wires caused enough impedence change that it contributed to the failure.

The engine likes the big fat spark of the copper plugs much better.

Check the wires before looking for something more complex.

K.I.S.S. (hasn't failed me yet)

[cbdo]

FWIW, a 560 running correctly will have a pretty smooth (though not quite watch-like) idle, will not stumble or snort under any condition, and will start immediately every time. (OK, it might have to crank a second or so once in a great while.) Anything else means there's an Opportunity For Improvement.

[bhatt]

Guys,

Thought I'd follow up with an update for those who were wondering how this turned out.

Dropped it off at my usual MB mechanic and he figured out that it was the Air Flow Meter (the large unit under the fuel distributor). This unit has a large plate that is moved by the air. On the side of the unit is a "rotary" type potentiometer. Resistance is varied based on how much air is moving (and therefore pushing on the plate).

Mine was apparently flaky. When the throttle was pushed lightly, the unit showed no resistance whatsoever (open circuit). It was causing the computer to "guess" how much fuel to use.

An apparent secondary problem was the O2 sensor. My tech says this type of problem can certainly kill an O2 sensor but it could have been something else also (ie. age, running rich in the past, etc.)

Since it's been changed, I've started it cold once and hot 3 times. I *never* needed to push on the gas pedal while starting (much less pushing it to the floor as I did before!) even when starting hot. The hot starts catch immediately (< 0.5 seconds) and the car doesn't stumble and runs perfectly. I'd say there is approximately 10% more engine power in the low end (if not more) than before.

Apparently this part is $1300 new and I was lucky that he found me a used one for $300.

Neal