Help! Why is my car stalling?? - PeachParts Mercedes-Benz Forum

bhatt · #1 03-24-2007, 12:21 PM

Hello all,

I'm stuck.. broke my car and have no idea what I did. Any help would be MUCH appreciated.. especially before Monday when I take it to the mechanic!

1990 300SL. Replaced the Mass Airflow Potentiometer a year ago. Ran OK, but never got it adjusted properly. Mechanic told me to get a new airflow meter unit with a pot attached and just swap the whole unit.

I did that this weekend. Pulled the old one off and found the new one wasn't exactly the same so I couldn't use it. Put the old one back on and reconnected everything as before.

NOW THE CARS STALLS AT IDLE!

I couldn't get the car to start at first (it would catch but die before you could even hit the gas to keep it running). Found that the "boot" on the bottom of the airflow meter wasn't connected properly around the throttle "pipe" so it was leaking air into the intake (ie. bypassing the entire airflow meter!)

Reconnected everything so the pipe has no leaks. Now the car starts and runs, but only over about 1000RPM.

If you let it get below 1000RPM, it starts to shake. Below 800RPM is in danger of stalling, and it definately won't run much lower than 700RPM. Need to keep your foot lightly on the gas otherwise the car stalls.

Any ideas??

I found a vacuum line I had cracked and replaced it. The only parts I touched were:

-2 vacuum lines (both reconnected and don't appear to leak)
-large round thing on the front of the airflow plate.. seems to be some kind of fuel control valve
-EHA on the back - unscrewed it and reattached it. Connector is attached to it firmly.
-Air cleaner including the air temperature sensor connector. Connector is reconnected firmly, not that this should make much of a difference
-Separated the fuel distributor and reconnected it. All 3 screws are tight, and the O-ring is in place. It leaks no fuel, either from the lines or between the air flow meter and fuel distributor body itself.
-Some thing that looked like a PCV. It's a bit cracked and the hose has a small split, but it appears to be full of "oily air" and I don't think it's critical like a vacuum line. I think it is a PCV but could be wrong.

Everything looks like it did last night before I started. Nothing was damaged and there are no extra parts

Any ideas??? It sounds to me like a vacuum leak but I can't find one. I did try spraying the 2 lines I disconnected with carb cleaner, but it didn't make a "big" change. It might have affected the car by 100 or 200RPM but I can't tell since I had someone with their foot on the gas at the time (to keep the car running) and that may have been a variation with their foot.

Any help would be appreciated!!

zcc · #2 03-24-2007, 01:00 PM

Check this link, I think it will be helpful for you.
http://www.peachparts.com/shopforum/showthread.php?p=1458133#post1458133

stevebfl · #3 03-24-2007, 01:01 PM

Well if it has anything to do with what you did then you have a mixture problem of some sort. could be false air (vac leak), could be the airflap is binding or it could be just not adjusted properly.

Properly should be done electronically and is simple but has been explained ad finitum. To determine what you have and get close try having someone start the car and hold it at 1000rpms (where it runs). Place a finger on the airflap and gradually let up on the throttle. As the engine slows and approaches dieing press gently on the airflap. If it makes it run better the engine is lean for one reason or another. Let the throttle all the way off. add pressure to the plate till the engine runs strongest. If the engine speed is above normal idle when doing this then you have an air leak (uncontroilled air).

If pushing down the plate causes the engine to stall quicker then you are already too rich. If that is the case back off the mixture with the 3mm mixture screw in the tower. If the engine ran good pushing the flap and ran at normal idle speed turn the mixture screw in till the motor runs good without pushing the flap. This only gets you a crude set-up the final adjustment needs to be done electrically which is simple read my article in DIY or others to achieve the total adjustment.

bhatt · #4 03-24-2007, 03:57 PM

Tried to have someone run the car at 1000RPM and let off the throttle while I'm holding the air plate in position. Letting off the throttle and pressing lightly on the plate caused the engine to stall.

So by the logic posted above, the car is running rich.

Loosened the 3mm mixture screw 2 turns at a time (went up to 10 turns total) and tried each time in between. Thought it was getting a bit better, but that may be in my head. By the time I hit ten turns though, it seemed to be the same.

Would the mixture need to be adjusted based on anything I did? I thought this adjusted the quantities of air & fuel, and that shouldn't have changed.

So I think we've proven that the mixture is OK. I didn't adjust the air plate at all and it seems to be opening/closing fine. It doesn't bind at all.

Any other thoughts??

stevebfl · #5 03-24-2007, 04:47 PM

Oh *****! Now that you are absolutely screwed up, you should be definitely lean. I should have said that one turn is a massive, huge, correction. Five degrees of rotation might correct a stalling condition. 360 degrees is huge. Ten times 360 is the moon.

I imagine the car doesn't run period now. Now you need to get a long stick and from a distance push the plate way down. You need to push it down till it runs. If it don't you need to quit all this and find out why you have no fuel. The point of the long stick is that you could easily have a back fire with mixture way off. You don't want your face down in there.

The fact you have turned it so far really doesn't hurt anything, but it absolutely should assure that you are lean. The point being that if there are no other problems (such as an inoperative fuel pump or an ignition that coincidentally took a dump while you worked on the fuel) then pushing down the airflap some amount will get you the right amount of fuel. same instructions follow.

You probably should eventually take it back ten turns if you are sure, but because the actually point is so sensitive you would do well to try what I have stated.

If done properly your massively misadjusted system will operate at some amount of depression (maybe an inch or two, maybe just a little press it down with your face away while some one cranks the engine. When I do this I keep a small throttle opening with one hand and depress the plate with the otherplaying each to achieve a good running motor. I let off the throttle while correcting the depression of the airplate with the other hand PLAY IT. Once I gain control I then start moving the 3mm with the free hand using the relationship we discussed before.

Remember the point is that if you have indeed turned the mixture screw 10 turns counterclockwise then you ARE lean. If you can't make it run by depressing the plate you have a different problem.

stevebfl · #6 03-24-2007, 04:51 PM

I might also point out that it could be a good idea to remove a plug and verify that they aren't fuel soaked. It is concievable that you could have a piston stuck up in the fuel distributor and be flooding the car. What you don't want is to fill the engine with fuel. if the plugs are dry then the above technique should work.

bhatt · #7 03-25-2007, 09:25 AM

The car still runs as it did before (only at 800+ RPM). I turned the screw 10 turns and counted each turn, so I was able to put it back in "roughly" the same spot where I started.

As I mentioned, it doesn't appear that the screw was doing anything.. The first time I turned it, it was 2 complete turns and the car acted exactly the same as before I touched it... isn't that really weird considering you said just a little bit should make a huge difference??

stevebfl · #8 03-25-2007, 10:22 AM

Sounds like the piston may be stuck up into the fuel distributor. Does the plate move real easy. This of course is a loaded question if you have never felt one that is working properly.

I guess one way of judging it would be whether the plate feels the same with the key off or when cranking. Normally if you have fuel pressure (which you won't unless cranking) the plate resists movement and pushing it down causes a slight screaming sound from fuel running through the injectors into the motor (something one doesn't want to do much of). If you were to get the fuel pump to work with the engine off and were to remove a single line from the dist to injector, it would be proper for no fuel to be coming out of the dist. If the piston is stuck down the fuel will flow when the pump runs even without the plate deflected. If you felt the plate in such a situation it would move with a breath to the point of contact to the piston and then get hard.

stevebfl · #9 03-25-2007, 10:32 AM

One thing I might point out. The whole level of observation I have been trying to express is only applicably if you actually caused the problem working with the fuel distributor. If I were evaluating such an issue in the shop it would have taken about one tenth as long to make these judgements and moved on than it took to write this.

I point that out because this wild goose wouldn't waste much of my time chasing, if the problem is totally in another area. What I have described is a simple efficient way of getting to a proper mixture in an otherwise working fuel system. It won't make up for deficient fuel pressure or any manner of flow restriction in the individual injector streams. it definitely won't help if the problem is ignition.

Uncle Acky · #10 03-25-2007, 06:24 PM

Hello,

Please replace any cracked hoses or tubes before you continue.

When adjusting the 3mm screw, did you push down while turning? You must push down to engage it while turning as just turning it without pushing down will have no effect.

Acky (Anthony)

stevebfl · #11 03-25-2007, 06:40 PM

Yes, what a good point. You must find the actual adjustment by pushing down against the spring tention and when you are thus engaged there will be some resistance to turning.

Really good point! It would explain a lot.

bhatt · #12 03-26-2007, 08:15 PM

Thanks so much for everyone's help (especially Steve!!)

To answer a previous question, I didn't know that I had to push down. Was wondering why there was a spring. Turning it the first time, I pushed down for about 2 of the 10 turns. Turning it back, I did the same thing just for the heck of it.. pushed down for the first 2 turns only, then just turned it normally the last 8.

So I dropped it off at the mechanic.. had NO idea what he was going to call me with or what it would cost.

He got it running and says it took him 1.5 or 2 hours to figure it out.

He says all he did was disconnect and remove all the parts I said I touched (fuel distributor, air flow meter, hoses, connectors, etc.). Made sure everything looked OK out of the car and nothing was broken. Didn't make any adjustments or clean anything. Put it all back together and put it in the car.

Of course I tried that TWO times myself and it didn't do anything. He does it once and everything works fine. He said he didn't bother diagnosing it and just put it together, as he figured I'd be happy that it runs (which I am !)

So we'll never know what I screwed up (twice!) but apparently he fixed it. He also said he set the mixture afterwards which is good since I forgot to tell him I screwed with it. My suspicion is that he got the car running and it was really rich or lean so he had to adjust it.

Thanks SO much for everyone's help! Good to know that I was doing everything right!

Neal