W108 dying at idle when hot

[Graham]

To find out if the AAV is really closing, get the engine fully warmed up by going for a long drive. Then with engine at idle, screw the idle screw in all the way - engine should stall. The screw is normally out about 1 1/2 turns, but this can vary.

As mentioned in a PM, with engine at idle and fully warm, you can check if AAV is closed by squeezing the exit rubber elbow fully closed. There should be little or no change in rpm.

That AAV bulb should be polished so it will respond as quickly as possible. There are also two temperature sensors that can benefit from bulb cleaning.

[craftysince86]

so it's still stalling, was holding on strong for i want to say longer than before the cleaning but not much longer. When she really warms up and starts giving out, she's giving out even in neutral and park now too.

[Graham]

Originally Posted by craftysince86

so it's still stalling, was holding on strong for i want to say longer than before the cleaning but not much longer. When she really warms up and starts giving out, she's giving out even in neutral and park now too.

Does it do that if you screw the idle screw out another turn or so? If so, then your problem may lie elsewhere.

For example, you may want to check the fuel pressure in the injector rails (supply header). In your climate, I would set it higher than spec. Maybe 32-34 psig.

These Djets are quite simple, but it usually takes going through all systems until you get it right.

[craftysince86]

the idle screw was the first thing i tweaked, i think since it started dying i have about 3-4 full revolutions of adjustment up.

i did notice recently my car has started to backfire once i let off the gas, someone said it could be running lean. Could that have anything to do with this?

[Frank Reiner]

At the risk of being overly pushy, may I direct your attention again to Post #9?

[Graham]

Originally Posted by craftysince86

the idle screw was the first thing i tweaked, i think since it started dying i have about 3-4 full revolutions of adjustment up.

i did notice recently my car has started to backfire once i let off the gas, someone said it could be running lean. Could that have anything to do with this?

One thing that happens at idle (or with foot off pedal), is that the timing gets vacuum retarded. But only if the coolant is below 100C and the A/C is off. This causes the rpm to drop and may cause engine to stall depending on how the timing is set. Do you have A/C and is it turned on or off when you are testing idle?

If your car backfires, you may need to set timing. If you do this, set it at about 27-30deg BTDC at 3000rpm. This will likely be about 5-10 BTDC at idle with vacuum disconnected and plugged. This is different from MB NA specs, but close to Euro specs. You need a timing light to do this. Let us know if you are going to try.

What Frank suggests is a good idea. By all means unplug all injector connectors, clean with contact cleaner and replug. Same with plug on side of distributor, on Throttle switch and on MPS. Can't do any harm. Also check the #1 and #2 temperature switch connectors. And as mentioned before, make sure you have fuel pressure somewhere above spec. 32psig or so.

There are probably no quick fixes. Hopefully you have access to the shop manual (it is in one of the links provided earlier). You will need to go through sections 7.4, 7.5 and 15. Test criteria for the various sensors are provided and are easy to check. The D-Jet section in Benzworld 107 forum stickies (EG) is something you should read.

Does adjusting the idle mixture screw on the ECU have any effect on how engine idles? If it makes NO difference, then the throttle switch and linkages may need adjusting. If it does make SOME difference at idle, then forget that for now.

My own car has been running a bit lean (on AFR meter) and spluttering a bit. Probably dirty injectors? I will try running some injector cleaner through and see if that clears things up (It has in past)

[_Dirty_]

Brining this thread back to life.


I am having very similar issues with my 66 250se.


The car is injected.


First, where would I find the AAV? or does my engine not even have one?
Second where would I find the idle and air flow adjustment screws?


Pics are very helpful!


thanks!


G

[kishg]

Originally Posted by craftysince86

She works! ...i didn't check t see if was working before though. i did;t think of doing that until after i cleaned it

hopefully this fixes the issue if not i'll report back.

thanks again everyone

Was it the AAV? Same issue here on a '72 300 SEL Will be adjusting valves, change plugs etc but wasn't sure on AAV. This is probably a stupid question but if the aav doesn't close fully, why would that cause the rpm drop while idling in drive?

[Todd Miller]

AAV= Auxiliary Air Valve.

On injected engines, extra "air" needs to be provided during cold running/warm up, in order to compensate for the extra fuel being provide by the ECU. The valve slowly closes as the engine warms up.

An engine that dies at idle, once warmed up, is most likely going to be either too lean, or too rich. Multiple causes for either condition. You can "probably" determine whether too lean or too rich, by standing at the rear of the car, and smelling the exhaust, while it's running at idle. If the exhaust smells acrid, sort of bitter, like maybe even burn your eyes; that's too lean. If the exhaust smells heavy like you want to choke; that's too rich. On some set-ups, if you hold the throttle somewhat open for a moment, and then quickly let go, if, when the engine returns to idle, the rpm's dip low, and it then recovers back to a higher (normal) idle rpm, that's an indication that it is running too rich. I'm not sure if this tuning trick will work on your 300SEL.

One thing you might try before doing a bunch of work, is to replace the air filter, unless you've inspected it and know it's good, and then run a can of SeaFoam through it, on a 1/2-3/4's of a tank of fuel. Take the car out on the freeway for 30-45min each way, and also if you can find a long grade to climb, where you can have your foot in it, but not to the point of the transmission shifting down. Climbing a long grade will allow you to make the injectors spray as wide open as they can, under a lot of load (hence why you don't want the trans to shift) and this will do a great job of cleaning them. It will also remove any carbon that's built up on the backs of the intake valves. You may find that after you run through that entire amount of treated fuel, you have an engine that starts easier and runs much nicer. BG44K is another great injection system cleaner, and you can usually find a good deal on the 3 can kits, on ebay. Pro level, and it is what we used prior to SeaFoam being on the market.

[rumb]

Hi,

It would be most helpful if you started a new thread with your particular issue.

[kishg]

Originally Posted by Todd Miller

AAV= Auxiliary Air Valve.

On injected engines, extra "air" needs to be provided during cold running/warm up, in order to compensate for the extra fuel being provide by the ECU. The valve slowly closes as the engine warms up.

An engine that dies at idle, once warmed up, is most likely going to be either too lean, or too rich. Multiple causes for either condition. You can "probably" determine whether too lean or too rich, by standing at the rear of the car, and smelling the exhaust, while it's running at idle. If the exhaust smells acrid, sort of bitter, like maybe even burn your eyes; that's too lean. If the exhaust smells heavy like you want to choke; that's too rich. On some set-ups, if you hold the throttle somewhat open for a moment, and then quickly let go, if, when the engine returns to idle, the rpm's dip low, and it then recovers back to a higher (normal) idle rpm, that's an indication that it is running too rich. I'm not sure if this tuning trick will work on your 300SEL.

One thing you might try before doing a bunch of work, is to replace the air filter, unless you've inspected it and know it's good, and then run a can of SeaFoam through it, on a 1/2-3/4's of a tank of fuel. Take the car out on the freeway for 30-45min each way, and also if you can find a long grade to climb, where you can have your foot in it, but not to the point of the transmission shifting down. Climbing a long grade will allow you to make the injectors spray as wide open as they can, under a lot of load (hence why you don't want the trans to shift) and this will do a great job of cleaning them. It will also remove any carbon that's built up on the backs of the intake valves. You may find that after you run through that entire amount of treated fuel, you have an engine that starts easier and runs much nicer. BG44K is another great injection system cleaner, and you can usually find a good deal on the 3 can kits, on ebay. Pro level, and it is what we used prior to SeaFoam being on the market.

Thanks. Yeah, it's definitely running rich. It's the same issue as the OP. The rpm drops after warm up when idling but not when it's in neutral, only in drive. I will try running injector cleaner first.

[Todd Miller]

Originally Posted by kishg

Thanks. Yeah, it's definitely running rich. It's the same issue as the OP. The rpm drops after warm up when idling but not when it's in neutral, only in drive. I will try running injector cleaner first.

Also, please be aware that I know nothing about the 6.9L V-8's or their fuel injection system. So I don't know how they manage fuel mixture, both during cold start and warm up, or after warm up. I'm guessing it's a Bosch injection system, and I've worked on all of those, but that was about 30yrs ago, in my 20's.

[kishg]

Originally Posted by Todd Miller

Also, please be aware that I know nothing about the 6.9L V-8's or their fuel injection system. So I don't know how they manage fuel mixture, both during cold start and warm up, or after warm up. I'm guessing it's a Bosch injection system, and I've worked on all of those, but that was about 30yrs ago, in my 20's.

Sorry, I should have been more clear. Mine is a 4.5L 300 SEL. M117.

[Todd Miller]

So it has Bosch D-Jetronic injection. I wonder if this vid on YT might help you? https://www.youtube.com/watch?v=3JMtaJojWRQ Something he comments on is that as the MAP Sensor (Manifold Absolute Pressure) ages and begins to fail, or fails, it drives the system rich.

[rumb]

Dirty hijacks thread and then kishg hijacks the hijack......


Hard to tell what is going on.


Please both of you start your own topic.