280SE- Engine dies when coming to a stop - PeachParts Mercedes-Benz Forum

ROLLGUY · #1 05-17-2021, 11:11 AM

I am finding that this is a common problem for many different models, but I have found nothing specific for a 280SE (W108). The car starts and runs fine, but when coming to s stop, the engine dies (stalls). It has a new coil and spark plugs, and a new fuel pump. The idle speed is adjusted perfectly. I don't know much about these older fuel injection systems for gas cars (I'm a Diesel guy). Anyone got any ideas?

Sugar Bear · #2 05-17-2021, 01:39 PM

Does it by chance have an EGR valve? If yes, they can hang open creating a large vacuum leak.

Is it running rich, any gasoline smell in the oil? There is a thermostat in the injection pump under the two 1/2" hoses toward the back.

Has it sat for an extended period? Does it stumble and die or abruptly shutoff?

Good luck!!!

ROLLGUY · #3 05-17-2021, 04:44 PM

Quote:

Originally Posted by Sugar Bear

Does it by chance have an EGR valve? If yes, they can hang open creating a large vacuum leak.

Is it running rich, any gasoline smell in the oil? There is a thermostat in the injection pump under the two 1/2" hoses toward the back.

Has it sat for an extended period? Does it stumble and die or abruptly shutoff?

Good luck!!!

I don't think it has an EGR, but I will check. It does smell rich at first, but is less smelly as it warms up. It just dies with no stumbling, and yes it has been sitting for about 5 months. Some great questions, Thanks!!

ROLLGUY · #4 05-24-2021, 11:18 AM

I figured out the problem after thinking about it for a while. I raised the idle with the adjustment screw on the throttle body (left side of photo), and lowered the idle with the (mixture?) screw (right side of photo). The engine goes back to idle with no stalling now. This makes sense, as the engine was not getting enough air through the throttle plate at idle.....Rich

Sugar Bear · #5 05-24-2021, 11:25 AM

Great, glad to hear this.

bracurrie · #6 05-31-2021, 10:19 AM

Quote:

Originally Posted by ROLLGUY

I raised the idle with the adjustment screw on the throttle body (left side of photo), and lowered the idle with the (mixture?) screw (right side of photo). The engine goes back to idle with no stalling now.

The adjustment screw on the throttle body does not adjust idle. It is adjusted so that the flap just closes at idle. Its the first step in the linkage setup and if changed probably has other unintended consequences. The screw on the right regulates the air into the system at idle. It is adjusted along with the idle mixture control on the MIP to adjust idle.

ROLLGUY · #7 05-31-2021, 12:25 PM

Quote:

Originally Posted by bracurrie

The adjustment screw on the throttle body does not adjust idle. It is adjusted so that the flap just closes at idle. Its the first step in the linkage setup and if changed probably has other unintended consequences. The screw on the right regulates the air into the system at idle. It is adjusted along with the idle mixture control on the MIP to adjust idle.

Well, all I know is the car now idles, and does not stall at a stop. It is clear to me that the flap was too far closed, so I adjusted it to be a little more open. That clearly solved the problem.

bracurrie · #8 06-01-2021, 11:58 AM

Quote:

Originally Posted by ROLLGUY

Well, all I know is the car now idles, and does not stall at a stop. It is clear to me that the flap was too far closed, so I adjusted it to be a little more open. That clearly solved the problem.

I am happy for you, but your workaround may mask other problems. I am not a tech, but I have been researching and directing efforts on these engines for five years now and have been around these cars since the 1970s when these cars were new. The venturi flap is a vital cog in the system for these engines.
If you restore the venturi flap to its proper position, which is just closed at idle, and the car will not idle then you should open the idle screw to introduce more air at idle. There is an iterative procedure for setting idle that involves getting the proper air to fuel mixture at idle then if idle is too low you first stop the engine, richen the MIP idle setting then set the fuel to air mixture by manipulating the idle air screw and observe resulting idle speed. Typically once these have been setup properly they do not go out of adjustment unless someone ignorant of how it works fiddles with linkages. If all bushings are good and linkages are as they were when the engine idled well then you do have another issue.
Also, if you haven't already you could check your ignition system for optimal setup. That can affect your idle speed stability and is the first step for the recommended procedure for idle optimization.
I have the information from the service manual and some other writeups describing how to set the idle. Let me know if you want me to PM that information to you.

ROLLGUY · #9 06-01-2021, 12:14 PM

Quote:

Originally Posted by bracurrie

I am happy for you, but your workaround may mask other problems. I am not a tech, but I have been researching and directing efforts on these engines for five years now and have been around these cars since the 1970s when these cars were new. The venturi flap is a vital cog in the system for these engines.
If you restore the venturi flap to its proper position, which is just closed at idle, and the car will not idle then you should open the idle screw to introduce more air at idle. There is an iterative procedure for setting idle that involves getting the proper air to fuel mixture at idle then if idle is too low you first stop the engine, richen the MIP idle setting then set the fuel to air mixture by manipulating the idle air screw and observe resulting idle speed. Typically once these have been setup properly they do not go out of adjustment unless someone ignorant of how it works fiddles with linkages. If all bushings are good and linkages are as they were when the engine idled well then you do have another issue.
Also, if you haven't already you could check your ignition system for optimal setup. That can affect your idle speed stability and is the first step for the recommended procedure for idle optimization.
I have the information from the service manual and some other writeups describing how to set the idle. Let me know if you want me to PM that information to you.

Thank you for that information. However, my friend picked up his car and drove it home. He is quite happy with it. If he has any more idle issues, I will forward this info to him. Thanks again, Rich

Frank Reiner · #10 06-01-2021, 12:31 PM

Why did the engine idle consistently after the throttle plate stop screw and throttle plate itself was moved a bit farther open? Was it additional air? No. When the throttle plate was moved, the injection pump, via the linkage, was also moved to deliver more fuel, i.e., a richer mixture. The engine was stalling because the mixture was a bit lean. After the adjustments the same amount of air was entering the engine as before, but more fuel!

ROLLGUY · #11 06-01-2021, 03:42 PM

Quote:

Originally Posted by Frank Reiner

Why did the engine idle consistently after the throttle plate stop screw and throttle plate itself was moved a bit farther open? Was it additional air? No. When the throttle plate was moved, the injection pump, via the linkage, was also moved to deliver more fuel, i.e., a richer mixture. The engine was stalling because the mixture was a bit lean. After the adjustments the same amount of air was entering the engine as before, but more fuel!

After adjusting the stop screw, I had to lower the idle speed with the mixture screw.

Frank Reiner · #12 06-01-2021, 04:15 PM

Quote:

Originally Posted by ROLLGUY

I don't know much about these older fuel injection systems for gas cars (I'm a Diesel guy). Anyone got any ideas?

Quote:

Originally Posted by ROLLGUY

and lowered the idle with the (mixture?) screw (right side of photo).

Roller, you questioned the function of the screw on the right side of the photo, and in the next post bracurrie explained to you that the screw is not a mixture screw; it is an air screw.

Quote:

Originally Posted by bracurrie

The adjustment screw on the throttle body does not adjust idle. It is adjusted so that the flap just closes at idle. Its the first step in the linkage setup and if changed probably has other unintended consequences. The screw on the right regulates the air into the system at idle. It is adjusted along with the idle mixture control on the MIP to adjust idle.

Quote:

Originally Posted by ROLLGUY

After adjusting the stop screw, I had to lower the idle speed with the mixture screw.

Again, you were able to lower the idle speed by reducing the amount of air entering the manifold by closing the air screw.

ROLLGUY · #13 06-01-2021, 09:43 PM

Quote:

Originally Posted by Frank Reiner

Roller, you questioned the function of the screw on the right side of the photo, and in the next post bracurrie explained to you that the screw is not a mixture screw; it is an air screw.

Again, you were able to lower the idle speed by reducing the amount of air entering the manifold by closing the air screw.

Whatever you want to call it, I adjusted both parts to fix the problem. That is all I know. The car is gone, and hopefully I won't see it again......Rich