99 e300d idle problem when hot
 

Hi all, nice to be back to this board, om606 newbie with an idle problem. Its a 1999 e300 turbo diesel with 198k mi and when its starts up in the morning it runs smooth at idle, even after a few runs around the block after the engine has come up to normal operating temp it still runs fine but after a long drive on the highway and you come to a red light the engine starts to rock from side to side as if it was running on 4 cylinders and the whole car rocks, this is more pronounced when the brake pedal is pressed. I tried adjusting the cable that operates the accel position sensor but this only makes it run at higher speed when the car is cold but when is hot it runs bad. The car runs like a rocket otherwise, where should I start?:confused:

Don't stop:), jk.

How old and how many miles are on the engine/transmission mounts?

Car has 198k but it was purchased recently so i dont know, but would think is not engine mount related since it idles fine when cold, the problem happens when the engine is hot and it gets worse when the brake pedal is pressed

Vacuum problem?

Bad booster or weak vacuum source causing vacuum to be diverted from the engine control systems when the brakes are applied.

Tee into the main vacuum line with your mity vac and rig the gauge where you can see it then go for a drive.

Just to satisfy my curiosity, when it's acting up, try getting under the hood and pulling the vacuum line off the EGR mixing chamber and see if the idle improves and/or if you detect vacuum at the line. Also see if the presence of vacuum changes when the brake pedal is pressed...

I will try doing that tomorrow but i noticed tonight I drove it and it had been sitting for 3 hours and the idle was fine, even with the brake pedal pressed and the a/c on it idled fine until it warmed up then it started again. Am wondering what component does the injection pump rely on to adjust the iddle speed and then I should see which of those are affected by heat. also when the problem occurs it makes this rattling noise that goes away when engine is revved so am thinking that comes from the engine mounts as the engine shakes so much from side to side.

I believe you maybe getting air in fuel delivery hoses. Mine would idle rough and then quickly die.

Good luck

The injection pump is completely electronically controlled. Well, except for the advance but anyway, vacuum should have nothing to do with fuel.

I agree on checking for bubbles forming in the fuel lines when hot. But it's a mystery how the brakes are effecting things.

the fuel lines are clear plastic and all were replaced by prior owner and I dont see a single air bubble in the lines.

Am trying to post a video of the car at idle so You guys can see. Today I tested the EGR control solenoid and it seems to function the way it should, at idle there is no vac being sent to the EGR valve but when raise the speed I can feel vacuum on the tube that goes to the EGR valve but as soon as engine speed returns to idle the vacuum cant be felt. One thing is weird though, I tried sucking on the line that goes to the EGR at idle speed to see if I noticed a change in engine behavior since this should have opened the EGR mixing port and allowed exhaust to enter the intake manifold but to my surprise it stayed the same. Could this mean the EGR is stuck open?

Hazard Frought Tools has a vac tester for $18 w/coupon... better than sucking the hose.

I still suspect that you have something that is drawing down your vacuum system. Stepping on the brakes amplifies the problem since you need so much vacuum for the brake booster it takes away from the other systems.

Note that besides the operational difficulties you are experiencing, it is also bad for your vacuum pump to operate in this manner, it is not meant for continuous operation.

That sounds logical but pressing the brakes only makes it worst but the problem is still there even without pressing the brakes.

Last night the check eng light came on and then when I turned it off and started it up again it went away then today I took out the intake air temp switch which was full of oil inside where the pins are and cleaned it and drove around but the idle did not improve. It sat for a couplle of hours and the check eng light is on again and it set code p0560 "system voltage malfuntion" and then when I turned on and off the check eng light goes out again. Any ideas? Could this be where the idle problem is? It cranks fine and no other electrical problem is present at the moment....

When you press the brake pedal and the idle roughness gets worse, are you pressing the pedal down firmly or does the idle get rough if you simply touch the pedal lightly enough to illuminate the brake lights?

Just lightly enough to illuminate the brake light, I dont have press hard at all.