Hi Guys, I searched this forum but I couldn't find the answer so I started a new thread. Thank you in advance for any feedback.

I acquired a used S350D few months ago. Had some hard time to replace the glow plugs since one was stuck in hold and wouldn't coming out. Eventually I left that one there, and I was able to start the car with five new plugs. But I noticed a new problem it didn't have before. When engine is cold, the idle speed is normal. But when the engine temperature got to a normal level, the idle level will goes up to 2000 RPM which make the car is not safe to drive any more. The idle was OK before replacing the glow plugs. Is it possible I broke any vacuum line? But it doesn't explain when idle was normal when it just started? I am completely lost. Can any of you give me some directors? Thank you!

My OM602 (1987 190D 2.5) Has an electrical idle control module on the back of the IP. It Idles a bit rough with it unplugged but nothing else is affected. If you have the same thing, maybe try unplugging it, see what happens.

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So I should be looking for some plugs at the back of the IP with wires connected? I will try that when I get back home. Can I drive the car with that cable disconnected if OM603 has one? Thank you.

The engine will run with the ELR disconnected but it might not idle.

Sixto
MB-less

Perhaps you have messed up something in the throttle linkage? Vacuum has nothing to do with idle on an MB diesel of that vintage. Vacuum is used for power brakes, to shut off the engine, modulate the quality of the transmission shifts, open/close the door locks, and vent control for the climate control system. There may be some other uses (EGR valve?), but idle control is NOT one of them. You should have mechanical throttle linkage all the way from the accelerator pedal to the injection pump.

There is a two-pin connector at the back of the injection pump, which is the electronic idle control. I've been driving my '87 300TD with that disconnected for years. I had to turn up the manual idle adjustment screw to set the idle at about 750. When you disconnect that plug, expect the idle to drop to about 500 rpm.

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Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

I thought about it too. But it doesn't explain why the idle was normal when engine is cold. Idle speed goes up when engine is warmer. If I messed up the throttle linkage, it will goes to 2000 RPM as soon as the engine is started, right?

Should be easy enough to disconnect the ELR connector on the back of the injection pump, and then you will know if the problem is in the ELR circuitry or in the mechanical linkages or internal to the injection pump itself.

__________________
Respectfully,
/s/
M. Dillon
'87 124.193 (300TD) "White Whale", ~392k miles, 3.5l IP fitted
'95 124.131 (E300) "Sapphire", 380k miles
'73 Balboa 20 "Sanctification"
Charleston SC

I disconnected the connector on the back of the IP. The idle went down to 500 RPM. Engine shakes a little bit but it stays and won't go up to 2000 RPM without press the gas. So I guess that ELR is the problem.

I cleaned the connector a little bit and put it back on. RPM goes to 1600 and stays, then dropped to 600-700 after about 10 seconds. I press the gas paddle and release it for few times. Not it stays at 600-700 now. I will leave it as is to see how it goes.

Thank you so much for all your help.

Other things to check:

- how much tension there is on the throttle cable during hot idle. Does idle drop if you loosen the cable?

- does idle drop if you separate the electrical connector to the cruise actuator behind the IP?

Sixto
MB-less