1985 380sl Idle Speed Too High

[tx380sl]

With a warm engine idle speed is 900 rpm. Electronic idle speed control unit has been changed, 16 degree centigrade temperature switch checks ok, idle speed control valve checks ok. Any idea what chould be causing the high rpm?

[ljmauricio]

I have same problem. In gear or out of gear engine idles @ 900rpm when warm. You would think it would be slightly lower in gear. Also, When I shift from "in gear" to "out of gear", the engine surges to 1300 rpm and drops back down to 900. This means I have to pause a few seconds when doing from drive to reverse in order to let the rpm's settle back down. Anybody have any idea what is causing this surge?

[ctaylor738]

If all the electronics are OK, then about all that's left is vacuum or air leaks.

[RickM]

There seems to be to swiches to check...at least from previous posts. Coolant and oil temp switch. Which is the 16 Degree C Temp switch?:

"You might check by making sure the engine is warmed up and then unplugging the oil temperature switch located down on the right side at the front of the engine, just above the oil filter can. If the idle goes down unplugging the switch, the swith is defective. The terminal on the switch should have continuity to ground cold and be open when warmed (above 16C) up. Your 1985 may have a pressure step switch, a round valve mounted near the coolant tank, I believe. You might try unplugging it and see if the idle drops. This switch is for enrichment while accelerating and may be stuck. If neither "unplugging" changes the idle, check for voltage at the idle adjuster connector, a two prong connector just to the left of the distributor cap when standing in front of the car. If you have approx 5 volts, the idle adjuster may be bad; if you have no voltage, the idle controller (behind glove box, to the left side of the box as you are sitting in the passenger seat) may be bad , or the connector may have fallen off, as happened on mine. Good luck!"

[MrThankU]

Originally Posted by ljmauricio

I have same problem. In gear or out of gear engine idles @ 900rpm when warm. You would think it would be slightly lower in gear. Also, When I shift from "in gear" to "out of gear", the engine surges to 1300 rpm and drops back down to 900. This means I have to pause a few seconds when doing from drive to reverse in order to let the rpm's settle back down. Anybody have any idea what is causing this surge?

Here I go reviving an ancient thread... this is the only place that has my problem explained exactly as I'm experiencing it. I've checked for vac leaks, checked the 16C temp switch, verified the IACV is working properly, but I still have this behavior. Anyone?

[ctaylor738]

See if the idle relay is working. Push the electrical connector on the idle valve back to expose the pins. Put a meter on the pins and see what voltage the idle relay is sending. If it's zero, you have a dead relay. If it's around 3.5-4V, the relay is trying to control the idle and you need to look for air leaks. Spray some brake clean around the idle air tubes and see if the idle speed changes.

Good luck,

CT

[MrThankU]

Originally Posted by ctaylor738

See if the idle relay is working. Push the electrical connector on the idle valve back to expose the pins. Put a meter on the pins and see what voltage the idle relay is sending. If it's zero, you have a dead relay. If it's around 3.5-4V, the relay is trying to control the idle and you need to look for air leaks. Spray some brake clean around the idle air tubes and see if the idle speed changes.

Good luck,

CT

Thanks, I had already run those tests but I should've clarified. Doesn't matter as I actually found the issue - the pressure step switch. It's only on 1985 models. Part number 0025452814 but they seem like unobtainium. With it unplugged cold or hot my idle lowers to where it should be.

It works with the 16C temp switch to decide when the engine has warmed up sufficiently to adjust fuel/air mixture and lower idle. With it stuck like this my car was running in the 60/40 warmup mixture constantly, increasing the idle.

[ctaylor738]

Ah yes, I don't remember it well, but I do remember it. Isn't it vacuum controlled, to switch off the lambda and get a bit more power under heavier acceleration?

Cheers,

CT

[MrThankU]

Right, to my understanding it does both by 'activating' the same 60/40 fuel mixture via a ground signal to the lambda system when (a) the 16C switch indicates oil temps are below 16C and (b) when vacuum drops, like under heavy acceleration.