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High idle

Tiny
DAVID BEGGS
  • MEMBER
  • 1990 MERCEDES BENZ 500SL
  • 5.0L
  • V8
  • 2WD
  • AUTOMATIC
  • 118,000 MILES
This vehicle starts up and runs great for three minutes, then the rpm's hunt 800-1100 rpm's then goes up to 1500 to 1800 rpm's. I have cleaned the Idle control valve with no help. I can apply a nine volt battery to it and the rpm's drop down and car runs good. I have no change in the rpm's if the ICV is unplugged. I unplugged the TPS with no change. However, I was picking up 11.3 volts on the two outside connectors. I have changed out the engine control module thinking that was it with no luck. I Just opened the wiring harness running along the engine and found no cracked or damaged insulation. I also checked for vacuum leaks by spraying carburetor cleaner and break cleaner and cannot find any leaks, vacuum is steady on my test gauge at 24hg. I have read several threads and trying to figure this out. Any help would be appreciated. I hope to have the wiring harness and components reinstalled this evening.

I also replaced the over voltage relay, alternator and battery. We bought this car and have not put two hundred miles on it, We first noticed high idle and the throttle block was sticking. I have cleaned the throttle block and replaced the connecting boot. The throttle no longer sticks but still have the high idle. Even when the linkage is disconnected.

I just did a continuity test of the wires from the TPS connection and the main plug at the end of the harness and I am picking up continuity on several stabs, I am getting this on both of the outside wires at the TPS. I cannot get into the back of the plug as it is tight. Not sure if there is a circuit board or individual stabs. Thinking that it has shorts but the wiring harness it self looks okay. Please help and follow my threads.
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Friday, September 15th, 2017 AT 9:54 AM

19 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 47,490 POSTS
Is there any warning lights on the dash? Also it sounds like the engine is getting leaned out which will make the idle high at times. Has the MAF sensor been replaced or cleaned lately?

Here are some guides that can help

https://www.2carpros.com/articles/mass-air-flow-service

Because cleaning doesn't always help

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Your car might have a mixture control unit that can do the same thing. There is a year split. This unit wears causing the mixture to lean out.

You can drill the small plug out in the hat and use a long 2.5 mm Allan to readjust the mixture.

I did this all the time in the shop, back in the day.

Please let us know what happens and upload pictures or videos of the problem.

Cheers, Ken
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Saturday, September 16th, 2017 AT 3:55 PM
Tiny
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Thanks for your reply, I have cleaned the Mass air flow sensor and it did not help. I am not getting any Warning or check engine lights. I may be going in the wrong direction but when I tested the wiring harness, because the voltages to the TPS had voltage on both sides, looking for a short or ground I was picking up continuity from the TPS terminals and on several stabs on the main harness plug like it was a ground. I then unplugged the plug that connects to the ignition control module and the grounds cleared. And I checked each wire and they seem OK. When I check continuity at the pins on the ignition control module I am picking up continuity on every pin, I am not sure but am thinking it is grounded. But I do not know if each pin is independent or not. The Module is a Bosch
0 227 400 722, I was getting battery voltage on both sides of the TPS, I should of only got 5 volts, less than a volt on the signal wire. Also I should of gotten 12 volts to the idle control valve and there was zero. But like I said earlier as conditions are now I can force voltage using a 9 volt battery to the idle control valve and the car runs smooth other than a slight hunt in the RPM's.
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Saturday, September 16th, 2017 AT 4:52 PM
Tiny
KEN L
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It sounds like the module is going out. Can you find a used one and plug it in? I think this will solve the problem.
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Sunday, September 17th, 2017 AT 11:47 AM
Tiny
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Well, Found one on E:Bay Will find out for sure maybe by Friday 22nd
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Tuesday, September 19th, 2017 AT 3:46 AM
Tiny
KEN L
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Im interested to see what happens :)
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Tuesday, September 19th, 2017 AT 10:15 AM
Tiny
DAVID BEGGS
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I changed out the ignition control module and that was not it. As for the Mass Air Flow sensor I do not believe this car has one, It does have a ambient air sensor that I cleaned. I just changed out the coolant temperature sensor and the Oxygen sensor and the high idle problem continues. One thing I have noticed is while the car is running `i can unplug all the sensors with no change and with them unplugged the check engine light does not even light up. It only lights when I first turn the key on then clears. Any suggestions would be appreciated as I have about exhausted my efforts.
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Thursday, October 5th, 2017 AT 10:13 AM
Tiny
KEN L
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It it kinda sounds like the injector hat is going bad, is the check engine light or service light on?
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Friday, October 6th, 2017 AT 10:16 AM
Tiny
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No I do not get any service lights, I unplugged the ICV / TPS / Coolant temp sensor and it still runs the high RPM's with no service or check engine lights. But when it first starts up it runs smooth, not a miss at all even as ramps up to 15-1800 rpm's after about 3 minutes.
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Friday, October 6th, 2017 AT 11:04 AM
Tiny
KEN L
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Just a thought have we checked for vacuum leaks? Here is a guide to help you

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Please let us know what you find.

Cheers, Ken
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Friday, October 6th, 2017 AT 11:25 AM
Tiny
DAVID BEGGS
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Yes I checked for vacuum leaks, using several cans of carb cleaner and I also used smoke from a fogger and had no leaks, it runs a constant 24hg when running with my vacuum test gauge.
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Friday, October 6th, 2017 AT 11:33 AM
Tiny
KEN L
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Its been a while since I worked one on of these. There is an idle control valve that sounds like is stuck or not working. Here is its locations so you can check it out.
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Saturday, October 7th, 2017 AT 10:53 AM
Tiny
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The Idle control valve was one of the first things I checked, It seems to be working good, It has been cleaned and will travel the full distance and if I apply a 9 volt battery, it closes and the car runs great. I am only getting 3.5 volts to it and when the RPM's go up to 1800 the voltage drops around 3.0 volts. When the car is running and I put a 9 volt battery on it it closes and I am measuring 4.3 volts with the battery applied and the car runs good. That was why I thought it was the computer control module and changed it out. Then the ignition control module, oxygen sensor, coolant temperature, over protection relay.
It's still weird that during all this the Check engine light does not come on when the car is running and the sensors unplugged. The Image you sent is correct for this vehicle. I did most all the simple testing I think one would do. Was wondering what the MASS computer module does on this car, Also what controls and tells the warning and check engine light to come on. Still trying to get car running correctly.
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Saturday, October 7th, 2017 AT 12:14 PM
Tiny
KEN L
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I have seen where coolant temperature sensors will cause a high idle as well. Can you unplug it while the car is running to see what happens?
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Monday, October 9th, 2017 AT 10:19 AM
Tiny
DAVID BEGGS
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Wow, I did and I replaced it and there was no change. Thanks but you are repeating everything I have already done. Please read the entire threads that I submitted. Right now I do not know what to do. No check engine light or warning light except when it is first started the red light comes on and clears, I can not force it on during its run time. The only other thing that is not right is the headlights come on when I start the car, with the switch off. And I replaced the switch. Maybe it could be a weak ICV but it closes and I think the voltage is to low going to it. I will try any suggestion that I am capable of doing. Right now, I am at a stand still. Thanks.
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Monday, October 9th, 2017 AT 8:13 PM
Tiny
KEN L
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Sorry about that, I get a lot of questions. Did you try adjusting the injector hat? There is a small plug that needs to be drilled out and adjusted which controls the idle. This adjust changes as the engine ages and the vacuum changes.
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Tuesday, October 10th, 2017 AT 11:05 AM
Tiny
DAVID BEGGS
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I drilled the plug and the very small screw was backed out or broken. I tried but can not get it to go back in. Not sure if it is broken or not, I can not pull it get it out to inspect it. Is it made into the Air flow position sensor or part of the fuel hat. I will have to remove the sensor to locate the part number in case I have to order one.
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Tuesday, October 10th, 2017 AT 2:40 PM
Tiny
MHPAUTOS
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There is a small gauze filter on the inlet pipe into the throttle body, check that this is often over looked, also has the engine back fired as if it has the air flow sensor plate can easily warp and cause massive issues that are hard to detect, as it can become stiff and not operate with inn range, just a few things extra to rule out
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Tuesday, October 10th, 2017 AT 3:44 PM
Tiny
DAVID BEGGS
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The vehicle has never backfired and the throttle body is in excellent shape, I had cleaned it and replaced the rubber boot. I ordered a new air flow position sensor 3 437 224 035.
It will be a couple weeks, was not but $12.00, Thanks and I will get back with you when I get it installed.
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Tuesday, October 10th, 2017 AT 4:18 PM
Tiny
MHPAUTOS
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OK, keep up the feed back so we can get on top of this problem.
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Tuesday, October 10th, 2017 AT 5:43 PM

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