87 300E Idle problem - PeachParts Mercedes-Benz Forum

dukeofbluz · #1 07-08-2007, 01:11 AM

Hi everyone, this is my 1st post.
I bought 87 300E and just love it. The other day the idle started to surge up then down. It goes from around 500rpm to 2000rpm with about 3 seconds between. I cant find any vacuum leaks. I had removed the idle control valve and cleaned it, still the same. So after standing there watching the idle surge up and down I unplugged the Idle control valve, the car started running fine, I took it for a drive and everything worked great, turned on the A/C and still ran great. Thinking the car would run fine without this valve connected I forgot about it for around 5 hours. Then I started and it would not stay running. I bought a used ( but tested ) idle control valve and installed it, nope same problem.
Any Ideas?
I have done a search but I hoped knowing the car runs great without the idle valve, maybe it would help make my diagnosis easier.
Thanks, Duke

J.HIDALGO · #2 07-08-2007, 09:17 AM

It could be a lot of things but, I would start with the OVP specially if you have some work done with the electrical system....

saumil · #3 07-08-2007, 03:38 PM

The resistance of the idle control valve should be between 7 to 10 ohms. If the resistance is o.k., check the current at idle, it should be around 600 ma, and see if it changes when the idle suddenly increases.

dukeofbluz · #4 07-09-2007, 01:21 PM

Just a shot in the dark here, what would happen if my daughter put in like 87 or 89 octane gas?

Duke

saumil · #5 07-09-2007, 11:53 PM

87 or 89 gas will most likely not cause the problem you are seeing, this is a classic symptom of a sticking idle control valve.

Another possibility is the air flow sensor potentiometer. With car off, check resistance. When the air flow sensor's plate is all the way to the top, you should get between 1 to 2K resistance. Now gradually push the plate down, the resistance should increase smoothly without any jumps, until the plate is well depressed. If you see jumps (specially high resistances), you need a new potentiometer, which now-a-days is hard to find, atleast a new one is hard to find. A high resistance jump causes the engine to rev up because the computer thinks you are getting a lot more air than you really are, and so sends in a lot more fuel, thus enriching the mixture.

dukeofbluz · #6 07-10-2007, 12:38 AM

air flow sensor potentiometer?

Would this be located in the air cleaner?

Thanks, Duke

saumil · #7 07-10-2007, 11:38 AM

You will have to remove the air filter box. Then you will see the air sensor assembly. If you look from the front, then there is a small black part attached to the assembly. Desperadosearch posted this picture of the air sensor potentiometer.

dukeofbluz · #8 07-10-2007, 01:24 PM

Thank you Saumil, 1 more question, once the engine warms up, it runs fine. would this point to one diagnosis or the other?
Duke

saumil · #9 07-10-2007, 01:53 PM

At idle, the air sensor pot should be in the same position whether the engine is cold or not. So I am not sure if this is the most likely cause of your problem. However, a wrong air metering signal could for sure cause an inapprorpiate air/fuel ratio.

Check also the coolant temp sensor, also called fuel injection sensor. On my 87, 260E, it is the last sensor on the engine, closest to the windshield. When engine is cold (20 degC), the resistance across the sensor should be about 2.5 to 3.5K and when the temp is about 80 deg-C, the resistance should drop to between 250 and 350 ohms. If this sensor is not sending a correct signal when cold, the air/fuel ratio could again be inappropriate.

david s poole · #10 07-11-2007, 10:36 AM

if it's fine when it warms up then you are most likely running in limp mode.the ovp relay supplies power to the main computer which runs the idle.suggest replace relay and make sure female sockets are clean and in good shape before plugging in new one.