zaphiri

Greetings folks. Many threads on the subject. I did a lot of reading and I have done some diagnosis already, but I will need your help and opinion.


When the car starts cold, the idle is perfect at 700 RPM. It stays there for some time until the engine temperature goes up at 50 F. I get the surge after that with peaks at 2000 RPM and minimum at 0, where the engine stalls. When the car is warmed up (80+ F) no major surges. Here is what I did.

1. New alternator voltage regulator (no change).
2. Replaced EC unit with a spare one. No change.
3. Replaced ICV with another. No drastic change. I feel that the ICV is not the cause of the problem but the result of the voltage it gets from the EC unit.
4. I did not replace the OVP. I had no ABS light, and the fact that the car starts when cold, is an indication OVP does its job.
5. I am about to swap the fuel relay, but I hold no high hopes, unless there is something intermittent caused by temperature in it.
6. Old plugs, relatively new rotor and distributor cap and reasonable cables. I evaluate the chances of these components being the cause is rather low.
7. Ignition coil. Some posts suggest that it can be the cause. I did not change it. Planning to get an old coil and test.
8. Crank position sensor as reported by posts. Not considered it. Could be the issue.
9. I am about to fill with 93 gas and add chevron techron in it.
10. MAF sensor. That is a hard one. Have not considered it, but it looks like a possible source.
11. throttle linkage. I sprayed it clean, but I need to go back and check.
12. Vacuum line test with WD40 spray. I am about to do it to see if the engine surges. A previous inspection showed no visible cracks in the lines.
13. Engine Temperature sensors. A very possible source. I am not sure which is the sensor that reports the temperature to the EC unit. Looks like the sensor that reports it to the instrument cluster is OK. Any help?

Have I left anything out? It seems that the key here is that when the car is cold, the idle is perfect, and when it is warm, it is steady, but the in-between temperatures might be the clue to the problem. Any help from the experts will relieve my headaches by a huge amount.


BTW, the car sat for a while before I put it back on the road, which is when the problem appeared. Many thanks.

Plutoe

You are wasting your time and money on things that do not affect the idle control---do your self a huge favor get a pulse counter and or search the web not the forums for MB idle speed control systems and controls!!

optimusprime m1

Temperature sensor as a lot to do with the cold start and running of the injection system . Its the sensor on the far end of the cylinder head .Some are a round shape with 4 pins ,others are green and two turrets on top..If it fails it sends the wrong message to ecu..Engine thinks its cold and sends in a rich mixture. or if in the other possision will weaken the mixture thinking its hot. Hope it helps you to understand the way temp sensor works ..Also if replacing parts they must be of the same type and number .Missmatch of parts wont get it sorted ..

QVHK

zaphiri,
Hope you have sorted the problem. One thing I learned is that when there is a problem do not throw different parts in trying to cure it as some or all of them may give rise to other problems so your original issue becomes blurred. Always back track to review what may have happened before the problem emerged, like recent change of air filter, etc. If the car has been sitting long, after warming the engine up, give it a nice cruising and full blast to wake up the machine and clearing any fuel, water and air channels that may have been narrowed or blocked. If the problem persists and you detect a pattern, then it is time to focus and narrow down the possible causes.

I have a few queries. What is the ambient temperature like when you said you stared the car cold? Normally on a cool morning like now the idling at start up may be higher than normal, around 1000 rpm and as the engine warms up and reaches normal temperature (80C) or so, the engine speed drops to between 650 and 700 (yours is M103 I believe, like mine). Anyway, from what you describe, it is best to make sure that the engine temperature sensor (on the engine block near the firewall) is working properly, as is the thermostat that regulates the opening and closing of the coolant flow from the radiator. Anyway, both the temperature sensor and thermostat are pretty cheap and there is no harm replacing them as a start. If the engine behaviour is still the same then keep on looking for the problem part.

I myself am also dealing with idling issues: engine hunting for idling during stand still, surging up or sometime stalling even at normal temperature. I took the car to the workshop which found that the potentiometer inside the airflow meter is failing due to wear and tear (yours and mine W124 are over 25 years). This may not be your problem but do find out more how your engine behaves after dealing with the temperature sensitive parts first.