I have a 1989 420 SEL with 155k.

About six months ago, I began to have problems when attempting to start the car - this was not dependent on temperature as our weather changed from about 20 degrees F to 70 degrees over the weeks to follow. The engine will not turn over without depressing on the accelerator. After the initial startup, I would need to keep my foot on the accelerator until the idle smoothed out. After that it would run just fine. If the car sat for a while, I would have the same problem again.

A local mechanic checked it over and said that the idle control valve was sticking. For $20.00 he cleaned the valve and everything went back to normal for a while. When it began happening again, I ordered a new valve. Once the old one was replaced, the car started like it should again but only for a short time.

Following the valve replacement, the battery light also came on. I checked the battery which was fine and was able to jump start the car. I checked the alternator and found that the voltage was low (even without any load) and this was confirmed by a local mechanic. I agreed to let him rebuild the alternator and again the car started fine for a short time. Since then, I have had the same problems when starting.

While searching this forum, I have found a number of possible problems. Unfortunately, I am not a mechanic and am not sure where to start or how to eliminate these as possibilities. They are:
1. OVP relay: can pulling it and checking for a 'rattle' determine whether it needs to be replaced?
2. fuel pressure regulator: where is it and how would I check it?
3. coolant temp sensor: I did not see this listed on the materials that I have for parts related to my vehicle...
4. adjusting the fuel mixture

To date the following have been checked or changed recently or with regular maintenance: plugs, wire set, air filter, fuel pump relay, idle control valve

Thank you to anyone able to offer suggestions.