Bad Idle Speed Control Motor?

plymouth
laser

#1

I’m an original owner of a 1990 Plymouth Laser RS 1.8L non-turbo (Only 74,000 miles)



Lately I’m having a hard time starting. It takes 4-5 turns of the key to get it going. Once it starts, rpm is a low 500. Sometimes it stalls before moving. After it warms up, idle is a constant 1000 rpm in park or neutral. (750 in gear when waiting at intersection)



Car starts good when engine is warm. When cold, above problem repeats.



Car has been checked out & mechanic thinks Idle speed control motor needs to be replaced.



What do you think? Should I replace? Does ISC really effect cold starting?




#2

Sounds reasonable. The ECU is the one that takes the temp info in and then it tells the IAC motor what to do. Your IAC is not being a good slave. It isn’t properly responding to the ECU’s instructions.

On old carburated engines, a seperate thing called a “high idle cam” would take oover the idle speed when the engine was cold, and the regular idle adjustment would take back control when the engine warmed up. Those days are history.

I’ve included a link to a good description of the IAC function. I’d suggest a visit.


#3

I would make sure to check the coolant temperature sensor before anything else. If the car doesn’t “know” it is cold then it won’t provide the right amount of fuel for cold starts.

If the IAC is cheap then it couldn’t hurt just to put a new one on, but I’d not necessarily replace it on a guess. The next time the car is cold, have someone hang around and give it some sharp raps with a rubber mallet or screwdriver handle. If it is hanging up this should jar it some & that will change the idle.

Someone should also have a look at the fuel pressure when the car is cold and watch what happens with it as you try to start it up.

Has your mechanic kept it overnight and done any trouble shooting at cold start?


#4

Coolant temperature sensor is good, (new one was installed & checked to specs). Car has original IAC. I believe it is suspect after wiggling pointed end, car started instantly but RPMS are now very inconsistent

Mechanic checked out car for a few days. It is getting plenty of fuel & spark. Car seems to drive good after it warms up, does not stall in traffic & starts right up when engine is still warm & not given chance to cool down.


#5

Thanks, I’m probably going to replace it, but will test some more before spending the $$.


#6

You could just try cleaning it first - spray it down w/ some carb or throttle body cleaner & clean out the intake passages.


#7

Pulled ISC from car & tested motor by itself. Plunger goes up & down when 12v battery is applied. Seems to be working out-side of car’s system, but not really sure if it’s totally good. I don’t understand why motor isn’t working in car. I’m not getting voltage to ISC when connected to car’s circuit.

When ISC connected to cars circuit, I get 9 -12 volts for a brief 2 seconds then nothing. Plunger twitches but does not advance. Why is voltage not going thru to ISC?