'86 SE starting weird... (Page 1/1)
CSM842M4 AUG 23, 08:56 AM
For the past couple of weeks or so, my wife's '86 2.5 has been doing a few goofy things on start-up. If it's been sitting for a while or overnight, it will fire up but run like it has a single-cylinder miss. It clears up in less than a minute and seems to run fine after. Sometimes, when the light is right, I can catch a glimpse of a puff of smoke from the tail pipe, but it's brief and, therefore, difficult to tell the color. It doesn't behave this way once it's warm, only after it's sat for a bit. Sometimes, when starting, it won't go to fast idle, goes to well under 1000 RPM and seems like it wants to die. Also, fuel mileage seems to be less than what it has been.
I haven't had the chance to do any diagnostics yet, but at first blush, my thinking is there may be multiple issues - fuel injector, fuel pump and IAC valve are my prime suspects. I'm not the diagnostician I wish I were, and don't want to shotgun parts at the car. Anybody experience these symptoms, together or separately, and cured the problem/s? What should I look into first? Am I even on the right track? Any insight or input is, as always, greatly appreciated. Thanks in advance - Chris
armos AUG 23, 05:29 PM
Your coolant temperature sensor might be giving false readings.

If the ECM thinks the engine is warmer than it really is, it would explain the low idle and also why it idles better after 1 minute.
You can just unplug the sensor and measure the resistance across the 2 terminals using a multimeter.
I don't remember if the 4cyl has an intake air temperature sensor, but if it does, check that also.
When you check the coolant temp sensor, make sure you're checking the right sensor. There's a separate sender that just goes to the gauge - that one won't affect how the car runs. The gauge uses a similar scale but it's not exactly the same.

1 minute is how long it usually takes for the O2 sensor to become active, which gives the ECM feedback to adjust the fuel mixture. Until that time, it's blind and just using the preprogrammed mixture based on temperature and manifold pressure. The preprogrammed mixture might be too lean (especially if it's reading a higher temperature than reality).

On the V6, factory programming tells the ECM to wait 1 minute before reading the O2 feedback. I don't know if it's the same on the 4cyl, but I doubt it would be much different.

[This message has been edited by armos (edited 08-23-2024).]

Patrick AUG 23, 05:38 PM

quote
Originally posted by armos:

I don't remember if the 4cyl has an intake air temperature sensor, but if it does, check that also.



The '84-'86 dukes do not have a MAT sensor. The '87-'88 dukes do have them.