87 Carreras missing backfiring cold

[captainlewis]

When I start in cold under 50 degrees will not keep running, stalls backfires, I have to feather the throttle to keep it running. After about 5 minutes of keeping 3500 rpm will smooth out run idles normal and runs great. Head temp sensor tests good with ohm meter. Likely suspected next step?

[steely]

Feel your Idle Control Valve for vibration, key ON, engine NOT running.
If none, chase that down, if yes, look up one of the threads on cleaning it.
CHT and O2 sensor are ignored I think until they warm up, but the ICV is involved until then.
Just one thought - good luck and let us know.

[cabmandone]

Steely makes some good points. The engine will run about the same with our without the O2 sensor so don't worry about that.
What doesn't make sense to me is that it sounds like a CHT problem but it clears up when the engine warms. As I understand the CHT, it makes the mixture richer when the engine is cold and leans out when the engine warms up.

[captainlewis]

Since original post have done full check of flow meter temperature sensor, air flow meter power and resistance test, air plenemnim is air meter, coil resistance test, replaced Dme relay and rechecked temp head sensor which is to spec. Also at Steeley's suggestion checked idle control cable for vibration which is does with key on. I am wondering if head temp sensor can still be bad with resistance check specs right but that doe not make any sense. When the car is missing and hunting until warm you can smell the gas also Back fires. Sound like ignition problem until it clears and warm up. It 85 where the car is now and did the same thing at start up but took less time to warm up and the ran great.

[cabmandone]

That really sounds like a CHT issue. I accidentally bumped my wire connector once and didn't notice it. The car ran rough and smelled like gas. I didn't wait for mine to straighten out so I can't speak to how it ran after warming up a bit. But I'd wait for others to chime in. Sorry I can't be more help.
I recall a thread on here a while back that owner had a car that displayed some of the same characteristics. I'll see if I can find it and post it here.

[steely]

The CHT, Speed and reference sensors do suffer after a long time, their insulation will crack and break and compromise operation, and you might get a good reading if they start going 'intermittent', but the CHT really doesn't affect running until the engine is warm. (My understanding anyway)

You could test by disconnecting at the connector (where you made the reading) and try running the car, and you'll see it primarily affects a warm idle.

You might benefit from a cleaning of the ICV, ICV removal & cleaning.

Your normal idle should be 800 or 880, I forget for an '85. What does your settle out to be?
The only other thing I can think of might be vacuum (leak), but my logic may be wrong.
And just in case, is your O2 sensor connected?

[MConn]

Cracked solder joints on the ECU under the driver's seat can cause lots of crazy symptoms.

Mike

[captainlewis]

I have read several other posts on other blogs that have had the same problem. 4 different posts state that after replacing all the Sensors and testing everything, including the ignition coil for resistance they replaced the ignition coil even though it tested to specs and it solved the problem. the ignition coil solved the problem. To make sure several of them switched the old coil back and the problem reoccurred. A symptom of a weak coil is cold engine miss and backfiring. I have ordered a coil as all the test indicate everything is good including the coil. I think it will fix it will post the results in a week.

[steely]

I kind of discounted ignition issues, partly out of ignorance, and partly since I assumed it was ok when things seem to run well 'when it warmed up'.
I forgot another item to be checked would be the idle switch itself - a micro switch on the LH side and behind the linkage of the throttle body (with its plug leads on the right of the TB), but I think it plays the same role when the engine is at warm idle, so that theory maybe questionable.
Best of luck and please let us know.

I had cold start issues when I and my car were new to each other our first winter. I was doing a general tuneup (filters, plugs, dist cap and rotor), but I also had a few vac leaks and a bad ICV driver in the DME. Also, the PO had disconnected the O2 sensor b/c he thought it ran better (and it did -in warmer weather). Just furnishing this as bkgnd.

[captainlewis]

Thank for all the suggestions, will let you know after coil installation and road test.

[captainlewis]

Update, replaced ignition coil. Before installing it tested new coil for resistance in both primary and
Secondary circuits. There was virtually no difference in the test results between the old and new and old coils. The car runs perfect now. The backfiring was the issue that I probably did not pay enough attention to. Almost always backfiring is an ignition problem. FYI this was the original coil and had 101,000 miles on it. I don't quite understand how the resistance checks could be good with a bad coil. I checked the resistance on both coils warm and cold and there was no difference. Anyway hopes this might save someone from replacing a lot of unecessary parts. The correct coil was only $85, thanks for the suggestions

[steely]

Way to go - and thanks for the update!

[cabmandone]

Glad you got it fixed. Seems weird that it acted up when cold but got better when it warmed up. Normally is would be the opposite for electrical components.

[steely]

Agreed - I would have never guessed the coil.