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Thread: Why Does My 3.8 Run Smooth When It's Cold Out and Rough when It's Hot Out?

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    Question Why Does My 3.8 Run Smooth When It's Cold Out and Rough when It's Hot Out?

    Can anyone tell me why the LG3 3.8 in my 86 Century always runs smooth when it's cold outside and rough in warm weather? Today it's -12 F and the engine is running as smooth as can be, but in warm weather it always runs rough.



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    Senior Member Keiths1976's Avatar
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    Sensors that have cooper winding inside . Such as ignition coils , Iac , throttle position sensor , ignition control module , sometimes a coolant temp sensor could be off on temp readings of the actual engine dumping more or less fuel as it?s thinking cold or warm soon as it starts start up . That also affects open and closed loop . As the winding get hot they can malfunction and cause havoc on the engine idle . I?m sure I forgot a few sensors that have them as well . If yours has the magnavox ignition coil system I?d start there on ohm testing them.

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    Quote Originally Posted by Keiths1976 View Post
    Sensors that have cooper winding inside . Such as ignition coils , Iac , throttle position sensor , ignition control module , sometimes a coolant temp sensor could be off on temp readings of the actual engine dumping more or less fuel as it?s thinking cold or warm soon as it starts start up . That also affects open and closed loop . As the winding get hot they can malfunction and cause havoc on the engine idle . I?m sure I forgot a few sensors that have them as well . If yours has the magnavox ignition coil system I?d start there on ohm testing them.
    I checked the ohms on the ignition coils and the coolant temperature sensor and all are within spec. The IAC is new.The differences in engine smoothness between hot/cold weather driving are most dramatic during acceleration.

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    Senior Member 85_Ciera_Rebuild's Avatar
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    If cold/hot starts are fine, and no fuel rich exhaust going out the exhaust pipe, this would rule out ECT replacement.

    Difference between cold and hot weather is air density, with higher density via coldness. I don't remember LG3 3.8 electronic setup...but MAF and MAP sensors would be prime suspects. MAPs are cheap to replace...I'm not current on MAF pricing. Make sure intake hose from MAF to intake is not sucking outside air.

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    Does the engine fully warm up and TCC lock when cold? Because if it doesn?t fully warm up and stays unlocked when cold it could disguise a roughness problem that might really be there all the time. And staying unlocked when cold could simply be a bad thermostat.

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    Quote Originally Posted by 85_Ciera_Rebuild View Post
    If cold/hot starts are fine, and no fuel rich exhaust going out the exhaust pipe, this would rule out ECT replacement.

    Difference between cold and hot weather is air density, with higher density via coldness. I don't remember LG3 3.8 electronic setup...but MAF and MAP sensors would be prime suspects. MAPs are cheap to replace...I'm not current on MAF pricing. Make sure intake hose from MAF to intake is not sucking outside air.
    My LG3 doesn't have a MAP, it has a MAF and an intake air temperature sensor. I measured the resistance of the IAT sensor at different temps and the were all at the correct ohms specified by the table. I've checked and rechecked for any air leaks. Prices for a MAF at Rock Auto range from $45 to $125.

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    Senior Member 85_Ciera_Rebuild's Avatar
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    Quote Originally Posted by T Type View Post
    Prices for a MAF at Rock Auto range from $45 to $125.
    Older MAFs will not perform the same as a new MAF...in 1986 era, GM had MAF issues. I've replaced a 'vintage' Bosch AMM (MAF), and there was a significant improvement.

    Either coils can't take the heat (ignition issue) or something is affecting air/fuel ratio. What is needed is a OBD-1 real time reader, like dealerships used.

    I'm not aware of O2 sensors causing this issue, but if its a single wire O2, these are cheap to replace.

    Older vehicles I buy get O2 replaced, ECT replaced, FPR replaced, injector o-rings replaced, sparks/wires....normally, this eradicates potential issues.

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    Senior Member 85_Ciera_Rebuild's Avatar
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    BTW...error code exist? If running in open-loop, this could explain issue. Does TCC lockup?

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    Dont forget to check the Fuel Injectors themselves. They also have copper windings, and can have some de-lamination going on internally.
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    Quote Originally Posted by 85_Ciera_Rebuild View Post
    BTW...error code exist? If running in open-loop, this could explain issue. Does TCC lockup?
    Always good idea to check for codes and monitor the shifting/TCC. But my 6000 did this for a whole winter, the TCC would keep staying unlocked on the highway due to a bad thermostat keeping it in open loop, and my mpg went down 30%. And never a trouble code.
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    The TCC seems to work fine. I checked for codes with a paper clip there were none. Plugs and wires are 3 years old. Coil and injector ohms all within spec. I sprayed the injector bases with throttle body cleaner last summer and there was no change in RPMs. Fuel pressure test was normal. I tried to see if I could replace the O2 sensor but even with the engine warmed up I could not budge it. I think I may have to bite the bullet and buy a new MAF.

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    Senior Member 85_Ciera_Rebuild's Avatar
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    Here's how one Bosch AMM (MAF) works:

    "The sensor inside the air mass meter consists of a wire which is maintained at 250?F (120?C), above the ambient air temperature of the air entering the engine. As the air mass passing over the wire increases, more current is required to maintain the correct temperature. The amount of current required is used to calculate the air mass taken in."

    Key Words - " above the ambient air temperature of the air entering the engine."

    On this engine setup, there is a thermostat that mixes hot exhaust manifold air with outside air to maintain a set air temperature going into intake manifold.

    Translation - If your incoming intake air temperature is off spec from OEM, then fix it. Since more current would be required with pure cold air, extra fuel would be added during open loop, and if O2 was not up to spec, there might be extra fuel in closed loop. Note - It depends upon how ECM was programmed to act.

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