Fluctuating Idle when cold
04-30-2003, 08:23 AM
#1
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Fluctuating Idle when cold
Previously my engine was stumbling on acceleration and had a fluctuating idle 700-900rpms. I have been looking for vacuum leaks, (replaced a few lines) and intake leaks (couldn't find any), and still had the same problem. However, I noticed that if I let the engine warm up in idle (3 mins or so) that all seems to go away. The idles gets steady around 1000 (in Park-- AT tranny) (normall??) and the acceleration stall or gasp is gone when I hit the pedal.
If the problem is only occurring during a cold engine idle could that be the result of a bad Cylinder Head Temperature (CHTS)? If not what else would anybody suggest should I look for?
Also there still seems to be a slight vibration or miss in the motor during acceleration but no gasp after it is warmed up (just a little rough). Could that be the timing is off a little? Thanks again for the help.
If the problem is only occurring during a cold engine idle could that be the result of a bad Cylinder Head Temperature (CHTS)? If not what else would anybody suggest should I look for?
Also there still seems to be a slight vibration or miss in the motor during acceleration but no gasp after it is warmed up (just a little rough). Could that be the timing is off a little? Thanks again for the help.
05-21-2003, 07:27 PM
#4
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Update on Cold Engine Idle
I recently did a timing belt job, and replaced the CHTS and harness thinking that would solve my problem of a cold erratic idle and stumbling acceleration. My problem would go away after about 3-5 mins warming up and would idle and accelerate fine.
awgalbraith On the z31.com board posted this in reply to one of my post on this issue: It is a lean surge condition that is chronic on many 84-85 cars. It is caused by the chts sending a signal to the computer that says the engine is warmer than it actually is causing the computer to lean out the mixture. After about 2 minutes the fuel system goes closed loop and the O2 sensor becomes the primary sensor for mixture and the engine then runs good. The fix is to fool the computer into thinking the engine is colder than it is so that it will enrichen the mixture under open loop conditions. You can do this by using the FTS as the warmup signal instead of the CHTS. Wire a three contact switch into the wiring so that you can select the FTS signal for warmup instead of the CHTS and then when it warms up you can throw the switch to the CHTS. Without the switch the engine will run a little rich afterwarmup. Several people have tried this mod and it has worked for every one.
He followed up with some more detailed info:
Use an old CHTS wiring harness for this. Run the ground wire to ground at a grounding point on the plenium, run the sensor wire to the FTS, plug it in to the main harness connector for the chts. Cold start your car and see if it fixes the cold start warmup problem. If it does, then wire in a 3 contact switch(radio shack has them) with one side for the FTS contact, the other side for the CHTS, and the center going to the sensor lead in the main harness. Then you can select the FTS for cold start and warmup and switch to the CHTS once warmed up. Put the switch under your dash. There is room for one in the recess for the steering wheel adjuster, just drill a hole to accomadate it and run the wires thru the firewall
Don't know if this is the same problem you guys are having but if it is hopefully this helps. I haven't done it yet but will post back after I do.
Sorry so Long, but thought it was good info to share.
awgalbraith On the z31.com board posted this in reply to one of my post on this issue: It is a lean surge condition that is chronic on many 84-85 cars. It is caused by the chts sending a signal to the computer that says the engine is warmer than it actually is causing the computer to lean out the mixture. After about 2 minutes the fuel system goes closed loop and the O2 sensor becomes the primary sensor for mixture and the engine then runs good. The fix is to fool the computer into thinking the engine is colder than it is so that it will enrichen the mixture under open loop conditions. You can do this by using the FTS as the warmup signal instead of the CHTS. Wire a three contact switch into the wiring so that you can select the FTS signal for warmup instead of the CHTS and then when it warms up you can throw the switch to the CHTS. Without the switch the engine will run a little rich afterwarmup. Several people have tried this mod and it has worked for every one.
He followed up with some more detailed info:
Use an old CHTS wiring harness for this. Run the ground wire to ground at a grounding point on the plenium, run the sensor wire to the FTS, plug it in to the main harness connector for the chts. Cold start your car and see if it fixes the cold start warmup problem. If it does, then wire in a 3 contact switch(radio shack has them) with one side for the FTS contact, the other side for the CHTS, and the center going to the sensor lead in the main harness. Then you can select the FTS for cold start and warmup and switch to the CHTS once warmed up. Put the switch under your dash. There is room for one in the recess for the steering wheel adjuster, just drill a hole to accomadate it and run the wires thru the firewall
Don't know if this is the same problem you guys are having but if it is hopefully this helps. I haven't done it yet but will post back after I do.
Sorry so Long, but thought it was good info to share.
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