Idling Funny With the AC On

[Rick Biarritz]

If I pull up to a light, and I'm running the AC, the car will start idling funny in 5 to 10 seconds -- surging and backing off, surging and backing off.  Sometimes the gas pedal flutters, sometimes not.  In the cold weather, the pedal flutters all the time for the first minute or two after starting the car.  I've been told this is the throttle position sensor acting up.  Is it cold sensitive?  Any ideas? 

[Otto Skorzeny]

That's what it sounds like to me. I had the same problem with a 1988 Dodge truck.

[J. Gomez]

Rick,

Do you have an Idle Air Control valve on yours? This could also cause similar symptoms and they are susceptible to gunk build up. Where you able to read any ECM error codes?

[Dave Shepherd]

This is a screw type isc, a skewed tps or an out of adjustment tps can cause this, so can a cam failing as it can cause an erratic idle and this causes the isc to try to compensate for the idle drop. Seen both conditions on the HT4100 many times.

[Rick Biarritz]

Don't know if I have that part.  83 Eldo, with the 4100.  Do you know?  Thanks. 

[Otto Skorzeny]

The same one he's been talking about on this forum for a year.

[Rick Biarritz]

83 Eldo with the 4100. 

[TJ Hopland]

Hmm he said 83 eldo a few posts above.

Cadillac seemed to hang onto the plunger idle control longer than other GM's.  83 for sure still had it.  Its just like the ones on the carbs of the era very similar to the idle solenoid that was used in the 70's except its variable not just on and off.  Other TBI's like the GM trucks starting in 87 used an air bypass valve built into a throttle body just like everyone else.  Its just a big box with a plunger sticking out of it that the throttle linkage hits.  They are still available new from parts stores for around $75.  They can be tested with a volt meter and a 9V battery.  I dont have the procedure handy but 2 pins are just a switch that senses if the throttle is touching the plunger that can be tested with an ohm meter and the other wires are to the motor so you can use a 9V battery to see if it operates smooth.  I think the motor may be 5V so you dont want to use 12v but for short tests the 9v is fine.  Its been a long time since I have dealt with one but I do remember always using a 9V.   In this case it seems the motor is operating so its possible that the switch portion is not working so its sort of hunting around looking for the switch to work.   Its also possible that the internal plastic gears are partially stripped or the grease is hard and sticky so again the system is 'hunting' for the response its expecting.

If the idle motor tests properly you need to make sure its set properly.   Also check to make sure the base timing is set properly.  Both require the computer to be put into a specific mode with jumpers or by unhooking specific harnesses.   If either the base idle or timing is off the computer tends to hunt for the correct settings but wont usually be able to find it.

[Guidematic]

The TPS will have to be set properly per the procedure in the manual. That is where I would start. Also, the manual will outline specfic swithc tests in which the TPS is one. That can be done through the ECC display

Surging idle is a fairly common issue with these cars, and most often it is caused by a misadjusted or defective TPS. The ISC is only responding to the commands sent by the ECM, which it gets its' inpus from the TPS among other sensors.

Mike

[Otto Skorzeny]

So let me get this straight;

The GM ISC recieves commands from the ECM which receives data input from the TPS. The TPS  may have taken a BM which leaves you SOL with an IOU to the local NAPA.

[TJ Hopland]

Yep and if you BYOB be careful not to get a DUI.

The odd thing about these is they are not stepper motors like the current designs.

I assume these TPS's just have slotted mounting holes?

How well do the throttle shaft bores hold up on these things?

[Dave Shepherd]

Yes the tps is slotted for adjustment. you should see .5v at dead idle with the isc retracted, usually the tb shafts were not an issue, but that should always be checked as mentioned.