ViciousZ

iTrader: (1)

OK, here's the problem: the car sometimes dies when it comes to a stop and idles really bad, fluctuates between 450-650 rpm. Also, if I'm driving down the highway and I shift into neutral, the engine stalls. I put in a new IAC (actually two) and that didn't solve the problem. BUT I know it's related to the IAC somehow because the problem is completely solved if I unplug the IAC. I did this by letting the car idle, turning on AC to raise the idle a little, and then unplugging the IAC with the car running. But this is just a quick-fix for the problem; I can't exactly go cruising around all winter with the IAC unplugged.
Also I've tried replacing the computer and checked all the connections/pin fittings to make sure they're tight. I don't know what else to do! What else could cause this problem? Is the computer commanding these incorrect IAC positions? Could a sensor be giving the computer bad info? How about the PROM? Or is the problem somewhere between the computer and the IAC? Help!!

thunderstick

Have you even checked your timing? Mine ran similar to yours once upon a time, but not anymore, if you catch my drift.

KAM

Vader

Vicious,

Check the TPS voltage. It should be at 0.54VDC at idle position for engine through 1989. Later cars may have a non-adjustable TPS, but the voltage at idle should still be below 1.0VDC.

You should also follow the procedure to set throttle minimum air position when installing an IAC (or any time you suspect an IAC problem).

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Later,
Vader
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rondubbs

are you getting any codes??
how old is your o2 sensor??!!

JP84Z430HP

Check codes, check for vaccuum leaks, timing, tune-up? Fuel pressure? TPS volts? Is it a mass-airflow setup? If so check for leaks between the sensor, and the throttle body. I'm about out of things to check! Most of these are pretty quick and easy checks if you've done mauch work on cars, if not the kind people here can really be of help!

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Working on:
'84 Z28 LG4 305
Added dual elec fans.
145 MPH IROC Speedo
Building 430 HP 350
using primarily GMPP parts.
Block is in shop!

Awaiting results from all 8 ASE tests.

Starting to look like the Kicker poster child!

ViciousZ

iTrader: (1)

I've checked all that stuff and more!
TPS: .60v @idle 4.35 @WOT
No codes
Doesnt' look like I have any vacuum leaks, got 17 in Hg, but I don't know what it should be at because I have an aftermarket cam (SLP .480 .487 lift 206 212 dur) I put a scan tool on it and my IAC read between 20 and 30 at idle, it would be lower if I had a vacuum leak, I think.
O2 was replaced about 6 months ago. Besides, it idles bad even when the car is in open loop.
Timing is at 6BTDC
have new spark plugs, wires, cap and rotor. Tried a different ignition coil and module.
Fuel pressure is set at 48 psi. I put a gage on it, and I'm not losing pressure when the idle drops.
Map is at about 1.5V at idle. I think this is normal.
There is no carbon behind the throttle plates or in the IAC passage.
I went to set the minimum idle and found something very interesting. The car is not idleing where it thinks it is. What I mean is, when setting the minimum idle, the tach and the scan tool both say the rpm is at about 650-700, yet by ear I can tell it's much lower than that. Probably around 450. So it seems that my computer thinks the engine is idleing about 200 rpm higher than it actually is. WHAT COULD CAUSE THIS? Like I said, I've tried a new coil, module, and computer. HELP!!!

Corry

I'd say bad reference pulses...but I have never had to deal w/ this problem (I learn something when my car forces me to!) But by everything you had said before, that's what I'd check...got a frequency counter or o-scope avail? If not do you know wnyone who does? Maybe someone else has another idea. I also guess like thunderstick said, it could be your timing, if the distributer is sending spark for 650-700 rpm, and the spark does nothing in the cylinder, that could be an issue. As far as I know the tach is based off the reference pulses from the distributer/MSD ignition module ( I know I had to run the tach through my MSD box when I put it in) As far as I know, the tach is not based on the output of the engine, but like I sdaid on the reference pulses.
Good luck
Corry