my car has been stalling a lot, especially when going downhill and I put the clutch in to coast. when ever i stop at a stop light or stop sign the idle, oil pressure and voltage all drop right when i come to a complete stop. If i have my stereo on ( i have a big system) the car will die no matter what - the car has been idleing bad since I had the engine rebuilt in jan but has not been stalling like it is now - everytime I reset the ecm the car stalls basically every time I stop, but then after a while it gets slightly better. it seems if im even rolling slightly the car isn't that bad but It's when i come to a complete stop.
although when i trough the clutch in when im on the freeway the idle jumps around tons. everytime I stop the car alomost dies but the ecm catches it usually and then jumps the idle up. I had just replcaed the tps in march and i've replaced the whole ign. system except the coil. the car also misses slightly at idle

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91 z28 lb9 5 speed - hooker 1 5/8 headers - high flow cat 3 inch exhaust - home made ram air - tpis airfoil - dual chamber flowmaster - car used to be fast - lt1 auto killer - had a stock re-build on the lb9 - and now it sucks a$$

 

So what happens if you are going down the highway and put it in neutral/clutch in? The idle jumps around you said? I know that if you have a lot of gunk on your throttle blades, it may stall at idle because even though your IAC valve is working, the sludge on the plates may be blocking enough air to get past.

 

Check the clutch and other parts of the transmission too. When my automatic got bad, (i.e. both sides of the pump bad, and torque converter almost dead) the thing would stall when coming to a stop because the transmission was not properly disengaging, brakes fought the engine and won, the car would stall.

Also, how drastic is the oil pressure drop? Verify the reading w/ a good accurate mechanical OP gauge. (if it is strictly an electrical problem this will show.) Also, if the oil pressure drops off drastically and the engine is not being properly lubricated, it can slow down and your voltage will drop, etc. Generally bad. That could be anything from clogged oil lines to main bearings bad or to oil pump bad. Hopefully none of the above

It can't hurt to check for a bad ground either, considering the system can cause the car to stall no matter what.

Finially, well maybe not in this order , check the alternator output.

Personally I am lazy so I would do it alternator out, ground, mechanical OP gauge, then trans.

Good luck
Corry

 

I just replaced the alternator last weekend so that rules that out - I even took the car into the shop and they didn't find anything either - its been doing this since I had the eninge replaced and when i did that I put on some hooker shorties and I had a 3 inch Y pipe made - my cat was also bad so i put on a 3 inch high flow - I still have the stock inter pipe 2 1/4 - does anyone think this could cause a problem - another thing is when The replaced my engine the shop dented up one of my tpi runners pretty good - the dent can't be bigger than 1/8 of an inch but im taking everything into consideration

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91 z28 lb9 5 speed - hooker 1 5/8 headers - high flow cat 3 inch exhaust - home made ram air - tpis airfoil - dual chamber flowmaster - car used to be fast - lt1 auto killer - had a stock re-build on the lb9 - and now it sucks a$$

 

Check your EGR valve. If it is a Positive Backpressure type of valve, which you'll find in various models from all the domestic carmakers, uses exhaust backpressure to regulate EGR flow by means of a vacuum bleed. The stem of the valve is hollow, so it allows backpressure to bear on the bottom of the diaphragm. This opens the valve and allows recirculation to occur during heavy loads. GM puts the letter "P" after the part number to tip you off. Obviously, anything that alters the pressure in the exhaust stream, from headers and glasspacks, will screw up the calibration of any backpressure system. Also check to see if there is any carbon build-up in the valve. Sometimes a small piece of carbon will get stuck on the valve an won't let it close all the way. I had the same problem.
Another problem might also be your fuel pressure regulator.
Check it out.