91 , TPI 5.7 eng.vin 8, Exhaust system is slightly modifyed cats replaced with twin 12" glass packs for a ressonater & a crossover tube with flowmaster mufflers. fresh tune up all OE parts. plugs & wires, cap & rotor, fuel pump & filter, pressure flushed injectors, cleaned throttlebody, new IAC, TPS, MAP. No vac leaks, EGR not leaking or stuck. No trouble codes. Doesn't surge untill after goes into closed loop operation, and ocasionally it will stablize itself, most of the time it will just die. Idle fluxuates when surging between 1000 rpm to 500rpm before it die's. When driving at low speeds below 20 mph, feel slite missfire. At cruising speed about 65 mph on slite decelleration ,feel slite missfire or roughness in engine. I don't see how the two are related or if even are.. Was wondering if need to get ECM reprogramed or new PROM. Anyone have any suggestions ?

91 , TPI 5.7 eng.vin 8, Exhaust system is slightly modifyed cats replaced with twin 12" glass packs for a ressonater & a crossover tube with flowmaster mufflers. fresh tune up all OE parts. plugs & wires, cap & rotor, fuel pump & filter, pressure flushed injectors, cleaned throttlebody, new IAC, TPS, MAP. No vac leaks, EGR not leaking or stuck. No trouble codes. Doesn't surge untill after goes into closed loop operation, and ocasionally it will stablize itself, most of the time it will just die. Idle fluxuates when surging between 1000 rpm to 500rpm before it die's. When driving at low speeds below 20 mph, feel slite missfire. At cruising speed about 65 mph on slite decelleration ,feel slite missfire or roughness in engine. I don't see how the two are related or if even are.. Was wondering if need to get ECM reprogramed or new PROM. Anyone have any suggestions ?

Surging is usually caused by a fuel/air mixture related problem only. Have you replaced your oxygen sensor lately? Because the oxygen sensor should be changed every 30,000 miles. Also, are you absolutely sure your IAC motor and throttle position switch has been adjusted properly? It sounds to me like your computer is getting bad information sent to it.

Surging is usually caused by a fuel/air mixture related problem only. Have you replaced your oxygen sensor lately? Because the oxygen sensor should be changed every 30,000 miles. Also, are you absolutely sure your IAC motor and throttle position switch has been adjusted properly? It sounds to me like your computer is getting bad information sent to it. I have a Snap On scanner which allows me to watch O2 sensor activity . it constantly reads rich / lean and info i have about TPS is not adjustable and IAC is self adjustable. I've always understood O2 sensors are recomended to be changed at 80,000 miles. Even at that they usually read or respond a little slow .. What i'm having a hard time understandin. Is why i have no trouble codes with this problem. Yes surging is usually a air/fuel mixture problem rather than a mechanical one. I will recheck my O2's. Thank you TOOBROKETORETIRE what is your mechanical background, I appriciate your input. Keep thinking people I need some help. I know there are a lot of smarter Tech's than me out there

Also, are you absolutely sure your IAC motor and throttle position switch has been adjusted properly? It sounds to me like your computer is getting bad information sent to it.

:iagree:

Heres an easy check for the IAC.
Pull the IAC off and reconnect the harness. Be careful. Place a plastic bag over the IAC to capture the pintle if the valve is functioning correctly. Jump the "service" pins on the ALDL connector and place the ignition switch in the run position. NOT the start position. If the valve is functioning, the pintle will unscrew itself and pop off into the bag. While the valve is out clean off the carbon from the pintle and its seat.

Readjust the pintle, install, remove the service jumper and start the engine.

GL

I was a heavy equipment mechanic all through the 70's and early 80's. Then I went on to become a mechanical engineer until I retired in '96.

My '82 shop manual tells me to change my oxygen sensor every 30,000 miles, so I assumed you needed to change your's that often as well.

I'd like to see what your fuel pressure is doing when your engine starts it's surging? Could you do that? Take a fuel pressure test? If nothing else, you could eliminate the fuel pump as being the cause.

My 91 will do that until it warms up,then its fine.

I was having a similar problem with mine. Tried changing all that you mentioned but to no avail. I took it to the dealer and found out that i had 2 bad fuel injectors . The mechanic informed me that the injectors are one of the reoccuring problems . Had all 8 changed runs like new. Try disconnecting one injector at a time and watch and listen to the engine for changes. Hope this helps you .