I'm running a M52 bottom end with OBD1 M50NV head/intake manifold/etc. I didn’t do the swap on it, but it has run great since I bought it from the previous owner a few years ago. Before I put the car into storage last year it developed a stumbling/stalling problem when coming off idle. It originally manifested itself while I was driving - the car began cutting out intermittently while cruising, until it died and would have trouble starting and running. I was able to limp it home with it dying a number of times on the way. What is curious about it is if I am able to keep the RPMs up, it doesn’t stumble at all - only when they drop to or around idle. If the car is cold it fires up and revs fine, but as it warms up (only about 10 seconds of running or so), the problems begin and continue getting worse until it will barely run at all. At idle, it’ll stumble and fluctuate; when I rev it, it just stalls and occasionally backfires through the intake (seems to go lean), then will return to a stumbly idle.
I've been picking away at it, trying to diagnose it. So far I have done the following:
-Replace fuel filter
-Replace in-tank fuel pump (my car only has one being a late model) - upgraded to Walbro 255
-Replace fuel pressure regulator at fuel rail
-Remove and clean idle control valve with carb cleaner (was a little gunked up but mechanism inside spins freely)
-Remove and clean mass airflow sensor with MAF cleaner
-Remove intake manifold and check vacuum lines (all OK)
-Check resistance of throttle position sensor - got 1.3k ohm at throttle fully closed, 4.15 at throttle fully open. Seems like this should be 1kohm/4kohm exactly, is this close enough? Maybe the elements are worn out right around the closed throttle position?
-Unplug MAF while idling to see if it changes - no real improvement
Next steps I am considering:
-Replace TPS
-Replace ICV
-Replace MAF (and return if it doesn’t help...$$$)
-Replace o2 sensors
-Replace crank/cam position sensors
-Replace DME (I have a red label on hand but it has some type of grounding issue I need to debug)
-Add octane booster or something else to help with potentially bad gas? (It's pretty old at this point but has had sta-bil in it)
Am I missing something? Any low hanging fruit I can try to fix this?? I plan to change over to OBD2 in the somewhat near future instead since I really want useful diagnostics and the ability to flash tunes once I turbo the car (later on down the road), so I’m trying to avoid throwing too many OBD1-specific parts at it if I can avoid it. Thanks for any input!
I've been picking away at it, trying to diagnose it. So far I have done the following:
-Replace fuel filter
-Replace in-tank fuel pump (my car only has one being a late model) - upgraded to Walbro 255
-Replace fuel pressure regulator at fuel rail
-Remove and clean idle control valve with carb cleaner (was a little gunked up but mechanism inside spins freely)
-Remove and clean mass airflow sensor with MAF cleaner
-Remove intake manifold and check vacuum lines (all OK)
-Check resistance of throttle position sensor - got 1.3k ohm at throttle fully closed, 4.15 at throttle fully open. Seems like this should be 1kohm/4kohm exactly, is this close enough? Maybe the elements are worn out right around the closed throttle position?
-Unplug MAF while idling to see if it changes - no real improvement
Next steps I am considering:
-Replace TPS
-Replace ICV
-Replace MAF (and return if it doesn’t help...$$$)
-Replace o2 sensors
-Replace crank/cam position sensors
-Replace DME (I have a red label on hand but it has some type of grounding issue I need to debug)
-Add octane booster or something else to help with potentially bad gas? (It's pretty old at this point but has had sta-bil in it)
Am I missing something? Any low hanging fruit I can try to fix this?? I plan to change over to OBD2 in the somewhat near future instead since I really want useful diagnostics and the ability to flash tunes once I turbo the car (later on down the road), so I’m trying to avoid throwing too many OBD1-specific parts at it if I can avoid it. Thanks for any input!
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