944-Spec#94 Posted August 22, 2009 Posted August 22, 2009 Ok, Now that Jim's is taken care of time for mine. Guys, Looking for some advice/experience so i don't need though parts at this issue. My car has a warm idle sumble. What happen is that I can start the car and it runs fine. Cold idle is good. It does idle about 100 rpm high, but nice and smooth. (88 motor, 88 controls, stock DME chip) So I take it out on track and run it when I pull in it still idles at about 1000 rpm. However if I let it idle for 5 minutes it will occasionaly stumble and stall. Next session of the day I pull pu on grid with a warm, but not fully up to temp motor and it occasionaly stumbles at ldle again. If I give it gas I can get it to recover some times. Most of the time it will restart, but on occasion it will crank, but not fire. Wait 1-2 minutes and it comes back. On track at full throttle the car run perfect with no misses and full power. I am thinking it is an electrical issue since it can work fine and then just die for no reason. DME temp sensor seems to be fine, but I have never measured temps when it stumbles. AFM is working fine too. I have replaced the O2 sensor with no effect and even run without the O2 will no effect. (remember this a non-street legal track car with no cat). TPS has been checked. So anyone else have warm idle random stumble? Did some additional diagnostic testing today. Swapped DME and DME relay. No change. I did only Idle testing. Anyway warm the car will idle just fine then drop idle then come back. I did alot of contact cleaning today and there were no stalls just the same issue. Also of note is the at DME relay clicks when the idle stumbles. I swapped relays and it did the same thing so I don't believe the issue is in the Relay and the swap DME rules out a bad DME. No driveablilty issues on track under power. Only at idle. I have replaced the O2, but maybe it is bad? Quote
Tim Comeau Posted August 22, 2009 Posted August 22, 2009 "Also of note is the at DME relay clicks when the idle stumbles." Joe, The thing that turns the DME (or fuel pump relay) on is the key going to position 3. After you release the key, it relaxes to position 2 and the engine runs. Ignition switch? I don't think so yet? Funny thing is it works at throttle under load. So it's an idle issue? Computer was swapped. It's an 88 engine? Check that PCV (positive crankcase ventilation) unit on the rear of the manifold. If that warms up and opens, it introduces a sudden vacuum leak? That could be it. Also, at idle, the TPS (thottle position sensor) tells the DME, "Hey, I'm at idle. Give me just enough gas to idle." Anything over 8 volts (partial throttle) from the TPS and it hands over fuel flow control to the AFM (air flow meter), until you hit WOT (wide open throttle) , in which case the TPS retakes control and sends the WOT signal to the DME for max fueling. I think that PCV valve cycles as needed? It only vacuums up the oil vapors in the crank case as needed? Under racing conditions, you'd have no change in vacuum. It would be at zero. Quote
944-Spec#94 Posted August 22, 2009 Author Posted August 22, 2009 It is not ignition switch related. I have the stock switch by passed and run through other switches. These are good. PCV? The motor is an 88 (block , head & pistons), but the intake and plumbing is from an 87. DME is an 88, but when swaped with an 87 DME the issue remains. TPS is not an issue. I tested good and unplugged or not the problem remains. Quote
Tim Comeau Posted August 26, 2009 Posted August 26, 2009 Ok. Only at idle. Like I said before, there's no vacuum issue at speed, but there is at idle. How about the aux air regulator, Joe? Quote
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