Quote:
Originally Posted by kslizzy
At the track yesterday and my car started misfiring halfway through my second run session. Going into a hard braking corner, turned, eased back on the throttle and crazy misfire. Limped up the hill towards the pit entrance and it started running fine. I thought it was maybe fuel starvation even though I had an indicated half tank. The turn leading to the problem is a fast, uphill right hander into a tight left hairpin. I ran another lap and again after a hard braking zone the car misfired. Got it to the pits and it would not stay running without throttle input and even so it was very rough. I waited, thinking and hoping it was a vapor lock in the 90 degree heat. 2 hours later it still wouldn’t run. I trailered it home another 2 hours. It started to get it in the garage and it at least ran on its own but not well at all.
I replaced the fuel pump, fuel filter, cap, rotor, plugs two winters ago, maybe 2000 miles. Wires are probably 7-8 years old, 10-15k miles. I checked and had positive connection at cap and plugs.
Car is a 3.2 with Steve Wong chip, cat by-pass and M&K exhaust.
Any ideas?
Thanks!
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I see issues like this quite frequently on cars when they come in to get dyno tuned. Although ideally when a car is brought in the car should be functioning 100%, half the time they are not, even fresh from the engine builder. The dyno tells everything, especially when the power is not coming in where it should or the, or the air fuel ratios are not coming in where they should statistically.
In no particular order, and not saying which is or isn't a possible cause on your car, this is what I see a lot. Some of these are very difficult to diagnose and take a lot of educated analysis and testing:
a) erratic part throttle misfiring and rpms cutting out at high rpms over 6000 rpm. Cause: 1 or 2 bad fuel injectors found as verifed by a IR temp reading off each header. I had 3 cars like this in the past 12 months.
b) high rpm cutout. Cause: due to overgapped flywheel sensor past 0.8mm, or aftermarket flywheel pin requiring a closer sensor gap to not cut out.
c) rpm misfiring and cutout at load and upper rpms. Cause: overzealous spark plug gapping causing the spark to not fire
d) random misfiring or cutouts. Cause: Failing/defective DME unit or DME relay causing intermittent spark or power
e) lean running condition and misfiring and cutouts at high rpms. Cause: Cars were an engine transplant with a non-Carrera tach converted by XYZ Speedometer that overdrove and blew out the IC in the Motronic DME - VERY COMMON!
f) same as above eventually blowing out the DME: Cause: Tapping into the upshift indicator wire from the DME for a tach signal for a aftermarket tach/upshift light/data logger etc, blowing out the DME
g) car running lean causing misfiring or maxing out the injector capacity: Cause: aftermarket fuel pressure regulator set below factory pressure
h) Twin plug motors or 3.6 engines not producing the power they should. Cause: one side of the twin ignition has failed. Failures include bad ignition modules, broken belts, and bad coils
i) 3.6 engine power lacking and peaking at 5500 rpm. Cause: faulty resonance valve operation
j) Dyno shows only 80% power and air fuel curve off. Cause: faulty WOT switch or car is not achieving full throttle, only opening 2/3'rd
k) air fuel ratios coming in at the rich end of the statistical range. Cause: faulty head temp sensor, including the newer two wire versions. Last one measured 1kohm hot, in the mega-ohms cold.
l) air/fuel ratios coming in at the lean end of the statistical range. Cause: air flow meter previously tampered with and internal clock spring tightened for whatever reason previous. Sometimes also done on grey market cars by the importer to lean out the mixture.
m) same as above but coming in very rich overall. The converse of the above where the air flow meter as tampered with and clock spring loosened, probably someone's attempt to 'tune' the car
These are just some of what I have seen and I don't remember them all and, but it happens so often it's frustrating for everyone as it drags out the day. But I can't always fault the owner or builder because a lot of this is hard to see or experience unless the engine is loaded such as on the dyno or being driven such as under racing conditions. Better on the dyno of course so the issue can be addressed rather than in on the road or track, but if you can't figure it out and don't find anything mechanically faulty, you might want to dyno it (preferably a Dynojet) and send the results to me to evaluate.