High speed missfire

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DaveBrooks

New member
I just ran pocono this weekend.Just installed a fresh motor,sohc.Fired right up in the shop,let it run in for a while. I figured everything was good. All I got was two laps then the motor started to missfire and loose power. This happened at about 5,000rpm. Poped the hood found a vaccum line off at the throttle body. I put it back on and it fired up and ran fine. Went back put for the race and it did the same thing again. I talked to my mechanic and he said that it could be the speed sensor. Possibly making the car thinking it was going a million miles an hour and that the speed limiter should cut in. So i changed that and the pcm, Car did the same thing again. Anyone have an idea.
 
Codes?

Some Ideas

Ignition wires
Engine Harness
Injector
Spark Plug
Valve Spring
coil pack
VSS
Cam Sensor
Crank Sensor


I have seen all of the above cause this on Neons. I listed them is order of probability

you can emaill me at [email protected]

Cheers
 
DaveBrooks said:
I just ran pocono this weekend.Just installed a fresh motor,sohc.Fired right up in the shop,let it run in for a while. I figured everything was good. All I got was two laps then the motor started to missfire and loose power. This happened at about 5,000rpm. Poped the hood found a vaccum line off at the throttle body. I put it back on and it fired up and ran fine. Went back put for the race and it did the same thing again. I talked to my mechanic and he said that it could be the speed sensor. Possibly making the car thinking it was going a million miles an hour and that the speed limiter should cut in. So i changed that and the pcm, Car did the same thing again. Anyone have an idea.
Click to expand...

You can also check the TPS with a ohm meter - it's a potentiometer. I've had issues with these at WOT in the past..... Though as the previous post has stated, the wiring harness is a known weak point - Since it's the most expensive - I'd try to rule out the others first.
 
does the idle change if you unplug the MAP/intake air temp sonsor?

if not, it(or the harness to it) may be the issue... should idle high with it disconnected.

will it rev past 5000? if so this may not be it...
 
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