Guys, PLEASE HELP
Thread Starter
On The Tree
Joined: Jul 2003
Posts: 140
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From: RPM
Guys, PLEASE HELP Our car keeps popping at random times and is never consistent...it has little "hiccups" per say at higher rpms....like anywhere from 5700-6100 where the car is pulling on the dyno fine and then hiccups and loses about 25-30hp and then picks right back up where it was before it happened. This is a solid roller 427ci with no knock sensors, so that is not the problem. We have been over everything it seems and cannot figure this out....any help would be MUCH APPRECIATED! Tuning in HP Tuners BTW.
TECH Senior Member
Joined: Nov 2001
Posts: 7,036
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From: Lake Zurich, IL
Does the AFR go real lean at the same time? (losing fuel pressure for some reason or an injector cutting out)
Does AFR go real rich? (spark issue)
Valve springs are definetly up to the task for the cam selected? (and the lifters are up to par?)
Key is what happens to the AFR when this happens ... if you have the scanner hooked up also look at the wideband (not sure what your exhaust setup is... i'm guessing true duals or open headers?) but if you still have the narrowband O2s operational see if 1 side becomes much different from the other when this hiccup occurs...
Basically I'm looking to see if the AFR on 1 side of the engine changes during the hickup while the other side remains the same (hard to tell with true duals (w/x or h pipe) or a y-pipe since the exhaust gas mixes ... if you can install the O2 sensor in the narrowband spot and force the car to run open loop via HPT, you can do a pull , 1 for each side, and see if either side changes when the hiccup occurs
Also w/the scanner hooked up (assuming you have a MAF attached) monitor the MAF readings... when it hiccups check to see if there's some reversion going on (MAF readings real high like 55/60lb/min then dropping then coming back up) ... if it drops like that it could be from air being forced back out through the intake manifold (out of the cylinders) which could be an indiciation of a broken valve spring or valve spring that's not closing the valve fast enough
Does AFR go real rich? (spark issue)
Valve springs are definetly up to the task for the cam selected? (and the lifters are up to par?)
Key is what happens to the AFR when this happens ... if you have the scanner hooked up also look at the wideband (not sure what your exhaust setup is... i'm guessing true duals or open headers?) but if you still have the narrowband O2s operational see if 1 side becomes much different from the other when this hiccup occurs...
Basically I'm looking to see if the AFR on 1 side of the engine changes during the hickup while the other side remains the same (hard to tell with true duals (w/x or h pipe) or a y-pipe since the exhaust gas mixes ... if you can install the O2 sensor in the narrowband spot and force the car to run open loop via HPT, you can do a pull , 1 for each side, and see if either side changes when the hiccup occurs
Also w/the scanner hooked up (assuming you have a MAF attached) monitor the MAF readings... when it hiccups check to see if there's some reversion going on (MAF readings real high like 55/60lb/min then dropping then coming back up) ... if it drops like that it could be from air being forced back out through the intake manifold (out of the cylinders) which could be an indiciation of a broken valve spring or valve spring that's not closing the valve fast enough
Thread Starter
On The Tree
Joined: Jul 2003
Posts: 140
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From: RPM
Ok, the a/f does change when this happens, but only minimal...if the a/f reading before the hiccup is 12.6, the a/f leans out the 12.8 for that split second. This car does not have O2 sensors at all, so we cant see bank to bank differences....also, this car is tuned in SD, so no MAF....hope this helps...
Thread Starter
On The Tree
Joined: Jul 2003
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From: RPM
Oh and also, we have ruled out the valvetrain completely....pulled the heads on the car last night and went over everything because we were getting a 300 cranking compression reading only in cylinder #1 when all other cylinders were 200-225...no broken spring, no collapsed lifter, not water in the cylinder, valves doing what they are supposed to...the only other thing we thought was the cam, so we changed camshafts. Start the car tonight on the dyno and the problem is just as bad if not worse. Also the car doesnt want to start everytime, when you let it sit for a few minutes, it starts fine. We are swapping wiring harnesses now to see if its a bad wiring issue (the original wiring harness has been modified).
