5 Cylinder GTP

[blueguy]

Ok guys...been scratching my head at this for awhile after we got my motor back in...we put in ALL new gaskets and such for everything...didn't skimp...only thing we reused from the old motor was my timing cover, water pump...and Valve Springs/Retainers. So we start it and it misses...random misfire on cylinder #2...changed everything after that...new plugs, MSD sent me a new wire, Swapped coil packs...even ICM's. Thought maybe it wasn't getting fuel but we pulled the plug and turned it over and it shot out fuel so that killed that option...did a compression test and it fluttered from 10-30psi...talked to Brian (Reptile), who talked to Dave (Trannyman95)...and they both said that even if there was a hole in the piston it should put out more compression than that...

So I've come down to a couple things...either...

1) Piston Rings...misfires when driving normal but if you beat on it I get full boost, no miss, shifts GREAT thanks to Dave...

2) Valve Springs...maybe one is sticking? Holding a valve open? Which is why when I beat the crap out of it she opens and closes...

Then there is the oil pressure issue...when I start the car it sits around 50 +/- 5 psi...I start driving it and as it warms up the pressure drops to about 25-30psi. The longer I sits the lower it gets...got down to about 10psi today or so...and when I give it gas she goes back up to about 25-30. I just know my last motor was always consistant and ran about 50-60 all the time and didn't fluxuate like this one does...Is it normal? Not sure, just didn't seem very promising Thoughts on that would be great along with the latter.

Nick

[comongoose]

Wow thats low compression. oil pressure normally fluctuates but I don't know how normal it is for it to get down to 10 psi. I have had cars with blow-bye that went down to OP of 10 psi though. so maybe its one piston ring, that saw too much corrosion. I am not aware of your motor replacement situation though. Dont know if this helps, but thanks for the help on my thread. The cps was the culprit, I guess the cut wires fried the sensor. NOw i just have to get that dang crank pulley back on. BTW did u do a compression test on all the cylinders or just #2. If so what was the compression of all the other ones?

[QwikGT]

i would compression test all 6 cylinders

[blueguy]

All the other checked out fine from around 135ish to 150...pretty normal for a used motor...before we put it in though we compression tested it and it was fine...thats why I am leaning more towards the valve springs...

[Sabrewings]

Could be a lifter is bad and overpumped. That would prevent the valve from closing all the way.

[blueguy]

Could be a lifter is bad and overpumped. That would prevent the valve from closing all the way.

Brand new lifters though...(LS7's)

[QwikGT]

do a leak down test

[comongoose]

i agree u should do a leka down test...you can get a kit cheap from harbor freight. its got to be something up top in the valve train if u had normal compression in that cylinder before u put the motor in. Especially with that low compression its gotta be a valve staying open. Not even improper gaskets could do that.

[minichopper6hp]

did you have it down to the shortblock? Also if you have a hole in the piston or valve it will likely have 0 compression. As for the oil pressure that is not normal. Spec is 60psi@ 1850rpm. Mine cold on startup is 65psi then after it gets really warmed up all the way is 40 psi idle, then when you hit the gas goes back up to 70ish and full throttle 80 psi. I would say to check out that bypass spring or the pump itself, if it is used. Im assuming youre using a mechanical gauge too, not an electrical one.

[BlackGS]

I just had a similar problem with oil pressure. At idle, it would drop to 10psi or less, and over 1500 rpms it would build back up to 50-60psi. The problem turned out to be a crack inside the timing cover right over the pressure spring. This was causing an internal leak and bleeding off the pressure at idle because the pump wasn't spinning fast enough to create enough pull.

You might want to look into the pickup tube gasket, pressure spring, front cover gasket, or oil pump to solve the oil pressure problem. The cylinder problem may be unrelated.