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Old 12-25-2012, 10:43 AM   #21 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

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However, I have a sneaking suspicion that I may have an valve or piston ring issue. Anyone know a good indicator of piston ring condition? I'm not about to remove the headers.
While some may reccomend a bleed down test I still prefer the dry/wet compression test. Pull all your plugs so it cranks easy and please please please disable your fuel pump. the numbers you get should be within 12% of each other, wet should bring those numbers up but not dramaticly, big increase usually means rings, low with no change is valves.

whatever happend to a simple vacuum test anyway, lol.
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Old 01-07-2013, 07:22 PM   #22 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

I finally got around to doing a compression test yesterday. I got the following numbers on the dry compression test.

Cylinder #1: 171psi
Cylinder #2: 160psi
Cylinder #3: 160psi
Cylinder #4: 170psi
Cylinder #5: 161psi
Cylinder #6: 175psi

This yields a max pressure difference of about 9%. I wasn't expecting these numbers to be so high. Therefore, I didn't even perform a wet compression test. Anyone think it's worth doing a wet compression test or leak down test?
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Old 01-07-2013, 10:09 PM   #23 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

the wet test would confirm/deny pressure loss was at the rings, pressure should come up slightly for good rings, more on bad rings, none for valves.
the n/a dohc 10:1 cr is in the 180-185 psi range iirc. so overall yours looks good per miles. under 150 is where i would worry, or a big difference hi/low.

A bleed down test will help you determine if it's the rings or valves and wich ones while the heads are still on the car.

I ask this, if i am going to pull heads for a valve job do i need to know wich ones beforehand ?
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Old 01-07-2013, 11:20 PM   #24 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

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the wet test would confirm/deny pressure loss was at the rings, pressure should come up slightly for good rings, more on bad rings, none for valves.
the n/a dohc 10:1 cr is in the 180-185 psi range iirc. so overall yours looks good per miles. under 150 is where i would worry, or a big difference hi/low.

A bleed down test will help you determine if it's the rings or valves and wich ones while the heads are still on the car.

I ask this, if i am going to pull heads for a valve job do i need to know wich ones beforehand ?
No. Most good machinists can feel which valves need new valve guides. All seats should be reworked while in the shop anyway. Don't forget to have them milled to insure flatness. Aluminium heads tend to warp when pulled. You can also do your own leak test with the heads off. Just fill the chambers with water and let sit for 1-3 hours. Leaks around the valve seats become quite apparent.

Doing this testing and documenting your findings will make you feel better once you get a report from the machine shop. If there findings match yours, you sleep at night. If they do not, take them somewhere else.

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Old 01-08-2013, 12:35 AM   #25 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

I know you are grasping at straws but the evidence is still in your timing light. A steady blink should be on all cylinders. If you cant find components bad there has to be a signal bad. I know you changed coils but what about plug wires? Sometimes the silliest things are staring us in the face when we finally find the problem.

Leakdown test is the BEST. Period. It tells you EVERYTHING you need to know about an engines internals. I do it on everything I wirk on just so I'm not chasing my tail when something doesnt work right.

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Old 01-08-2013, 12:26 PM   #26 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

I know you said you tested the PTU, but i've had a lot of problems with semi-bad PTU's. One day they work fine, the next day it runs fine until it warms up and it will randomly start misfiring. Of course the PTU bench tests fine b/c that would be too easy.

Over the years any ignition problem I've had was a bad PTU.

Since going back to stock injectors/ ecu made your problem seem better could be another indicator.

Maybe with the bigger injectors not being tuned properly yet its harder for the weakened PTU to supply enough voltage to fire the plugs.

Also look carefully at the PTU connectors. I've seen wires semi pulled out of the connector that was making a bad connection.

I would also check to make sure your timing marks line up.

Cams are defiantly in order? I vs E
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Old 01-08-2013, 01:21 PM   #27 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

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Originally Posted by soccermania688 View Post
I know you said you tested the PTU, but i've had a lot of problems with semi-bad PTU's. One day they work fine, the next day it runs fine until it warms up and it will randomly start misfiring. Of course the PTU bench tests fine b/c that would be too easy.

Over the years any ignition problem I've had was a bad PTU.

Since going back to stock injectors/ ecu made your problem seem better could be another indicator.

Maybe with the bigger injectors not being tuned properly yet its harder for the weakened PTU to supply enough voltage to fire the plugs.

Also look carefully at the PTU connectors. I've seen wires semi pulled out of the connector that was making a bad connection.

I would also check to make sure your timing marks line up.

Cams are defiantly in order? I vs E
you think an ignition amplifier would do the trick?
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Old 01-08-2013, 06:02 PM   #28 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

Thanks for the words of advice everyone. I'm gonna do a leak down test this weekend, and I'll post what I find.

speedy25 - The spark plug wires were replaced with ACCEL low ohm wires.

soccermania688 - I've replaced everything in the ignition chain, except ECU PTU and wiring harness. When I warmed it up for the compression test, I threw on the timing light, and some of the other cylinders were misfiring more than usual. I just ordered a new OEM PTU, so we'll see if that fixes the problem.

kelvinTurbo - From what I've read, our stock ignition coils are rated higher than most aftermarket ones. At most, I think an amplifier would only compensate for another problem.
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Old 01-08-2013, 06:31 PM   #29 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

If I'm not mistaken, numerous members have had issues with the 'quality' of the Accel's in the past.
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Old 01-08-2013, 07:47 PM   #30 (permalink)
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Default Re: Mystery Misfires after 1-2 min of running

Do you have OEM wires you could test out?

Kelvin- i wouldnt put any aftermarket parts like that on my car until it was running right with stock parts. Unless your above stock HP and trying to stop spark blow out. Your having the same problem as the OP?
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