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Old 04-24-2010, 12:06 PM   #1
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Default Misfire at high RPMs

My car has been misfiring at higher RPMs recently, starting around 6000rpms or so, and seems more prominent in higher gears (not noticeable in 1st and rarely in 2nd). I also noticed that this seems to coincide with a significant drop in oil pressure on my stock '90 gauge. I'm curious how the 2 could be related. I know the gauges aren't known for their accuracy and it doesn't seem like a misfire would be a symptom of an oil pump failure (maybe I'm wrong). I'm wondering if it's possibly something electrical?

This is on my turbo'd '90 with a '95 motor, MSPNP.

Things I've done since the problem emerged:
replaced fuel pump
changed the fuel filter
changed plugs
regapped plugs
pulled timing
leaned out and richened fuel curve
installed a larger battery
pulled and reseated injectors
added coil on plugs
flashed firmware on megasquirt and loaded basemap
checked as many grounds as I can find
removed the wastegate actuator so it runs NA
adjusted the boost from the wastegate (~8psi) to 15psi
pulled the motor to install rods, shaved head and new headgasket.

The problem continues after each thing has been changed, nothing affected the miss at all. I figured I'd post up, see if anyone had any ideas or suggestions. I'm kinda running out of things to try. I'll go out and datalog it to post up in a few min.
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Old 04-24-2010, 12:07 PM   #2
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What plug gap are you running?
--Alex
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Old 04-24-2010, 12:18 PM   #3
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Quote:
Originally Posted by pdexta View Post
I also noticed that this seems to coincide with a significant drop in oil pressure on my stock '90 gauge. I'm curious how the 2 could be related.
Well a drop in OP suggests detonation to me detonation would be more prominent under greater laod and higher gears.
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Old 04-24-2010, 12:23 PM   #4
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Quote:
Originally Posted by magnamx-5 View Post
Well a drop in OP suggests detonation to me detonation would be more prominent under greater laod and higher gears.
^^^ +1
Referring to the early NA's OP acting in a det detection gauge. You're *probably* not actually loosing pressure, the gauge just flutters when knock occurs.
--Alex
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Old 04-24-2010, 12:40 PM   #5
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Quote:
Originally Posted by magnamx-5 View Post
Well a drop in OP suggests detonation to me detonation would be more prominent under greater laod and higher gears.
That had crossed my mind, but I pulled 5 deg of timing across the entire map and it continues to do the same thing. Also did the same thing on the basemap which is very conservative. Plugs show no signs of detonation and when I pulled the motor to put the rods in everything looked fine as well.
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Old 04-24-2010, 12:59 PM   #6
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Current msq and datalog attached.
Attached Files
File Type: msq 4.24.msq (35.4 KB, 76 views)
File Type: xls datalog201004241121.xls (284.8 KB, 77 views)
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Old 04-24-2010, 01:28 PM   #7
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Quote:
Originally Posted by AlexO35 View Post
What plug gap are you running?
--Alex
This.
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Old 04-25-2010, 01:42 AM   #8
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Currently running the plugs gapped at .034, I've had them as low as .024 and it didn't help. Also, the problem was happening before and after COPS.
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Old 04-25-2010, 10:36 AM   #9
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I didn't see the word "alternator" anywhere in this thread. could be a weak point as well.
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Old 04-25-2010, 11:08 AM   #10
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I didn't see the word "alternator" anywhere in this thread. could be a weak point as well.
That might explain the voltage drop under load in the log.
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Old 04-25-2010, 12:36 PM   #11
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Thanks for the advise. I put a new alternator on this morning, but it's still doing the same thing. I really appreciate the suggestion though. I think you guys may be onto something with the voltage drop, I went through some older logs and didn't see that same kind of drop.

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If anyone thinks of something else to try, please throw it out there.
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Old 04-25-2010, 01:39 PM   #12
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One of the times I had such symptoms; one of them it was the VAF and the other one a bad distributor.

IMO the alt swap is a waste of money and time, but of course I could be totally wrong.
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Old 04-25-2010, 02:24 PM   #13
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Quote:
Originally Posted by mazpr View Post
One of the times I had such symptoms; one of them it was the VAF and the other one a bad distributor.

IMO the alt swap is a waste of money and time, but of course I could be totally wrong.
Excuse my ignorance, but what is the VAF? Also I'm pretty sure we don't have a distributor, and I've already replaced the coils/plug wires have already been replaced with coil on plugs.

I had an extra alternator already so there wasn't any cost and after all the things I've already tried, I was more than willing to spend 30 minutes swapping it out to see if it helped.
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Old 04-25-2010, 02:26 PM   #14
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Quote:
Originally Posted by mazpr View Post
One of the times I had such symptoms; one of them it was the VAF and the other one a bad distributor.

IMO the alt swap is a waste of money and time, but of course I could be totally wrong.
Excuse my ignorance, but what is the VAF? The air flow sensor? If that's it, I'm using a MAP sensor and it appears to be working fine. Also I'm pretty sure we don't have a distributor, and I've already replaced the coils/plug wires have already been replaced with coil on plugs.

I had an extra alternator already so there wasn't any cost and after all the things I've already tried, I was more than willing to spend 30 minutes swapping it out to see if it helped.
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Old 04-25-2010, 03:18 PM   #15
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run a ground from the battery to the botom of the alternator that cured my voltage drop just fine. if it works maybe voltage drop was causing your coils to not have enough juice.
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Old 04-26-2010, 01:27 PM   #16
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Ran the ground to the alternator and it is still doing the same thing, thanks for the suggestion though.
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Old 04-26-2010, 01:33 PM   #17
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maybe try running less than 10:1 AFR in boost?
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Old 04-26-2010, 01:35 PM   #18
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What sort of coils are you running? You may need an upgrade.
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Old 04-26-2010, 01:37 PM   #19
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looks like he went to toyota coil on plugs.
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