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Discussion Starter #1
I've searched for something similar in the history, but no luck. CPS' and these two coils have been replaced and I found a post-MAF vacuum leak on the same bank, but it still persists.

The misfires only happen when the car is fully warmed up, and after I pull over and restart the car it will re-occur after anywhere from 1 to 20 minutes.

Before I go too far down the road with A/B testing -- is there any significance to the fact that cylinders #1 and #3 always fail together? I'm not familiar with exactly what is shared between cylinders, but this symptom makes me think that it could be an electrical problem.

Thanks,
Mark
 

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Discussion Starter #2 (Edited)
One more thing to mention - the car seems to run perfectly until the computer shuts off the fuel to these two cylinders. C4 & C6 are the Peake codes for misfires with fuel shutoff. I believe this happens whenever there are 3 misfires within 200 revolutions of the engine.

It seems like these two cylinders always misfiring together (but never #2 & #4) would rule out some if not most of the possibilities, like fuel injectors or the fuel pump.
 

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and these two coils have been replaced
Did you do any other testing or moving of the other coils? Misfire detection can be a little funky with this engine. My guess is your trouble cyl is really 6. If it was an injector issue on 6 and that camber had too much gas left in it that would slow the opposite cylinders fire, which is 1 and 3 fires after 6 so a bad fire on 6 might get picked up on 3.
The easy place to start would be to move(to cyl 4) or replace coil 6 with one of the coils you took out. Might be coil but I suspect that will not do much but it is easy. Then I would do a static fuel pressure test and watch how fast the pressure drops when you turn of the fuel pump. Next you would run the pump and immediately pinch the line from the tank to confirm the pressure lose is not back to the tank which is common. Just depends how fast the pressure drop is. If it is not too fast you will pinch the return line also. The problem is there is no easy way to tell which injectors are leaking as they leak on closed valves so you have to pull both raills and set them up so the pump can be run and you can then see which are leaking.
 

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Discussion Starter #5
Thanks for the kind replies. You guys are rock stars.

There is one more piece of useful information: the previous owner let the car sit for about 6 months with minimal driving before selling it.

Unfortunately, the car must have seen mgdmb and Sailor24 on this thread and gotten scared -- the misfire w/cutoff code hasn't happened since Friday morning (4 days). I let it sit over the weekend (snow storm) but I have driven it quite a lot the last 2 days.

Next time I have the plenum off, I'll try the fuel pressure tests.
 

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Next time I have the plenum off, I'll try the fuel pressure tests.
It is better to do it under the car. So the gauge and all the lines are together so you can pinch a line and see the gauge. You could save yourself some work by throwing in a bottle of techron and then following it with a double dose of Lucas Upperlube. If neither of those has any effect then it might not be injector related, it still could be broken but since it seems intermittent that makes it more likely cleaning then lubing the injectors would have some effect.
 

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In the same tankful of gas, or 2 separate treatments?
Frankly not positive the two products are compatible so I guess for safety sake in separate tanks. I can't see there being a problem, but I am not a chemist. I have re thought this a bit though from something you said and something that came to mind with an old 540 that used to get stored for long periods of time. Maybe try some Lucas upperhead lubricant first. The 540 would have the injectors stick from sitting and the Lucas was an instant(5 min of run time) cure. So maybe try the Lucas first tank then the techron the next. The Lucas comes in a big bottle and is meant to be used in every tank so after the techron tank go back to the Lucas. I have always used Lucas in every car to avoid injector problems and it keeps deposits from forming so I never need cleaners. I also never clean injectors I just replace them so I am content with using the Lucas to extend the injector life, if it does. Not saying it is the right way just the way I do it.
 

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Discussion Starter #9
OK thanks! After your post, I found some of your old messages about replacing fuel injectors. Very informative.

Also - congrats on joining the 10,000-posts club. You might even be the founding member for all I know.
 
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