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Discussion Starter #1
I have a 90 gtst that has the common misfire problem. It happens at 40-100% throttle around 4000-5500 rpm. I can take the car all the way to redline at 25% no issues. The car holds boost all the way even when the car is misfiring/sputtering. The car idles fine and runs fine otherwise., and occasional i can do a full pull 100% throttle, however it is random. I have tried three different spark plug gaps and two types of plugs and i have replaced the coil packs and the issue still remains. The car idles fine and it is running on all 6 cylinders, so i do not think it is the igniter. I am now stumped because i thought this was a spark issue and now i am thinking fuel or maybe a bad maf?
 

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Sounds like coils to me. Are they New?

To rule the maf out you can unplug it when when the engine is running and see if it behaves differently.

Fuel filter is easy (and cheap) to change too.

Is your fuel pump original?
 

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Also worth doing a compression test. Worst case it can be Ring Lands !

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Discussion Starter #5
Sounds like coils to me. Are they New?

To rule the maf out you can unplug it when when the engine is running and see if it behaves differently.

Fuel filter is easy (and cheap) to change too.

Is your fuel pump original?
I replaced them with new coil packs and it did not change anything. I have a buddy who is going to to give me a new maf, but i will try unplugging it. I also, will check the fuel pump.
 

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Have you also checked the coil wiring and the igniter, these can all give trouble,
Are there any engine mods? what grade plugs have you got fitted?
IF its fuel the first thing you need to check is the FPR and make sure the vacuum line is connected and not split.
 

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Discussion Starter #8
Have you also checked the coil wiring and the igniter, these can all give trouble,
Are there any engine mods? what grade plugs have you got fitted?
IF its fuel the first thing you need to check is the FPR and make sure the vacuum line is connected and not split.
The coil pack harness is crusty, but it is all the plugs are in tact and the wiring going into them in not frayed.
The car has a 3in dp but the stock elbow, and it has a CBE other than that it is stock.
The hose to the FPR has been replaced with a new one.
Today when i was driving the cut was much harder and at 3000 rpm. The car was still in boost but it was fluctuating up and down when it was cutting.
 

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What plug gaps are you running?

I was running 0.8mm and had similar issues. Reduced the gapping to0.6mm and its fine now.
 

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Discussion Starter #10
Have you also checked the coil wiring and the igniter, these can all give trouble,
Are there any engine mods? what grade plugs have you got fitted?
IF its fuel the first thing you need to check is the FPR and make sure the vacuum line is connected and not split.
So for the past month i have been trying to work out this issue still. The problem i am having now is my missfire/sputter/whateverthisis is different. I have dropped the fuel tank, cleaned it out, replaced the fuel pump, put in a new fuel filter. Occasionally my car will hit the 4k sparkcut missfire, other times i cant even get into boost and go over 40mph, and lastly the car just sometimes works. I am really stumped at this point.
 

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My money would be on the maf, before trying a new one why don’t you clean the old one? Remove it spray a little bit of carb cleaner on the wire, let it dry and refit


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