Stumped by my misfire!

[mold]

Hey guys, as the title says, I'm having some issues with misfiring.

I have just replaced the coils in my 1990 gtr with some Yukon coils (AC Delco d585). This replaced the coils, wiring, and ignitor. The reason I replaced the coils in the first place was due to a miss. I tested the factory coils and they were arcing from the sides. After the new coils It still had a miss, so I replaced the old copper plugs with some platinums. It then ran fine for about 3 weeks, then started missing again.

I then replaced the plugs again with some iridium plugs. 2 weeks later and its missing again! The Iridium plugs i used were NGK BKR6EIX-11

All of the plugs that I have taken out have looked fine. No fouling, no cracks, no visible signs showing anything wrong. I also ohm testing the "bad" plugs and they are within spec.

Every time I replace the plugs the miss goes away for a bit only to return. I am running out of ideas here and don't have an unlimited money supply to keep throwing parts at the car hoping for a fix.

Other things recently tested/ruled out:
CAS
MAFS
Fuel Pump
Fuel Injectors (all firing but not really ruled out yet)
Boost/vacuum leaks
Fuel filter (new less than 5k kms ago)
Compression

I have yet to check the ignition timing and will be doing so this weekend. Could this cause the problem I am having?

TPS has a dead spot on it and needs to be replaced, but I don't see how this would be temporarily fixed with new plugs. I have one on order but it is at least 2 weeks away. (Just so everyone knows a 1996 maxima tps is NOT the same! haha It fits but the .5-5v signal is flipped so you have 5v closed and .48 wide open.)

Any ideas would be much appreciated! Even if they are a long shot, I NEED to get this fixed as my GTR love is going for a **** at the moment!

Thanks!

[ARC240]

When does the miss occur? If it's under boost that it begins to misfire, you have to look at: timing, ignition system (unless you're sure you can rule this out), fuel delivery (next paragraph).

Have you checked your fuel pressure with a gauge? Hook one up, rule out the engine starving of fuel. This could be an FPR issue, as those are commonly intermittent, especially if this problem is occurring under boost. Could also be the fuel pump not flowing enough. Hook up a pressure gauge and drive around with it, monitor pressure, it should rise under load, if the needle dips under load while the misfire happens, there's a fuel system issue.

I'd run coppers, not iridiums, iridiums are known to break their fine spark electrode off inside the cylinder, typically resulting in a cylinder failure. There's no advantage from a properly gapped and good condition copper plug to an iridium aside from extended service life with iridium.

[r33_gtst]

When does the miss occur? If it's under boost that it begins to misfire, you have to look at: timing, ignition system (unless you're sure you can rule this out), fuel delivery (next paragraph).

Have you checked your fuel pressure with a gauge? Hook one up, rule out the engine starving of fuel. This could be an FPR issue, as those are commonly intermittent, especially if this problem is occurring under boost. Could also be the fuel pump not flowing enough. Hook up a pressure gauge and drive around with it, monitor pressure, it should riseder load, if the needle dips under load while the misfire happens, there's a fuel system issue.

I'd run coppers, not iridiums, iridiums are known to break their fine spark electrode off inside the cylinder, typically resulting in a cylinder failure. There's no advantage from a properly gapped and good condition copper plug to an iridium aside from extended service life with iridium.

a common problem for our cars is the fuel pump is not directly wired to the battery and when upgrading things like fuel pump and turning up the boost, the **** there isn't enough power to the fuel pump. my feel pressure was dropping 10 to 15 psi under heavy boost but was not causing a misfire for me. when my fuel pump was wired directly to the battery in the car pulled stronger and made an additional 35 wheel horsepower. I also went through a ton of issues with misfire the first year I had my car. Got it all taken care of when my tuner re did my ground wires and a few extras

[mold]

The misfire is at low rpms, around 2000-3500. Once i get above that it goes away until I hit around 4500-5500. It seems I can power through the misfire until I get into the higher rpm range then it misses badly. Fuel pressure tested fine with fpr vacuum plugged in and unplugged. Have not checked fuel pressure under driving conditions. I will do that soon.

Checked timing and it was at 30 degrees! I changed it back to 20 and it is still missing. I also switched back to copper plugs and no change there either.

The guys on gtroc seem to think it's the ECU. but I am going to eliminate all other possibilities first. When I get the new ECU I don't want to start tuning if there are other issues with the car already.

Thanks guys.

[Frankyy]

platinum plugs are crap, use bkr7e-7 if you have a fairly stock setup

second, the ecu is tuned for the good gas in japan, my car had a miss and I couldn't eliminate it at all, the ecu got tuned and the miss went away it was nothing else but the stock tune on the ecu


I strongly... ssttrroonnggllyy suggest scheduling a tune with Robin Almasi in Montreal, definetly worth it, your miss will go away and you will make a few more hp of the stock setup

[DAAPGTR]

Had same problem. Ended up being a small pin hole crack in one of my injectors...

[Skym]

Yukon coils (AC Delco d585)

Did you adjust the dwell settings on ECU to suit the aftermarket coilpacks???

I then replaced the plugs again with some iridium plugs. 2 weeks later and its missing again! The Iridium plugs i used were NGK BKR6EIX-11

The misfire is at low rpms, around 2000-3500. Once i get above that it goes away until I hit around 4500-5500. It seems I can power through the misfire until I get into the higher rpm range then it misses badly.

Why I ask, as I read this and that relates to ignition breaking down due to weak spark at higher rpm. Can regap sparkplugs to 0.8mm or smaller (be careful of touching the centre electrode part thus damaging it) to stop the missing until can sort out the coilpack dwell settings on ECU (someone on here found the stock dwell settings are good to around 20psi before ignition breaks down).

Also missing (engine stutters, similar to coilpacks missing) can be caused by airleaks around MAF's, so double check that there isn't any airleaks infront of MAF (hits boost cut on ECU), behind MAF (2x airsources makes MAF + ECU make engine run rich?).

[caliber676]

It's easier to simplify things rather than get a bunch of random ideas around the board. Combustion has three parts. Air, fuel and spark. You seem to have done all your work with the fuel system so if your testing is accurate you can rule that out. Now onto air, check for vacuum leaks or "false air behind the maf, use brake clean to source leaks, test the maf as this regulates air. Now ignition related you've tested timing so your spark is happening at the right time. A quick check I always do is look at the story of the plugs. They will show you if it's a lean misfire, rich misfire, etc. go back and check this against a picture chart to make sure.

Now you've said that there is a flat spot on the tps, I would replace this before further diagnosis. You've found a problem, it needs to be fixed, it's possible that it may not fix it, but cars act strange and if it does, you'll be wasting a lot of time overlooking that.

If your spark is fine and your air supply is ok then you will have to dig deeper into electrical and pin point testing. Think what can cause a misfire, ignition related, that is constant even after replacing the ignition system. It's not plugs, it's not coils, it's not the ignitor. What that leaves you with is voltage supply to the coils, grounds and ecu.

In cases like this where you don't have a labscope to dig into the electrical system with detail, i would take it to a compotent shop. Tell them your issues, and what has been replaced. If they're good they will fix it the first time with the proper tools and you'll be happy.

[Dragon Humper]

My CAS was my miss issue. Popped on another used one and all was well.

Did you test it with a new/different one?




Jon.

[nick-nismo]

i have thoes coils and i ran into some miss firing issues untill i did these 2 things.

thard wire the coil packs via relay to direct batter voltage with some good 12 guage, thoes puppies need a lot of power
or if you haven't done so already pick up some msd or ngk wires to go along with thoes coils,

bkr7e for the win