jimep

I've been working on my truck for 3 weeks straight now trying to figure out this problem I'm having and nothing I've done seems to make much of a difference. It's still doing the same bucking thing like it's hitting fuel cut but the check engine light doesn't come on and no codes are thrown. My stoich meter runs ritch when it first starts building boost but when I hit 2,000 plus rpms with boost it goes way past the lean side and thats when it shuts down and occasionaly backfires. I tried it all from presurizing the system and eliminating all the small vacuum/boost leaks I could find to checking and adjusting the tps, checking/swapping coils, ignitors, afm's, distributors, air valve, egr valve, and blocking off anything vaccum operated like the brake booster, bvsv, charcoal canister, pcv valve, and everything else you could possibly think of. Bought new replacement fuel pump and fuel filter when I did the build less than a month ago. Checked the timing. +12* then falls back to +5* when terminal is jumped. Did a compression test and it seems ok. ( my brothers pressure tester is realy beat) Also tried replacing the factory fuel press regulator with an adjustable mallory unit but that only seemed to make it run worse especially at lower pressures. I just tried driving around with a voltmeter hooked up to the fuel pump + wire and it was getting voltage even when the problem started. Are the turbo trucks fuelpumps shut off by the ecu through the ground or the positive wire? My main question is whats the limitations on the factory (replacement) fuel pump. Will it push enough fuel for my build or am I hitting fuel cut even though the computer isn't throwing any codes? Currently running ct 26 turbo, intercooled, re head with os valves, doa c270 cam, rc 370 injectors, topline turbo spec pistons, .066 cometic hg (as close to factory thickness as I could get), and 2 1/2 inch exhaust from turbo back. Sorry this is so long but its at the point where I'm about to burn my truck or beg for all the help I can get. Any advice or tips would be immensely appreciated!!!!

annoyingrob

It doesn't sound like fuel cut, it sounds like a severe loss of fuel pressure. Get a fuel pressure gauge, and hook it to the rail, drive around and see what you're getting. It's entirely possible that the replacement pump you're running isn't big enough, especially if it was a replacement for a non-turbo truck.

At idle you should see around 25-30psi, hitting about 38psi by the time you hit 0psi in yoour intake, then it should rise linearly with boost. If you're boosting 8psi, you should see 46psi of fuel pressure for example.

If you need a new pump, I would suggest getting a hold of a 7M-GTE fuel pump (MKIII Turbo Supra). I've flowed 400whp out of them with stock fuel lines.

jimep

Thanks man! I just needed someone to confirm what I suspected. I did some more checks today and found some more things on my own. The fuel pump will still run without the ground wire from the plug connected (pump's grounded to tank) so it isn't hitting fuel cut just like your saying. I already have the pump out and am going to see what I can pick up tommorrow when the parts stores open up. and Thanks for the recomendation on the 7mgte one cause that was going to be my next question. Wish I still had those spare parts from my old supra. I wonder if the old lady would notice if I took the one out of my lexus (her daily baby hauler). Hmmm!

Dirt Driver

Have you completely ruled out the ignition system as a problem? How about a rich missfire? Missfires will show a lean mixture on an AFR gage whether they are caused by too little fuel, too much, or the ignition itself.

annoyingrob

One way to check would be to pull the plugs. If it's running that righ there will be a lot of carbon on them, and likely some raw fuel.

But the fact that he said it got worse when he lowered the fuel pressure tends to lead me towards not wnough rather than too much. If there was too much fuel, dropping the fuel pressure would make ir run better.

You are correct that both will show up as a lean condition though. The best way to set it up would be with a wideband o2 sensor, that way you could watch it approach lean or rich.

Dirt Driver

That does make sense. I overlooked the part about lower pressure making it worse, but I also thought it was suspicious that it would start rich at boost onset and immediately swing lean and start missing. Usually running out of fuel manifests itself as a more linear transition into a lean mix.

Jimep, if it runs worse with lower pressure, how does it run with 50 or 60psi of base pressure? 370cc injectors are enough for at least 200bhp at their rated pressure, but they can support 250+ if you turn the screws to them. Also, how much boost do you have by 2000rpm when it breaks up?

jimep

Sorry to bring up an old post but i just realized I never responded with the answer to my problem. I think my old laptop took a crap on me around the same time... Thanks again to annoying rob for helping me figgure this one out because I was seriously tempted to light my truck on fire... the issue was junk in the fuel tank that i didn't see when changing the pump was plugging up the prefilter and causing me to lose fuel pressure.... now if anybody does a search on intermeitent bucking, occasional backfireing or just random issues that seems to come and go along those lines this could be the issue.

Dirt Driver

At least it was something simple, right?

annoyingrob

Yaaay! I solved a problem!

abecedarian

And for the record, we need to say that the "fuel cut" is not the ECU cutting the fuel pump but rather is the ECU not firing the injectors.

jimep

ahh! learn something new every day! Thanks for the correction Abe

annoyingrob

yeah, if the ECU cut the pump, it would drop the pressure, and would cause a lean condition that could damage the motor. Little fuel is a lot worse than no fuel.

The alternative is to just not fire the coil, but I don't believe the ECU does it that way, it's bad for emissions.

Dirt Driver

Ignition cut can/will also nuke the cat converter. Another reason why manufacturers use fuel cut even on NA engines.

Ignition cut makes fun noises if there is no cat, though.

annoyingrob

Yeah, the aftermarket ECU in my Surpra does that. No cats, and ignition cut, I can blow fireballs out my tailpipe at the rev limiter.

abecedarian

My 78 VW Scirocco would do that, but no ignition or fuel cut. Just a 2.25" exhaust pipe that, when I revved it out then closed the throttle, the injection couldn't cut the fuel fast enough and it would hit the converter and go bang. A friend of mine almost dumped his old GPz when he was following me too closely.