General [RESOLVED] Fuel cuts and misfires

[2Gracing]

Having an issue with my 96 GSX, cold starts are rough with an idle of around 500-600 for the first few minutes and once it starts to warm up it’s goes to about 1000. Recently while driving and getting into boost I get a fuel cut (hitting 7-8 pounds) also after the car is fully warmed turning it off and letting it cool for about an hour and then starting it will result in a bad misfire every single time and will go away once you drive away.

Not sure if this is part of it but when the AC is turned on the idle drops down to a bumpy 300-500 and the car try’s to kill it self when revved or when shifting. Things like fuel filter, fuel pump, MAP sensor, coil packs, spark plug wires, and boosts leaks have all been tested and are normal.

Any help is greatly appreciated.

 

[pauleyman]

Having an issue with my 96 GSX, cold starts are rough with an idle of around 500-600 for the first few minutes and once it starts to warm up it’s goes to about 1000. Recently while driving and getting into boost I get a fuel cut (hitting 7-8 pounds) also after the car is fully warmed turning it off and letting it cool for about an hour and then starting it will result in a bad misfire every single time and will go away once you drive away.

Not sure if this is part of it but when the AC is turned on the idle drops down to a bumpy 300-500 and the car try’s to kill it self when revved or when shifting. Things like fuel filter, fuel pump, MAP sensor, coil packs, spark plug wires, and boosts leaks have all been tested and are normal.

Any help is greatly appreciated.

If its consistent misfire after a shutdown I would check this. Shut it down and wait awhile. Pull the plugs and smell them. If any are strong with fuel that cylinder has a leaky injector. If yiu have a fuel pressure gauge you would also notice it drops pressure on shutdown. Simple test anyway. Would also check coolant temp sensor. Any recent work done?
I know you said you checked it but has every sign of a boost/vac leak.

 

[steve]

Welcome to the site. Much of what you describe sounds like issue with the idle circuit and potential vacuum (and boost) leaks.
Please tell us more about your car, what's been done to it, perhaps with some pictures to help us figure out where to start.

Creating a profile for it would also be a great idea.

 

[dwb]

Turning on the AC should increase the idle. If does not, it's very possible you have an issue with the ISC. There's some good writeups how to test it.

Any chance you have ECMLink and can post a log?

 

[2Gracing]

Thank you for the replies, around Feb-March the engine was rebuilt and fresh sensors were installed, I replaced the ISC because I thought it was going bad so that should be good. The rebuild was a stock re build and I do not have ECMLink. The only thing changed about the rebuild was a balance shaft delete and a lightweight flywheel. When the engine was freshly rebuilt it used to start perfectly and over time started to get worse and worse until now with these issues.

Other than exhaust, boost gauge, oil pressure gauge and some miscellaneous mods like a short throw shifter and motors mounts the car is pretty much stock. I should also note the car has a EGR and charcoal canister delete. Spark plugs were replaced around 2 years ago, and idle is relatively good when the car is WARM at around 900-1000.

 

[nice97civic]

That idle is pretty high for a stock motor, or any motor for that matter. If plugs are that old, would be worth a look at them and if the motor falls flat on its face once it gets to 7-8psi then as suggested, sounds like a boost leak.

Just for fun, how did you test for boost leaks?

 

[2Gracing]

I tested leaks with a smoke machine and a pressure test. I should also say the fuel cut does not happen every single time, it is actually very random when it happens.

 

[pauleyman]

Elaborate please. Pressure how? From where and to what pressure?

 

[dwb]

I replaced the ISC because I thought it was going bad so that should be good.

Still a good idea to test it. You could also have broken wiring or an issue with the ECU (broken trace or blown driver chip). It's also important that the idle adjustment (BISS) is set according to the manual.

Misfire under boost can often be spark plugs and especially spark plug wires, even if they visually appear OK. What spark plugs are you running and at what gap? For a stock DSM, NGK BPR6ES are a good choice gapped at .28. Boost leaks are another possible culprit. Keep in mind a smoke test won't reveal all boost leaks, so focus on a boost leak test.

 

[2Gracing]

I’m gonna start by trying the ISC I have a spare Mitsubishi one I can swap in. I’ll get with my friend to see if I can use his boost leak tester I believe last time I used it it was hooked up to the turbo at around 20 psi.

 

[steve]

As dwb mentioned there are three parts to the ISC function, the ISC itself, the wiring between the ECU and the ISC, and the ECU driver IC's and sensors like the IPS that tells the ECU if it should be managing the idle or emulating a dashpot with the ISC. Each needs to be verified.

Lightweight flywheels are problematic and can often result in stalling but the other idle issues sound like a bad ISC or ECU drivers or the ECU not knowing it needs to manage the idle.

 

[2Gracing]

Thank you all for the help! Ended up being the ISC motor I had bought a new one from autozone thinking it would last. Took it apart today and the motor was seized so I replaced it with an oem one and all my issues are gone!