I've got an LS6 that's slightly bored, slightly stroked. It's got forged pistons with about 11:1 compression, forged connecting rods, stage III ls1 heads, titanium valves, stiff springs, a street/strip cam that goes to 6900 RPMS, rocker arms, 32ft/lb fuel injectors, and a few other aftermarket components I can't exactly remember. It's got about 22000 miles on the engine.

I have a recurring fuel problem. Firstly it comes and goes, remember this when trying to diagnose it. Basically it feels like I lose about 60% of my power, it sounds as if somethings hitting inside the engine and feels that way too (double the vibration, rough sounding noises, etc). It comes and goes, meaning sometimes it starts up fine and comes on, sometimes it starts up rough and goes away. For an example, going 70 in 6th gear it will come over the engine, upon shifting into 5th and bringing it to a higher RPM for a few minutes it seems as though I can force the problem to disappear but it comes back sometimes for no apparent reason and sometimes upon accelerating or down-shifting. All fuel problems go away upon going full-throttle (at any RPM).

I've spend about 600 dollars trying to fix this problem. I've changed my spark plugs, wires, fuel filter (about three times), fuel pump (which actually fixed the problem for about three months), a few coils (the cylinders that had blacker sparkplugs), and my gas cap (which was apparently not holding vacuunm). My front o2 sensors are disabled and my rear o2 sensors are disconnected.

I'm not a mechanic. I've been to friend's houses, a number of shops, and even asked a few strangers, but anything somebody comes up with doesn't seem to solve my problems.

If anybody has any advice to offer me it'd be greatly appreciated. I'm sick of spending all of this money for no reason especially considering I'm selling it in a few short months. Please get back to me.

Sam

 

Just to follow up..

I was thinking it could be some sensor somewhere throughout the ignition system that relays information about when to fire or send fuel and the problem is becoming more apparent now that it's summer and the oxygen is leaning out of the air. I'm in Texas. The problem seemed to be gone when I changed the fuel pump but that could have just been the compensation it needed for the colder air at the time. It seems like it's basically missing at mid-rpm when the symptoms are apparent, and just so you know the car has always pretty much ran rich. I'm just brainstorming with possibilities here, it could be something about the ignition mapping.

I figure the dealership could diagnose the problem with a diagnostic test, although I'm sure it won't be cheap.

 

Hmmm...sounds like it could be detonation. You could have too much timing. The motor will pull some timing based on feedback from the knock sensors (if they detect knock). However, there's a limit to the amount of timing they're allowed to pull. Find somone who can tune with HPTuners or EFILive and have them go for a ride with you (preferably someone with a wideband). Then, they can see if it's really a fuel problem or a spark issue. You might want to find out what HPE, Texas Speed or MTI would want ($$$ wise) to work on it. The dealerships won't be able to do jack for a modified car...

 

I figure I can replace the part myself or with the help of a friend. The problem is diagnosing it and I think with the right diagnostic tool/tuner I could figure out where my problem is. I wouldn't want to go to MTI or any of those shops, I don't have that kind of expendable money especially when I might be able to eventually figure it out on my own.

 

Well, I've been learning to tune for over one year now. If you think you can figure it out before the thing blows up, more power to you. What if it's in the PCM? What if your tune is wrong. Those people would/should be able to tell you. You can't just replace the PCM and have it work like you could a lower control arm. I don't expect anyone to have an expendable amount of cash just sitting around. But I keep telling people....

"If you can't afford the premium gas, why did you buy the sports car in the first place?"

Things need adjusting when they're modified. If you can't afford those adjustments, don't slightly bore, stroke and up the compression of the motor.

 

Here's a list of some things that could cause your problems.

Bad or dirty MAF or wiring
bad IAC or wiring
Bad coolant temp sensor or wiring
Bad crank or cam position sensor

You say that you have all the O2 sensors unhooked. I assume that you have had it tuned for open loop operation??

Is the SES light on?

 

Yes the SES light is on. I'm fixing the rear o2 sensors tomorrow afternoon; the front o2 sensors were programmed out.

Thanks for the sensor list, I'll check those out.

 

If the SES light is on.....Try a scanner and that will tell you where your problem is coming from. Go to autozone or somewhere and the will be able to scann your ecu for codes usually for free, I know they do down here in savannah. Thats the easiest why to find out what your problem is. Start there it may save you money in the long run. It sounds to me like either the IAT sensor or your MAF is F*****.

 

This doesn't make any sense to me. The front sensors tune the motor in closed loop operation. The rear sensor do nothing but monitor the catylitic converters and have nothing to do with how the car runs.

It is normal to have the rear sensors programmed out when there are no cats.
And fixing the rear O2s will change nothing.

 

You're right, I had it backwards.