truecontr3

I need help. I have a 1996 z28 lt1. Long toob headers 306 can 44lb injectors 58mm BBK throttle body everything was fine then it wasn't . it started the dreary surging but no codes. I've replaced the tps,the omnispark,the plugs, the plug wires the IAC tjemaf ,the temperature control sensor and o2 sensors and now she idles roughly. I put a new fuel pump and fuel regulator in as well.still no change. Can someone please help before I go insane and broke?

Injuneer

Have you had the ICM tested?

Did you test the fuel pressure before/after replacing the pump and FPR?

What are you scanning it with? Should have codes for misfires. Does the problem only exist at idle (rough), or does it run poorly out on the road under load?

JakeRobb

Stop throwing parts at the motor and hoping for luck. As you've seen, that's a recipe for driving yourself crazy! Run tests to find out exactly what's broken.

truecontr3

Yes my fuel pressure was bad . but now it holds solid at around 47psi. Other than random misfire no other codes I understand the 300code its throwing is because of the can.

truecontr3

I also replaced the ICM.

Injuneer

Is it 47psi with or without the vacuum compensation line attached to the fuel pressure regulator?

At idle, vacuum compensation line disconnected, fuel pressure spec is 43.5psi, with GM accepting a range of 41-47psi. Reattach the vacuum compensation line, and the pressure should drop proportional to intake manifold vacuum, or about an 8psi drop from the no vacuum measurement.

What brand are the injectors - possibly Accel?

truecontr3

Yes. It runs fine in open loop but once it goes into closed loo it has a surge. Oven had the injectors tested they're fine . you can hold her around 2500rpms and she fine at first but after that she stumbles. My Lt ftrm is around 15.6-23.7 which is really rich that's why I changed the o2 sensors with no change.

Injuneer

If you mean the LTFT is +15.6% to +23.7%, that means the PCM is adding a significant amount of extra fuel to correct what it sees as a very lean condition. Could be a "true" lean (= engine fueled correctly with high LTFT's), or could be a "false" lean (= engine running rich due to unneeded fuel). Are the percentages the same on both banks of the engine? Which Cells are those readings from?

Misfires and exhaust leaks before the O2 sensors can cause a "false" lean. At idle, a high overlap cam can cause a false lean. But as RPM increases, that effect goes away. That's why knowing which cells your reading came from is important.

truecontr3

Both banks run about the same, 15.6 and higher. Once she's past about 3000rpms she seems to level out but if you hold her around 2500rpms shell stumble , fall then shoot back up. In open loop she runs decent. But I want her back to running right in closed loop .

truecontr3

What would cause my lt1 to retard the timing to -44* every time its above 2000rpms

Injuneer

You are reading the scanner wrong. -44* indicates that the ignition timing ADVANCE (not retard) is 44-degrees before TDC. Some scanners show this is a negative, some show it as a positive. That's typical of stock program normal ignition timing above 2,000 RPM, under low to moderate throttle (low MAP).

The stock program has a maximum knock retard of 15*, and it is indicated in a separate field called "knock retard".

I have a scanning document that explains all of this stuff. Send my your e-mail address via Private Message (left click on my screen name for option to send message) and I will send you a .pdf copy.

truecontr3

My e-mail is. Bayouboys143@yahoo.com.
Thanks for the help. I started my z28 this morning and it started up fine but all of the sudden it acted as if it was flooding and died now it won't start.. I have 50psi for fuel, good spark , it will only chug once or twice then backfire but will not start. It has a new mad opt, new plugs, wires, coil, ecm,tps, IAC o2 sensors , new map new starter new alternator, fuel pump, fuel regulator, filter and may. So what the hell is wrong???!!!!!

Injuneer

50psi fuel pressure is above the factory acceptable range. See my post #6. You never answered my question whether you were measuring the fuel pressure with or without the vacuum compensation line connected. Never answered the question about the brand of the injectors.

You appear to have a scanner. Any codes since the new problem?

Formula Steve

I'm kind of having the same problem. My thread probably won't help...but maybe worth a look??


https://www.camaroz28.com/forums/lt1...d-help-883100/

RobbyRob

Also did you have any tune done for your mods? As Injuneer asked what brand injectors and what size. 50# fuel pressure and incorrect tune for fuel can cause your problems.