Sidewaysc4

Okay so i have a 1996 lt4 obviously 6 speed... i did a cat delete x pipe in place of the resignator and magniflow mufflers about 4 months ago, and i get a constant check enginle light because of the cat delete, but it wasnt causing any problems so i wasnt worried about it. But about 2 months ago my icm went out and in the process of trying to figure out the problem of my car not starting i replaced the opti, all new plugs and wires, the coil, and then the icm last

But none the less the car fired up and ran, but i now have a studder right at 5000 rpm and i cant figure out the problem. Its almost like it hits a rev limiter at 5000 rpm in any gear every time, i cant go past it.

And yesterday i replaced a broken roller rocker hoping it was the problem, and it not only didnt fix the studder problem i now randomly stall when i come to a stop... the car idles way down to around 300 or 400 rpm and just dies, but fires right back up and i can drive away fine.... the stall problem is random, somtimes it does it and somtimes it doesnt.... but the studder at 5k its consistant and i can't figure it out...

SOMEONE PLEASE HELP ME!!!

antfarmer2

Did you use the proper thermal grease on your ICM. Clean your throttle body and IAC. How old are your o2s?

Sidewaysc4

I used the grease that came with the icm, i will try cleaning the iac, and the o2s are the same ones on the car when i baught it.

Sidewaysc4

What should i clean the IAC with?

antfarmer2

Any good cleaner but be very gental. How many miles on it?

Sidewaysc4

Its pretty high... 180k

antfarmer2

You do know the o2s are normal maintenance item's. And don't always show a code. I would pull the knocks while under there and do a proper flush. Just pinch the connectors with your fingers to remove.

Sidewaysc4

Should i clean the o2s, or are you thinking they probably just need to be replaced?

antfarmer2

Replace if your tight on money start with the front two but I would do them all to match.

Sidewaysc4

Okay, will do. ill let you know if it works... do you think that will help with the idle issue? Or cleaning/ replacing my IAC is that problem?

antfarmer2

Could be both do not reuse gaskets and make sure the IAC passages are clean.

Sidewaysc4

I will do both next weekend and let you know.. thanks!

Sidewaysc4

***UPDATE*** i havnt looked into the o2s yet, but now the car will die if i even try to drive it and most of the time wont start... i cleaned the IAC very well, and found a couple vacuum leaks using starter fluid. I replaced the bad line, and any others that i thought could be a problem... didnt help at all... at idle it runs like crap and will die. When it does die it wont start back up, i have to wait for a while then it will fire up and repeat...

pcolt94

At 5000 RPM the throttle is probably close to wide open and the O2s are out of equation. They are not used for A/F calculation at high power. You have to have it idle first before you worry about the high end. Have you checked the EGR for leaks and proper operation with a vacuum gauge and the vacuum release checked.

I would get a professional smoke test and make sure the manifold is sealed properly as well as the other areas. I would also have a good scanner connected to check the IAC counts, fuel trim and o2 voltage swings.

aklim

I would do a WOT run to see if it holds pressure. IF so, the pump is good. After that, I'd check the IAC counts. If it holds at 20-30 counts when it is at stable operating temp with everything but the motor off, I doubt there is a leak. OTOH, if he can't do that simple thing, it means there is a leak somewhere and the ECM won't let the IAC open which explains why he would turn the screw all the way out and still have 0 counts.

grandspt

Lt4 engines do not have EGR valves. I would look at the new optispark you replaced, there are a lot of new defective/inferior ones out there!
I agree that the O2s won't cause a problem at 5000RPM (I doubt this could be your problem). If you have good fuel pressure 40-45psi at idle, I suspect your new optispark.

RichS

Are the rockers still the stock 10mm stud, non adjustable ones or were they replaced with adjustable 3/8 ones? If they are adjustable and you over tightened it it will cause a rough condition.

Sidewaysc4

Thanks for all the responses guys! The 5k studder acurred after i changed the opti, icm, ect... other than not being able to go past 5k it ran fine in the bottom end. My new problem (dying, rough idle) acurred right after i replaced the roller rocker, but the car sat for a month waiting on the rocker...

They are stock studs and non adjustable, but my concern is the rocker i baught is not an oem, and it came with an adjustable nut (with a set screw in it)?? I just used my stock nut from the other rocker... i could not find a direct replacement? Could that be a problem? If so where can i find an oem lt4 1.6 ratio rocker?
Thanks guys

bjankuski

This sounds like an ignition problem.

pcolt94

Your right, I forgot, it was just a general brainstorming thought.