almost stumped: LT1 problems
05-16-2008, 08:27 AM
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almost stumped: LT1 problems Well here's the deal, I have a 94 Z28 LT4 H/C it's been running great up until my knock sensor failed and threw an SES, I replaced it and the SES went away but the car has the same problem. What happens is when I start the car up from a cold start it runs great for about 5 minutes then it will start misfiring and backfiring. It seems to be a heat related problem because it makes no difference if the computer is in open/closed loop. The longer I let the car cool off the longer it will run good till it starts misfiring and backfiring. Also I see that when watching the scanner it shows the knock retard increase up to 10 under load and knock count will exceed 10,000.
Here is a list of things that I have changed in attempt to repair with no result
O2's
Coil/ICM (not a known good, could be overheating coil)
Fuel Injectors (not a known good, possibly sticking when warmed up?)
Fuel Filter (pressure is between 45-55)
Fuel pressure regulator
Knock sensor (put a second brand new one in)
MSD cap and rotor
MAF
Here is a list of things that I have changed in attempt to repair with no result
O2's
Coil/ICM (not a known good, could be overheating coil)
Fuel Injectors (not a known good, possibly sticking when warmed up?)
Fuel Filter (pressure is between 45-55)
Fuel pressure regulator
Knock sensor (put a second brand new one in)
MSD cap and rotor
MAF
05-16-2008, 09:01 AM
#2
How do you know its not an open/closed loop issue? IIRC the car will not go into closed loop until the o2's reach the appropriate temperature. The longer you let it cool off, the longer it will take to reach temperature and switch to closed loop.
Have you tried leaving the o2's unplugged and driving it around?
Have you tried leaving the MAF unplugged and driving it around?
You could try installing the LT4 knock module.
Have you tried leaving the o2's unplugged and driving it around?
Have you tried leaving the MAF unplugged and driving it around?
You could try installing the LT4 knock module.
05-16-2008, 09:04 AM
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I have tried driving with the O2's unplugged, no change
I have tried driving with the MAF unplugged, no change
And have have installed a new pcm with a new LT4 Knock module, no change
I know it isn't an open/closed loop issue because with the o2's unplugged it stays in open loop with no change in engine performance
:-/
I have tried driving with the MAF unplugged, no change
And have have installed a new pcm with a new LT4 Knock module, no change
I know it isn't an open/closed loop issue because with the o2's unplugged it stays in open loop with no change in engine performance
:-/
05-16-2008, 09:28 AM
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Do you have Datamaster? I STRONGLY suggest you invest in a cable download Datamaster (it comes with 20 free collections). If you collect data with it I am sure we will find the problem quickly, and you will buy it that's why he gives free trial.
From your description I would check EGR, for vacuum and exhaust leaks. It sounds like you are running LEAN. As you said could be injector, to eliminate that get a can of Seafoam add it to ~6 gallons of gas, put a 1/3 of the can in oil.
Go for a ~50 mile round trip interstate drive collecting with Datamaster. The cost of a cable was ~$50 and WELL worth it.
Good luck
From your description I would check EGR, for vacuum and exhaust leaks. It sounds like you are running LEAN. As you said could be injector, to eliminate that get a can of Seafoam add it to ~6 gallons of gas, put a 1/3 of the can in oil.
Go for a ~50 mile round trip interstate drive collecting with Datamaster. The cost of a cable was ~$50 and WELL worth it.
Good luck
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05-16-2008, 09:36 AM
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I have eliminated the EGR with a block off plate, I have yet to test for vacuum but I will do that today. There are no exhaust leaks, I have hooker s/c's with copper gaskets and they are as tight as can be with no noise.
05-16-2008, 09:42 AM
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I had EGR block off plate and the gasket leaked caused my car to have split BLMs. Best bet is to get cable and car ECM data will tell you what is wrong.
05-16-2008, 10:02 AM
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I had a 94 Caprice that was OBDI computer and had an OBDI connector (supposedly 94/95 have OBDI COULD have been an exception sold). A friend had a 94 Caprice that had an OBDII connector. So the answer is it depends.
It is best to look at your vehicle's port and make sure you have one style or the other OR I believe AKM cables offers a cable with both style connectors if I were buying a cable today I would buy the dual style.
That way you can collect/monitor most OBDI LT1 vehicles. I say most because 92 Corvette and 93 Camaro have a chip version that is OBDI but won't talk to Datamaster and Tunercats it uses Speed Density (No MAF).
Hope that helped. I recommend AKM cable he is a great guy and his product is HIGH quality.
05-16-2008, 10:19 AM
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I have had 2 fail right out of box rebuilt isn't always better. I have had them give no indication of error, hi lo resolution never got set. I have had some LT1s indicate bad hi/lo res and never have a bad opti.
Opti's are often blamed and often guilty just hate to jump to say it is the opti, since it is soo difficult to be certain it is the problem until you have checked and verified everything else is ok.
I use Delteq to avoid MOST opti problems but they still have failed thankfully mine has a lifetime warranty and easy to get to on my vehicles. I always seal the opti when I install and keep a close eye on leaks. An opti is a poor swimmer and if water gets close seems it wicks it in and dies.
