LT1 won't rev??
Thread Starter
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Joined: Dec 1969
Posts: 10,745
From: Buffalo, New York
LT1 won't rev??
I am helping a friend work on his '96 (LT1, OBD2). It has a new 383 with a Procharger D1. Runs fine up to ~3,800rpm, then starts to run very rough. Occurs under load or in neutral. Here's some info.
No codes except "High MAF".
Good AF ratio.
Replaced Opti.
Put in known good MAF.
TPS checks out fine.
New plugs and wires.
Intake is not collapsing.
Replaced MSD box.
Fuel pressure is fine.
Anyone have any ideas? I don't have a clue.
Rich Krause
No codes except "High MAF".
Good AF ratio.
Replaced Opti.
Put in known good MAF.
TPS checks out fine.
New plugs and wires.
Intake is not collapsing.
Replaced MSD box.
Fuel pressure is fine.
Anyone have any ideas? I don't have a clue.
Rich Krause
Thread Starter
Moderator
Joined: Dec 1969
Posts: 10,745
From: Buffalo, New York
Originally posted by madwolf
Does he have cats still? How about timing, are you sure it isn't too much? (in case he doesn't have tuning/boost retard box)
Does he have cats still? How about timing, are you sure it isn't too much? (in case he doesn't have tuning/boost retard box)
Rich Krause
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Joined: Jul 2001
Posts: 4,577
From: DeKalb, IL
Oh yes, if they're clogged, they will definetely cause something like that and gradually get worse.
I don't know how the air fitting are on an OBDII car, but you could try unbolting the pipes from the manifolds and run the car like that. It will be loud, but if the problem goes away, the cats are bad.
I don't know how the air fitting are on an OBDII car, but you could try unbolting the pipes from the manifolds and run the car like that. It will be loud, but if the problem goes away, the cats are bad.
Thread Starter
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Joined: Dec 1969
Posts: 10,745
From: Buffalo, New York
Originally posted by madwolf
Oh yes, if they're clogged, they will definetely cause something like that and gradually get worse.
I don't know how the air fitting are on an OBDII car, but you could try unbolting the pipes from the manifolds and run the car like that. It will be loud, but if the problem goes away, the cats are bad.
Oh yes, if they're clogged, they will definetely cause something like that and gradually get worse.
I don't know how the air fitting are on an OBDII car, but you could try unbolting the pipes from the manifolds and run the car like that. It will be loud, but if the problem goes away, the cats are bad.
Anyone else?
Rich Krause
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Joined: Jul 2003
Posts: 374
From: Great White North
Mechanical timing
If the motor is fresh and the cam has just been either installed or reinstalled then I might suggest checking that the cam timing is correct. If its off by much that could be the problem. Either or degreeing and or off one tooth could do it. Good luck.
Vince
Vince
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Joined: Feb 1998
Posts: 383
From: Ft. Wayne, IN
If he still has cats on his car (they need to be off anyway 
) I would bet 99% that is your problem. My buddies cat clogged on his low mileage '94 Z28 (34,000 miles) and his car would rev up to redline, but would take forever to get there. The only other suggestion I would say is fuel pump not keeping up with demand, but you said you already checked fuel pressure. Good luck!
Jeff
) I would bet 99% that is your problem. My buddies cat clogged on his low mileage '94 Z28 (34,000 miles) and his car would rev up to redline, but would take forever to get there. The only other suggestion I would say is fuel pump not keeping up with demand, but you said you already checked fuel pressure. Good luck!Jeff
Registered User
Joined: Jul 2001
Posts: 4,577
From: DeKalb, IL
Originally posted by Seal
cats could be it, but it wont make that much of a diffi think...
are you making any mechanically weird noises? have u atapped it to see whats going on when it gets rough?
cats could be it, but it wont make that much of a diffi think...
are you making any mechanically weird noises? have u atapped it to see whats going on when it gets rough?
Registered User
Joined: Jun 2002
Posts: 31
From: Austin,TX
Just some ideas: Valve float, collapsed lifters, worn valve springs?
We had a dyno day recently with our local club and a guy there had similar problems ~4000 rpms. Car ran fine before that but after was real rough. The general consensus was he had valve float, or possibly a collapsed lifter due to high milage on his heads. I know you said it was a new 383 motor, but what about his valvetrain?
We had a dyno day recently with our local club and a guy there had similar problems ~4000 rpms. Car ran fine before that but after was real rough. The general consensus was he had valve float, or possibly a collapsed lifter due to high milage on his heads. I know you said it was a new 383 motor, but what about his valvetrain?
Thread Starter
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Joined: Dec 1969
Posts: 10,745
From: Buffalo, New York
Originally posted by clb95z28
Just some ideas: Valve float, collapsed lifters, worn valve springs?
We had a dyno day recently with our local club and a guy there had similar problems ~4000 rpms. Car ran fine before that but after was real rough. The general consensus was he had valve float, or possibly a collapsed lifter due to high milage on his heads. I know you said it was a new 383 motor, but what about his valvetrain?
Just some ideas: Valve float, collapsed lifters, worn valve springs?
We had a dyno day recently with our local club and a guy there had similar problems ~4000 rpms. Car ran fine before that but after was real rough. The general consensus was he had valve float, or possibly a collapsed lifter due to high milage on his heads. I know you said it was a new 383 motor, but what about his valvetrain?
Rich Krause