TPI mods, complete w/MISFIRE that WON'T QUIT!!!
04-08-2007, 08:31 AM
#1
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Join Date: Oct 2000
Location: Middle River MD
Posts: 133
TPI mods, complete w/MISFIRE that WON'T QUIT!!!
A quick history....
Car is 85 Corvette, L98 TPI
Did head, intake, rocker arm, & longtube header swap.
Car should easily make 275rwhp/375rwtq, probably more.
(It made 239rwhp/359rwtq w/dyno issues highlighted below)
Car originally had some vacuum issues, since taken care of.
Car wouldn't go into closed loop.
Traced it to bad injector connection in header tube that had O2 bung welded into it. Fixed it.
Car ran excellent.
Took car to dyno and only change was filling up gas tank for the drive.
Find 40hp in my buddy's blower tune, can't wait to caveman tune mine (FP and timing).
6 passes on dyno, all of them at mid to high 12:1;s, but then with severe misfire/very erratic AF readings at 3300 rpm's, then by 4000, over 14:1 AF and no power. Car does NOT lose fuel pressure when it goes/shows lean.
I had some brand new injectors anyway (24's), so I swapped them out. No difference.
Even though most items were changed during head swap, I swapped (again) the plug wires, plugs, cap, rotor, coil, fuel filter. I even re-adjusted valvetrain. Originally at 1/8 turn past zero, now 1/2 turn past zero. (Hydraulic flat tappet stock cam w/1.6's)
Car still misfired and wound up melting catalytic converter guts.
Opened exhaust up, car screamed...for a day or two...again.
Misfire returns...
Swapped TPS for known good one, no change.
Tried advancing and retarding base timing, no change.
Upped and downed base fuel pressure, no change.
When it misfires, it backfires and/or pops through intake and exhaust at the same time. I can best describe it as the sound of a blower car getting spark blown out, or a wiped cam. If I ease into throttle, up to 5800 rpm's, it doesn't misfire. If I rapidly open throttle, it misfires at least half the time, maybe more since it seems to be getting worse.
Charging system is fine. (14.2 volts at idle)
Fuel pressure doesn't fluctuate at all at WOT (38psi)
Timing at stock 6*
Misfires only at WOT (or at least that's when I can feel/hear it.
I know this isn't a Camaro, but was hoping a TPI/L98 enthusiast could possibly shed some light....
Any help greatly appreciated
Car is 85 Corvette, L98 TPI
Did head, intake, rocker arm, & longtube header swap.
Car should easily make 275rwhp/375rwtq, probably more.
(It made 239rwhp/359rwtq w/dyno issues highlighted below)
Car originally had some vacuum issues, since taken care of.
Car wouldn't go into closed loop.
Traced it to bad injector connection in header tube that had O2 bung welded into it. Fixed it.
Car ran excellent.
Took car to dyno and only change was filling up gas tank for the drive.
Find 40hp in my buddy's blower tune, can't wait to caveman tune mine (FP and timing).
6 passes on dyno, all of them at mid to high 12:1;s, but then with severe misfire/very erratic AF readings at 3300 rpm's, then by 4000, over 14:1 AF and no power. Car does NOT lose fuel pressure when it goes/shows lean.
I had some brand new injectors anyway (24's), so I swapped them out. No difference.
Even though most items were changed during head swap, I swapped (again) the plug wires, plugs, cap, rotor, coil, fuel filter. I even re-adjusted valvetrain. Originally at 1/8 turn past zero, now 1/2 turn past zero. (Hydraulic flat tappet stock cam w/1.6's)
Car still misfired and wound up melting catalytic converter guts.
Opened exhaust up, car screamed...for a day or two...again.
Misfire returns...
Swapped TPS for known good one, no change.
Tried advancing and retarding base timing, no change.
Upped and downed base fuel pressure, no change.
When it misfires, it backfires and/or pops through intake and exhaust at the same time. I can best describe it as the sound of a blower car getting spark blown out, or a wiped cam. If I ease into throttle, up to 5800 rpm's, it doesn't misfire. If I rapidly open throttle, it misfires at least half the time, maybe more since it seems to be getting worse.
Charging system is fine. (14.2 volts at idle)
Fuel pressure doesn't fluctuate at all at WOT (38psi)
Timing at stock 6*
Misfires only at WOT (or at least that's when I can feel/hear it.
I know this isn't a Camaro, but was hoping a TPI/L98 enthusiast could possibly shed some light....
Any help greatly appreciated
04-08-2007, 01:25 PM
#3
Registered User
Thread Starter
Join Date: Oct 2000
Location: Middle River MD
Posts: 133
Stock type HEI Coil is Standard Motor Products replacement, and is new
Distributor is stock, but I am trying to locate a used one for troubleshooting purposes.
04-08-2007, 07:33 PM
#7
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Thread Starter
Join Date: Oct 2000
Location: Middle River MD
Posts: 133
Stock for the 85 is 34-39psi, but having it set as high as 45psi base didn't make any difference. I'm still using stock ecm.
04-09-2007, 02:39 AM
#9
Registered User
Join Date: Jan 2002
Location: Cleveland
Posts: 73
The reason I asked about the burnoff is because when I was still running the 85 ECM I had a bad one without it throwing a code it would do weird stuff like misfire out of nowhere and stall while turning etc. If you think you need one get a hold of me I have 3 of them(1 is bad -2 are good I just dont know which one is bad because they got put into a box togetther). I also believe that they are not available anywhere. I also have a 85 Maf and ECM that i am not using anymore.
04-09-2007, 05:42 PM
#10
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Thread Starter
Join Date: Oct 2000
Location: Middle River MD
Posts: 133
I tried to datalog today using the WinALDL program. Laptop kept shutting itself off for some unknown reason...
Here's what it sounds like...does it now even sitting still just revving it up:
http://www.youtube.com/watch?v=hD4R08CVQDw
Wacky Tower, is your MAF for an 85 Corvette?
Here's what it sounds like...does it now even sitting still just revving it up:
http://www.youtube.com/watch?v=hD4R08CVQDw
Wacky Tower, is your MAF for an 85 Corvette?
04-09-2007, 09:01 PM
#11
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Join Date: Dec 1969
Location: Phila., PA
Posts: 1,150
If it was runnign strong before and suddenly problems came up it's not your tune or your selection of parts. SOMETHING BROKE.
A burned plug wire would be almost too simple, wouldn't it? Lean O2 readings often result when you're blowing unburnt mixture into the exhaust because it's still loaded with unused oxygen. O2 readings plunge and you see it as dead-lean, plus all the popping and snarling that goes with it.
If I had to give a general diagnosis over the internet I'm going to lay $20 on it being something electrical. Ignition, ECM, pinched wire somewhere, bad MAF sensor, SOMETHING like that. If it was mechanical it would not exhibit the kind of "lightswitch" behavior you're describing, short of catastrophic valvetrain failure.
I know that isn't much help, but don't start changing your basic combo of parts- that AIN'T the problem. You aren't going to tune your way around this. Stick with your current combo and keep diagnosing until you hit "ah HAH!". Whatever it is, it's likely a single point of failure (given the sudden and obvious onset of problems) and when you fix it you'll be right back to cranking out bottom-basement 12s again.
Sherlock Holmes is the way you need to think right now. Not Conan The Destroyer.
A burned plug wire would be almost too simple, wouldn't it? Lean O2 readings often result when you're blowing unburnt mixture into the exhaust because it's still loaded with unused oxygen. O2 readings plunge and you see it as dead-lean, plus all the popping and snarling that goes with it.
If I had to give a general diagnosis over the internet I'm going to lay $20 on it being something electrical. Ignition, ECM, pinched wire somewhere, bad MAF sensor, SOMETHING like that. If it was mechanical it would not exhibit the kind of "lightswitch" behavior you're describing, short of catastrophic valvetrain failure.
I know that isn't much help, but don't start changing your basic combo of parts- that AIN'T the problem. You aren't going to tune your way around this. Stick with your current combo and keep diagnosing until you hit "ah HAH!". Whatever it is, it's likely a single point of failure (given the sudden and obvious onset of problems) and when you fix it you'll be right back to cranking out bottom-basement 12s again.
Sherlock Holmes is the way you need to think right now. Not Conan The Destroyer.
04-09-2007, 09:22 PM
#12
Registered User
Thread Starter
Join Date: Oct 2000
Location: Middle River MD
Posts: 133
If it was runnign strong before and suddenly problems came up it's not your tune or your selection of parts. SOMETHING BROKE.
A burned plug wire would be almost too simple, wouldn't it? Lean O2 readings often result when you're blowing unburnt mixture into the exhaust because it's still loaded with unused oxygen. O2 readings plunge and you see it as dead-lean, plus all the popping and snarling that goes with it.
If I had to give a general diagnosis over the internet I'm going to lay $20 on it being something electrical. Ignition, ECM, pinched wire somewhere, bad MAF sensor, SOMETHING like that. If it was mechanical it would not exhibit the kind of "lightswitch" behavior you're describing, short of catastrophic valvetrain failure.
I know that isn't much help, but don't start changing your basic combo of parts- that AIN'T the problem. You aren't going to tune your way around this. Stick with your current combo and keep diagnosing until you hit "ah HAH!". Whatever it is, it's likely a single point of failure (given the sudden and obvious onset of problems) and when you fix it you'll be right back to cranking out bottom-basement 12s again.
Sherlock Holmes is the way you need to think right now. Not Conan The Destroyer.
A burned plug wire would be almost too simple, wouldn't it? Lean O2 readings often result when you're blowing unburnt mixture into the exhaust because it's still loaded with unused oxygen. O2 readings plunge and you see it as dead-lean, plus all the popping and snarling that goes with it.
If I had to give a general diagnosis over the internet I'm going to lay $20 on it being something electrical. Ignition, ECM, pinched wire somewhere, bad MAF sensor, SOMETHING like that. If it was mechanical it would not exhibit the kind of "lightswitch" behavior you're describing, short of catastrophic valvetrain failure.
I know that isn't much help, but don't start changing your basic combo of parts- that AIN'T the problem. You aren't going to tune your way around this. Stick with your current combo and keep diagnosing until you hit "ah HAH!". Whatever it is, it's likely a single point of failure (given the sudden and obvious onset of problems) and when you fix it you'll be right back to cranking out bottom-basement 12s again.
Sherlock Holmes is the way you need to think right now. Not Conan The Destroyer.
04-10-2007, 01:18 PM
#15
Registered User
Join Date: Feb 2003
Location: Virginia
Posts: 1,601
There has to be a burned wire or vacuum line somewhere. Stuff like that really screws with the older computers and they go haywire.
Did you replace the gaskets in the TPI when you put it back together? If not,one of them might have cracked or broke on reassembly and that would cause a vacuum leak.
Just some things to try,hope this helps man.
Did you replace the gaskets in the TPI when you put it back together? If not,one of them might have cracked or broke on reassembly and that would cause a vacuum leak.
Just some things to try,hope this helps man.