My 86 is driveing me crazy
Thread Starter
7th Gear
Joined: Aug 2008
Posts: 7
Likes: 0
My 86 is driveing me crazy
I have a 86 L98 alum. head Corvette. It will fire right up and idle fine, but as soon as you hit the gas it starts backfireing through the intake. I have replaced the maf sensor, the MSD 6AL box, the Accell super coil, plugs ,wires, the whole distrubitor, engine control computer. It has a Edlebrock high flow fuel pump that is about 6 months old. I have also replaced the fuel pressure regulator, egr valve, fuel filter, and the EST module, and knock sensor. Now they are telling me it may be the Super chip prom. I dont know how old it is. Could this be my problem.
Burning Brakes
Joined: Apr 2004
Posts: 762
Likes: 2
From: Huntington ma
Back to the Basics. A backfire in the Intake is a symptom of the motor going very Lean. Old school would say look for Vacuum leaks - that is still valid.
I'm assuming here that you are Certain that the Timing advance isn't doing anything stupid, that the Camshaft is "in time" (that the timing chain isn't in trouble, or something), that there aren't any Valve issues (compression check?). Again - all sort of "basic" mechanical stuff that has to be right for the thing to Run properly.
In the TPI world "vacuum leak" can mean manifold/plenum/runners, it can mean Throttle body and it can mean the tube between the MAF and the throttle body. It can mean any of the vacuum Lines going to the HVAC, Brake booster, Charcoal cannister, Cruise solenoids, etc.
Beyond actual Leaks, intake backfiring can also be a symptom of a faulty MAF reading. You've replaced That - does it do the same thing with the MAF sensor actually disconnected (unplugged)?
Another possibility is the EGR valve. Again, you've replaced it. Unhook it and make Sure that it is closed/closing all the way.
I'm assuming here that you are Certain that the Timing advance isn't doing anything stupid, that the Camshaft is "in time" (that the timing chain isn't in trouble, or something), that there aren't any Valve issues (compression check?). Again - all sort of "basic" mechanical stuff that has to be right for the thing to Run properly.
In the TPI world "vacuum leak" can mean manifold/plenum/runners, it can mean Throttle body and it can mean the tube between the MAF and the throttle body. It can mean any of the vacuum Lines going to the HVAC, Brake booster, Charcoal cannister, Cruise solenoids, etc.
Beyond actual Leaks, intake backfiring can also be a symptom of a faulty MAF reading. You've replaced That - does it do the same thing with the MAF sensor actually disconnected (unplugged)?
Another possibility is the EGR valve. Again, you've replaced it. Unhook it and make Sure that it is closed/closing all the way.
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Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
backfiring
usually when the engine backfires through the intake, there is something wrong with an intake valve.
Try adjusting the valves. If the rocker nuts have harmonically rotated and tightened, a valve might be open. It's possible that you have a spot of carbon under a valve. Change any rocker nut that seems to have lost it's tension and it's ability to be a lock nut.
Try another distributor cap. The one you bought may be cracked.
Try straightening out your spark plug wires.
try....try...
Try adjusting the valves. If the rocker nuts have harmonically rotated and tightened, a valve might be open. It's possible that you have a spot of carbon under a valve. Change any rocker nut that seems to have lost it's tension and it's ability to be a lock nut.
Try another distributor cap. The one you bought may be cracked.
Try straightening out your spark plug wires.
try....try...
Thread Starter
7th Gear
Joined: Aug 2008
Posts: 7
Likes: 0
Thanks for all the help so far. I adjusted my valves and changed my intake manifold gaskets. I also checked for any vacum leaks, but found none present. The car will run and now if you go easy on the rpms it will go all the way to redline. But as soon as I stab the throttle I get the same backfire. I love this car like it is one of my kids. I am just running out of ideas. As far as " throwing parts at it" goes I dont mind as long as I get my baby back. I have rebuilt the engine and tranmission in this car and it has alot of my blood sweat and tears in it. All the advice you guys have given me is great and thank you for the help. I wish I could tell you it was fixed instead of pleading for anymore ideas.
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
This will cost you nothin':
Sounds like the valves are just a bit tight (too much preload). Back all your valve nuts up a 1/4 turn put the valve covers back on and try to rev it up again.
Good Luck
Sounds like the valves are just a bit tight (too much preload). Back all your valve nuts up a 1/4 turn put the valve covers back on and try to rev it up again.
Good Luck
Instructor
Joined: May 2001
Posts: 136
Likes: 0
From: Raleigh NC
A rounded off exhaust cam lobe will cause this problem. Been there, done that. At least it will on a carburated engine. Pull the rocker covers and crank the engine (disable ignition) and look for a exhaust that moves a LOT less than the others.
When I had this problem on a 350 engine the exhaust lobe was almost round.
When I had this problem on a 350 engine the exhaust lobe was almost round.
Burning Brakes
Joined: Jun 2005
Posts: 1,191
Likes: 15
From: Wallingford Vermont
I was getting ALOT of popping with my iron headed '86. Well first and formost, I had some bad injectors. New injectors runs MUCHO better.
Still can get it to pop just alittle but I still have a charcoal evap solenoid to replace...cross your fingers.
Still can get it to pop just alittle but I still have a charcoal evap solenoid to replace...cross your fingers.
Advanced
Joined: Jul 2008
Posts: 51
Likes: 0
"A rounded off exhaust cam lobe will cause this problem"
As will a broken pushrod or rocker. If the cylinder fires and cant exit through the exhaust valve, it will exit through the intake valve instead, causing a... backfire!
Usually a lean run condition will give you one or two "pops" when you hit the gas, then resume normal operation. An exhaust valve not opening will be give you a steady pop pop pop.
As will a broken pushrod or rocker. If the cylinder fires and cant exit through the exhaust valve, it will exit through the intake valve instead, causing a... backfire!
Usually a lean run condition will give you one or two "pops" when you hit the gas, then resume normal operation. An exhaust valve not opening will be give you a steady pop pop pop.
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
also check to see that the timing chain isn't so slack that the cam timing isn't off in regard to the crank This will give you late cam timing.
Last edited by coupeguy2001; Aug 17, 2008 at 02:21 AM.
