Backfire during acceleration on L98
Thread Starter
Racer
Joined: Aug 2012
Posts: 279
Likes: 2
From: Las Vegas Nevada
Backfire during acceleration on L98
Yup, I'm back again asking for more help.
Sorry, but you guys have more experience than me.
I'm getting about 2 or 3 backfires occasionally when I'm WOT at about 3k-4k rpm. It's probably been there since I got the vehicle, but I've only recently started to give it some hard acceleration to about 60 max to see how it's performing.
I want to say the sound is coming from the engine, not exhaust. I've read up that it could be any number of things, so can't fixate on one issue because just about everything in the engine has been said to be the culprit. Blown intake gasket, blown head gasket, bad timing, worn valve seals or springs, worn pushrods, bad EGR, bad spark plugs, bad wires, bad distributer, and a few others.
First... I just replaced the intake gaskets and it no longer leaks. That's out. And second, a compression test revealed between 160 and 170 on all cylinders, with the new plugs, wires, and distributer. It still idles fine and has good power. Also out.
I have a feeling it's the valve springs or timing because the backfiring is right in sync with the engine, and it only does it at higher RPM with full throttle.
Help and advice is always appreciated. Thanks.
Sorry, but you guys have more experience than me.
I'm getting about 2 or 3 backfires occasionally when I'm WOT at about 3k-4k rpm. It's probably been there since I got the vehicle, but I've only recently started to give it some hard acceleration to about 60 max to see how it's performing.
I want to say the sound is coming from the engine, not exhaust. I've read up that it could be any number of things, so can't fixate on one issue because just about everything in the engine has been said to be the culprit. Blown intake gasket, blown head gasket, bad timing, worn valve seals or springs, worn pushrods, bad EGR, bad spark plugs, bad wires, bad distributer, and a few others.
First... I just replaced the intake gaskets and it no longer leaks. That's out. And second, a compression test revealed between 160 and 170 on all cylinders, with the new plugs, wires, and distributer. It still idles fine and has good power. Also out.
I have a feeling it's the valve springs or timing because the backfiring is right in sync with the engine, and it only does it at higher RPM with full throttle.
Help and advice is always appreciated. Thanks.
Safety Car
Joined: Aug 2009
Posts: 3,751
Likes: 275
From: Melbourne, Victoria, Australia
Yup, I'm back again asking for more help.
Sorry, but you guys have more experience than me.
I'm getting about 2 or 3 backfires occasionally when I'm WOT at about 3k-4k rpm. It's probably been there since I got the vehicle, but I've only recently started to give it some hard acceleration to about 60 max to see how it's performing.
I want to say the sound is coming from the engine, not exhaust. I've read up that it could be any number of things, so can't fixate on one issue because just about everything in the engine has been said to be the culprit. Blown intake gasket, blown head gasket, bad timing, worn valve seals or springs, worn pushrods, bad EGR, bad spark plugs, bad wires, bad distributer, and a few others.
First... I just replaced the intake gaskets and it no longer leaks. That's out. And second, a compression test revealed between 160 and 170 on all cylinders, with the new plugs, wires, and distributer. It still idles fine and has good power. Also out.
I have a feeling it's the valve springs or timing because the backfiring is right in sync with the engine, and it only does it at higher RPM with full throttle.
Help and advice is always appreciated. Thanks.
Sorry, but you guys have more experience than me.
I'm getting about 2 or 3 backfires occasionally when I'm WOT at about 3k-4k rpm. It's probably been there since I got the vehicle, but I've only recently started to give it some hard acceleration to about 60 max to see how it's performing.
I want to say the sound is coming from the engine, not exhaust. I've read up that it could be any number of things, so can't fixate on one issue because just about everything in the engine has been said to be the culprit. Blown intake gasket, blown head gasket, bad timing, worn valve seals or springs, worn pushrods, bad EGR, bad spark plugs, bad wires, bad distributer, and a few others.
First... I just replaced the intake gaskets and it no longer leaks. That's out. And second, a compression test revealed between 160 and 170 on all cylinders, with the new plugs, wires, and distributer. It still idles fine and has good power. Also out.
I have a feeling it's the valve springs or timing because the backfiring is right in sync with the engine, and it only does it at higher RPM with full throttle.
Help and advice is always appreciated. Thanks.
The 85 had 24lb injectors standard and ran less pressure but you should have at least 30 psi fuel pressure, the electrical connectors on the fuel injectors can get damaged easily pulling them off by the wires .. a injector or spark plug problem will usually show up as a shudder/misfire. Fuel pressure, timing or valve lash as back firing.
Hope this info helps you
I have my original but checked out ok heads on my 383 engine and that has spun well over 6,000 rpm a few times by accident, i would say that your valve springs are not your problem...
Last edited by gerardvg; Jan 25, 2013 at 03:24 AM. Reason: More info
Thread Starter
Racer
Joined: Aug 2012
Posts: 279
Likes: 2
From: Las Vegas Nevada
I checked the injectors when I had the intake off last weekend and all were still in new condition, and performing well with no leaks at the O rings. Previous owner had them changed in 2011, but I'm not sure which kind because my fuel pressure will sit around 50 to 60psi during driving.
Thanks for the quick response. I hope the weather's nice down there
Thanks for the quick response. I hope the weather's nice down there
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
50 to 60 PSI for fuel pressure...much too high for 24 pound injectors on a 85. Are you sure (
) your not reading oil pressure off the dash. You need to hook up a fuel pressure gauge on the Schrader valve on the passenger side fuel rail to get reading. You can tape the gauge to the windshield while driving to see where it is at.
Try opening the hood with the engine running on a dark night and see if you are getting any arcing from any of the plug wires.
) your not reading oil pressure off the dash. You need to hook up a fuel pressure gauge on the Schrader valve on the passenger side fuel rail to get reading. You can tape the gauge to the windshield while driving to see where it is at.Try opening the hood with the engine running on a dark night and see if you are getting any arcing from any of the plug wires.
Team Owner
Joined: Aug 2004
Posts: 21,543
Likes: 3,216
From: Park City Utah
X2 on that. If that information is accurate, something is way off. Confirm your fuel pressure. If FP is in fact correct (under the WOT load where the condition exits -checking at idles/driving around isn't checking), your plug wires aren't arcing (shorting to ground) and plugs are good, then I'd pull my rocker covers, crank the engine and look for a flat lobe (one or more rockers not cycling as much as the others).
Safety Car
Joined: Apr 2011
Posts: 3,660
Likes: 22
is this a misfire or a "back fire"? big difference.
misfiring can come from almost anywhere. Backfiring can only come from a path thru the engine back to the intake..like an intake valve thats stuck or burnt. If you say comp test were equal on all cyl, to me that is saying that you have a misfire. There is a small chance that a broken/bad spring could cause it at higher rpm where the weak spring makes the valve slow or late closing but it would also come with some clatter from the rocker flopping around waiting for the valve stem to rise...
The electrical is the heart & soul of the C4. Go thru the plugs on all sensors, inj, the ground bundle on the block and the jumper post hot wire stack.
If that yields no results then go to the ign and go thru it. A store brand ICM will cause this. Thats DELCO ONLY for the ICM. Cheap wires will also cause a misfire.
misfiring can come from almost anywhere. Backfiring can only come from a path thru the engine back to the intake..like an intake valve thats stuck or burnt. If you say comp test were equal on all cyl, to me that is saying that you have a misfire. There is a small chance that a broken/bad spring could cause it at higher rpm where the weak spring makes the valve slow or late closing but it would also come with some clatter from the rocker flopping around waiting for the valve stem to rise...
The electrical is the heart & soul of the C4. Go thru the plugs on all sensors, inj, the ground bundle on the block and the jumper post hot wire stack.
If that yields no results then go to the ign and go thru it. A store brand ICM will cause this. Thats DELCO ONLY for the ICM. Cheap wires will also cause a misfire.
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,009
From: Texas
Good advice from all the people above. The fuel pressure is waaaaaaaaaaaaaay off, assuming an otherwise stock engine. FWIW, at 60psi you are pushing the limits of the fuel pressure regulator (FPR) diaphragm. Assuming the 60psi is at cruise, that means WOT fuel pressure is even higher. If it ruptures, 60+psi will easily blow the vacuum line off the FPR spraying raw fuel all over the engine. On a nice hot engine while driving, you will then get to see flames shooting out from under the hood while watching the car burn to the ground.
Another "esoteric" thing that can cause backfires is timing. The base timing could be off...which also puts you at risk for burning valves. The computer controls the timing. However, if the connector between the ECM and distributor is disconnected, you will only be running on base timing...also a bad thing. FWIW, a scanner will not show this issue since it only reports what the ECM is commanding regarding timing versus what is actually happening. The single wire connector is, IIRC, on the driver's side near the rear of the engine.
Another "esoteric" thing that can cause backfires is timing. The base timing could be off...which also puts you at risk for burning valves. The computer controls the timing. However, if the connector between the ECM and distributor is disconnected, you will only be running on base timing...also a bad thing. FWIW, a scanner will not show this issue since it only reports what the ECM is commanding regarding timing versus what is actually happening. The single wire connector is, IIRC, on the driver's side near the rear of the engine.
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Thread Starter
Racer
Joined: Aug 2012
Posts: 279
Likes: 2
From: Las Vegas Nevada
Yes. Fuel pressure readings were off the dash. I'll see what I can pull together for the gauge.
I'll check the plugs tonight in the dark, but I don't think it's the wires.
Still leaning toward the valve springs since it sounds right... I'm also getting a clicking through the covers so this might be the issue. When I was reading through other forums, another 85 had a similar issue, and it was solved with new springs after he tried everything.
I'll check the plugs tonight in the dark, but I don't think it's the wires.
Still leaning toward the valve springs since it sounds right... I'm also getting a clicking through the covers so this might be the issue. When I was reading through other forums, another 85 had a similar issue, and it was solved with new springs after he tried everything.
Thread Starter
Racer
Joined: Aug 2012
Posts: 279
Likes: 2
From: Las Vegas Nevada
As of about 3 minutes ago, I think my fuel pressure sensor is busted because it's showing 80 psi, and not moving. I guess that's added to the list for autozone today...
Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,009
From: Texas
Btw...if it's the oil pressure sender, it's not unusual for them to leak oil when they fail. Check for an oil leak at the rear of the intake.
Le Mans Master
Joined: Oct 2008
Posts: 9,758
Likes: 49
From: Houston Texas
If you pulled the intake your timing could be off. Check your timing and make sure you unplug the EST wire. After setting timing, shut down, plug in EST wire, disconnect battery and then reconnect battery.
Thread Starter
Racer
Joined: Aug 2012
Posts: 279
Likes: 2
From: Las Vegas Nevada
Oil pressure... my mistake. I'll check the rear of the engine at lunch. Also getting my timing done after work. I'll try to remember /\ that /\ when my mech is doing the timing. Wish I could do it myself, but I really have no knowledge on timing, and it's better left to the semi-pro. hah
Thread Starter
Racer
Joined: Aug 2012
Posts: 279
Likes: 2
From: Las Vegas Nevada
Ok, my mechanic went on vacation before I could get back to him earlier today... figures
Ended up buying a cheap light and changing the timing myself to 6* but it didn't feel right when I drove, so I started adjusting it between 2* and 8*. Finished with 8* because it's running much smoother in the mid rpm range. But, it's still backfiring at higher rpm.
Timing is off the board so I need to keep looking.
John, I'll take you up on that another day. Thanks.
Ended up buying a cheap light and changing the timing myself to 6* but it didn't feel right when I drove, so I started adjusting it between 2* and 8*. Finished with 8* because it's running much smoother in the mid rpm range. But, it's still backfiring at higher rpm.
Timing is off the board so I need to keep looking.
John, I'll take you up on that another day. Thanks.
Last edited by Ziggy91; Jan 25, 2013 at 08:55 PM. Reason: changed info because I'm dumb
Le Mans Master
Joined: Mar 2004
Posts: 5,107
Likes: 171
From: Dublin CA
Did you unplug the brown/black wire next to the wiper motor? You have to do this when you set the timing. When you do this (unplug the wire) you will end up with a code 44 (I think, it has been awhile). If you didn't set a code when you set the timing you have done it incorrectly.
You have to unplug the brown/black which disables the computer from advancing the distributor timing. Then set you timing. Turn off the car, plug the wire back in. Disconnect the negative cable from the battery to erase the code.
You have to unplug the brown/black which disables the computer from advancing the distributor timing. Then set you timing. Turn off the car, plug the wire back in. Disconnect the negative cable from the battery to erase the code.
Safety Car
Joined: Aug 2009
Posts: 3,751
Likes: 275
From: Melbourne, Victoria, Australia
The ticking noise you are describing is usually a result of camshaft lobe wear, that is a common issue on the early C4's.
My engine never seemed to be affected by lack of performance and the wear was obviously visible when i removed the camshaft.
Valve springs are hardy things and when used with standard camshafts are never under much stress, usually the only symptom of "valve bounce" is a tapering off in performance rpm limiting no back firing and at way further up in the rpm range.
As for the fuel pressure 30 psi standard and i have an adjustable one on my 85, (45 psi the car really likes) any more and she will be way too rich at idle and yes the injectors can still work at around 60 psi but is that way over the top.
Get a gauge to measure your fuel pressure that is your number one most likely issue, be it a blocked fuel filter or bad fuel pump and or pulsator.
There have been several posts on this forum about backfiring, that were traced to bad fuel pressure and failing fuel pumps or bad fuel pump pulsator that was leaking .... see link or 101 projects for a corvette...
http://books.google.com.au/books?id=...lsator&f=false
My engine never seemed to be affected by lack of performance and the wear was obviously visible when i removed the camshaft.
Valve springs are hardy things and when used with standard camshafts are never under much stress, usually the only symptom of "valve bounce" is a tapering off in performance rpm limiting no back firing and at way further up in the rpm range.
As for the fuel pressure 30 psi standard and i have an adjustable one on my 85, (45 psi the car really likes) any more and she will be way too rich at idle and yes the injectors can still work at around 60 psi but is that way over the top.
Get a gauge to measure your fuel pressure that is your number one most likely issue, be it a blocked fuel filter or bad fuel pump and or pulsator.
There have been several posts on this forum about backfiring, that were traced to bad fuel pressure and failing fuel pumps or bad fuel pump pulsator that was leaking .... see link or 101 projects for a corvette...
http://books.google.com.au/books?id=...lsator&f=false
Last edited by gerardvg; Jan 26, 2013 at 01:50 AM. Reason: More info
Thread Starter
Racer
Joined: Aug 2012
Posts: 279
Likes: 2
From: Las Vegas Nevada
Flushed the engine today, changed the oil, changed the knock sensor, and also the mass air flow because it was missing a few...... peices. No backfires after driving for 10 minutes, and it's running much smoother. Progress has been made, but I'll see of it helps in the next few days.