What causes this? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Deltaskier · 03-03-2004, 12:02 AM

On an 83 with a 350 & flowmasters, what makes it backfire every time I let off the gas? I know an exhaust leak could cause it, but the truck also rumbles & sometimes backfires when it's compression braking.

I hate it that every time I get on it, it backfires three or four times when I let off the gas & then makes that rumbling sound with some smaller backfires as it coasts.

Timimg? Exhaust leak? rich? Lean? I want to stop it, but I don't know where to start.

87 STEPPER · 03-03-2004, 12:46 AM

If your still runing your EGR valve it's either inoperative or it's not connected properly and it could cause this problem.

If you have removed the EGR. Then it could be a timing issue.

Deltaskier · 03-03-2004, 12:49 AM

No EGR & an edalbrock 1406.

**crossy** · 03-03-2004, 02:20 AM

before any other info comes in I'm going to say rich, make sure your choke opens all the way. DAVE

Deltaskier · 03-03-2004, 02:31 AM

Can't be rich. The exhaust is glowing from being too lean. Already today I've gone from stock, #1 on this chart http://www.edelbrock.com/automotive/eps_1406.html to number 19 & that seemed to quiet it sown a little, but it's still there.

I'm not changing metering rods to solve the backfire. I'm trying to solve the exhaust glowing red after a freeway test drive.

BladeX · 03-03-2004, 02:57 AM

rofl yup im done that. flyin bout 60 let of the gas and it backfires like 5 times.

2 diffrent times.

1st i was missing a bolt in the manifold air injection hole.
2nd was when the o2 sensor bolt in the manifold fell out.

J-Spec C10 · 03-03-2004, 03:17 AM

still have convertors on the truck? could there be a blockage?

Deltaskier · 03-03-2004, 03:46 AM

No converters. Just flowmasters.

I've wondered, since I noticed the glowing exhaust, if my problem could be that none of the smog equioment is hooked up but the smog pump is still blowing into the exhaust throught the air tubes. I don't know crap about emmissions though, so I'm not even sure if it blows into the exhaust or sucks out of the exhaust through the air tubes.

The exhaust never heats up with normal in town driving, just freeway driving. I paln to try rejet the secondaries richer tomorrow to see if that helps any.

bigd65 · 03-03-2004, 09:37 AM

My 55 chevy in high school did that and it was an intake vacuum leak. I believe i traced it to the PCV.

**crossy** · 03-03-2004, 11:43 AM

re-jetting secondary side will not help. Your problem is at cruise RPM where your secondaries are not even working. Severely reatarded timing can do the same, If your going into the carb again it needs to be something related to primary/part throttle.
EGR's can affect this also. I know your emissions stuff is disconnected but if you still have an EGR in place try hooking it directly to one of the ported vacuum ports on the carb. the ported ones will not havea vacuum at idle. An EGR can really fatten up a mixture. DAVE

Deltaskier · 03-03-2004, 01:10 PM

The EGR isn't there. I probably should have said 1406 carb & performer rpm intake.

BladeX · 03-03-2004, 09:10 PM

If ya dont have air pump check valves on each branch of air pipes that could make it backfire too.

CHEVYE · 03-03-2004, 10:54 PM

couldn`t it be a timing issue?

*mike reeh* · 03-03-2004, 11:05 PM

Quote:

Originally posted by Deltaskier
the smog pump is still blowing into the exhaust throught the air tubes. I don't know crap about emmissions though, so I'm not even sure if it blows into the exhaust or sucks out of the exhaust through the air tubes.

that is your problem. If ANY air goes thru those tubes during deceleration, it will cause a backfire. Remove the tubes and put pipe plugs in each hole, and you problem will be solved.

mike

**crossy** · 03-04-2004, 12:08 AM

Mike's prolly right ,come to think of it I had to do that to the tubes on a street bike that I had modified the exhaust on.

CRRoy71 · 03-04-2004, 12:12 AM

There should be check valves on each air tube. You can get them from the dealer if you don't have them. I put mine on, and haven't had a problem.

Deltaskier · 03-04-2004, 12:26 AM

This is what I've got. I'm assuming there's a check valve in there somewhere.

Deltaskier · 03-04-2004, 12:28 AM

THe backfiring is annoying, but it doesn't bother me as much as the glowing exhaust. I would like to be able to drive the truck. I can drive it with it backfiring, but not when the exhaust is melting.

BladeX · 03-04-2004, 01:37 AM

yeh you have air pump checkvalve on that pic.

MylilBowTie · 03-04-2004, 05:04 PM

I thought that the backfire sound was from exhaust pipes that are to small for the engine. I would look into the timing. Has the balancer slipped or anything that you noticed? Has anything else been replaced besides the carb and intake?

Deltaskier · 03-04-2004, 05:42 PM

I had a friend who builds engines come over & check out the timing. We thought it might have slipped as well. We pulled the plug, found TDC, set initial timing & full timing, & made sure the vac advance was working properly. As far as I know, it's not timing related.

I am planning to swap out the distributor soon just to rule that out. It's the only other thing that I can think of that would cause the exhaust to glow since I've tried everything else.

I'm not ssure what size my exhaust is. They're probably 2". Could that be where my restriction is that's causing my exhaust to glow?

*mike reeh* · 03-04-2004, 10:25 PM

how does the engine run? What do your plugs look like?

based on what you've told us, I would bet $ that

A) your backfiring is caused by malfunctioning AIR injection
B) your red hot exhaust is caused by a lean condition and might possibly be causing the backfiring also.

just a thought, though.

mike

03-03-2004, 12:46 AM	#2
87 STEPPER STILL PLAYS WITH TRUX Join Date: Nov 2002 Location: Melbourne, Fla. Posts: 2,764	If your still runing your EGR valve it's either inoperative or it's not connected properly and it could cause this problem. If you have removed the EGR. Then it could be a timing issue. __________________ ~~Bruce~~ MY 87 STEPPER MY 99 2D/WD TAHOE

03-03-2004, 02:20 AM	#4
crossy Oh,you can't buy that new Join Date: Apr 2000 Location: Pennsville,N.J. 08070, USA Posts: 2,039	before any other info comes in I'm going to say rich, make sure your choke opens all the way. DAVE __________________ "been there, done that, ruined the T-shirt". 2007 LBZ GMC Sierra, ECSB 2006 LM7 RCSB Silverado 98 Vortec project. 94,3500,6.5L 4L80 81 SWB GMC POS finally gone 73 Pinto, stock, w/CragarSS's-eww LOTS of Cummins trucks.

03-03-2004, 03:17 AM	#7
J-Spec C10 Whats this wire for?.... Join Date: Dec 2002 Location: Barrie, Ontario, CANADA Posts: 378	still have convertors on the truck? could there be a blockage? __________________ Kyle CANADIAN CONNECTION '82 C10 Resto-Custom--> * 2 1/4" manifold back dual exhaust with Hooker Aerochambers & High Flow Cats Taylor Ignition Wires Centerline 15" Telstars with 32"x10" mudders

03-03-2004, 09:37 AM	#9
bigd65 Registered User Join Date: Mar 2000 Location: Lake Jackson, TX. Posts: 1,930	My 55 chevy in high school did that and it was an intake vacuum leak. I believe i traced it to the PCV. __________________ 2004 Silverado Crewcab (Dark Spiral Gray)

03-03-2004, 11:43 AM	#10
crossy Oh,you can't buy that new Join Date: Apr 2000 Location: Pennsville,N.J. 08070, USA Posts: 2,039	re-jetting secondary side will not help. Your problem is at cruise RPM where your secondaries are not even working. Severely reatarded timing can do the same, If your going into the carb again it needs to be something related to primary/part throttle. EGR's can affect this also. I know your emissions stuff is disconnected but if you still have an EGR in place try hooking it directly to one of the ported vacuum ports on the carb. the ported ones will not havea vacuum at idle. An EGR can really fatten up a mixture. DAVE __________________ "been there, done that, ruined the T-shirt". 2007 LBZ GMC Sierra, ECSB 2006 LM7 RCSB Silverado 98 Vortec project. 94,3500,6.5L 4L80 81 SWB GMC POS finally gone 73 Pinto, stock, w/CragarSS's-eww LOTS of Cummins trucks.

03-03-2004, 12:02 AM	#1
Deltaskier Registered User Join Date: Jan 2004 Location: Montgomery, TX Posts: 73	What causes this? On an 83 with a 350 & flowmasters, what makes it backfire every time I let off the gas? I know an exhaust leak could cause it, but the truck also rumbles & sometimes backfires when it's compression braking. I hate it that every time I get on it, it backfires three or four times when I let off the gas & then makes that rumbling sound with some smaller backfires as it coasts. Timimg? Exhaust leak? rich? Lean? I want to stop it, but I don't know where to start.

03-03-2004, 12:49 AM	#3
Deltaskier Registered User Join Date: Jan 2004 Location: Montgomery, TX Posts: 73	No EGR & an edalbrock 1406.

03-03-2004, 02:31 AM	#5
Deltaskier Registered User Join Date: Jan 2004 Location: Montgomery, TX Posts: 73	Can't be rich. The exhaust is glowing from being too lean. Already today I've gone from stock, #1 on this chart http://www.edelbrock.com/automotive/eps_1406.html to number 19 & that seemed to quiet it sown a little, but it's still there. I'm not changing metering rods to solve the backfire. I'm trying to solve the exhaust glowing red after a freeway test drive.

03-03-2004, 02:57 AM	#6
BladeX Registered User Join Date: Dec 2003 Location: Kentucky, Ft. Campbell Posts: 287	rofl yup im done that. flyin bout 60 let of the gas and it backfires like 5 times. 2 diffrent times. 1st i was missing a bolt in the manifold air injection hole. 2nd was when the o2 sensor bolt in the manifold fell out.

03-03-2004, 03:46 AM	#8
Deltaskier Registered User Join Date: Jan 2004 Location: Montgomery, TX Posts: 73	No converters. Just flowmasters. I've wondered, since I noticed the glowing exhaust, if my problem could be that none of the smog equioment is hooked up but the smog pump is still blowing into the exhaust throught the air tubes. I don't know crap about emmissions though, so I'm not even sure if it blows into the exhaust or sucks out of the exhaust through the air tubes. The exhaust never heats up with normal in town driving, just freeway driving. I paln to try rejet the secondaries richer tomorrow to see if that helps any.

03-03-2004, 01:10 PM	#11
Deltaskier Registered User Join Date: Jan 2004 Location: Montgomery, TX Posts: 73	The EGR isn't there. I probably should have said 1406 carb & performer rpm intake.

03-03-2004, 09:10 PM	#12
BladeX Registered User Join Date: Dec 2003 Location: Kentucky, Ft. Campbell Posts: 287	If ya dont have air pump check valves on each branch of air pipes that could make it backfire too.

03-03-2004, 10:54 PM	#13
CHEVYE my gas saver Join Date: Sep 2003 Location: Clarksville, TN Posts: 2,046	couldn`t it be a timing issue?

03-04-2004, 12:08 AM	#15
crossy Oh,you can't buy that new Join Date: Apr 2000 Location: Pennsville,N.J. 08070, USA Posts: 2,039	Mike's prolly right. come to think of it i had to do that to the tubes on a street bi Mike's prolly right ,come to think of it I had to do that to the tubes on a street bike that I had modified the exhaust on. __________________ "been there, done that, ruined the T-shirt". 2007 LBZ GMC Sierra, ECSB 2006 LM7 RCSB Silverado 98 Vortec project. 94,3500,6.5L 4L80 81 SWB GMC POS finally gone 73 Pinto, stock, w/CragarSS's-eww LOTS of Cummins trucks.

03-04-2004, 12:12 AM	#16
CRRoy71 Registered User Join Date: Feb 2003 Location: Mont Belvieu, TX Posts: 852	There should be check valves on each air tube. You can get them from the dealer if you don't have them. I put mine on, and haven't had a problem. __________________ Chris '92 ext. cab s/b 4.3L "Sparky" '04 crew cab 2500 4x4 6.0L



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03-04-2004, 12:26 AM	#17
Deltaskier Registered User Join Date: Jan 2004 Location: Montgomery, TX Posts: 73	This is what I've got. I'm assuming there's a check valve in there somewhere.

03-04-2004, 12:28 AM	#18
Deltaskier Registered User Join Date: Jan 2004 Location: Montgomery, TX Posts: 73	THe backfiring is annoying, but it doesn't bother me as much as the glowing exhaust. I would like to be able to drive the truck. I can drive it with it backfiring, but not when the exhaust is melting.

03-04-2004, 01:37 AM	#19
BladeX Registered User Join Date: Dec 2003 Location: Kentucky, Ft. Campbell Posts: 287	yeh you have air pump checkvalve on that pic.

03-04-2004, 05:04 PM	#20
MylilBowTie Right turn Clyde Join Date: Nov 2002 Location: Cincinnati, Ohio Posts: 2,911	I thought that the backfire sound was from exhaust pipes that are to small for the engine. I would look into the timing. Has the balancer slipped or anything that you noticed? Has anything else been replaced besides the carb and intake? __________________ 1974 C25 LWB: 454 4bbl Carb / TH400 / 3.73 1992 Camaro RS 305 TBI

03-04-2004, 05:42 PM	#21
Deltaskier Registered User Join Date: Jan 2004 Location: Montgomery, TX Posts: 73	I had a friend who builds engines come over & check out the timing. We thought it might have slipped as well. We pulled the plug, found TDC, set initial timing & full timing, & made sure the vac advance was working properly. As far as I know, it's not timing related. I am planning to swap out the distributor soon just to rule that out. It's the only other thing that I can think of that would cause the exhaust to glow since I've tried everything else. I'm not ssure what size my exhaust is. They're probably 2". Could that be where my restriction is that's causing my exhaust to glow?

03-04-2004, 10:25 PM	#22
mike reeh 5 day ban, learn to behave. Join Date: Nov 2002 Location: san diego, ca Posts: 684	how does the engine run? What do your plugs look like? based on what you've told us, I would bet $ that A) your backfiring is caused by malfunctioning AIR injection B) your red hot exhaust is caused by a lean condition and might possibly be causing the backfiring also. just a thought, though. mike

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