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post #1 of (permalink) Old 03-21-2004, 11:57 PM
nxcj
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Lean Backfire??

I'm stumped!

I've got an '83 3/4t Silverado. On a 300 mile round trip last weekend, it started to miss and backfire through the exhaust. I replaced the plugs on the road and this cured it. So I get to my destination and pick up my truck camper (reason for trip).

Cruising toward home and the ignition cuts out again; longer this time as when it caught, it blew both mufflers apart! Hit and miss until I stopped and replaced the cap and rotor (I think the rotor was grounding out as it was REAL ugly).

It ran ok for the last 100 miles home; hard to hear though due to said exhaust eruption.

Fast forward to today: Exhaust is fixed so I can hear. It sounds like a lean backfire coming through the carb. It is definately popping through the carb when you give it throttle. Starts and idles fine.

I've replaced cap/rotor/plugs/coil/wires/module/fuel pump; pick-up coil tests fine. Ran a compression check: great in 7 cylinders but #7 is at 90. (others are 160-165) That said, all plugs look the same. Pulled plug wires one by one while running, trying to isolate the culprit cylinder; no difference in killing any one cylinder.

Has a full 12V to the HEI. Also has a fairly new Quadrajet.

Tried to fire up my buddies "circa 1985" Allen scope yesterday, but 20 years of dust was causing us to truobleshoot the troubleshooter


Any advice before I give up my shadetree antics and drop her at the shop???

Thanks in advance - Nick
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post #2 of (permalink) Old 03-22-2004, 11:48 AM
Way Outta Control
 
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Re: Lean Backfire??

Pull the plug wire and ground it from that low compression cylinder - drive it like that - bet it'll stop backfiring.
Ground it to prevent crossfire - misleading you.

Sounds to me like the cam's going away on that cylinder - exhaust valve lobe. It doesn't open enough to let the exhaust out, misfires, and backs up fire into the intake - backfire.
The misfire also fills the exhaust with gasoline - ignites, blows the mufflers off.
Pull the rocker cover on that side, look at the valve train movement - use a caliper or scale - eyeball won't always see it. Also look at the valve spring - may be broken.

But could be an intake valve going away too.

Too bad about the Allen scope - I spent many years with Allen selling and training them - it may be old, but it still beats anything on the market now.
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post #3 of (permalink) Old 03-22-2004, 03:50 PM
nxcj
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Re: Lean Backfire??

Thanks for the reply Rich. I read a couple of posts near to my problem lately, and you had a lot of great input on both.

By the way, it's a 454 with 130k so the cam going south wouldn't be too odd.

I have pulled every plug wire trying to isolate it. I even pulled them in several paired combinations thinking that an intake leak might be affecting two cylinders at once. Still popped.

I ran acetylene near the edges of the intake and I think I found a leak near #6 and #8.

I pulled the odd bank valve cover and turned the engine over with a remote so I could watch the valves on #7. The pushrod guides keep a shiny spot on the pushrod and both valves are moving through this range. Brake booster was inhibiting my base and inicator setup. Too dumb to grab a caliper!

I hated to do it, but I dropped it off at a shop a little while ago for diagnosis. I trust them and explained where I had been so far. Hopefully they'll find something I missed. Likely though, I'll be pulling the intake and cam...

I'll post what I find out. [img]images/graemlins/confused.gif[/img]
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post #4 of (permalink) Old 03-24-2004, 12:34 AM
nxcj
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Re: Lean Backfire??

Bad cam was the correct diagnosis. I pulled it tonight and #2 and #6 exhaust lobes are nice and ROUND. Hopefully I'll have her up and running tomorrow night!

Feeling pretty stupid for not pulling the even bank valve cover now....... Nice reminder to follow through on all diagnosis methods. It would have been real obvious as there is zero lift on these two lobes. [img]images/graemlins/tongue.gif[/img]
post #5 of (permalink) Old 03-24-2004, 09:33 AM
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Re: Lean Backfire??

Exhaust lobes - what happens is AF mixture gets in, cylinder compresses it, plug fires it, but it can't get out.
Next time the intake opens, the fire flashes back into the intake, burning everything in there - starving other cyls from getting their charge.
Often a compression test won't see it.
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