90 GMC S15 4.3 6cyl, manifold red hot

Ok here goes...90 GMC S15 6 cyl 4.3, fuel injected.

last week all of a sudden my exhaust manifolds have started to run very hot and at night you can see a red glow.

I am in Toronto Ontario Canada, do not have a good mechanic and have been to many shops and $1700cdn later this probem is still not solved.

Truck for the most part runs and drives as usual....and I only found out about the manifold problem when I was hunting for this burning smell that started all of a sudden and sure enough a night test drive confirmed the manifold was glowing.

Since last Thursday I have been to several shops and each one has claimed to have fixed the problem and here is what has been done:

new map sensor, wires, plugs, cap, rotor, complete new exhaust, the problem still continues.

I talked one shop into putting it on the scanner and the mechanic said I should replace the engine because it would take too long and cost too much to diagnose the red hot manifold condition, but I did manage to take a peek at the scanner when it was hooked up and saw the o2 sensor switching from lean to rich and back but staying on lean longer and it wasn't switching that fast.

I am concerned because of the fire hazard and that it the heat is caused by it running too lean then I have probably already damaged pistons.

The truck is not using more gas so I fear it is in fact running lean.

one person did suggest a fuel pressure regulator but I am looking for more ideas from any mechanics here.

Shops these days only want to swipe my gold visa card...not fix the problem....$1700 in 7 days and counting and the problem is still not fixed.

any help from mechanics would be appreciated.

mark

Reply to
mark rozitis
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Bad O2 sensor causing it to run lean?

Or a leak in the manifold or throttle body base gasket maybe?

I use carb cleaner spray to find leaks. I spray it around the TB and the gasket where the manifold hits the engine block or heads. If the idle changes, I have found a leak.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

mark rozitis wrote:

Reply to
Mike Romain

I'd be curious what scan data would show. I'll bet the Block Learn Multiplier (BLM -- "LTF" to you OBD-II afficionados) is not right. BLM starts out at 128 (no a/f mixture correction) and should fall as the engine heats up (once closed-loop operation starts) -- this is because intake air density's decreased. I wonder whether you have an air injection system; if so, are the switching valves stuck partially in the upstream direction (thus dumping air into the exhaust manifold, causing the computer to detect, incorrectly, a lean condition, thus causing additional fuel delivery, causing the fuel to be burned in the exhaust manifold)?

Another thing that comes to mind is that ignition timing could be REALLY retarded (e.g., timing belt jumped a few teeth). This, too, would cause fuel to burn in the exhaust manifold.

Somebody smarter than me can probably come up with better ideas.

Regards,

Bohdan Bodnar

mark rozitis wrote:

Reply to
B. B0dnar

Mark, you mentioned completely new exhaust system--did that include ALL pipes. Often a pipe's inner wall can collapse, raise back pressure, & make it run hot as you described. HTH & good luck. sdlomi PLEASE report back with your findings to help us all!

Reply to
sdlomi

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