1992 GMC Suburban Tranny replacement

STEIGERHOUSE

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1992 GMC SUBURBAN
V8
2WD
AUTOMATIC
65,000 MILES

Help! I have a '92 Fleetwood Flair sporting a GM chassis with a 7.4L big block Chevy engine. Last fall on a trip down south I lost my transmission (input shaft broke). The tranny was a 4L60E. I had a new GM tranny installed at a cost of a little over $3000 by a GM dealer in Red Springs NC. They put in a 4L80E. They said the 4L80E would hold up longer and maybe get better gas mileage. They had to place a new wiring harness because of differences in the wiring of the 4L60E and the 4L80E. I got a 1 year 50,000 mile warranty on the new tranny.

Since then I have had a rapid backfire intermittantly when the transmission downshifts on a grade or pull. This never happened before the transmission replacement. I called the dealer in Red Springs and they say this could not have anything to do with the new transmission. They suggested I had a bad plug or plug wire or distributor or clogged fuel filter. I have replaced fuel filter, plugs, wires, rotor cap, rotor, rotor module. Still have the problem.

I bought a Chilton manual for this engine and it shows that with the two different engine/transmission combos, the fault codes are entirely different. This tells me that if they got a wire misplaced with the new harness, it could be causing something to misoperate. Does anyone out there have any knowledge of this type of problem?

I would greatly appreciate any help.

7 Replies

KEN L

MASTER CERTIFIED MECHANIC
48,363 POSTS

Hello, does you sub have air injection for the exhaust, If so it sound like the backpressure check valve near the exhaust manifold has failed, remove to check the one way air valve. Check and let me know.

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Saturday, December 27th, 2008 AT 7:48 PM

STEIGERHOUSE

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Boy, I really was hopeful that you had hit the nail on the head, as this seemed to fit the condition exactly. First, let me say that my vehicle is a motorhome. It is on a 1991 p37 chassis when I decifer the VIN number. The backfiring I am experiencing is a rapid burst of misfires in a group on acceleration, usually when the transmission is automatically shifting to a lower gear, such as when you start to lose speed going up a hill. It sounds as if it is in the exhaust manifold or the exhaust pipe, close to the engine, rather than at the muffler or catalytic converter. Anyway, no air pump. It is crazy, but though the VIN says it is a 1991 chassis, the drive belt configuration matches a 1993 in the Chilton manual. It has a TBI carburator and electronic distributor with an external coil. While I was looking for the components associatiated with an AIR system, I did discover a set of two vacuum hoses from the carburator body below the throttle plate to the air purge canister were disconnected at the cannister. I thought WOW, this is probably the problem. I fixed the connection and went for a test drive, but no luck. It is still backfiring, although it did seem a little harder to make it happen. Maybe that is just my imagination though. Any other ideas would be greatly appreciated. Thanks, Ron.

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Sunday, December 28th, 2008 AT 3:54 PM

KEN L

MASTER CERTIFIED MECHANIC
48,363 POSTS

Hi, is there a chance when installing the transmission the distributor got moved a little and the ignition timing is now off, if the timing was off that would make it backfire.

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Monday, December 29th, 2008 AT 1:02 AM

STEIGERHOUSE

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I was told by a GM service dept manager that sometimes when they remove the transmission in an RV, the back of the engine drops down and the distributor heats the firewall and cracks the rotor cap. I replaced that part and it did not help. I will try to check the timing soon, maybe today.

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Monday, December 29th, 2008 AT 8:46 AM

KEN L