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Old 01-05-2019, 05:01 PM   #1
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Workhorse Backfires

I have a 2002 workhorse 8.1L. All fine until I have the cruise set and the rig heads up a hill and goes into passing gear. The revs go high and the engine backfires. It doesn't do it if I gently go up the hill. I have had the plugs and wores changed. I had the codes read thinking it would be a coil....no codes. Mechanic stumped. Any ideas?
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Old 01-05-2019, 05:09 PM   #2
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We had a problem like that on one of our work trucks where I used to work and it was a cracked intake manifold.

Also the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust system and causes a loud bang.

Change the oxygen sensors, stop air leaks on your intake manifold and also you might check your catalytic converter.
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Old 01-05-2019, 05:20 PM   #3
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I had a 2003 Pace Arrow that did the same thing going up hills. Repair shop put in new plugs and wires and problem solved.
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Old 01-05-2019, 05:31 PM   #4
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Check the vacuum hoses and advance on the distributor. Also double check the sparkplug wires to make sure they're in correct order.
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Old 01-05-2019, 05:44 PM   #5
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I don't think it has a distributor.
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Old 01-05-2019, 06:14 PM   #6
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I wasn't aware that the Vortec 8.1 had a distributer, vacuum advance, or a mass of plug wires to cross up, or a way to manually adjust the timing.


OP, try getting one coil pac and replace yours one at a time and then drive it with the new one on each cylinder and see if it clears up, then you will know which one is bad.


Also, there are many, many threads in the Workhore forum on 8.1s with a power on miss.
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Old 01-05-2019, 08:41 PM   #7
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Low fuel pressure/volume will create a lean condition under load, resulting in backfiring.

Taking the hill slowly without cruise control seems to verify that you are able to manage the amount of power to the available fuel. Cruise control continues to try to maintain speed with more throttle, while you are trying to minimize the backfiring by reducing throttle.

This is a typical symptom of a failing fuel pump that will leave you stranded somewhere as the ability to deliver fuel continues to drop off. Fuel pressure and volume should be checked.

The fuel filter should be replaced if you can't remember or don't know the last time it was done.
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Old 01-05-2019, 10:19 PM   #8
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I had a similar problem when going up hills with my GM 454 motor, in a Winnebago Suncruiser, during high torque hill climbs. I took it in and after showing them how to reproduce the problem, they located the issue. It turned out it was a wiring short. When the motor had high torque, the motor would lean over and pinch a wiring harness, making it sputter, and occasionally backfire. It was manageable to drive home because it was only under high torque on very steep hills in low gear, which is a rare occurrence. The motor mounts were good, as it was a new RV. The problem was the wiring harness wasn't located in the right area in the motor compartment. They said it was easy to relocate the harness, and the wires weren't damage much, so those were easy to fix.

Another unrelated backfire situation I had was another wiring short, same RV. This one occurred when cruising down the freeway, flat, no hills, and all of a sudden the RV starts to backfire, really loud! The RV started slowing down. I made it to an off ramp fortunately and as soon as I got off the freeway the motor quit running. I'm thinking.... what the heck? I opened the hood and I happen to put my hand on the battery cable. It was super hot, so I disconnected the hot wire to the battery ASAP. I followed the battery cable back to the starter under the RV and found it shorted to the transmission cooling line. I could smell burned plastic. That cooling line apparently got so hot it melted the battery cable insulation. The battery wire was actually arc welded to the cooling line, a major short. After carefully wiggling the cable back and forth, hoping not to break the cooling line, the cable broke loose, and no leaks! Yay! I wrapped the battery cable with several wraps of electrical tape and re-routed the cable with nylon tie wraps so it couldn't touch the cooling line. What surprised me was when I reconnected the battery cable to the battery, the RV started right up. I got lucky finding that problem. It only took me 15min to fix, however I had to replace the cable when I got home, couple of hours for that.
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Old 01-06-2019, 07:08 AM   #9
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A lean condition under load will cause an engine to backfire. It could be caused by low fuel pressure/volume , vacuum leak, EGR valve or MAF sensor. Possibly the Oxygen sensor , but it should show a fault code . I would start by connecting a fuel pressure gauge and driving it under load to see if the pressure is in spec.
A plugged Catalytic converter may also be the problem.
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Old 01-06-2019, 09:28 AM   #10
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Quote:
Originally Posted by Craig36 View Post
I had a 2003 Pace Arrow that did the same thing going up hills. Repair shop put in new plugs and wires and problem solved.
Exactly! Forget the other stuff for now. Your problems will probably go away after plugs and wires are replaced. Been there, done that...
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Old 01-06-2019, 09:32 AM   #11
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Is it backfiring out the tailpipe or out the intake manifold?
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Old 01-06-2019, 11:08 AM   #12
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Get a scan tool and look at the O2 switching under load. It should initially go full rich on load and then start regulating before it backfires.
If it stays lean, then look at fuel delivery problem like dirty filter (my guess of highly probable as others have posted).
If you haven't replace the filter recently. Start there with no further diagnosis as it should be done anyway.

After that, look at ignition like others have posted.
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Old 01-06-2019, 11:30 AM   #13
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Check the fuel pressure. Fuel pump failure on early 200x 8.1’s is relatively common.
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Old 01-06-2019, 12:11 PM   #14
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Originally Posted by Mudfrog View Post
Exactly! Forget the other stuff for now. Your problems will probably go away after plugs and wires are replaced. Been there, done that...
Just to be clear to others, the OP already said he had done those two things.

When the exhaust valve opens, the mixture isn't done burning so it ignites the unburned fuel in the exhaust system and causes a loud bang. ... As with backfiring, coughing or spitting can also be caused by a bad ignition system, such as cross firing, which sends a spark to a cylinder that has the intake valve open.
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