Backfire under load.

[Soppy]

Could be the ECM . The backfire is either lean air fuel or timing . The ECM controls both. Possibly the Oxygen sensor , but not likely .
Are any codes being set ? You could disconnect the Oxygen sensor which forces the system in open loop & try that. Open loop uses a fixed set of values & only a few of the sensors.

[Charlie 5320]

Originally Posted by sheepofblue

Just fixed a back fire under heavy load on my 8.1. I had one plug wire that was burned or broken internally. Put a meter on it and it had the same resistance as the other 7 until you moved it, then poof it would jump to a meg ohm.

That said mine has zero to due with shifting. The 7.4 should have a non-electronic transmission I think which means you have a vacuum modulator. Thus every time you shift the vacuum changes. So I would look for more vacuum leaks and even climb under to check at the modulator. If there is any fluid in the line change the modulator, it will also shift better if that is the problem.

The 95 P30 chassis DOES have an electronic shift trans, no modulator. Don't think they ever put a 700R4 trans in a motor home.

[rockymt]

Mist the plug wires with water - can you see. Sparks - does the engine miss ? Replace the wires

[scdiver]

Another update.

We are blessed in this town to have a auto shop in our high school with the best teacher and mechanic that I have ever seem. Frank can rattle off the specs for just about any car, specially Chevy’s. Franks stays in touch with certain ex students and my youngest is one.

When I called Frank to get his suggestions on these problems he was convinced I had a broken or weak intake spring. Using a spring tester I found all springs 80 lbs or slightly more. We then did a leak down test and found all cylinders with 10 to 15 percent leak down. Specs call for 40 percent leak down before this engine starts to be bad. So what I see is a like new engine.

Frank then suggested that after I replaced the plugs with iridium autolite xp25 plugs that I set the timing to 8 degrees btc. The plug in number 5 had hard deposits on the electrodes that I think glowed under certain conditions causing incoming fuel to ignite. Since my tag axle brake vacuum draw is at number 5 I think the deposits came from when I overhauled the tag axle brake master cylinder.

As for the pinging, I purchased a cheep smoke machine off Ebay and smoked my engine and found two rubber boots with small holes in them. A cheep fix.

I left St. Louis Saturday Aug 19th to see the eclipse in Columbia Mo. and now we are traveling to Dodge City Mo. Boy is this rig running good. It has not run this good in the four years that I have owned it.

It is hard to believe that deposit on a plug, two rubber boots and timing is all that was wrong. I am hoping that this trip does not require major repairs when we return home like every trip before.

[boatman10]

there are LOTS of counterfeit AC iridium plugs out there.....I would NOT buy the cheapest listed on EBAY or AMAZON..........only from chevy dealer[ouch] or respected parts place[and that's dicey maybe].