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Engine Misfire [message #357408] Sat, 01 August 2020 17:14 Go to previous message
TR 1 is currently offline  TR 1   United States
Messages: 348
Registered: August 2015
Location: DFW
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So I took the coach out for a final run earlier today after doing some pre-trip maintenance and I noticed the engine not running quite as smooth as normal... It was minor, and I had not run the coach for a few weeks, so thought it just needed to run a bit. Getting it out on the highway, under heavy load I got a decent backfire, which the coach has never really done before.... My AF meter also showed a lean condition after the backfire. I don't believe it was showing lean before. As a limped it back home, the backfires started getting worse, though with smooth, light throttle inputs it would run ok, though still lean per the afm

My coach runs a Howell system, but no EBL... Standard valvetrain, and the o2 sensor for my afm is on the left bank. I started poking around, one of the things I did was to pull the fuse to the engine computer so I could turn the coach over without it firing. When I did this, I heard was what sounded like compression puffs coming back up through the intake. Opening the throttle butterflies I could also feel the puffs and hear them get louder the more I opened the throttle. I also have an engine vacuum gauge, and I could see small jumps on the gauge that tied to the puffs in the intake.

I started pulling plugs one at a time to see which cylinder made the puffs quieter, and I believe the problem is number 2 cylinder. (Puffs got quieter when I pulled this plug)
Assuming my understanding of how the Howell fuel injection works is correct, my assumption is if #2 is not firing, and dumping raw fuel into the exhaust, the Howell will try to lean out the engine. Since my afm is on the left bank, I would just see a lean reading, since the raw fuel was going to the right bank.

So does anyone have any idea what would could cause compression puffs to go into the intake? The only thing I could think of was a broken intake valve spring, or maybe a dropped valve? I would have thought if it were a dropped valve I would hear the valve banging around... But the only thing odd I am hearing out of the ordinary are the compression puffs.

About to go out and pull the valve covers, but does anyone have any ideas for other areas for me to check? The kids are bummed as we were supposed to get on the road today, and I kinda doubt this is going to be something I can fix quickly and salvage the trip, so any help, thoughts, etc. would be very appreciated by me and the kids Wink

Mark S. '73 Painted Desert, Manny 1 Ton Front End, Howell Injection, Leigh Harrison 4bag and Rear Brakes, Fort Worth, TX

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