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I have a problem that I hope someone can help me with. I have a 1984 Isuzu P'UP with a 1.9L 4 cyl. It has a backfire in between shifts, already blew one muffler out. With the air pump disconnected it doesn't backfire anymore, but with it disconnected it burns up the catalyic converter. I would like to get the emissions system back to stock, and in normal working order. Thanks

is it just a faint pop and not a loud bang? If it just kinda pops im sorry to say it might be normal. My 90 trooper does that from time to time as does my LUV whith the air pump on. I dont realy have a bunch of insight into this but you might make shure all your hoses are are in perfect condition and if they dont look realy good youll probly need to get new stuff(any leak in these hoses might act like a manifold leak). Well good luck, and welcome to the site

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1980 LUV 4x4, going under re-construction...

It's a loud bang, thanks for the welcome!

hummm, well now ya got me
still check all your lines, i would also check the mixtrure on the carb (1.9L is carbed right?). After that i have no idea.

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1980 LUV 4x4, going under re-construction...

When mine was stock I band-aided the problem by removing the belt to the AIR pump (aka smog pump). The back firing always went away, but it wasn't the proper way to fix it obviously.

I seem to remember something about replace the diverter relief valve at some point? Not sure on that one.

Could the egr valve be hanging open or closed during shifts causing the backfire?

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Hammer to fit, paint to match.
If it aint broke, don't fix it.
There's always room for duct tape and bailing wire.

WATCH YOUR PIEHOLE!!!!!!

Sounds like it might be the Vacuum controlled anti-back fire valve gone defective. This device shuts off the air to the exhaust at high vacuum.
Good luck Fitzworld

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Fitzworld
A driveway full of Full size American 4x4 iron and one
LUV Diesel long bed

Yeah there is a valve that controls the air pump that I think is the usual source of these problems. Look for a vac line that crosses from the carb side to the exhaust side of the motor. That line should hook into the valve. You might be able to get it working better again with just some cleaning, maybe some carb cleaner or something will unstick it. Check that the hose and lines going to it are in good shape too, might have a slight leak keeping it from working.

The EGR valve can cause some emissions problems as well and seems to like to clog up. You pretty much have to pull the intake manifold to get at all of it though. My 77 still pops even with no air pump on it at all, but nothing like the backfire you describe. It has a leak on the EGR pipe at the exhaust manifold. When I patch that hole, almost no pops, but it burns just about anything I've tried to fill it in with. I'm just living with it until I get time to swap a manifolda and EGR pipe I've already got sitting waiting.

The last tip would be to look at the carb adjustment and general tune up stuff. You could be running fairly rich if there is enough unburnt fuel to give a big bang and/or burn up the cat. Might be time for a rebuild kit for the carb.

mine did this after i accidently crossed 2 air pump hoses. it backfired real loud everytime i shifted. try switching the 2 big air hoses that go into the air pump. when i did this it stopped mine from backfiring.

There is a valve up front right next to the smog pump that is a high replacement item. I have one on the 73 that has rusted away, so you should look into replacing it.

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