Back fire problem

I have a 97 Jeep wrangler with a 4.0 engine, I had a new stainless exhaust system, stainless manifold, stainless steel cat and an AEM brute force air intake with the fuel injector spacer installed. The gas mileage and performance improved a whole lot. It feels like a race car. The problem I have if I drive the car, and then park it in the sun the car back fires before I take it off. Then the car is ok. Would you know what the problem may be ?

Your problem is that you have no back pressure. You may be able to fiddle with the timing to get it to a point in which it doesn’t backfire and still runs okay. Otherwise, you’ll probably need a different air intake.

The “car backfires before you” WHAT! Before you take what off? Your vest? Your hat? What?

To answer your question, it backfires only once, after it sits in the sun and I restart it and take off. After it back fires once it is ok.

You might want to check to see if there’s a leaky fuel injector.

As the vehicle sits in the sun, the pressure in the gas tank increases. If there’s an injector that’s stuck slightly open, it will drip more gas into a cylinder under hot conditions rather than cold conditons. If there’s excess fuel in any of the cylinders when the engine is started, not only does the fuel that’s being injected into that cylinder ignite, but also the excess fuel sitting on top of the piston. This then causes a backfire once, and afterwhich the engine operates normally.


Sounds like you nailed it, Tester.

I agree, sounds like you got it, this never happens to me in the colder weather. My question, would this cause any engine damage. My mechanic said no, since it happens only once. I do get around it by revving up the engine before I put it in gear… Thanks HenryC

Probably won’t cause engine damage, but with repeated backfires damage to the catalytic converter might occur. Better to fix it than replace a cat.


TESTER, thanks for your help it is appreciated !! … HenryC

An indication, not proof, that the fuel injector(is) is leaking, could be done with a fuel pressure gauge installed, the fuel return-to-tank line plugged. With the engine OFF, and the fuel gauge showing pressure, wait 15 to 30 minuets to see if the fuel pressure drops. If it does, it may be from a leaking injector(s).

A more direct way, without a special tool, is to remove all the fuel injectors still attached to the fuel rail. Tie plastic bags around the injectors, Turn the ignition key ON to pressurize the fuel injectors, wait a few minuets to see if any fuel leak from the spray nozzle of the fuel injectors. A pressure cleaning of the fuel injectors may help ones which leak.