back fires out of throttle body

[justinmm1987]

i got my z34 running but it back fires out of the throttle body and i dont know why. it starts fine and it runs fine but when you step on the gas and it gets to about 3000 rpms it pops fire balls. i think it might be vaccum lines, spark pulgs or wires,or coil pack. any ideas?

[tblake]

3.4L V6 I'm guessing? Check the timing belt, backfiring through the intake is a common indication that the timing has skipped a tooth or so.

[RahX]

they use a timing chain and to see one of those go out is pretty uncommon. you might have have someone see if there are any codes. ive seen cam sensors cause injector spray at the wrong time (intake valve opening) which can also mess with the ignition timing as well and you get a nice fireball. but its also good to check fuel pressure and such too. but i would check for codes first.

[tblake]

chain on the 3.4 dohc's? I dont think so. Anyone?

[corning_d3]

The DOHC uses a belt, which needs to be replaced every 60,000. Backfiring thru the intake is a good sign of jumped timing..

[RahX]

well in my defense, it does have a timing chain coming off the crank but a belt driving the cams. i was thinking of the wrong 3.4 ><

[justinmm1987]

and i think it is only fireing on the front 3 cylinders beacuse i can un plug the back ones and the car does not really run any diffrent.

[tblake]

there ya go, the belt skipped on the timing gear on the back cam. To make sure, grab a compression testor from anywhere, and check the compression.

[justinmm1987]

i do not get it though because the motor ran just fine in a diffrent car. i have to put gas in the throttle body to get it to run and when it does fire it runs really rich. the car was sitting for about a year with gas in it so i thought it was just bad gas in the lines or something. i put new gas in and it doesnt even make a diffrence. when i put the peddle to the floor the motor surges but it will not rev over 3000 rpms.

[justinmm1987]

could it be a vaccum leak ?

[tblake]

backfiring through the intake is often times a more serious problem than just a vacuum leak. Once you hit the gas, the vacuum drops out anyways, so I dont see it having anything to do with how the motor runs when its being revved. Vacuum leaks often times cause a stumble or miss at idle, or high idle.

[justinmm1987]

anyone else got any imput it would help

[corning_d3]

Originally Posted by tblake

there ya go, the belt skipped on the timing gear on the back cam. To make sure, grab a compression testor from anywhere, and check the compression.

I agree. I've had a few vehicles jump time just by shutting the engine off. I'd do a compression test, then you can rule out skipped timing 100% and go on

[txroda]

If disconnecting the back 3 spark plugs doesn't change how the engine runs, then it sounds like a fuel injector or spark issue to the back cyclinders. They aren't firing every time and fuel builds up in them, then they occassionally fire and have way too much fuel (running rich) because they don't fire every time. This is probably what's happening...but WHY is the question. Maybe the rear spark plug wires are bad or the spark plugs themselves. Because they are harder to get to, I've seen where mechanics replaced the front 3 but not the back. Or maybe they don't get the plug wires on good. Or they messed up the back plug wires when pulling them off the spark plug.

[corning_d3]

Still no compression test. The cam that controls the valves on the back cylinders still sounds like it's slipped timing.