Backfiring Viciously

1991 318, 3spd. LA engine.

Hey gang. I've been trying to tack down the source of my backfiring, which came on, while at Van Nationals. It was mild then, once in awhile, but now, whenever I accelerate, it backfiries. It also backfiries when I pump the gas, while in park.

What I've done so far. Changed the plugs, air filter, PCV, checked and reset timing. I had previously reset the idle, which seems fine. I checked the transmissions throttle rod, which also seems fine.

Plans for today are to check the exhaust to see if there is blockage. I currently am not running the air pump, and have no cat. converter. Also I want to buy a compression tester.

Also, with regards to timing, my label says 10 degrees, is this the best setting, or should I consider a different one.

I haven't yet check ed the exhaust, but I did fill my empty gas tank, and tried disconnecting the EGR, both with no change. Tomorrow morning I'm going to pull exhaust off, have look inside, and drive the van down the road, and see if any change.

Any thoughts on this subject?? I thank you in advance... http://www.dodgetalk.com/forums/images/smilies/1zhelp.gif

Oh, also, I'm trying to do everything possible, prior to doing a compression test. I want to rule out every possibility first. I also have plans to change my MAP and TPS sensors. My o2 sensor has already been changed.

The engine has about 145,000 miles on it.

The three "easy fix" things that come to mind as possible culprits are the TPS, a vacuum leak, or low fuel pressure (pump or filter problem).

The more involved possibility is a stretched/worn timing chain set which is throwing off your valve timing.

A bad plug wire will cause a backfire if it has an air gap. If you have insulated plug wire pullers you can disconnect one wire at a time to find the bad cylinder. Or disconnect one wire at a time start and find where it sounds different. You should have a good idea which side is causing the problem.
Once a problem cylinder is found pull the valve cover and check for a problem valve spring or lifter.
Once a bad spring is found pull up the valve put some small rope in the plug hole. Then roll the engine up till the piston is pushing up on the valve. Change the bad spring and back off the piston and pull out the rope.

Thanks to both of you for info. I am pretty sure that the timing chain has jumped a tooth, as I can not even see my timing mark, on the damper, when I try setting timing. I tried bringing the #1 piston to TDC, and then checked to see if rotor was pointing at #1 sparkplug, and it wasn't. It was about an inch and a half around, clockwise. So if my #1 plug was sitting at 12:00, then the rotor was sitting at 2:00. I set the timing by ear, and feel, to the best of my ability, but it still backfires under hard acceleration. And it also seems to miss a little at higher RPM's.

your back firing could be your valve timing being off or ignition timing being off, or simply since you do not have a catalytic converter also can cause this to happen.

Changing a timing chain isn't that hard you should be able to do it in a day's time. It isn't very expensive if you do it yourself. I put a short block in my Dakota and had a valve job done. This took a little longer.

rhandwor, thats my plan. It's just a bit of a pain in the a$$, as I will have to tear out most of the front end. I am planning on changing my water pump, and belts while I'm in there. Will probably be starting to find the parts I need tomorrow (monday). Thanks for your input.

rhandwor, thats my plan. It's just a bit of a pain in the a$$, as I will have to tear out most of the front end. I am planning on changing my water pump, and belts while I'm in there. Will probably be starting to find the parts I need tomorrow (monday). Thanks for your input.
For my money I would expect the timing belt. I had the same issue on a 318. It jumped one tooth and I lost power. It jumped two teeth and I got the back fire. I checked it by loosening the distributor hold down and rotating the distributer as far as it would go. The enngine began to run normally.
I changed out the timing belt and gear after resetting the timing. and the vehicle performed normally.
A mechanic told me that had the belt gone one more tooth, there would have been a good chance that the valves would have hit the pistons.
Good luck on your repair.
bigbaribob

Well, I got my timing chain changed, along with a new waterpump, and thermostat, and the problem was still there. I changed all plugs, and no change. I was sitting in driveway with engine running, when I pulled off a sparkplug wire from distributor, and then held it near, just checking spark. It was fine, so I tried a couple more wires. On the third try, I got a wild shock, so I figured maybe I had at least one bad wire. Replaced all wires, and problem solved. The old wires were put on only 5000 miles ago, and they were 8.8mm Accel wires, so I didn't suspect them. At least now I know my timing chain, and waterpump are good for quite a while.

Thanks for input here, guys. I appreciate it...Brad

That's darn frustrating to do all that work only to have the problem still be there...I feel your pain! :)

Well like you said, at least you know the timing chain will be good for a while.

I've found many bad crimp jobs on aftermarket plug wires at the plug end. I have a tool for crimping plug wires and a box of staples from Napa. I just cut the end off about 2 or 3 inches put in a staple and recrimp. I usually check them with an ohm meter but use my scope if I have a problem which is hard to find.