94 grand caravan...replaced fuel pump...now backfires!

[highmileage]

Hi,

I've got a 94 Grand Caravan LE with 3.8, 175,000 miles.
Last week you guys really helped me out by diagnosing my bad fuel pump. I spent Tueday afternoon dropping the tank and installing a new pump. Van started right up and seemed to run fine. About the second trip out, though, I heard a loud banging sound coming from right in front of the driver side (It really sounded like something banging in the transmission). I popped the hood today and found that its backfiring throught the air filter.

I've got three theories:
#1
Before I replaced the fuel pump, I had put in STP gas treatment and fuel injector cleaner. I refilled the tank with some of the old gas after replacing the pump (filter was changed, too). So, maybe that messed up the fuel system?

#2:
It's very possible that it is coincidence and I have bad wires & plugs. My wife said she noticed the engine bucking and lacking power on rainy days before all this happened. I've got two plugs in the back that have NEVER been changed since I couldn't get to them. They have to be shot by now.

#3
Is it possible that I might have bought a defective fuel pump? I don't see how I could have installed it incorrectly. Would it cause the engine to backfire?

I'd appreciate any advice.
Thanks,
Dave

[GTP Dad]

I seriously doubt that the fuel pump is causing you problems. I would change the rear plugs along with all the others, wires too. When you change them do a compression check on the engine. This sounds more like a bad valve or stuck lifter than anything else although the plugs may be the entire problem. If you find low compression then you will have to diagnose further.

[highmileage]

Originally Posted by GTP Dad

I seriously doubt that the fuel pump is causing you problems. I would change the rear plugs along with all the others, wires too. When you change them do a compression check on the engine. This sounds more like a bad valve or stuck lifter than anything else although the plugs may be the entire problem. If you find low compression then you will have to diagnose further.

Yep, you were right! I went ahead and put six new plugs and wires on today and the van runs like a champ. The backfire really had me fooled, though. It sounded just like metal hitting something under the hood from the driver's seat. Not until I revved the engine from the engine compartment did I realize the sound was coming from the air cleaner.

Thanks again for your advice!
Dave