I have a 1997 F150 with about 150,000 on it. It has a 4.6 liter engine. Yesterday, I took the truck to the store and when I came out from the store I started the truck and it started backfiring through the intake manifold. After doing a few checks, I determent one of the coil packs were bad so I replaced it. The truck still is backfiring through the intake and has hardly any power. I also notice that the brakes were very hard to push, so I pull the vacuum line off the brake booster and notice it was not getting any vacuum. Then I pulled the big vacuum hose off the intake and the engine does not seem to making any vacuum. I check the engine very carefully but I did not find any vacuum leaks. I hooked up my OBDII scanner and it pull a PO102 code.

I checked the compression ratio and all the cylinders are good. The plugs and wires are also good. I checked for any type of vacuum leak and the manifold looks good. I went to the auto parts store and got a new MAF sensor but it did not do any good. While the engine is idling, it will still pop back through the intake manifold.

I put a timing light on some of the spark plug wires to make sure I was getting spark and found something weird. The timing light is pulsing very fast. It is doing it on a number of different plug wires so I think the popping sound it related to a timing issue. Does this sound like a crank or cam sensor going bad to cause the sparking to be excessive? Would a faulting MAF senor cause the engine to have no vacuum? Is there something I am missing? Did the backfire do something else bad?

Thanks for the help

Mark

 

the EGR system might not be functioning properly if your sure of no vacuum leaks

 

I have seen these engines show no vacuum before . Remove the o2 sensor and see if the truck runs better. It will be loud . The one we found to have no vacuum had a clogged catalytic converter. Engine could not breath. Hope this helps.

 

Like the poster above Stang06 said, check your upstream O'2 and see about that first.
If you can get it to run long enough to heat the cats take a digital thermometer and point it at both ends of the cats and get a reading of temp. There should be over a hundred fifty degrees difference and if there are not the cats are bad. (This only works on the older cats as 2010 up to work a lot better)

Backfire is associated with the timing of the fuel and spark event. The only way popping in the intake can occur is if the plug's fires with the intake valve open even a small amount and the fuel injector adding fuel to the problem. Too much air and not enough fuel causes a backfire to occur in the intake manifold.
Check the CAM sensor to see if it is plugged in or getting volts. (Front of engine right side facing towards the top of the timing chain cover, that big V looking thing sets the Cam sensor just under the valve cover).
On your plugs and wires: make sure that they are not jumping fire from one to the other. You should have one box for each head.

 

Backfiring through the intake is due to one or more spark plugs firing while an intake valve is still open. It's most likely your timing chain(s) slipped and the valve timing is now retarded.