Hello,new to the site here,i have a 97 f150 4.2 that is backfiring through the throttle body,i have put a new coil pack on it,changed plugs and wires twice,changed the oil twice,i dont know what is causing this,it does it only on takeoff and it runs fine after that or if ur in park and goose the throttle it will do it.thx for any help

 

Today i cleaned the throttle body,maf sensor and iac sensor its still doing the same on take off,it jusr sounds like somebody pit a little bit of popcorn in there and its poppin when you take off,after cleaning all that i do see significant difference in it when it runs,i just wish i could figure out what is causing the backfire,ive had this truck for two years and have had no probs til now,its always been maintained,please if anybody can help it would be greatly appreciated,i just dont want to take it to a garage,i like workin on it myself

 

are all your vaccum lines good?

 

Inspect each plug wire for corrosion/damage. Test continuity with a VOM. Check plug for the Tranformer Capacitor where it plugs into the Ign Coil. Look for obvious heat damage to these parts. All good? Those were the obvious and cheaper repairs. Do a compression test to test for leaking valves. Look for a Vacuum leak at intake or elsewhere with vacuum test and inspection of hoses and PCV + the inside of its hose. Spray with carb cleaner or use unlit propane torch near suspected leak. If idle pick up its leaks to motor. Does all your vacuum equipment work?

After testing and eliminating things, intake valves gradually fail and the pop back thru the throttle body happens some times intermittantly sometimes full time. Tight valve clearence, build up of carbon on the underside, sticky, bent, cracked seat or head all happen. Have you overheated it?

These motors occasionally have manifold leaks, vacuum or coolant, + bolt problems.

 

Found one vac line off,its located under, the plenum,i cant find where it goes to so i just plugged it,ot has never overheated in the 3 years we have had it,i will do a vac test today and see if that helps,thx for the replies

 

Was that vac line you found at the front or the back of the plenum? If the front then it should plug in right below the throttle body. If its the back then there should be a connection under the plenum but you may need a mirror of some kind to see it.

 

Thx 97xl i will check that out,it must plug in under neath the plenum it wont reach the throttle body at all,but i believe i did find my prob on what is causeing the backfire thru the throttle body,it was a broken valve spring,number 2 on the driverside,have to wait on the new one for two days which sux big time nobody has them in stock and salvage yards wont sell you one unless you buy the whole engine,i really hope this fixes it,i have put alot of blood and sweat into this,i really love this truck and cant wait to get er back on the road,plus i have learned alot about engines and how they work

 

Broken valve spring was the problem got a new one put on and runs like brand new thx to all who replied,now working on powersteering pump not gonna be an easy job