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View Full Version : What exactly causes Intake Backfires??

eric91xj
July 23rd, 2003, 18:39
I know to some this might be a stupid question and i'm probrably just not thinking hard enough but what causes intake backfires? i get them sometimes if i'm in park and just very quickly hit the gas and let off, the motor will stumble and i'll get a pretty loud intake backfire. It happens occasionally while just driving around, not too often though. Mainly just while i'm stopped in park and hit the gas really fast.

Thanx,

Eric
eric91xj
July 23rd, 2003, 18:41
If it could be from a worn cam (225k), could i put the new one in with the old bearings or is that a big no no because from what i've heard those bearings are pressed in and i ain't pulling the block for that.

Eric
Eagle
July 23rd, 2003, 18:48
Intake backfires aren't usually loud -- are you sure it's popping thru the intake, and not the exhaust? I was getting some soft "popping" last summer about this time. Finally figured out the O2 sensor was failing -- didn't respond fast enough, and when I'd let off the gas it kept pumping a rich mix for a few micro-seconds and the excess fuel was burning in the header or the cat instead of in the engine.
eric91xj
July 23rd, 2003, 19:03
i'm sure it is because i have a cone filter and its blown it off before. its not necessarily loud like an exhaust backfire would be, i cause it kinda sounds like when you take a paper lunch bag or a little baggie, fill it up with air and pop it with your hands.

Eric
Eagle
July 23rd, 2003, 20:00
Check the oxygen sensor.
pec50
July 23rd, 2003, 20:22
Also a possibility your coil is beginning to fail.
eric91xj
July 23rd, 2003, 20:25
hmm, i will check the o2 sensor, dang i just replaced that thing about a year ago too, oh well. What type of reading should i be getting out of it if it is bad, and if it is good? Sorry for the questions but i lost my FSM and don't have any specs on this sorta stuff. Also regarding the cam bearings question, are they infact pressed in and could i just re-use the old ones if i eventually put in this new cam that i got laying around wasting space?

thanx,

eric
Bones
July 23rd, 2003, 21:18
Sorry for the questions but i lost my FSM and don't have any specs on this sorta stuff.

Eric,
Get another one:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2424818048&category=34224
http://www.ixl.net/bella/manual56.jpg

or

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2424417010&category=34224
http://ebay0.ipixmedia.com/abc/M28/_EBAY_25e6b5ded75b975bf7d7d639446f231e/i-1.JPG

Bones :skull1:
NAXJA #6 and eBay junkie:anon:
eric91xj
July 23rd, 2003, 21:21
i wish i could man but i've been unemployed for over a month now and the funds are'nt there at all.

eric
Essayons
July 31st, 2003, 11:17
A new oxygen sensor just cured mine, and the one that came out looked liek it hadn't been in long and came out easily.
MarkdaleXJ
July 31st, 2003, 14:20
Does anyone else think this sounds like a timing chain problem or a burnt valve? Just a hunch:anon:
J.R.
eric91xj
July 31st, 2003, 18:24
thats a good possibility on that burnt valve and timing chain, or a toasted cam lobe possibly too. i dont' think its a sensor or anything, i really think its deaper than that.

eric
Matthew Currie
July 31st, 2003, 19:08
When my 87 did that it was a bad Throttle position sensor. It would hesitate, nearly stall, then spit, often blowing the crankcase vent hose off. Of course 87's are a little different, but I enter that as another possibility.