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  1. #1
    Member jeep59's Avatar
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    Question Backfire through the intake when reving??

    So i finished my engine rebuild. RHS heads, a regrind Hughes 212/218 cam with 114 lsa cam, 21lb injectors, 52mm throttle body , and sct from hemifever. The engine runs rough like a 110lsa cam and is pretty shakey like misfiring almost. When I rev it up it backfires through the intake. My wideband sensor reads good ratios but I'm thinking I may have a timing issue maybe. I just set the cam on dot to dot TDC. Maybe it needs to be advanced or retarted? Thanks
    5.9l ,Indy RHS-X heads, 52mm Throttle Body, Ported Keggar, 210X Cam, K&N FIPK, Doug Thorley headers, Front 6" lift Rear 4" lift, Fox shocks 12 inch travel, bilsteins rear ,IRO front long arms, IRO trackbar, Fiberglass front fenders, 4.56 gearing, MSD wires, 180 t-stat, 8k HIDs, Tint.


  2. #2
    Member dukefromthecave's Avatar
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    Did you set fuel sync?

  3. #3
    Member jeep59's Avatar
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    Yes I set fuel sinc first thing. I can't think of what else could cause a problem. I know I don't have a intake manifold leak or any kind of exhaust leak. Miss fire counter reads no misfires but there is a lot of shaking going on makes no sense
    5.9l ,Indy RHS-X heads, 52mm Throttle Body, Ported Keggar, 210X Cam, K&N FIPK, Doug Thorley headers, Front 6" lift Rear 4" lift, Fox shocks 12 inch travel, bilsteins rear ,IRO front long arms, IRO trackbar, Fiberglass front fenders, 4.56 gearing, MSD wires, 180 t-stat, 8k HIDs, Tint.


  4. #4
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    What does hemifever say about it?

  5. #5
    Forum CONDUCTOR Man Z88Z's Avatar
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    I'm gonna assume you double checked your plugs wires, firing order etc.

    Any chance a valve could have bent or maybe not be seating properly? Might want to do a compression check to eliminate that as a possibility.

  6. #6
    Member dukefromthecave's Avatar
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    backfire through intake means spark with intake valve open. Either wrong firing order or improper engine timing (crank to cam). Are you positive you went dot to dot on #1 comp stroke tdc? I've done a couple cams but I always need to quadruple check myself, it is so easy to make a mistake.

  7. #7
    Member jeep59's Avatar
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    Thank you Guys! Yes I checked my wires and all and as far as TDC I checked it with a screw driver and brought it up to the top but perfect TDC probably not. I'm thinking it's the the timing chain. Need to be advanced or retarded a bit. I think my timing chain only has retard 4 or advance 4 degrees. Which way should I try first idk? Hemifever didn't have much input for me.
    5.9l ,Indy RHS-X heads, 52mm Throttle Body, Ported Keggar, 210X Cam, K&N FIPK, Doug Thorley headers, Front 6" lift Rear 4" lift, Fox shocks 12 inch travel, bilsteins rear ,IRO front long arms, IRO trackbar, Fiberglass front fenders, 4.56 gearing, MSD wires, 180 t-stat, 8k HIDs, Tint.


  8. #8
    Member dukefromthecave's Avatar
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    I would check to see if your distributor is 180 degree off before pulling motor apart. Remember there are two #1 TDC positions. #1 and #6 are at TCD together but only one is at TDC between compression and power stroke. This is where you need spark. try flipping dist 180 but leave body in same position for same plug index.

  9. #9
    Member jeep59's Avatar
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    I flipped it when I first started it. The jeep wouldn't start before I flipped it.

  10. #10
    Member dukefromthecave's Avatar
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    Do you have wideband O2 sensor? Could be lean.

  11. #11
    RallyJeep GO
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    Quote Originally Posted by dukefromthecave View Post
    Do you have wideband O2 sensor? Could be lean.
    Could definitely be. Not enough acceleration enrichment (accelerator pump shot in carb terms) can cause a backfire through the intake when stabbing the throttle suddenly.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

  12. #12
    Member SilverXJ's Avatar
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    Advancing 4 or retarding 4 wouldn't cause or fix that usually. It has to be pretty far off to do that. I would find a degree wheel and degree the cam in.

  13. #13
    Member jeep59's Avatar
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    Well crap. I guess I really have to go through every step and figure it out. Just sucks out of all the engines iv built at work mine seems to not run correctly. Lol
    5.9l ,Indy RHS-X heads, 52mm Throttle Body, Ported Keggar, 210X Cam, K&N FIPK, Doug Thorley headers, Front 6" lift Rear 4" lift, Fox shocks 12 inch travel, bilsteins rear ,IRO front long arms, IRO trackbar, Fiberglass front fenders, 4.56 gearing, MSD wires, 180 t-stat, 8k HIDs, Tint.


  14. #14
    Member SilverXJ's Avatar
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    It happens. I had installed a cam shaft on a 4.0l dot to dot. Luckily I degreed it in.. it was one crank tooth off. 13*. Even after fixing that it looked dot to dot. After that I always do a quick degree check.

  15. #15
    Member dukefromthecave's Avatar
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    I have a degree wheel kit you could use but shipping it back and forth would probably cost 3/4 of it's worth. (I'm in AK)
    Don't feel too bad. I just rebuilt my motor and managed to blow a head gasket on the test drive.

  16. #16
    Member jeep59's Avatar
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    ya i should have degreed it, a few thousand bucks and 2 years of not driving it and it still runs like crap. im going to change my plugs incase any are fowled, what plugs are best i have auto lites right now. and ill double check my wire routing while im at it. probably change all sensors on the intake and throttle body. if nothing works jeeps forsale lol
    5.9l ,Indy RHS-X heads, 52mm Throttle Body, Ported Keggar, 210X Cam, K&N FIPK, Doug Thorley headers, Front 6" lift Rear 4" lift, Fox shocks 12 inch travel, bilsteins rear ,IRO front long arms, IRO trackbar, Fiberglass front fenders, 4.56 gearing, MSD wires, 180 t-stat, 8k HIDs, Tint.


  17. #17
    Member dukefromthecave's Avatar
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    How do you know you don't have a vacuum leak? I just fired mine up and left something off by accident.

  18. #18
    Member SilverXJ's Avatar
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    A vacuum leak wouldn't cause back firing through the manifold.

  19. #19
    RallyJeep GO
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    Quote Originally Posted by SilverXJ View Post
    A vacuum leak wouldn't cause back firing through the manifold.
    Correct. Being a MAP based setup and not MAF, a vacuum leak won't make it run lean or anything. Worst thing that happens is that the IAC can't reduce the airflow enough and it might idle high.
    1998 ZJ 5.9 Limited - Deep Slate
    Mods: Big trans cooler, 231 swap, Indy MA-X heads prepped by IMM, Comp 20-744-9 cam, 1.7 HS roller rockers, 52mm TB, Airgap manifold, DT headers and full 3" exhaust, SCT tune homebrewed by me, Martin Saine valve body, B&M tranny pan, magic suspension made from unicorn tears, power steering cooler, lots of lighting mods

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