1987 fox with some intake backfire - Forums at Modded Mustangs
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post #1 of 4 Old September 30th, 2019, 12:55 PM Thread Starter
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Unhappy 1987 fox with some intake backfire

I've freshly built all of my engine and I'm trying to get it dialed in, i took it down the road for the first time and i noticed that after first or second at high RPMs it would pop and bang like hell before it shifted- I think it's coming from the intake. sometimes i can rev it at the throttle body and notice the intake backfire at me, I don't think it's an exhaust backfire at all.
I know I still need to properly set the idle, check some sensors, convert to MAF, but backfiring like this seems like another issue... I think I've sorted vacuum leaks. would this be a timing thing? injectors not keeping up? Bad o2 sensors? Timing advance issues with my EGR/smog pump delete? I just did a valve job with new springs so I'd be hurt if that was somehow the issue with a stuck valve. I've tried a lot of different timing setups and some have no power at all but dont seem to backfire, and some seem good on power but backfire badly.

It's a regular 5.0 with speed density as of right now, 19 lb injectors.
I have a trickflow stage 1 cam, GT40 heads with trick flow springs, explorer upper and lower intake, BBK shorty headers and X-pipe, Smog pump and EGR delete, comp cams high energy pushrods and rockers (stock ratio)

Thanks in advance, any ideas help!
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post #2 of 4 Old October 8th, 2019, 11:55 AM
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Sounds like timing, or if it feels like a rev limiter after a certain rpm, it could be floating the valves.
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post #3 of 4 Old October 10th, 2019, 10:24 PM
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I wouldn't think it was anything but the timing. Check to see if the wires are in the right order. If that is correct then check to see if the number 1 plug is in compression stroke. Try that, then get back with us. Good luck.

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post #4 of 4 Old October 10th, 2019, 11:22 PM
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Sorry this post is long winded.I overdue it sometimes with my thoughts/opinions.

It may be acting up because of being a SD setup with an aftermarket cam??
However,Ive read mixed reviews of the TFS-1 cam & SD combo,where some said it worked and others said it didn't.SD and a aftermarket cam usually equal a problem because of the change in vacuum and overlap.However some cams dont create this issue.The other bolt-ons normally wont cause an issue with SD.

As far as timing goes,you basically have two chances of getting it wrong.

One= valve timing
Did you set the cam & crank gear at 6&12 o'clock??

Two= ignition timing
Did you drop the distributor in the block while the number 1 piston was @ tdc on compression & the 0 balancer mark was aligned with the timing pointer and the rotor was pointing @ the number 1 post on the distributor cap?? If you're not positive,mark the 0 & 10 btdc marks with Witeout and do the things above. After you're done,barely loosen the distributor bolt,start the engine,let it run for a few minutes,then remove the spout plug from the tfi harness,aim the timing light at the balancer,then rotate the distributor til the 10 mark is aligned with the timing pointer.Tighten the bolt,recheck timing with the light to make sure it didn't move while you tightened the bolt,reinstall the spout plug and you're good to go.
So backfiring issues could be related to timing,but they could also be related to the cam + SD combo too.

As far as the egr & smog system goes, deleting those components wont cause any driveability issues or a cel (check engine light) on your year model.When the egr valve is opened,the ecu leans the fuel mixture and advances timing.If at anytime,a fault or code pops up for any egr system component,it will trigger the ecu to delete egr system function in the ecu's logic & the egr system components wont work and the fuel and timing will remain at normal values.However,if the egr system components are still in place with the vacuum hoses still connected & the egr valve is stuck open due to carbon buildup or a defective evr solenoid,the egr gases will still enter the system,but since the egr function was deleted in the ecu logic,the ecu's lack of leaning fuel and advancing timing will likely cause driveability issues.If the egr components were removed completely,you'll have no driveability issues related to their deletion as long as the vacuum line that was connected to the intake which fed the evr solenoid is removed & the intake vacuum port is capped.The same goes for the smog components.Just make sure all vacuum hoses are removed for both systems.It also wouldn't be a bad idea to pull codes.All you need is a jumper wire and a 12v test light.The second link below will show you how to run the tests.Use the test lite connection method in the link diagram since you don't have a functional cel.The codes will be flashed through the test light.If you run a cylinder balance test after the code tests,it may help you locate a suspect injector......

The Hot Rod Garage - Mustang EFI Information


91 LX 342ci (10.5:1)
TFS 190cc (stage ll)
TFS Track Heat
Lunati 61011(.549"/.565")
Flowtech 1 3/4" Shorties
QA1 Coilovers
Weld ProStar XP'S


Last edited by wbrockstar; November 4th, 2019 at 12:28 AM.
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