8-year project build -- Backfiring through intake, not idling.

bossdrb

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Oct 27, 2018
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Hi there. I have a 1988 Ford Mustang LX fox that has a 5.0 converted from speed density to mass air. This has been a long project for my dad and I, and recently we initially got it started for the very first time at zero degrees timing. It idled pretty well without stumbling around 1k, but backfired through the intake. Now, it's not even wanting to stay running.

Steps taken:

-We've verified the MAF pin-outs are in the correct pins (https://www.allfordmustangs.com/forums/5-0l-tech/138277-diy-speed-density-mass-air-conversion.html)
-Found TDC by using a pick in cylinder 1 spark plug location and rolling the piston all the way up, which matches exactly with zero degrees on the harmonic balancer, then putting in the distributor with the pointer closest to the cylinder 1 wire.
-Verified the marks on the camshaft and crank under the timing cover match up.
-Tested the MSD SF coil and box.
-No spark-jump markings on the wires.
-Set the correct valve lash.
-Pulled the fuel rails and injectors and primed the lines by turning the key into the "On" position before starting. No leaks or drips, so presumably no stuck injector.
-Initially we had an intake manifold gasket leak (sprayed brake clean around these areas, it idled up when it ran and we replaced this and it wasn't a problem).
-Plenty of fuel pressure on the rail gauge, so that shouldn't be an issue.
-All vacuum lines are new, and we've sprayed this extensively without any issues.

It ran at one point but backfired through the intake at idle. Now it seems as if we've taken steps back and it won't stay running, and it's running very rich. The new manifold gasket has a bit of damp fuel in the middle, and the plugs (which are new) blackened already. The only thing I can think to verify is that we are truly at TDC on the compression stroke by using the thumb trick and setting the timing to zero and trying again.

Again, this is a long project with basically all new/remanned parts. I'd love to hear any suggestions or field questions. I'm getting married next Sunday and my fiance really wants to drive away in this car, and we've been busting our tail every day after work to basically build it from scratch. Thank you!
 
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a91what

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Apr 6, 2011
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Post pictures of the engine, also did you disconnect the MAP sensor on the firewall?? leave it plugged in but the vacuum line should be removed from it, it is now a BARO sensor leaving this hooked up will cause it to run rich.

What firing order did you use?? Should use the firing order for an HO 5.0
 

bossdrb

New Member
Oct 27, 2018
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Virginia
Post pictures of the engine, also did you disconnect the MAP sensor on the firewall?? leave it plugged in but the vacuum line should be removed from it, it is now a BARO sensor leaving this hooked up will cause it to run rich.

What firing order did you use?? Should use the firing order for an HO 5.0
I'll take pictures tomorrow morning, and check to see if the MAP sensor is unplugged from the vacuum tree.

I'm using the H.O firing order, 1-3-7-2-6-5-4-8.
 

RangerJoe

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Apr 26, 2010
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I'll take pictures tomorrow morning, and check to see if the MAP sensor is unplugged from the vacuum tree.

I'm using the H.O firing order, 1-3-7-2-6-5-4-8.
The engine and computer are designed to operate at 10btdc with the spout out. Advance the timing to 10 degrees and try again.

Joe
 
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