Engine Foxbody 5.0L idling rough/backfires after HCI swap

MurderPony5.0

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Jul 4, 2018
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Ohio
Hey everyone, new to the group here so bare with me.

I’m currently working on my project 1988 foxbody mustang gt that here lately has been giving my tons of fits after swapping my heads and cam.
The mustang is a 5.0L stock bore 302ci EFI ( speed density ) car with a AOD transmission. In the last few weeks I have swapped out my old e7 heads and stock cam for some gt40p heads with stock rocker arms and some port work and a Ford Racing E303 cam. The car also has a gt40p upper an lower intake with a engine works 70mm throttle body (I believe) running stock 19LB fuel injectors and a racetronix 255 LPH fuel pump (installed about a year ago) also has BBK under drive pullys, shorty headers, CAI blah blah blah. Anyways, the problem I am currently dealing with after the swap is I can’t get the car to run very good at all. It will barley idle and at times it will die or it will backfire out of the exhaust badly. The car is also running very rich and if near it to long while idling it will make your eyes water. I have fully set timing on the harmonic balancer and distributor countless times setting the balancer at 0 degrees making sure I’m on the compression stroke of cylinder #1 an TDC of cylinder #1 and setting the distributor at 0 degrees with the rotor facing #1 plug on the cap. I made sure my cam/timing gear marks were all lined up before installing and torque everything from that point up to spec. A list of things I’m thinking could be causing my issues are possible vacuum leaks, fuel related issues (have not checked fuel pressure yet) or maybe even my stock computer has gone bad? Am I thinking in the right direction here? This thing has had me really frustrated the last couple weeks an all I’m trying to get to is that test drive!! If anyone has had similar issues after swapping heads and cam any advice is greatly appreciated!!
 
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Get back to basics, sounds like your confident with your firing order and timing, check fuel pressure, make sure your fuel pressure regulator is not leaking, check valve lash, see if the computer will communicate by checking for codes, if the engine is acting the way you describe the computer likely will have codes that are all over the place put your just looking to see if it is working. Go to the 'cranks but no start' checklist in the technical/how to threads on the 5.0 tech forum, it covers many things that you may have missed when you swapped the h/c/i like grounds, those are some of the most often missed things when swapping intakes and heads.
Is there any other things that you have changed on the car?
 
The E303 cam is not compatible with Speed Density computers See https://performanceparts.ford.com/part/M-6250-E303 for more information

M-6250-E303 *1

NOTES: (1) On EFI engines, performance camshafts work only with mass air induction systems. Will not work with Explorer EEC-V EFI or speed density EFI systems.

Computer system differences in 86-95 Mustangs.

Revised 15-Jan-2018 to add requirements for larger fuel system components to support large changes in airflow through the engine.

Speed Density uses Manifold vacuum (MAP), Throttle position (TPS) and RPM, & Air Temperature (ACT) to guess how much air the engine is pulling in. Then it uses all of them plus the O2 and ECT sensors to calculate the air/fuel mixture. It is dependent on steady manifold vacuum and minimal changes in airflow from the stock engine configuration to maintain the proper air/fuel ratio. Change the airflow or vacuum too much and the computer can't compensate for the changes, and does not run well. Forget about putting a supercharger, turbocharger or monster stroker crank in a Speed Density engine, because the stock computer tune won’t handle it. Every time you seriously change the airflow through the engine, you need a new custom burned chip to make the engine run at peak performance.

Mass Air uses a Mass Air Flow meter (MAF) to actually measure how much air is being pulled into the engine. The computer uses this information and inputs from the O2, TPS, ACT, ECT, RPM and Barometric Pressure (Baro) sensors to calculate the proper air/fuel ratio. It is very tolerant of changes in airflow and vacuum and tolerates wild cams, high flowing heads, and changes in displacement with minimal difficulties. Just remember that large changes in airflow require more fuel than the stock fuel system can deliver. At that point, you will need larger injectors and a larger fuel pump to make the engine run like it is supposed to. Larger injectors can be used with either an aftermarket calibrated MAF or a custom dyno tune. This makes it possible to use the stock computer with engine displacements from 302-408 cu in, and make many modifications without a custom dyno tune chip. Put a new intake manifold on your 331 stroker and the computer figures out how much more fuel to deliver without having to have a new chip burned to accommodate the extra airflow.
 
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