Engine 91 mustang 302 Low acceleration car backfires pops and acts like it doesn’t want to take fuel

[1991Fox5.0]

I have a 1991 Mustang 5.0. I have a bad hesitation at low acceleration and the car pops, backfires and acts like it doesn’t want to take fuel. Holley systemax intake, accufab 70mm throttle body, EGR delete, ecam and Estreet heads, 24lb injectors, pro m mass air, MSD coil.
Car runs great at idle and full throttle.
We have replaced or checked TPS, IAC, Coil, distributor, plugs, wires, fuel pump, no vacuum leaks. cleaned and checked 10 pin, new A9L computer, checked fuel pressure, changed Mass Air meter, timing advanced 12degrees, we’ve checked grounds and ran codes. Only codes are 31,81,82,84,85 and pretty positive we have ruled those out.
At a loss right now on what it could be. Anyone have any ideas?

 

[General karthief]

Fuel filter?
What's the fuel pressure at idle?

 

[gkomo]

https://www.corral.net/threads/93-5-0-idles-very-rough-pulled-code-82-help-me.1272387/

This thread talks about how if the smog solenoid isn’t there it can cause some issues with cats that can cause some symptoms you are describing.

 

[AeroCoupe]

You can use this to simulate the EGR if you do not want to research and find how to build the simulator yourself.

EGR Simulator Eliminator / Block Off Plate Kit for 1986-1995 Ford F150

Product OverviewEliminate EGR system hassles with this CNC-machined EGR Simulator Kit, designed for Ford F150/F250/F350 and Mustang models (1986–1995). Crafted from aircraft-grade 6061-T6 aluminum, this kit prevents Check Engine Lights (CEL) while maintaining emissions compliance for...

www.egrperformance.com

 

[1991Fox5.0]

Fuel filter?
What's the fuel pressure at idle?

Without vacuum it’s around 39

 

[1991Fox5.0]

https://www.corral.net/threads/93-5-0-idles-very-rough-pulled-code-82-help-me.1272387/

This thread talks about how if the smog solenoid isn’t there it can cause some issues with cats that can cause some symptoms you are describing.

No cats. Off road Hpipe

 

[AeroCoupe]

Assuming the MAF is calibrated for the 24’s? Did you buy the car like this or did this just all of a sudden happen or ???

 

[1991Fox5.0]

Assuming the MAF is calibrated for the 24’s? Did you buy the car like this or did this just all of a sudden happen or ???

Car was good until I blew rear main out. Automatic with GT40 Tubular intake. Fixed that problem and put motor back in. Started having trouble then. We swapped motor to the Holley Systemax intake and 5 speed T5. Still same problems. We are just stumped at this point and we know it has to something simple we are overlooking because the car runs so good at idle and full throttle.

 

[Noobz347]

You have a vacuum leak.

Either pull it apart and reseal or run a smoke test.

 

[nickyb]

You went from an auto trans to a manual, could it be some wiring issue? Speed sensor or some other .

 

[AeroCoupe]

O2 wiring harness is pinned different between an auto car and a manual car. Just verify you are good there.

Electrical Thread 'O2 Harness Question??? Swap From Auto To Manual'

Apr 2, 2014

After I did my AOD to Manual trans conversion almost 10 years ago, I have swapped the Auto trans harness to a manual one. But I have always kept the '87 Auto trans O2 harness in place. In that time I have gone between using a C3W auto ECM and a A9L manual trans ECM. I have yet to experience an ECM failure with the A9L still using the '87 auto trans O2 harness.
So is it really necessary for me to swap to a manual trans O2 harness when I use the A9L to make the trans swap complete?

• larrym1961
• Replies: 3
• Forum: Fox 5.0 Mustang Tech

Oxygen sensor Harnesses - Manual/Auto differences and...

I have been getting tons of pms about this so I figured its time to make a post about it. Yes the Oxygen sensor harness has changed alteast 4 times that I know of and it can cause issues such as frying the trace off pin46 on manual ECU's or leaving you unable to pull codes in KOER if...

www.corral.net

These ECU’s are 30+ years old and we are starting to see them fail in numbers. Most send them to ECU Exchange to have them gone through. Not saying this is your issue but food for thought.

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECU Repair Service

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECUEngine Computer Repair ServicePlease fill out the R&R form here once you've placed your order.A Trusted Product with LIFETIME WARRANTY QUICK TURN AROUND TIMEDomestic and International Shipping available! Our Team is Available Monday - Friday 9am to 5pm...

www.theecuexchange.com

If you are confident in the motor mechanically then you need to start looking for a vacuum leak like Noobz said.


View: https://youtu.be/crc6CJ_ni2w?si=FMNCeMgShFc4Dosp


I just finished one and used 26 ga 316 stainless steel wire. Ended up using approximately 3 feet as that was right at 4 ohms on my DVM so that’s a 3 amp load on a 12V system. Only found one small leak but it worked.

 

[1991Fox5.0]

You went from an auto trans to a manual, could it be some wiring issue? Speed sensor or some other .

We put same motor and transmission back in after rear main fix. Issues started then. Decided to do auto to manual swap and still same trouble.

 

[1991Fox5.0]

O2 wiring harness is pinned different between an auto car and a manual car. Just verify you are good there.

Electrical Thread 'O2 Harness Question??? Swap From Auto To Manual'

Apr 2, 2014

After I did my AOD to Manual trans conversion almost 10 years ago, I have swapped the Auto trans harness to a manual one. But I have always kept the '87 Auto trans O2 harness in place. In that time I have gone between using a C3W auto ECM and a A9L manual trans ECM. I have yet to experience an ECM failure with the A9L still using the '87 auto trans O2 harness.
So is it really necessary for me to swap to a manual trans O2 harness when I use the A9L to make the trans swap complete?

• larrym1961
• Replies: 3
• Forum: Fox 5.0 Mustang Tech

Oxygen sensor Harnesses - Manual/Auto differences and...

I have been getting tons of pms about this so I figured its time to make a post about it. Yes the Oxygen sensor harness has changed alteast 4 times that I know of and it can cause issues such as frying the trace off pin46 on manual ECU's or leaving you unable to pull codes in KOER if...

www.corral.net

These ECU’s are 30+ years old and we are starting to see them fail in numbers. Most send them to ECU Exchange to have them gone through. Not saying this is your issue but food for thought.

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECU Repair Service

Ford Mustang 5.0 A9L A9P C3W A3M X3Z ECUEngine Computer Repair ServicePlease fill out the R&R form here once you've placed your order.A Trusted Product with LIFETIME WARRANTY QUICK TURN AROUND TIMEDomestic and International Shipping available! Our Team is Available Monday - Friday 9am to 5pm...

www.theecuexchange.com

If you are confident in the motor mechanically then you need to start looking for a vacuum leak like Noobz said.


View: https://youtu.be/crc6CJ_ni2w?si=FMNCeMgShFc4Dosp


I just finished one and used 26 ga 316 stainless steel wire. Ended up using approximately 3 feet as that was right at 4 ohms on my DVM so that’s a 3 amp load on a 12V system. Only found one small leak but it worked.Just got the A9L back from ECU exchange so we know it’s good It was doing it with the old computer as well. We put same motor and transmission back in after rear main fix. Issues started then. Decided to do auto to manual swap and still same trouble.

 

[1991Fox5.0]

Just got the A9L back from ECU exchange so we know it’s good It was doing it with the old computer as well. We put same motor and transmission back in after rear main fix. Issues started then. Decided to do auto to manual swap and still same trouble. Smoke test is next on the to do list then pulling injectors.

 

[Mustang5L5]

So you are using the A9L with the manual, but the car used to be auto? Did you repin the O2 sensor harness jumper to a manual? Not doing this will fry the ECU pin 46 which makes the car run poorly. Not saying this is the cause, but if you aren't sure I would double check this.


I do think you should check for vac leaks. lean backfire when applying load (and good fuel pressure) is a strong sign of a vac leak somewhere.

 

[1991Fox5.0]

So you are using the A9L with the manual, but the car used to be auto? Did you repin the O2 sensor harness jumper to a manual? Not doing this will fry the ECU pin 46 which makes the car run poorly. Not saying this is the cause, but if you aren't sure I would double check this.


I do think you should check for vac leaks. lean backfire when applying load (and good fuel pressure) is a strong sign of a vac leak somewhere.

 

[1991Fox5.0]

Bought what we needed for a smoke test and going to check pin. Thanks everyone for the information. I will update after that is done!