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Old 05-20-2009, 20:14   #1 (permalink)
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91 5.0 Mustang pops and hesitates

I have a 91 Mustang GT 5.0 that has had a flat spot at about 3,500 rpm. It also pops out of the intake and has done this for some time. The popping and hesitation is more pronounced now that it is warmer outside. It happens when accelerating slowly at about 3,000 rpm and then launches out of it. Acceleration also stops in 3rd gear at 100 mph as fuel seems to cut off or lean out.

Everytime I upgrade to more horsepower, it becomes more pronounced. The only codes I have are for the removed items. I have removed the smog system and the EGR.

Upgrades: K&N cone filter, Pro-m 76mm mass air, Powerpipe, 65mm Accufab with EGR delete block, Edelbrock Performer, 24lb injectors, 1 5/8 Mac full length headers, Bassani x-pipe, Aeromotive Fuel Regulator.

TPS has been swapped and it is adjusted at .97v

The IAC has been swapped with a clean one.

Base idle is fine and at 600rpm

Base timing with spout off is 10*

Firing order is correct.

I have new plugs and wires. Gapped to factory.

Swapped MAF from 24lb C&L to Pro-M

tried different fuel regulator and adjusted from 38-44

Anyone have any ideas?
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Old 05-21-2009, 11:47   #2 (permalink)
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Re: 91 5.0 Mustang pops and hesitates

have you changed the fuel filter in a while.Also have you brought it to a dyno shop to have it tuned to see if it's running lean or rich?

Tony
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Old 05-21-2009, 19:40   #3 (permalink)
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Re: 91 5.0 Mustang pops and hesitates

Thank you. Yes, the fuel filter has been replaced 3-4 tankfuls ago. I have not had a dyno tune yet. As I can recall, I bought the car 1.5 years ago with 62,000 with no mods and it showed some faint signs of the problem. I now have 67,000.

I was hoping I could wait on the dyno tune till I install the AFR's which will be next year. I suppose this is my last resort.

I just had a friend recommend that I upgrade the stock fuel pump.
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Old 05-22-2009, 10:49   #4 (permalink)
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Re: 91 5.0 Mustang pops and hesitates

Quote:
Originally Posted by 1meanGT View Post
Thank you. Yes, the fuel filter has been replaced 3-4 tankfuls ago. I have not had a dyno tune yet. As I can recall, I bought the car 1.5 years ago with 62,000 with no mods and it showed some faint signs of the problem. I now have 67,000.

I was hoping I could wait on the dyno tune till I install the AFR's which will be next year. I suppose this is my last resort.

I just had a friend recommend that I upgrade the stock fuel pump.
lack of fuel could very well be the culprit. You have a return system on that car.Do a 255LPH in tank pump. What ever the motor does not use will return right to the tank.

The motor's probably starving with those injectors and a stock pump at that RPM.
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Old 05-26-2009, 04:29   #5 (permalink)
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Re: 91 5.0 Mustang pops and hesitates

Thanks Tony. I am starting with the TFI and then possibly the coil. I already checked the wiring. You will see why when you read below. I agree that the fuel pump needs an upgrade. I was hoping to do all this in the winter and just enjoy driving it.

I have an interesting turn of events. While visiting the cemetary, I shut the car off. Upon return, the car would pop while turning over. When it would start, it would immediately die. It sounded like it was out of time or missing. The exhaust was smoking with unburnt fuel. The fuel pump would turn on every time I cycled the ignition. I had it towed home. Couple of hours later it started as if nothing had ever happened.
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Old 05-26-2009, 12:44   #6 (permalink)
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Re: 91 5.0 Mustang pops and hesitates

My 92 did this to me a few years ago,but it was just bad gas from it's long winters nap.Has been fine one the old gas was out.There are no unplugged vacuum lines hanging around right?
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Old 05-27-2009, 05:22   #7 (permalink)
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Re: 91 5.0 Mustang pops and hesitates

Thanks for writing back. It is back to not starting at all. It has fresh gas. I have drove it over 1,000 miles in the previous 3 weeks. I have checked all the vacuum lines. I could start capping all of them off on the engine.

I replaced the TFI Ignition Module and Pickup capacitor inside the distributor. The cap, rotor, and plug wires are also new.

Still- there are no codes.

I only had 20lbs fuel pressure when the engine was spitting and sputtering trying to idle. I will order a new 255lph fuel pump and see what happens.
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Old 05-27-2009, 10:34   #8 (permalink)
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Re: 91 5.0 Mustang pops and hesitates

Know what try posting over at MuscleCarResto.com Muscle Car Forums - Ford - Chevy - Dodge - Chrysler - Buick - Powered by vBulletin

There are some heavy duty fox-body guys on the board,and also this guy Dana that owns his own dyno shop.He may have a solution for you.Just hate to see you start buying parts that you don't need.
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Old 05-31-2009, 14:30   #9 (permalink)
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Re: 91 5.0 Mustang pops and hesitates

Found the problem! Fuel pump was not the cause- but moving towards replacement of the pump lead to finding the solution.

Upon removing the gas tank and the external ABS plastic cover, I found a very large portion of my gas tank pushed up. It also pushed my fuel pump housing up too. This caused the engine to be starving for fuel even though the pump was working and the tank was 1/4 full. It also explains why filling the gas tank would make the problem disappear. I needed the higher capacity fuel pump anyway and would have upgraded it since the tank was down. It was great to find a problem. Now- just need to find a complete tank........

Thanks for your help.
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Old 06-01-2009, 12:13   #10 (permalink)
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Re: 91 5.0 Mustang pops and hesitates

You're welcome.if you need a gas tank here's a link to them on our site.

Ford Mustang Gas Tank
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