Engine Backfire Through Intake / Low Power at WOT

TOPDowNMUSTANG

New Member
Dec 4, 2013
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1991 LX 5.0 automatic convertible. 100% stock top to bottom end to end. 50k miles on the engine, 250k on the transmission and body.
No EEC codes.
Runs well at idle, at cruise, at normal throttle opening (normal driving around town and on the highway). They only time I have a problem is at quick throttle opening and at WOT.

Symptoms:

1. If I got to WOT quickly from a stop (or if I "blip" the throttle in park) it will backfire through the intake one or two times. It is more pronounced when the engine is cold.
2. If I go to WOT from any speed and hold it at WOT (floored) the engine struggles to increase the RPM's past 3,000. It feels like it simply has no power - it falls flat at 3,000 RPM. It will get to 4,300 and the transmission shifts, The RPM's drop because of the next gear, start to climb, then it falls flat at 3,000 RPM again.

I has a recent tune up. Fuel pressure is good. It does not run like it has a vacuum leak.

Do I have two problems or is it the same problem causing both issues?
I am starting to wonder if the catalytic converters are clogged. As far as I know they are they original converters (30 years and 250k miles).
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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polk county florida
You're jumping to conclusions that are not based on research.
Let's start with the codes, you said you didn't get any, like no codes right? You should be able to run it through the system check sequence, go to the how to/technical threads and make sure you did the code read thing correctly, the problems you are encountering may have a code that could point you in the right direction.
Also you implied the fuel pressure is good, what was the readings?
These machines have the ability to send you in the right direction to a successful repair experience.
I'm starting to sound like a f'n commercial, gotta lay off the red cool aid. :doh:
 

TOPDowNMUSTANG

New Member
Dec 4, 2013
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You're jumping to conclusions that are not based on research.
Let's start with the codes, you said you didn't get any, like no codes right? You should be able to run it through the system check sequence, go to the how to/technical threads and make sure you did the code read thing correctly, the problems you are encountering may have a code that could point you in the right direction.
Also you implied the fuel pressure is good, what was the readings?
These machines have the ability to send you in the right direction to a successful repair experience.
I'm starting to sound like a f'n commercial, gotta lay off the red cool aid. :doh:
You're right! I have run out of things to check and now I am grasping at straws. I have no codes. ECM returns 11's. Fuel pressure 36-41. Tune up is recent. It runs well around town. The only time it has issues is under rapid throttle opening and WOT. That leaves me with:

Partially clogged catalytic converters which would allow normal exhaust flow but would restrict high WOT flow.

Fuel pump. I have read here that the rubber hose in the pump can split and cause a 40% reduction in fuel volume. I have never replaced the pump in 15 years that I have had the car.

Both the converters and the pump are quite possibly 30 year old original components with 250k miles.

I am going to replace the fuel pump because it is probably going to fail at any moment anyway. It nothing else it will be peace of mind. It that doesn't solve the problem I will try the converters.
 

Blue_67Vert

Member
Sep 25, 2016
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I had very similar issues just recently. Do you still have good fuel pressure under throttle? I had good pressure at idle but it dropped under throttle. The problem was my fuel filter was clogged. It was new but at some point got clogged again.
 

Mstng93SSP

You have a nice rear end there Dave.
15 Year Member
Nov 29, 1999
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Mililani, Hawaii
Check all of your vacuum lines and make sure none are broken, cracked or have come off your upper intake.

Chris