[js1990gt]

I have had my 1990 mustang GT for about a year and have always had an issue with it cutting power and bucking only when it is at 2500-3000RPM. It is a 5.0 AOD car with speed density. I have changed my map sensor, removed and blocked my EGR, replaced my fuel pump and filter, replaced the distributer, cap, and rotor, spark plugs, wires, ignition coil, and ran the codes. Most of these parts needed to be replaced anyways as they were original. The issue I have been having is that once the car reaches 2500 RPM it cuts power and jerks, bucks, and backfires forward until it reaches 3000RPM then it gets back into power. Once it downshifts and gets above that 3000RPM mark it pulls hard and feels as though nothing is wrong it literally only has issues in between 2500-3000RPM. I have a feeling it is something electrical simply because it is only during that 500RPM range. There are two attached videos that show the issue I'm having. Please give me any advice you have so I don't keep spending money on parts that aren't necessary.

Video 1 Video 2

 

[Saleen414]

Does this happen regardless of throttle position? At WOT, or light throttle? At steady state rpm?

 

[js1990gt]

Does this happen regardless of throttle position? At WOT, or light throttle? At steady state rpm?

Yes, whether it is WOT or light throttle right when I hit 2500RPM it cuts power and will continue to do so until it reaches 3000. if I drive very conservatively the car will shift at 2500 right before this issue occurs and does make the car drivable but not very enjoyable.

 

[Saleen414]

Can you install a vacuum gauge and determine if this issue is when the engine is at or near 0 manifold vacuum?

 

[js1990gt]

Can you install a vacuum gauge and determine if this issue is when the engine is at or near 0 manifold vacuum?

yeah ill give that a try during this week and ill post the results

 

[sour]

How does your timing look?

 

[Bird76Mojo]

Look up the video on testing the TFI/PIP modules that Eric the Car guy has on Youtube. It's the best video on it I've ever seen. Very easy to do and it will eliminate those items as an issue.

 

[js1990gt]

How does your timing look?

I just reset the timing to 14 but the car smells a little rich

 

[js1990gt]

Look up the video on testing the TFI/PIP modules that Eric the Car guy has on Youtube. It's the best video on it I've ever seen. Very easy to do and it will eliminate those items as an issue.

I'll take a look at that thank you

 

[kjb302ho]

where's indy when you need him?

put on a fuel pressure gauge and drive it

 

[js1990gt]

where's indy when you need him?

put on a fuel pressure gauge and drive it

I'm going to test this after I test the vacuum and ill repost what I find

 

[AldeanFan]

My car had the same symptoms,
Turned out to be a cracked spark plug. They were new plugs so I must have cracked or putting them in

 

[js1990gt]

My car had the same symptoms,
Turned out to be a cracked spark plug. They were new plugs so I must have cracked or putting them in

I have taken them out and checked them once after I originally installed them but I have been having the issue since before all of that ill take them out and take a closer look I may have not noticed it.

 

[sour]

I just reset the timing to 14 but the car smells a little rich

Is it steady though? (I mean with the SPOUT unplugged) or does it wander ? From a sloppy timing chain causing sloppy timing or from a sloppy harmonic balancer preventing you from setting timing clean to begin with?

 

[Blueknights75]

Could literally be a hundred things…in my experience dealing with these same issues and more I found out a few things the hard way. First, always try and use Motorcraft parts for anything electrical related. Went through three different TPS sensors and TFI modules (non Motorcraft) and finally put a Motorcraft TPS sensor and TFI module. Same exact thing with the MAP sensor. Check your ACT and ECT sensor…bring the timing down to between 10-12 tdc. These SD cars hate vacuum leaks also. Is your belt tensioner good? Took me awhile to figure mine was bad…. These are just some of the things that I ran into trying to diagnose all of my gremlins. But finally got mine running like a champ…

 

[Blueknights75]

And O2 sensors!! What codes are you pulling? Do you still have smog equipment connected?

 

[js1990gt]

Is it steady though? (I mean with the SPOUT unplugged) or does it wander ? From a sloppy timing chain causing sloppy timing or from a sloppy harmonic balancer preventing you from setting timing clean to begin with?

I didn't consider this that is a possibility the harmonic balancer does have a slight wobble to it but I didn't think it would be note worthy, once I set it it does stay steady to my knowledge

 

[js1990gt]

And O2 sensors!! What codes are you pulling? Do you still have smog equipment connected?

I just installed brand new 02 sensors, I have no smog equipment left in the car now. The belt tensioner is working properly, I am going to start with the vacuum gauge test while driving to eliminate that first and ill also adjust the timing again to 10-12. Then I will start looking into the sensors you mentioned thank you so much!

 

[stangjumper]

I didn't consider this that is a possibility the harmonic balancer does have a slight wobble to it but I didn't think it would be note worthy, once I set it it does stay steady to my knowledge

You balancer sounds bad.

First thing is check if the tdc mark on balancer lines up at tdc with #1 piston

 

[charlenew1]

I have had my 1990 mustang GT for about a year and have always had an issue with it cutting power and bucking only when it is at 2500-3000RPM. It is a 5.0 AOD car with speed density. I have changed my map sensor, removed and blocked my EGR, replaced my fuel pump and filter, replaced the distributer, cap, and rotor, spark plugs, wires, ignition coil, and ran the codes. Most of these parts needed to be replaced anyways as they were original. The issue I have been having is that once the car reaches 2500 RPM it cuts power and jerks, bucks, and backfires forward until it reaches 3000RPM then it gets back into power. Once it downshifts and gets above that 3000RPM mark it pulls hard and feels as though nothing is wrong it literally only has issues in between 2500-3000RPM. I have a feeling it is something electrical simply because it is only during that 500RPM range. There are two attached videos that show the issue I'm having. Please give me any advice you have so I don't keep spending money on parts that aren't necessary.

Video 1 Video 2

Clogged fuel filter did that same thing on my 91 Ranger. Not saying that's your problem though.
but it might be something to think about.