1989 Mustang 5.0 Throttle problem

Can't you clear those codes with a custom tune? Maybe the computer is running in safe mode or whatever thinking your cats and smog pump is still there. Theres got to be a way to clear these codes. Unless the cpu is like the OBD I on my 89 Chevy Truck. Its as dumb as a rock, once in a great while the check engine light comes on, but then I cant get any codes because it isnt storing them, the light simply goes off and doesnt come back on for months sometimes. lol

Welcome and GL fixing this.

test your egr valve, check your vacuum lines and make sure u have the right vacuum line plugged up to the egr valve...it makes a huge difference in my 1990 mustang gt, when the mechanic gave me my car back with the egr valve hooked up to the wrong part of the vaccum tree located in the passenger side back corner of firewall (NEEDLESS TO Say that mechanic NO LONGER GETS MY BUSINESS...lol). also check the big salt and pepper shaker connectors in between motor and firewall...check to make sure they are clean and have good connection (there will be 2 of them should have a bunch of wires and be the main harness connections). Is this an automatic or a five speed and if it is an auto when you decelerate and just let the motor pull down does it surge decelerating or only when accelerating? have you checked tps sensor?

depending on what part of the vacuum tree the egr line is plugged in it can run really crappy. Mine had the same thing happen...kind of a dead spot where it would just stumble and it was due to the egr line in the wrong spot on the tree...and since you said it has the smog pulled off this could may well be your problem as I have no smog, cats, etc on my car either. Try unplugging the vaccuum line on the egr valve and run it (with the end of the line taped so it isnt sucking (WOT you have very little vaccuum present)...if it runs better then it is the line is hooked wrongly in the vaccuum tree.

here is a link of how crappy my car was running with a wrongly hooked up egr line in the vaccuum tree and timing off. it sounded like the car was missing.

YouTube - 1990 Mustang not running right

It is a five speed and the EGR valve has been replaced. I am going to replace the EGR position sensor as I have read some people have replaced that and seemed to help. Again the car ran great and strong for a long time. Hell it got me down the 1/4 mile 12.5 sec at 108mph which was awesome for me. Now the last serveral months I have been tring to fix this 1/4 throttle problem and just not able to get it right. I will double check the Vacume line but pretty sure its in the right spot as I have had no trouble in the past. I am also going to change the fuel filter as it could use it but but I don't think thats my problem.
Again thanks for all the tips and hints.. If I or we get it figured out I will be sure to post it and let other veiwers know what I all did.
Thanks

check for vacuum leaks where the intake manifold bolts to the cylinder heads by sprying some carb&choke cleaner in that area when the engine is running if the idle smooths out or picks-up you have a vacuum leak and the manifold gasket is bad. This happen to me, the fel-pro gasket I used cracked by some of the intake ports and was sucking air. Was told that this happens sometimes when using the fel-pro 1262 gasket, good luck.

5.0Mustang1989

I am running Ford Sportmen Short Track Heads (m-6049-N351) (64CC)
I have ran several diffrent plugs and never found the best for it yet! I think you maybe on to something. I am running AC DELCO 1 plugs.
They ran good for a maybe now a little while.. I do remember that when I did a tune up I did wires,plugs, and New MSD Dist. I am going to look up what plug I should use for that head. If you know that would be cool as well.. Thanks for that info you have got me pumped to check this out!!!!
Thanks Again

MC.Mustang said:

5.0Mustang1989

I am running Ford Sportmen Short Track Heads (m-6049-N351) (64CC)
I have ran several diffrent plugs and never found the best for it yet! I think you maybe on to something. I am running AC DELCO 1 plugs.
They ran good for a maybe now a little while.. I do remember that when I did a tune up I did wires,plugs, and New MSD Dist. I am going to look up what plug I should use for that head. If you know that would be cool as well.. Thanks for that info you have got me pumped to check this out!!!!
Thanks Again

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Did you find out your problem! Having the same issu and its getten worse!