Odd engine problem

[TheRookie]

My 91 is acting a little funny. Has been doing this for a while but its getting worse. Sometimes it feels like the engine stalls for a split second then back to normal again. Almost feels like getting rear ended lightly. I think its probably related to the power loss I have. Here is what I can tell you it isnt. it is NOt, tranny, transfer case, plugs, wires, maf. I know it isnt any of these cause they have all been changed since it started.
I think it may be a clogged cat. they are original with 125 k on them. Have not yet done fa fuel pressure test but its an unlikely cause. The other 2 mechanics at my job are getting tired of my truck.

 

[jimbo74]

fuel filter? what about IAC? what about the other emissions systems? you dont have an EGR system

 

[TheRookie]

oh IAC has also been changed. No egr, all vac lines are good. I have done a cat back on it also.

 

[TheRookie]

fuel finler isnt too old but it is a possibility i suppose

 

[MrShorty]

Why do you think fuel pressure/delivery is an unlikely cause? I could see this being easily explained by an intermittent fault in the fuel pump circuit. If the fuel pump occasionally and briefly lost power, there would be a brief moment when the engine had no fuel and would stumble.

 

[TheRookie]

Update. Mr shorty may be right but I think its an injector or a couple. Here is why.
Fule pressure test came back good. Changed cats just cause I can. Hooked up my wonderous snap on scan tool and got a few codes. KOEO said EGO not switching. EGO is ford for o2 sensor. Canged it and noticed the wires were kinda fried. I bought this new. Also had a Bank 1 lean code. Ran a KOER test and got 2 codes. One was bad TPS and one was bad air tewmp sensor. Changed both with parts from my parts truck. (I know the ones from the parts truck to be good. RE scanned both KOEO and KOER and got no codes even after lots of driving. However it still had the problem. Finaly getting tired of this I called over another guy named Phanh. I call him master phanh cause he really knows damn near everything. He took the truck down the street for a couple mins and told me its an injector. In his 30 years as a tech he has seen this a few times on fords. Mostly 4.0's including on his aerostar. Switching the injectors from the parts truck is going to be a little work. I gotta decide if I want to change over the heads too because of my rocker noise. REmoving the lower intake manifold I will be like 1/3rd of the way there.

 

[Blastingcaps]

could still be a fuel pressure problem if pressure regulator is not changing with reduced vacume .. pressure should increase with reduced vacume acting like an accelorator pump.. it drops back down when vacume stabilizes.

 

[TheRookie]

right, however a propper fuel pressure test would show that. Propper test also includes driving with the gauge.

 

[Bexplorer94]

Ive had the same problem about a year ago. Almost acted like a the engine was stalling but then came back up to speed. i let the problem go for almost a month, untill the engine sounded and felt like it was giving out. i did numores test's with the snap on comp. and even took it to a friend that is a ford mechanic. we couldnt figure it out for the life of us. So i finally gave in and took it into the ford dealership and it ended up being 3 fuel injectors and the coil pack. that put another hole in my arse. hope your not having the same luck i had!

 

[TheRookie]

thanks for the tip. sorry to hear your trouble. i bet that wasnt cheap at the dealer

 

[Billy177]

a clogged cat would not be intermitent check the fuel pump voltage and pressure

 

[Tat2dExploder]

any update i am having close to the same problem... i am gonna try a new coil pack, but i'm prayin its not the injectors

 

[TheRookie]

No update. I did change the coil pack and it didnt help any. This is making me nuts. My solution may end up being swapping the motor.

 

[Tat2dExploder]

your not makin me feel any better lol
i've had this problem for like 8 months now...

 

[Opera House]

So, does the engine go into a violent shudder at a stop light and then start running good? There is some history of the FPR sticking. Some springs are not centered and a bevel forms on one side of the seat. There is a tendency for the valve to stick open at conditions of high vacuum when the valve is open the widest although it can happen at other times. This will not show up on a pressure test unless the engine problem happens at that time. I finally pulled the fuel pump fuse and ran wires into the cab and drove around. Any time it would start to stall, the current would slowly drop. Cut open the FPR to verify that was the problem and found the bevel worn on one side.

Not real keen on that injector story. Likely your parts car regulator is not the same for a swap.

 

[TheRookie]

no it doesnt shudder at a stop. Idles a little rough but not a lot. It does however go into a bad shudder at low RPM at highway speed. I also dont believe the reg off my parts truck will be the same. Thanks for the idea, I may try your wire technique