Whats wrong with my 91 C2500

[Edmduncan]

Wondering if someone may have had a similar problem.
So had a problem pulling out travel trailer,
Have a 350tbi with 4l80E transmission, 3:73 rear.
Problem is :
Only when pulling the trailer the truck will get up to 110kmh then slowly drop down to 100kmh and thats where it will hang and will not go any faster.
Today pulled the trailer again with same symptoms, doesn't seem to effect it until it gets up to 110, and sits for a few minutes then it drops slowly and sits with the peddle to the floor at 100kmh.
Doesn't over heat, no engine lights or anything of warning.
Attempted fixes tried:
-took to tranns shop , transmission is cleanest he has ever seen and its been rebuilt with shift kit. Differencial is clean and good.
- took it to mechanic and fuel pressure was ok..maybe.(explain later)
Everything else checked out they said as well.
- so then realized maybe the cat was plugged as it was still present.
Put new exhaust from y-pipe back, and open the engine up a bit when not pulling.
- full TBI service, all new gaskets and cleaning. Jets flowing fine for pattern good while parked, that I could see.
-New tps, Map, vacuum tubes, pcv valve, AIC valve, new EGR valve.
- Fuel filter replaced last yr.

So with all that done, the truck accelerates well and performes well without a towing load on it.
Just when the trailer is on it it has the issue.
Trailer - 5200lbs dry.

There is a thought the fuel pump maybe under performing with a load on it. It very well could be the original factory fuel pump.
Could this be the issue even though the fuel pressure was checked and was supposidly ok?

Truck starts fin when warm, takes a few cranks/turn overs when cold. Idles pretty good, and no hesitation on acceleration or chugging.

Im at a loss as I believe the engine is still strong as its not passing , no smoke or leaks. Not over heating or anything saying its having issues.

Can someone maybe offer advice or have a similar issue.

 

[Hipster]

Im at a loss as I believe the engine is still strong as its not passing , no smoke or leaks. Not over heating or anything saying its having issues.

You can have plenty of blow-by and never see smoke out the tail pipe or have mad oil consumption. Do a compression and leak down test. Verify instead of guessing.

You have it to the floor and the trans is not shifting back into third?

What are your rpm's at 110kmh? A 350tbi is going to have a hard time maintaining speed in od pulling 5k unless you're running flatlands.

 

[Edmduncan]

Im at a loss as I believe the engine is still strong as its not passing , no smoke or leaks. Not over heating or anything saying its having issues.

You can have plenty of blow-by and never see smoke out the tail pipe or have mad oil consumption. Do a compression and leak down test. Verify instead of guessing.

You have it to the floor and the trans is not shifting back into third?

What are your rpm's at 110kmh? A 350tbi is going to have a hard time maintaining speed in od pulling 5k unless you're running flatlands.

Im at a loss as I believe the engine is still strong as its not passing , no smoke or leaks. Not over heating or anything saying its having issues.

You can have plenty of blow-by and never see smoke out the tail pipe or have mad oil consumption. Do a compression and leak down test. Verify instead of guessing.

You have it to the floor and the trans is not shifting back into third?

What are your rpm's at 110kmh? A 350tbi is going to have a hard time maintaining speed in od pulling 5k unless you're running flatlands.

Transmission is shifting fine, ran the truck in both od and drive and no difference.
Don't know what the rpms are as its auto and no tac. Thats on my list to install as I have it just haven't wired it in yet.
I was towing on flats not a single hill.
I am going to replace the fuel pump as even when not towing ive been noticing weak performance on hills. Can Flore it and not see a strong pull up hills.
Have a few buddies that figure its fuel pump as it may still be the original pump. I dont see rough idle or and consistent issues that would equate to bad compression, missing or a bad cylinder.

 

[Edmduncan]

Transmission is shifting fine, ran the truck in both od and drive and no difference.
Don't know what the rpms are as its auto and no tac. Thats on my list to install as I have it just haven't wired it in yet.
I was towing on flats not a single hill.
I am going to replace the fuel pump as even when not towing ive been noticing weak performance on hills. Can Flore it and not see a strong pull up hills.
Have a few buddies that figure its fuel pump as it may still be the original pump. I dont see rough idle or and consistent issues that would equate to bad compression, missing or a bad cylinder.

The truck is a C2500 350 5.7 with 4l80e and has never had an issue pulling anything as its rated for 9000 with the towing package as well. It has pulled a 33' park model almost the same weight up huge hills with no issues so the pulling problem is recent so to speak.

 

[Chewy11B]

So the question was asked, is it downshifting?? Also have you made sure with help that the throttle is in fact opening all the way with pedal to the floor?? Is engine audibly struggling, I know you said no tach, but does it sound like it's higher up in the rpm range??

 

[alpinecrick]

An outside chance, but the muffler may be in the process of collapsing. I have seen similar behavior before..

 

[Edmduncan]

Exhaust was just replaced from y-pipe back literaly 2 weeks ago because thought cat maybe plugged. Opened it up a bit.
In drive and od it will shift. I get all 3 in drive , and will get into 4th in od.
It doesn't seem like its screaming or doesn't heat up. It's a little loud but has a load on it, mos of the noise I hear I believe is from the fan.
The transmision guy said ( without a load) it was amazingly clean and solid tranmission. So its not slipping. Its not reving high .
I guess it sounds like its working to pull the trailer as it never really gets to that lower kick down cruising volume.
Im going to twst the compression tomorrow and change the fuel pump and inline fuel filter.