manvstaco

Recently, my 93 suburban with a 91 cummins engine and 47rh trans seems to be having an issue but only when I try and go faster than 60mph. It will cruise around town just fine but once I'm on the freeway trying to go over 60(OD AND TC engaged) it craps out on me and I can feel a LOT of vibration in the gas pedal. Things chill out once I let off the throttle and cruise 55 mph or so, not sure what's causing the problem. I did use some diesel fuel treatment 2 tanks ago so I thought maybe I had clogged the fuel filter from all the gunk in the tank and lines moving forward so I replaced that, I also thought I might have had a weak lifter pump so I replaced that as well. only around 10 lbs of boost and 650egts during this problem each time. Still have the same problem, engine hates life over 60 mph any help would be appreciated

Specs:

first gen 91 5.9l 6bt engine swap no tachometer 144k on the engine 202k on the body
stock h1c turbo with no intercooler
stock head bolts
Denny t fuel pin and boost spring
KDP fixed
3.75" exhaust, no cat or muff
stock injectors
47rh automatic transmission with TC switch and OD switch on dash
37" humvee tires

oliver foster

With those low boost pressures and EGT #s I am thinking low fuel delivery.
What is your fuel pressure at WOT?
I would suspect fuel filter, in tank screen on the pick up, and even the new lift pump might be bad too.
I would try to get some fuel pressures at idle and then at speed when you step on the pedal.
I am guessing it will read about 0 PSI at WOT.

j_martin

If that checks out, pull the valve covers and check the springs.

manvstaco

Roger that, I ordered a fuel pressure gauge 2 days ago it should be here next week. I'll get things installed and report back

manvstaco

What would I be checking the springs for? Noticeable damage? I need to swap out the valve cover gaskets so I might do that sooner than later if it helps us diagnose things

j_martin

What you describe is de-fueling or loss of compression at speed. The vibration bit makies it sound like specific cylinders are dropping out, not likely in de-fueling. Vlave float from a weak or broken spring would cause that, and might run normal at low speeds.

How 'bout the IP experts here. Any possibility it could de-fuel specific cylinders at speed?

manvstaco

Well I got the fuel pressure gauge installed. I'm getting around 6-7 psi at idle, 4-5 cruising around, and got it down to 2-3 going 45mph up hill. I have yet to try and cruise down the freeway as I feel like I might have found the problem. What do you guys think, drop the fuel tank and clean out the screen filter? I already replaced the lift pump and still has the same symptoms so I feel like that's not the problem, although it's possible. It still has the original 45 gallon gasser tank, I bet when they did the engine swap they didn't delete the screen and the pump in the tank. I'll have to drive it around a bit more as I still have over 20 gallons in the tank before I can drop it and take a look.

manvstaco

drove it down the freeway today. Engine starts stumbling just over 60mph, fuel pressure maintaining 4-5 psi, boost around 10 psi, egt's around 700
Should I still drop the tank and take a peak or do you guys think this is something else?

BHD

Does it stumble in neutral with no load like it does while you are driving it? If it does unhook the intake line at the lift pump and run a piece of hose into a can of good clean fuel and run it, that may save you from dropping the tank.

I take it there are no visible fuel leaks since you didn't mention any, however if you have steel lines and one is rusted the pump can suck air and not leak a noticeable amount of fuel. I know that sounds crazy but it happened on my '90.

I agree with J Martin on a cylinder specific issue since you complain of a vibration with power loss, I had that issue with an injector line working loose at the pump after I swapped IP's on my RamCharger but it was spraying fuel like a garden hose. Keep us posted.

manvstaco

just tried it in neutral and park exceeding the rpms it's usually at when having the problem on the freeway. No vibrating noticed, sounds good. It does seem a bit more shaky when I was driving it earlier, almost like I'm driving on a shitty pair of tires just going 35-45 mph, no noticable fuel leaks, idk if that helps any.

thrashingcows

What happens if you unlock the converter and take it out of OD when it does this? Does it get worse or better?

Just a shot in the dark...try removing the fuel cap and go for a drive. Perhaps the cap/tank is not venting and it's creating a vacuum situation in the tank that the lift pump can't fight against?

Might also be an internal IP situation as well. I have read about people who have had the 2 big springs behind the IP head break and the truck would still run. Believe they had issues at higher RPM's due to the lack of spring pressure on the cam plate and injection piston.

The broken springs are tough to diagnose with the IP on the motor. If the IP is removed you can grab the front input shaft...where the gear attaches...and if you can move it in and out move then a hair, then the springs are cracked and broken. You might be able to do the same thing with it still hooked to the motor, but have never had a chance to try this myself.

manvstaco

Well I think I can rule out the fuel tank or fuel lines(return and main feed). I spent the last few hours blowing them out with an electric air pump, no restrictions, could never build any pressure, verified the fuel cap is vented. Still having the same problems when locked up and in over drive trying to go faster than 60mph, I feel like the engine starts vibrating a bit around 55mph, then when I try to exceed 60mph it shakes a bunch and losses power till I easy off the pedal. I did try a test, I mashed it with the TC locked up and kept out of overdrive, could get it going faster than 60mph with 22 lbs of boost, 900 degrees on the egt gauge , fuel pressure never went below 7psi(better now that I cleaned the lines). No tachometer yet but was obviously much higher rpm's than when attempting to go faster than 60mph in OD. I'm stumped, any ideas?

mknittle

we once had a rubber fuel hose that had a bad spot in the inside lining fuel would get behind the lining and close off the line. the way we found it was air would go through one way but not the other.

manvstaco

Since I was able to maintain fuel pressure around 6-7psi during this problem today would that rule out a faulty rubber fuel line? What do you guys recommend I do next?

manvstaco

just ordered the fuel heater delete nipple, not having any visible leaking around it atm but thought it would be a good idea to get rid of it since it's not in use.