elway12

All of sudden my 91 1st gen. will only go 30mph max.! I immediately replaced the fuel filter. It runs great except won't go faster than about 30mph. Transmission is fine. I'm assuming it's a fuel related problem. Any ideas? Fuel pump...Sending unit? I had my fuel injection pump R&R professionally replaced a year ago. Any and ALL help greatly appreciated.

ofcmarc

Is it a non-IC 91 or an IC 91.5?

If it's an IC model, Check the intercooler boots. I had one blow one day and couldn't manage to get over 35mph....

elway12

Unfortunately it's not an IC

bigragu

Did you check all your throttle linkages, accelerator cable, etc? Do you have a boost gage? If so, what type of boost readings are you getting?

Also, check that boost line, the hard line that goes from the head to the AFC. Those have been known to crack. Although you have a non IC model, you still have a duct that runs from the turbo to the intake. Check all the connections there.

j_martin

Trash sucked into the air intake/filter?

MAC702

Any white smoke?

BigIron70

Check the fuel shut down valve I have seen them go bad and not open all the way. Easiest way to check is remove the valve and spring then reinstall without the valve. What warranty do you have on pump?

elway12

I haven't has a chance to test it on the road yet. If I do see white smoke what is that an indication of?

I have to look up the warranty. It's on my list of things to do!

Thank You for all of your responses. If you think of anything else to check send it my way. I will give an update as soon as I find the issue and fix it!

bigragu

White Smoke- Air in fuel, coolant being burned, injector pintle stuck and flooding the cylinders with fuel(recently experienced that one, but no loss in speed for me. Just tons of white smoke), and possibly timing way too retarded.

With all the suggestions being offered to you for loss of power, all signs have pointed to an air intake problem, boost leak, or throttle linkage issue. You never said you were stumbling or cutting out, which would point to a fuel problem. Sure, a plugged fuel filter would cause a power loss, but not limit you to just 30mph. Still, if you are unsure of the last time you changed it, change it out anyway. They're cheap.

A thought occurred to me yesterday, regarding this issue, and I'm going to throw this out there because I, too, would like to know if this is even possible. Is it possible for the woodruff wedge key that slots into the VE pump gear and pump shaft, locking the two as one, fall out and cause this?

Angry Johnny

It is very possible. I screwed my key up the first time I put my pump back in. It wasn't really very tight fitting and it slipped partially out.

MAC702

My experience is that it will let me cruise on a flat, but immediately limit me to 25 MPH when I hit a hill and needed that extra flow.

bigragu

Good to know- thank you

elway12

UPDATE 10/10. Have replaced 2 fuel filters. After each one the truck starts and drives great for about 3-4 miles at 45mph+ then starts not going above 30...no power same as described above. In my original post I forgot to mention I have just started running b99(biodiesel) thru it. Obviously I know this can cause havoc on fuel lines etc. that can't handle the solvent like bio. I have run it on bio for over 10 years at different times. I noticed black particles that came out of the fuel filter(s) seem to be black rubber pieces?...Where could those be coming from? I thought I had all metal lines and never have had this problem running bio thru it in the past. Is the fuel lift pump "before" the fuel filter? Maybe that is breaking down on inside? Does it have rubber in it?? Again Thank ALL of you for your help.

edwinsmith

One problem with BioDiesel is that it contains methanol. Methanol attacks some types of rubber lines and seals. Older systems are not equipped with methanol resistant rubber lines and seals so my guess is that the rubber parts everywhere are flaking off and clogging up things. No. You do not have all metal lines. There are rubber pieces between the tank and the lines. There is also a rubber piece in the filler pipe. Mine has a rubber lines from the metal line to the lift pump and a rubber line from the return line to the metal line going back to the tank. It would be nearly impossible to not use rubber lines in places that have to flex.

Also the methanol has a solvent effect on sludge in your fuel tank which could also be finding it's way to your filter. You'll either have to upgrade your fuel system or discontinue the BioDiesel and clean everything out.

Edwin

j_martin

I used to train field engineers for a computer company. The hardest thing for them to grasp was this:
1. You walk up to a running machine.
2. You do something to it.
3. The machine does not run.
4. It's what you did.