I’ve been having this issue for awhile. I thought it was related to the fss it’s been replaced. The pre-heater assembly has been completely removed. The issue is the truck will not stay running without throttle. I’ve tried raising the idle, bleeding the injectors and I took the bed off to get a look at the fuel module. My only ideas of what it could be now are an air leak on the suction side, fuel solenoid out of adjustment or my lift pump is failing. It’s a manual and in neutral the rpm’s fluctuate even after raising the idle to 1k.

 

Adding a fuel pressure gauge could help. Even just one under the hood is useful as a diagnostic tool for situations just like this. Few thoughts:

Fuel filter is new-ish?
When you say you've removed the 'pre-heater assembly', do you mean just removing the preheater from the prefilter, or removing the whole pre-filter mess too?
Leak on suction side is not unlikely. You can jam some shop rags and an air compressor hose down the fuel filller neck and get a few PSI of air pressure into the tank. This will often reveal leaks on the suction side.
Could be the OFV going out too. Clamp off the return line with a pair of vise-grips and see if the situation improves.

 

I’ve removed the whole prefilter and heater assembly. I would like to put a fuel pressure gauge on there. I just replaced the fuel filter on the 1st. I have installed a torktek ofv a while back as well. I think pressurizing the tank is my next move.

 

When you removed the prefilter assembly, you did install a spacer under the LP to replace it, correct? Definitely check the lock nut on the TorkTek OFV too -- it's not common but they've been known to come loose.

 

I got the work done at a shop over a year ago, but the kit that I purchased did include a spacer. I’ll take a look at the overflow valve and double check they used that spacer.

 

Have you checked the throttle return springs? Not sure if you have 2 like my 95, but if 1 is off it will act up similar to what you have.

 

Did this start all of a sudden? Or is this a, 'it was like this when I bought it' type situation?

 

Given that it’s an aftermarket mod from the previous owner and you’re unsure of its condition, a stuck exhaust brake is certainly a possibility.

An exhaust brake that’s stuck partially closed could absolutely cause erratic idle and stalling issues. It restricts exhaust flow, creating backpressure that messes with the engine’s ability to breathe properly. Even if the wiring is disconnected, the mechanical component (like a butterfly valve in the exhaust) might still be stuck due to carbon buildup, rust, or a failed actuator. Here’s how to check:

• Inspect the exhaust brake: Crawl under the truck or remove the exhaust piping near the brake to visually confirm the valve’s position. It should be fully open with no power applied (assuming it’s a vacuum or pneumatic type common on these trucks).
• Test it: If you can access the valve, try moving it manually (with the engine off) to see if it’s seized. If it’s stuck, freeing it up with penetrating oil or replacing it might solve your issue.
• Temporary bypass: If you suspect it’s the problem but can’t fix it immediately, disconnect the exhaust downstream of the brake and run it open to see if the idle stabilizes.

Other possibilities might be Air Intake issues, Injection Pump, Fuel Quality or if the adjustment screw on the P7100 has been tampered with or is out of spec, it could explain the fluctuation.

Also, even with your bypass to a diesel can, air could still be sneaking in if there’s a loose fitting or crack in the line. Bleed the system again at the injection pump to confirm no bubbles.

But I think starting with the exhaust brake first is a good call.