Intermittant Crusader 8.1l

Gibson 5000, twin 8.1's. Port 8.1 sometimes runs fine at 2000 but starts hunting at 2500. Back off to 2000 and runs fine. HOWEVER, SOMETIMES I shut off the engine, wait a bit (30 sec to a minute or so), restart the engine and it runs fine past 2500. Sometimes this does not work. This happens every two or three days. Pre and FCC filters are new. New raw water pump. New OEM thermostat. Backflushed heat exchanger and oil cooler. So where else do I look?

Sounds like fuel starvation--bad news on pistons!

What level of filtration are you using? (The water separator should be something like 30 micron, and the final filter after the fuel pump about 10 micron.) If you are running finer filtration that that they might be clogging up on you (which happens a lot with 10 % ethanol fuel).

Jeff

If you have the "returnless" FCC be sure you have good raw water flow to cool the fuel control cell. It should be cool to the touch. Those small water lines clog easily and cause vapor lock. I was ready to cut the decks and remove the tanks before I stumbled on to the problem.

I believe this is an injected engine. As suggested fuel starvation is certainly a leading candidate. The first thing to do is get a fuel pressure gage and see if the pressure is dropping below spec when the problem occurs. If so, proceed to chase it down. If not, and if you have an MEFI4 controller (or even an earlier ECM version) you need to get a diagnostic manual, which will cover the electrical possibilities as well as the fuel system. I downloaded mine (free) at a hot rod website (Pirate 4x4) and they are also available elsewhere. Several hundred pages of really good technical information and trouble shooting guides. You could have a bad MAP sensor (swap between engines and see if the problem moves). Or the "famous" cam retard setting could be off (you need a scanner to adjust). Either of those could also cause the rpm hunting.

2X on starting with a fuel pressure check.... As far as potential items - could be the anti siphon valve too.