GMC 6.5 Bad Running
 

Having issues with an 1994 GMC 6.5, it has a moind of its own. Runs perfectly 99% of the time then for no apparent reason will start but not ifdle correctly, sounds as if missing cylinders and will rev but not clear. Turn it off and restart it amnd it runs perfectly. No indication of any problems. Had injectors done, fuel tank good and pump running ok. Scratching head about this one !

I know nothing about the electronically controlled engines; hopefully someone knows more than me and can make some suggestions. But if it were mine, I would read up on how the fuel system works, and figure out what parts could possibly do that, then start swapping in new parts, starting with whatever seems most likely. Electronics are great until they start breaking, which is why I prefer the older mechanical-injection engines. If you don't have a factory service manual, this would be a good time to get one.

Thank you for the reply.
Most of the fuel system has been swapped out or tested. No electronics come into play on idling anfd itys that intermittent its hard work. I am beginning to think it could be a mechanical issue with either the IP or valve gear but unable to accertain why its not continous issue.

I was surprised to see an idle air control valve listed for your engine. I have no idea what an IAC valve would do on a diesel, but if it actually has it, there are electronics involved in idling. If it doesn't have it, there's a fuel injection control module, which I'd guess is involved whenever the engine is running, and no doubt has various sensors that will screw it up if they get flaky. And, correct me if I misunderstood, but it sounded like it's not an idle issue; when it happens, it also happens at higher RPM. All that, and the intermittent nature, make me think electronic, not mechanical, which brings us back to my original suggestion: Learn all you can about how the fuel system works, and figure it out. I've found that's the only way to fix the electronics on the few electronic engines I've had in vehicles, because no one else knows or cares why it has a problem but would be happy to take the rather large pile of cash I'd have to throw their way to motivate them to try to fix it, whether successfully or not. Hopefully someone familiar with the 6.5L will chime in at some point, but as dead as this forum is, you probably want to just get started learning about it yourself. Sorry I can't be more help.

I would start by getting a code reader for OBD1 (94-95) years. Maybe it can steer you in the rite direction.
Willis

my first place to look on my 6.5 is the notorious PMD. I keep 2 mounted under the hood behind BTY.#2. If it acts up I switch the PMD
wire to the fresh PMD, and go for a ride. In almost all cases of acting up the PMD has been the problem. Some PMD's last years, some hours.
You will need to have a spare at some point, so get one mounted where the wires can be switched over in seconds,.