Cummins 12-valve stalls when hot

[towbar]

The saga continues.... presuming that it's still the same issue

I had the P7100 rebuilt, the shop said it was in pretty poor shape (1995) with a cracked flyweight casing, several shot pistons etc. When I received it I installed the existing (adjustable) overflow valve, ran it extensively, no issues.

Last week the engine lost power and gradually stalled again, about 10 minutes after startup. I saw some air in the lines, purged it, etc. No cigar. THEN by happenstance I tried to prime with the lift pump to see if any air would still be coming back into the tank, hit a PROBLEM: the pump wasn't pumping. It was hard as rock and no fuel was coming back which immediately made me suspect a stuck-closed overflow valve in the P7100.

But I have never heard of such a valve being able to stick closed at all! Look at the design, how would this be possible? I *think* that with the valve stuck closed internal pressure builds up and defeats the pump completely. What would make it stick, not enough lubrication with the new no-sulphur fuels?

With the valve partly unscrewed the engine started and there was fuel pissing all over the place but the engine ran like it was new. So for now I have the valve installed with the spring and ball removed, waiting for a new valve. No problem runnigthe engine at any envelope rpm but I think it would not be a good idea to extend this mode of operation without a new valve.

Any ideas, anybody ever seen such a PIG of a INTERMITTENT snag? I have no regrets having had the pump redone (it was due at the least) but it may never have been THE cause of the stalls.

 

[terex herder]

Have you checked the vent in your fuel tank? Pull cap as soon as engine stalls. Any air sucking in is bad.

 

[towbar]

For over a year now I've been operating with a transparent plastic bottle and hoses to keep an eye on air. Plenty of venting. Just came back form a heads-up with my machinist/mechanic to order a new outflow vale: the Cummins price is $700cad (meaning they don't wanna be bothered)! Ordered an aftermarket non-Bosch for $105cad. I could get one stateside for $25us but that would require about that much again shipping plus "$45 customs brokerage even if there are no customs charges so nothing gained.

He said he's never seen an outflow valve stuck hard closed. So there is now a question of why

1 - I felt a LOT of resistance at the manual priming boot, no priming return to the tank at all
2 - why the engine ran just fine with the valve removed (pissing outflow all over)

It could be a coincidence he says, that the lift pump is intermittent locked up. Well, this one "I" have never seen.

It's gonna snow this afternoon and since we both agree that running the p7100 without an overflow restriction cannot really hurt it I'll do my snow thing this way and see 'if anything blows up'. I don't know what internal pressure can normally build up to inside the pump where outflow originates but the valve body with just a 1/16" hole is a restriction by itself even with the spring and ball removed. The reason I say this is that when I ran the engine for a few seconds with the valve removed the amount of dumping fuel was HUGE (I was really surprised) so if I visualise that hitting a 1/16" orifice that WILL cause some back pressure as is.

 

[towbar]

Just did 2 hours of snow-blower running 2200 rpm without a single issue. I wonder if THIS had been the source of all the associated problems?

 

[towbar]

I have since installed the OEM Bosch overflow valve, very little snow removal done but no issues. Assuming that this had been the problem I cannot for the life of me figure out how this design of a basic spring and ball regulator valve could possibly jam hard closed!

 

[thepumpguysc]

Just throwing it out there..
Being aftermarket, there’s no telling what the clearance between the ball and body is..
Seeing everything in the fueling circuit is lubricated by the fuel, it may require a lube in the fuel.??
I know that on the older mdl P7100’s, they require 25 psi to hold the amount of fuel necessary for all the pumping plungers to have enough fuel to pump properly..
On the P8000 series, they raised it to 30-35 psi..
I’ve taken both overflow valves on a service call.. the higher psi overflow valve seemed to cure a lot of running problems..

 

[towbar]

Just throwing it out there..
Being aftermarket, there’s no telling what the clearance between the ball and body is..
Seeing everything in the fueling circuit is lubricated by the fuel, it may require a lube in the fuel.??
I know that on the older mdl P7100’s, they require 25 psi to hold the amount of fuel necessary for all the pumping plungers to have enough fuel to pump properly..
On the P8000 series, they raised it to 30-35 psi..
I’ve taken both overflow valves on a service call.. the higher psi overflow valve seemed to cure a lot of running problems..

I can see problems with the lack of lubrication in the new fuels, that's another HUGE topic on its own (ignored because a lot of people are making money off the results)! I can see some weak possibility of a valve jamming shut too but it may not be the end of the story. Let's assume the valve sticks closed, exactly how would that defeat the pump seeing that the output side going to the injectors is not only completely isolated but also under higher pressure by orders of magnitude? One would have to test for this by inserting a plug in the port, not about to go there before the end of the season