fyremanbill

I have a missfire problem that I can't figure out. The truck always starts, but has an occasional missfire. Sometimes once every 10 minutes, sometimes almost continuous. It feels like the entire fuel system shuts off for a fraction of a second. Doesn't feel like just 1 cyl, it's too harsh for that. Other than the sporadic misfire, it runs fine.
The condition lasts for less than a second, usually won't stall the engine. Pedal responds fine and seems less likely too occur with more throttle. Tach and gauges remain working when it occurs. I've already:
Checked codes........none stored in computer.
Checked fuel flow at IP... >1gal minute.
Disconnected edge EZ....runs the same
Looked at all fuel lines....no leaks, nothing wet.
Replaced the fuel in tank....no change.
Left fuel cap loose....no change.
If it has good fuel delivered in an adequate amount to the IP which isn't setting anerror code, whats next?

Number47

just because you have flow doesnt mean your pump can build any pressure. yes you pump is moving fuel but is it moving enough fuel to feed the IP. change fuel filter and get a pressure gauge. there should be no less than 5 psi at WOT. i personally would like to see 8 psi. kinda sounds like the lift pump is on its way out or you have an air leak on the suction side of the fuel system. i know the m11 and n14 are real pick about air in the fuel system

fyremanbill

Yep, already changed filters. Pressure is 12PSI on my mechanical gauge at idle...and the problem occurs at all throttle positions.
I've made a clear air trap by connecting a see thru fuel filter to the discharge port of the main filter housing. I then capped the end of it with a valve, bled out the air with it running. Now if it is passing air, it will eventually collect bubbles in the trap. After running for about an hour, no air.
I'm starting to think that it's electrical. It sounds like you shut off the key for a fraction of a second. The IP definately stops demanding fuel because the fuel pressure spikes after it misfires.

RickCJ

12 psi at idle doesn't mean a thing you need psi reading at WOT!!! I've seen several read 12 or more and a vacumn under load at wot.

Number47

my lift pump holds 17 PSI at idle but wont support WOT after 1800 RPM. i have a Holley blue and a Detroit diesel fuel filter base that I'm installing tomorrow to fix my issue. not sure on your fuel gauge setup but tape it to the windshield if not cab mounted and go for a drive see what pressure you have. if you have good fuel pressure at Wide Open Throttle, we will have to go into more detailed diagnosis

nmen

How many miles on your injectors? And, How many miles on the VP?

At what mods have you done to the truck?

fyremanbill

OK, but the problem is at idle, and everywhere else. If fuel pressure were the problem, it wouldn't act up with 12 PSI.
Also, it's clearly shutting off fuel to the injector for a fraction of a second, because the fuel pressure rises when it misfires.
Seems I read on here that there's a test tool that plugs into the IP which totally eliminates the trucks electrical system. The test tool drives the pump independant of the vehicle electrical system. Anybody heard of it?

fyremanbill

I filled out a profile on the truck, but don't know how to get it on the bottom of page the like you have.
It has the MWFI 300hp injectors, put in about 18mo ago. Ip was new when I got the truck, about 3yrs and 70k miles age. Total miles on truck are 280k. Runs great and fast other than eratic misfire.

nmen

GO to User CP then to edit signature and fill in the blanks.

So the Injectors are probably good, the LP pump might be acceptable because there is some flow and pressure and this doesn't necessarily happen at WOT, and the mileage on the injector pump is 70K, which some may fail, Have you tapped the pump wire? Do you think the pick-up tube in the tank has pick-up some crap? And you did you an ODBII scanner for the no stored codes?

fyremanbill

No tap on the IP wire, and no codes stored. Fuel flow is more than 1 gal/minute at IP inlet.

nmen

This one is interesting, so I think out loud. One has to give a KEY OFF command to just shut power off momentarily. I'm thinking if the ECM, or PCM for that matter, lost power, or a sensor, transducer, VP, or what ever else, then it would trip a fault. So I would lean toward the mechanical side of things. I'm also presumming that you haven't noticed any other power glictches across the dials.

Also no air on the LP side, and presumming no HP leaks and it will run 10 minutes or so at road load before it strikes.

The fuel pressure increase at this phatom shut down would indicate that there is an upstream (of your gauge) stop of flow. Pointing to the VP. You could pop the VP off and have it tested under endurance for nominal operation. Pricey and time consuming, but cheaper than a new injector pump.

mishkaya

I'll chime in to say that yes the VP44 can die at any time without throwing any codes. But this case seems a bit peculiar. I would almost be tempted to think it is a bad connection issue somewhere. I know from experience that bad grounds can cause havoc on these trucks. Personally I would check all connections and grounds to make sure they are fine. I would also try running some sort of fuel additive at maximum dose for one tank to see if that makes any difference at all. It sure is hard trying to do a diagnosis remotely....
mishkaya

dozer12216

Perhaps a crank sensor could do this if it hickuped. Also the grounds and power wires need checking for quality connections. I'm sort of with the electrical issue crowd.

fyremanbill

No obvious power problems, tach and gauges remain working when it occurs. No high pressure side leaks.

The misfire can occur any time. Sometimes when I start it, sometimes not. 2 days ago, I cold started and drove about 30 mi. It happened once. Next morning, I cold started and it was so bad I didn't think I'd get to work. Leaving work it was fine for a few minutes, then started acting up, then was mostly OK again.
In my experience, mechanical things are usually more consistent. This happens hot or cold, on long and short trips, sometimes acts up, sometimes doesn't. It seems to make no difference if it's at highway speed or idle, and occurs in any gear.
I'm afraid that the removal of the Ip and bench testing may be the only way. I would like to know if there is a test tool that will simulate idle conditions on the truck with the trucks electrical system disconected from the IP.

nmen

To add to the last post, a re-mate of the VP connector is easy, so you could pull it and check pins, wire, and cleanliness.

According to the wiring diagram, there is no power pin to the VP, just 2 data lines + -, cam ps sensor, knock sensor, grnd ,fuel pump relay out, and the fault.

So it would be the loss of a data link, and that would I think trip a fault.

Hopefully, someone else will offer suggestions too.

There is a cummins data buss reader, but $$$. Is the a Bosch dealer worthwhile in your area?