Engine/Transmission (1994 - 1998) 12v idle miss

[JMcKee]

Well i dont usually wander over to the 12 valve forum but i figured you guys could help me out here. Im working on my friends 95 2500 5 speed with 200k on it (a hard 200k). Last week we looked at it trying to figure out why it was missing at idle, cracked injectors trying to narrow it down and found 1 and 3 leaking/not responding well to being starved of fuel. After removing all the injectors they did not look particulary clean but that is expected for 200k i assume. Installed a new set of DDP 75 hp injectors this evening and got rid of all leaks. The truck runs good when cold but after a test drive and warming it up the miss comes back and it is very noticable. All 6 cylinders seem to be making some power (cracking lines doesnt reveal an obvious dead hole). Valves were reciently adjusted, fuel filter was reciently replaced and injector cleaner was added. This miss only seems to be at idle the truck runs ok up through the rpm range but i just feel it should be making some more power with these injectors. The rig is stock besides a strait pipe, bhaf and gauges (fp gauge not hooked up). Is there anything else obvious to be checked out before major teardown? He did mention messing with the star wheel underneath the pipe plug on the back of the injection pump, what does that control and can it cause a miss?



Any suggestions would be well appreciated to save me from tearing this thing down looking for a problem easier addressed in an evening.



thanks for the help!

 

[Joe G.]

The idle may be too low.

 

[mfburns]

It is a 95 not one of the newer ones. You may be trying to compare it to your's. Look at another older one and compare them. I have a 94 and it sounds rough compared to the new ones. On my 94 the idle should be at 750-800. If its lower then that it runs rougher but it still runs down to around 600-650, but rougher all the time. Hope this helps.

 

[JMcKee]

The miss is definately not normal because the truck never used to do it and it is getting worse, even when you idle it up to 750-800 rpm it still will not smooth out. I am going to check all valve lash one more time then the only thing left i know of is a teardown.

 

[JonWatson]

have you checked, delivery vlaves, orverflow valve, maybe a worn plunger on the pump, how many miles on the pump?

 

[JonWatson]

mine has a miss at idle as well but onyl whens its been driven alot and then sits for a min, and my diesel man says possible worn plumger inside the pump, my pump on my truck also has about 484k on it so a lil wear i expected

 

[Missouri Mule]

The afc starwheel controls the amount of fuel the p7100 will get at low rpms. Turning it toward engine (clockwise, someone will correct me if that is wrong) will increase fuel.

75 hp injectors mite be two big for 175 hp engine without a fuel plate. Does it stumble at rpms higher than idle.

 

[Joe G.]

The afc starwheel controls the amount of fuel the p7100 will get at low rpms. Turning it toward engine (clockwise, someone will correct me if that is wrong) will increase fuel.

75 hp injectors mite be two big for 175 hp engine without a fuel plate. Does it stumble at rpms higher than idle.

Actually, the AFC controls fueling depending on boost not RPM. Boost is what moves the diaphragm to allow more movement of the governor follower. Of course, there is more boost (if everything is working right) as RPM increases.

 

[Missouri Mule]

Your right Joe thanks for the correction.

 

[JMcKee]

fixed

Well after scratching my head i rigged up a fuel pressure gauge following the FAQ design and found 5 psi at idle and 20 psi at appx 2000 rpm, pinched off the return line and pressure increased and truck ran smooth. New overflow valve and everything is working great!!! Runs strong. Thanks for the pointers.

 

[MAllen]

Where is everyone buying new overflow valves from?

 

[JMcKee]

Found this one from a local diesel shop in stock, pricey little piece.

 

[BSMcCauley]

Where is everyone buying new overflow valves from?

Hey, what's the part number on that?

 

[DValentine]

TST, PDR.
64 dollars to my door from TST. 160 from Cummins.

D

 

[Vaughn MacKenzie]

Wow glad to hear this 12-valve miss story had a good ending. Sometimes a cure is hard (or impossible) to come by. There's been a number of instances (like with my '96) where a idle and low-RPM miss could not be cured with new injectors, OF valves, injection pumps, all new fuel lines, lift pumps, valve jobs, new cylinder heads, etc. Spent a couple grand trying to cure the miss on my '96 to no avail, DDP injectors made it tolerable.