Running rough...sometimes

[GreyHound6]

Ok so. The truck starts rough when its cold. Also after sitting 8 hours at work. Ive read that could pissibly be a leaking injector. Noted and put on the list. So when I get it started, I keep rpm at 1000 and about every 5 seconds it will chug and bog down like its running out of fuel, and occasionally die. I just today changed the fuel filter and installed a fuel pressure guage. We're reading about 20 and it does climb a little at 2000 rpm but still chugging. Now I noticed that my tank is filling itself up the filler neck. It seems to rise an 1/4 inch every time the motor chuggs and drops 1/8 after idle smooths out. So now its over flowing. I took out the overflow valve for inspection. Spring was not broken but looks like there are 2 shims (previous owner?) Reinstalled. Same problems. Not sure where to go from here. Plan to redo fuel lines with hose and delete the preheater to be sure there are no air leaks, but have not yet. Any suggestions would be appreciated!!!!!!

 

[GAmes]

Filler neck? You mean the fuel level is going up and is now overflowing? I've never heard of that so can't help you there. Your fuel pressure is below spec, it should be 17-22 at idle, 25-30 at 2500 rpm, no load. Pinch the rubber return line briefly while some watches the gauge. The pressure will spike if the OFV is bad. There isn't anyway to determine visually if it is good. If the pressure stays the same (low) the lift pump is bad. Search Joe Gs writeup. All you will ever need to know about the system.

 

[SSage]

My lift pump went out recently, it started out with an odd lopey idle, rpm at idle dropped twice over time till it started running really rough.

 

[GreyHound6]

My lift pump went out recently, it started out with an odd lopey idle, rpm at idle dropped twice over time till it started running really rough.

How much time and miles were between the very first symptom and the pump replacement? Where was your fuel pressure reading? My fuel pressure is 20 at idle. Right on spec but doesn't increase to 30 at 2500 rpm. So I imagine lift pump might be on its way out. But how can I test that before throwing parts at it?

 

[SSage]

How much time and miles were between the very first symptom and the pump replacement? Where was your fuel pressure reading? My fuel pressure is 20 at idle. Right on spec but doesn't increase to 30 at 2500 rpm. So I imagine lift pump might be on its way out. But how can I test that before throwing parts at it?

I didn't have a fuel pressure gage when it failed. White smoke, loss of power, low idle rpm are commonly noticed when the pump gets weak. I first noticed that the boost topped out at 30 psi, normally I see 45. My pump actually lost prime if it set a week, one day I couldn't get it to prime manually anymore. My first oem pump died much slower, just lost power and it leaked.

My 12 valve truck mainly pulls trailers occasionally, so it acted up for a year till I finally got a lift pump ordered. It's not a daily driver at all, maybe the sitting got the 3 year old pump. I don't think the Chinese lift pumps are that durable though, I'm on my second one in the 1995 truck. This new one is supposedly improved, get the latest cummins pump if you do. About $69 via poor man's diesel on eBay.

 

[GAmes]

So I imagine lift pump might be on its way out. But how can I test that before throwing parts at it?

One more time.

Pinch the rubber return line briefly while some watches the gauge. The pressure will spike if the OFV is bad.

If the pressure doesn't go up the lift pump is bad, which I doubt to be the case.

 

[GreyHound6]

Ok. I might try a pump in it. Because ive adjusted up the idle once already. And it does put out white smoke on cold starts after sitting over night and 8 hours at work. Seems like it looses prime. But it still primed manually just fine.

 

[SSage]

Ok. I might try a pump in it. Because ive adjusted up the idle once already. And it does put out white smoke on cold starts after sitting over night and 8 hours at work. Seems like it looses prime. But it still primed manually just fine.

If the fuel hoses are old, replace them. I used the diesel spec hose Foster Truck kit, aka LarryB. I got my strainer kit from Geno's and I took out the fuel heater like most folks. Genos has the oem preheater hose too and the lift pump with studs to make the install easier. It's a pain to do it, thankfully good quality hose last years and years. The hoses may solve it, if it's loosing prime it could be an air leak, burnt up fuel heater, cracked hose etc. I replaced my hoses before the lift pump, I thought my 3 year old pump couldn't be it...

 

[BigPapa]

The truck starts rough when its cold. Also after sitting 8 hours at work.

What is "cold"? Are you waiting for the heater grids to cycle?

So when I get it started, I keep rpm at 1000 and about every 5 seconds it will chug and bog down like its running out of fuel, and occasionally die.

Sounds like when the heater grids kick on the alternator is pulling the idle down.

 

[GreyHound6]

Cold as in between 5F and 20F. 40 starts a lot better. But that seems like were getting somewhere. Yes I wait for grid heater light to go away. But it keeps cycling on after truck starts right? I can see the guage move and lights dim a tad. Btw.. 2 new batteries and new alternator a week ago.

 

[GreyHound6]

Ok. So to update. Ordered the LarryB fuel kit. Also the preheater to pump elbow. Going to delete preheater as well. And a tork teck OFV. We'll see where that gets me. But im sure the lift pump is old too, so its on the chopping block..

 

[BigPapa]

The grid heaters will continue to cycle until you reach somewhere @ 20-25MPH, I believe. I don't think they'll come back on until you shut the truck down.

 

[GreyHound6]

The grid heaters will continue to cycle until you reach somewhere @ 20-25MPH, I believe. I don't think they'll come back on until you shut the truck down.

Ok so grid heater shuts off at speed? (Sensor that controls that?) Do they also shut of with temp? ( Sensor that controls that?) What computer would all that info run through if any?

 

[SSage]

If I remember right the intake heater is off at 14 mph. There is a simple ecm as usual, right on the passenger side firewall I think. I've got a few vehicles I work on, but I remember the computer being near my BHAF.

 

[BigPapa]

Ok so grid heater shuts off at speed? (Sensor that controls that?) Do they also shut of with temp? ( Sensor that controls that?) What computer would all that info run through if any?

The speed related shut-off is done by the PCM. As SSage noted, it is on the passenger side firewall.

The Intake Air Temp sensor is in the intake manifold. I believe it is supposed to turn the heaters on at any temp below 50*. Mine work all the time at any temp. I have changed the sensor with no change. It may just be a hiccup with my conversion. I just disconnect the relays when it warms up. I need to put a manual switch on them and be done with it.

 

[Missouri Mule]

At that low of temp your Cummins is going to blow white smoke even if the grids are working. Once you get the engine up to operating temps exhaust will clear up.
Tank making fuel is probably the overflow valve spring getting weak.( The overflow valve regulates the pressure in p7100) The spring is set around 20 to 22 psi (senior moment which I to young to have). The fuel then flows back to the fuel tank.
Your pre filter mite plugged or you had or are getting large pieces of dirt in the filter.
Also you mite have big pieces of dirt in tank or the black crud ( starts with an A and know s what it ends with) that gets on screen of tank module where the pick up tube ( supply line from tank to pre filter/ heater.
When I clean the pre filter screen I use good old carb cleaner ( spray can)!
Just fuel to ponder.

 

[GreyHound6]

Update. Got LarryB's FH99K kit on. Pre heater delete now, and tork tek over flow valve. No issues at all now. But it did get up to 83 degrees. We will see how it goes in a week or two, as I am sure below freezing temperatures are not done yet