Ok, this will take a little explaining and ill try to keep it in order and logical for ya.

Last week my truck was acting like the pmd was going bad, misses, stumbles dies. It would always start back up after a few minutes and be fine. So i read some more of the faq's to see what i might have missed before. One thing was to check the lift pump/oil pressure unit.

I checked the pressure at the T-valve...Nothing...hmm. With the truck running I got underneith the truck and felt the lift pump vibrating, i have read where it can still seem like its working but fail to pump. So it seemed like if i accelerated very slowly and didnt get on it hard it would be ok. I have a friend that works at a parts store back home so i told him i was coming this weekend and to order it and have it there friday.

Now heres where the story gets interesting. My wife and I started for back home (about 100 miles) about 3:00pm friday. She said i was stupid for trying to take my truck home and it was gonna quit on me. I assured her that i could make it as long as i didnt get on it. She drove her truck to as we had a lot of stuff to do back home getting ready to move so we needed to take both trucks.

Needless to say about 25 miles later im having major problems. Truck died 4 times in about 3 miles. Got it started and out of town, 35-40 mph is as fast as it would go, bucking and surging the whole time. I finally gave up and jumped in with her to finish the trip.

Now about 4:00 pm me and a buddy are going back down with the new lift pump to pick up the truck and fix it. We get down there I put the new lift pump on, purge the air fire it up, all is good. Opened the T-drain valve and it just shoots out, good deal. Drove great as it ever had for about 30 miles, then started stumbling/surging again.

Pulled over and checked the T-drain, again nothing coming out. Got it started and told my friend im gonna try to limp it the rest of the way home.

Well I could only get up to about 50 mph and then it would start slowing down till i got to 30 then it would die. We pulled over and i decided to check the fuel filter. Pulled it out and there was only about an inch of fuel in the housing. I told him to turn the key to on and the lift pump would only pump a tiny bit of fuel in. I did this several times till the bowl was full, reinstalled the filter, bleed the air and fired it up. It would run fine for about 2 miles then back to the same thing, 50 mph to 30 then die.

I did this about 3 more times then the last time when i pulled the filter the lift pump would not pump any fuel in. Also i need to say that i would take the fuel cap off thinking maybe there was a huge vacuum in the take, but that was not the case.

So now we are sitting ducks, call my fatherinlaw to come get me with his gooseneck. We didnt get home till midnight friday night.

Saturday morning we pulled the line from the front of the lift pump and turned key to on...fuel just barely dribbled out (this is with brand new lift pump to). So im thinking bad lift pump, so i pulled lift pump off put the inlet in a cup of diesel and put power to it, it shot diesel about a foot out. My friend said blow air thru the line back to the tank. Did that and he was standing at the filler neck, he said heard a difference. Put it all back together started it up and went for a drive, all was good.

I went about 75 miles that day and it started to do it AGAIN:mad:. Limped it back to my inlaws and decided i was gonna drop the tank. Since i dropped the tank last month to clean it it wasnt that bad of a job. Had to put in a new sending unit last month to because i twisted the metal lines off from them being rusted. Anyway, nothign in the tank noticable. Looked clean, didnt look like there could have been anything to clog the lines. The strainer had just a very little bit of crud but nothing that would have made it starve for fuel. So i blew out the line from the tank to the lift pump really good and put it all back together AGAIN.

Ran great again. So now its sunday night and me and the wife are getting ready to come back to carbondale (fairfield is home town), I'm just thinking man i really hope i can make it with no problems. Well i go almost the entire way with no problems, about 1/2 mile from the house it startes to do it all over again, bucking, surging, stumbling. At this point its midnight and i say screw it, it will stay alive long enought to get to the house. It did but.... heres the question What in the world is going on?

My only thought is there is a pin hole drawing air. Is it true that you can have a hole that sucks in air but fuel Wont leak out of?

Very sorry for the long post just thought it better to explain well in the first place, might save some questions later.

check all your rubber lines..sounds like to me one of them is collapsing and cutting the fuel off

OPS, PMD or grounds would be my guess, have you gone through diagnostic list yet, if not please go through it & post back we've not had many collapsed hoses, the lift pumps aren't that robust, was replacement pump one for a 6.5 or a universal pump also used on a gasser

Had the same thing happen to me, but not quite as bad. Turned out to be bad fuel. Replace the fuel filter 3 times in two weeks and it finally went away as I ran down the fuel and replaced it with "good" stuff.

TD, did all years run the LP throught the OPS-I thought I read somewhere in this forum that only a few years ran it that way and other years ran it through the PCM and relay. Please correct me if I am wrong.

Definitely check the OPS

If you can catch it when it is acting up, check voltage at your lift pump. If you don't have full system voltage, it is a power to the pump problem.

Run the power directly to the pump and see if it starts working again. Include a good ground as well as power if the pump has a seperate ground wire.

TD, did all years run the LP throught the OPS-I thought I read somewhere in this forum that only a few years ran it that way and other years ran it through the PCM and relay. Please correct me if I am wrong.


If I read/interprit text correctly, all years run via OPS, just a matter of when; OBD-I it happens with rpm being X during start; in OBD-II with key on the PCM starts lift pump and co-ordinates it's cycling & continued running with glows & ECT, once truck is running OPS controls lift pump giving protection in a crash lose OPS lose fuel, which one actually carries power to lift pump in OBD-II switch or PCM I'm fuzzy on with the schematics, & descriptions in manual how it works aren't 100% clear .

Does anyone have an opinion on whether it is a good idea to bypass the OPS with a KEY-ON powered relay?

I DO; BAD IDEA

You do not want to continue pump fuel to a fire in event of a bad crash, unsafe for you, passenger, firefighters, person you ran into if you get knocked out in that crash or in a panic forget to shut it off.

OPS does not fail that often, or you can put a relay in the circuit controlled by the OPS to carry the high current to the lift pump, that will make the OPS contacts last longer and still have crash protection

Shouldn't hurt anything, just not very safe. If you wreck, you'll have a fire, as the fuel will keep pumping out, provided the batteries and wiring are intact.

Must have been typing at the same time.

TD, I'm sorry that I wasn't descriptive enough. That's what I meant by using the relay-NOT to totally bypass the OPS, just bypass the high current by using the the ops as the energizing circuit for the LP relay. My mistake. Do you know of any schematic for rewiring the OPS to isolate it from high current

Look for member Howie E he has some links of his retrofit in his signature, others as well but that is quickest one I know of

Camel413, I'm sorry for highjacking this thread. Sometimes a simple though de-rails what we are trying to do here-helping out our fellow 6.5ers

No problem, its easy to do.

As an update i just went outside started truck and opened T-drain valve and a good amount of fuel flowed out. Truck hasnt been started since sunday about midnight. Im think the collapsed line might be a possiblity. Given the time it has had to sit the pressure/vacuum had time to adjust, not sure.

Camel by all means eliminate the simple/cheap stuff 1st, but your symptoms are very familiar, and collapsed hose does not fit the picture very often, is there any way you can get an inexpensive test gage like autozone or other carries, & some long hose, one end on T drain, other to gage in the cab 9take care to not pinch off the hose trapping pressure falsely), go for a test drive if lift pump pressure does not fall below 2 psi on hard accel & maintains 4 or so while cruising it isn't a lift or collapsed hose issue

Update: i have a vacuum/fuel pressure gage, it reads up to 30 in vac and 7psi in fuel pressure. I did as td suggested. I hooked it up and put it in the cab with me.

With the wait to start on the lift pump was purring building about 3 psi pressure. At idle it runs at 2 1/2.

On normal acceleration it drops to about .5-1 and runs about 1 at 55 mph. If i get on it it will drop to 0. This is when it would stumble/stutter.

If i really laid into it and held it i could get it to read 5 vacuum.

Once back at the house (I only went for about a 3 mile test drive) i popped the hood, took out the fuel relay and jumpered across to get the lift pump to run continous. If i shut the truck off and let the lift pump build pressure it would go up to a little above 4 psi. As soon as i started the truck it slowly drop back to ZERO. For some reason it just will not suck fuel from the tank.

Ok another question. I have checked that there IS power to the lift pump with a test meter. I can jumper the relay to make the pump run continous. When jumpering the relay does it still go thru the oil pressure sensor? Can the pump hum and vibrate with a low load but not pump. I have read that some times when you test if there is voltage there will be with a test meter but not enough to run the pump. It just seems that if the pump humms/vibrates that its getting enough power.

Im still at a loss and to make matters worse my other truck, the S-10 that i had been driving to work took a dump on me today. Fan came off the water pump and destroyed the radiator and shroud. Lost the water pump pulley and fan spacer and belt some where on the highway.

Today has been a day from HELL

To test your line to the tank:

Get a 5 gallon can and fill with diesel. Get about 5 ft of fuel line and hook one end to the pump, and stick the other end down in the 5 gal can.

Set the can as high as you can in the bed, bleed the line and check fuel pressure with your gauge. Take for a short drive and watch pressure. If pressure is ok, change the line from the truck task to the lift pump and you are done.

I'd jump around the whole lift pump electric system myself wire the pump direct (JUST FOR THIS TEST), before I'd try to figure out the extra fuel tank deal. Everything stay the exact same except you put power directly to the lift pump. should take ove 15 minutes to set up. Just cause you removed your relay you might still be going thru the OPS before you get to the lift pump. Another question I didn't see the answer to was, is this pump designed to be the OEM replcement or just a one size fits all fuel pump?

It is an oem

Great, You could also just replace the OPS instead of all the testing, cost should be reasonable cheap.

Does you truck have the Diesel fuel cap, or a Gas fuel cap? The next time you have this problem, try leaving the fuel cap loose. That's the only thing I can think of that would get enough pressure to collapse a fuel line (it that IS the problem).

Yes it is a diesel cap, and i have tried to just leave it completly off thinking the same thing, but it didnt help. But i do have some good news...

Update again: I tried the "extra" fuel tank idea. Got 5 gallons of diesel and fuel line hooked it up to lift pump, and plumbed my fuel pressure gauge so i could see it going down the road. I went about 5X the distance that i normally go before it acts up, this time no problems.

Two things i wasnt thinking about tho was i never clamped my line off of the T-valve and at one point it came off going down the road, diesel fuel spraying all over the hot engine, not good.

The other thing was when i got back i let it idel for a few minutes while i was unplumbing my fuel pressure gauge and it stalled and died. I thought great now what. Went to the bed and seen how much diesel i had left in my diesel can and it was empty. That stumpted me at first. I thought how did i use roughly 5 gallons going no more than maybe 10 or 15 miles. Then i relized that the return fuel was still going to the tank on the truck.

Ok so i now beleive that the problem is the fuel line from the tank to the lift pump. Since it is a rubber line that goes to metal can i just use a diesel compatible rubber line from the sending unit to the lift pump? Just push the rubber line over the end of the metal line on the sending unit and put 2 clamps on it?

Also thanks for all the help and great replies, another happy and proud member of the diesel place!!!

Apperantly the rubber line didnt fix the problem, well it did for about 140 miles, so i thought all was good untill today. Still haveing the same problem any other suggestions except for wiring the lift pump direct to battery (for experiment only). That is the only thing that has been suggested that i havent done yet. I am beat from a hard hot day at work so i will try the lift pump thing tomorrow.

My truck had your symptoms to a t about a year and a half ago. Even drove it a 150 miles home while taking it easy on it. No fun going up hills though!! It ended up being the OPS...and i think a weak lift pump as well which I changed out a few months later. I also replaced the IP recently but that is a differant story. Truck ran great until a couple of days ago. I think my lift pump is out again.

Well i cant seem to get fuel from the tank up to the lift pump. Although it has a brand new sending unit (new strainer that is clean, even dropped the tank again to check it) new fuel line from sending unit to lift pump and new lift pump. Lift pump purrs and vibrates but wont suck the fuel out of the tank, even with fuel cap on or off. Tomorrow im gonna direct wire lift pump to a battery and see what happens.

Update: i have a vacuum/fuel pressure gage, it reads up to 30 in vac and 7psi in fuel pressure. I did as td suggested. I hooked it up and put it in the cab with me.

With the wait to start on the lift pump was purring building about 3 psi pressure. At idle it runs at 2 1/2.

On normal acceleration it drops to about .5-1 and runs about 1 at 55 mph. If I get on it it will drop to 0. This is when it would stumble/stutter.

If i really laid into it and held it i could get it to read 5 vacuum.

Once back at the house (I only went for about a 3 mile test drive) i popped the hood, took out the fuel relay and jumpered across to get the lift pump to run continous. If i shut the truck off and let the lift pump build pressure it would go up to a little above 4 psi. As soon as i started the truck it slowly drop back to ZERO. For some reason it just will not suck fuel from the tank.

Ok another question. I have checked that there IS power to the lift pump with a test meter. I can jumper the relay to make the pump run continous. When jumpering the relay does it still go thru the oil pressure sensor? Can the pump hum and vibrate with a low load but not pump. I have read that some times when you test if there is voltage there will be with a test meter but not enough to run the pump. It just seems that if the pump humms/vibrates that its getting enough power.

Im still at a loss and to make matters worse my other truck, the S-10 that i had been driving to work took a dump on me today. Fan came off the water pump and destroyed the radiator and shroud. Lost the water pump pulley and fan spacer and belt some where on the highway.

Today has been a day from HELL

My apology for not responding sooner Ole TD has had a full plate of stuff lately, Those numbers make me suspect a lift pump "fixin to" go bad healthy lift pump should be 4-6 psi idle, maintain 3+ on cruise, dip to 1 maybe on accleration for short period and then come back to cruise psi.

Replace with OEM pump, or look at Heath's h/o pump a little more than cost of stock pump but it will support performance requirements of most guys.

Remember fuel is what cools the driver on the IP so starving of fuel to IP can impact that cooling ability, plus do bad things to IP innards, as fuel also lubes the IP

TD, lift pump is new oem. I just went to work on it and I took the fuel selondoid out and put a jumper wire in so the lift pump would run continous. I let it idle for about 15 minutes, as soon as i started it i opened t valve and fuel flowed out pretty good. At the end of the 15 minutes fuel barely flowed out.

Took it for a drive and got about 1/2 mile down the road and it died. Since i had lift pump jumpered even with engine not running lift pump was, so i crawled underneith and disconnected the line from the front of the lift pump, no fuel came out. I then took my tshirt off and a short peice of fuel line and stuck it in the filler neck, wrapped shirt around it and blew my lungs out. Bled the fuel filter and got it back home the 1/2 mile.

Pulled in the drive way and it died. I again disconnected line in front of lift pump, VERY little fuel came out. I completly removed lift pump, blew on the line back to the tank untill i could hear air coming out of the filler neck. Hooked lift pump back up to the tank line but not the line going to the filter. Plugged lift pump back in and fuel shot out, i even tried to put my thumb over it to block it but it had so much pressure i couldnt contain it and it sprayed everywhere. There has to be somthing cutting/blocking the fuel but i have new sending unit, line and lift pump. The path from fuel tank to the lift pump is all brand new, i just dont get it.

Sorry for being such a pain i just cant seem to understand this problem, and i really apprecieate all the help and advise greatly.

What is your gauge telling you though the numbers you posted still say you have a lift delivery issue, I suspect your "New" lift pump is bad from data you posted. I would say PMD but even the last reply you report no fuel thru filter which is before a PMD issue could be intervening, could possibly be a bad filter as well. Where did you get the new pump, is it for sure a OEM pump, which is suppoded to be a flow on fail pump, unless your IP is weak also and can't pull it's own fuel.

TD thanks for all the help. I am "pretty" sure its oem, i know that when i first got it before i installed it i blew thru one end and couldnt blow thru the other end so i would say that its a flow thru design.

After i posted my last message i went back out and decided to drop the dang tank again (4th time in less than a month). Every thing looked ok, the strainer had a tiny amount of crud on the strainer but nothing significant. This got me thinking. I didnt have any of these problems untill i put the new sending unit on. So i got my old sending unit back out (i find it very hard to throw old parts out, even completly junk ones) and the strainers are a little different. The new one is all plastic and not really round. The old one is plastic and metal, with one end being a metal round plate.

This got me thinking, so i took the new strainer and tried to collapse it as much as i could then i sucked on it, I could barely suck air thru it. So i took the old strainer and tried the same thing, i could suck air just as easily as i could if i did nothing to it.

With this new info i installed the old strainer and went for a another test drive and so far it is fine. I guess ill try driving to work tomorrow and see how it does.

This is the second time that i have blew air thru the line back to the tank and it has "fixed" the fuel flow problem.

Could be the new filter is too fine a mesh maybe a gasser one by mistake, sender is a sender and when they boxed it maybe did not pay attn of screen/sock micron rating, or maybe you are growing algeae in your fuel and it's plugging the sock, do you run fuel treatment? one that kills "bugs"

i run the white power service and a little 2 stroke oil.