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Here’s another one for the newbie’s out there.

After frequenting the boards for awhile, you’ll notice there seems to be a few things that come around, and around, and around again. One of those things is the Lift Pump, official title: Fuel Lift Pump. So, what is a lift pump, and why does it lift fuel? The lift pump is an in-line fuel pump mounted on the frame rail under the driver’s seat. This is a small electric pump that “helps” bring fuel out of the tank and up to the injection pump. This is to help prevent the expensive injection pump from having to “suck” it’s own fuel all the way up from the tank, which it can do, but can cause damage to itself and it's driver module - more on that in another section*.

Now, that’s pretty straight forward isn’t it? So, why so much fuss about a lift pump anyway? Only because it causes so many issues* when it’s not working.

Will my truck run without the lift pump? Yes it will. How well? Well, to some it will be very noticeable when it’s not working. Some may not even notice.

If I don’t notice that my lift pump isn’t working, I’m okay right? Not exactly. That means that your expensive injection pump is doing all the work in the fuel system. This is NOT a desirable thing. Plus you are setting yourself up for problems*.

How can I tell if my lift pump is working? Do the famous “lift pump test.” How do you accomplish this highly scientific experiment you ask? Very simple. Pop the hood - you’re half way there. Find on the front top of the engine the thermostat housing. You should see a little T-handle valve standing proud there in front of you. (If you have a van, you’ll have a schrader valve down deep behind the oil fill) That T-valve is calling your name saying “turn me.” Pay attention, ‘cause there’s a hose on the end of the T-valve. That hose should have diesel fuel come out of it when the engine is idling and you open the T-valve. When you open the T-valve and the engine is idling, and no fuel comes out, you’ll hear your engine cough, sputter, and die within 30 seconds. If it doesn’t cough, sputter and die with the T-valve open and no fuel coming out, something is plugged up in your fuel system between the fuel filter cannister and the t-valve, perhaps even inside the fuel filter cannister.

If the engine does die, it has emptied the fuel cannister and run out of fuel, so you will need to close the T-valve, then troubleshoot and repair the lift pump system, as follows in the text below.

Now, if you get a continuous stream of fuel out the hose, then close the T-valve and open the plastic air-bleed valve on the top of the metal filter cap - if fuel spurts out there, then congratulations! You are the proud owner of a working lift pump. Not everyone is as blessed as you are at this moment. If you're not, keep reading.

To those not as blessed, pickup reading here. You must determine why you have no fuel supply coming to your injection pump. There are two main culprits to the demise of a lift pump. Either it’s out to lunch, or it’s in the morgue. See, one means it still might work, the other means it’s dead. How do you tell? Check to see if it has power.

First, find the lift pump under the truck. With the engine idling along, pull the plug for the lift pump. You can either use a test light, or a meter to see if you have voltage at the pins. Make sure you get a good connection, otherwise you may condemn the wrong thing. Sometimes it is difficult to get a probe to meet up with the pins inside the plug, so make double sure you’ve got it.

If you have voltage, *chances* are you have a dead lift pump. Not always, but could be a dead lift pump. You may have voltage present under a "no load" condition. This means that voltage may be present when the lift pump is not attached to the circuit, but once the lift pump is connected, the load exceeds the amount of power the circuit can provide because the OPS contacts are creating a high resistance. For more information on this see this thread on electricity and how it can affect your voltage readings with loads and no-load situations.

Now, if you don’t have voltage, you’ll have to verify upstream from there why there is no voltage present. What is upstream? The infamous OPS (Oil Pressure Switch), or fuse.

What does the OPS have to do with the lift pump? Doesn’t sound right does it to have oil pressure tied to fuel does it? Well, some think that it is a fail safe that in case your engine ever lost oil pressure, it would shut off the lift pump so that engine would stall. Guess what? If you read a little bit ahead of this paragraph, you’ll find 'dat ain’t so'. The real reason why this circuit even exists is because of an accident. In case you should be in a wreck, the last thing you would need is to have your lift pump going to town pumping out that precious, expensive, fuel all over the accident scene. After all, should you be bleeding and having a thumpin’ head - the last thing you’ll be thinking is “Hey, I just paid $2.50 a gallon for that, somebody get a shop-vac!”

Why would the OPS not allow my lift pump to work? Well, from time to time, or shall we say in the corporate world, from dime to dime, some decisions get made. Some for the good, and some for the good of the keepers of the money. The OPS has a set of contacts inside to power the lift pump. This set of contacts are not heavy enough to carry the pathetic amount of current to the lift pump. Sad, I know, but true. So, what ends up happening is your lift pump works fine, but the OPS gets smoked, and then it quits. You think the pump is bad, but it’s not.

Can I just eliminate the OPS? Well, be careful, it is a safety device in some people’s eyes, and to the rest of us, a pain in the rump. Eyes, rump, pick your part. Anyway. If you just “jump past” the OPS, then your pump will run all the time. Not just all the time, but ALL THE TIME. Christmas and Easter included. It will stop when your batteries are dead. Because, while you are grocery shopping, that pump is running. While you are down for a long winter’s nap, it’s pumping. Doesn’t matter if the key is on or off, it’s pumping.

You can make up another circuit and repower it some other way, but be careful how you do it. Some methods have kept the truck running after the ignition is shut off and keys in pocket.

Back to our little friend the lift pump.

The lift pump may fail in a variety of different ways. It may fail “open” meaning that the coil driving the pump no longer has continuity. It may fail mechanically where it is just frozen and nothing moves. It may even (not as often) fail to pump. This means, it makes noise, but doesn’t actually pump.

Does the lift pump make noise? Yes it does. The model year of your truck will determine if your lift pump is powered during the Wait To Start period. That’s the time where that light is on before actually starting your engine. '96-up OBD2 models pre-run the lift pump during WTS and during START, '94-'95 OBD1 models pre-run the lift pump only during START.

The lift pump is characterized by a kind of “purring” noise heard beneath the truck. That noise is quickly drowned out by the engine, once started. When you shut your engine off, you will hear that pump run briefly, maybe only a quick second or two. If you shut your engine off when cold, you will hear the pump run much longer, sometimes up to 30 seconds or more.

A loud clacking noise would indicate no fuel from the tank, or the lift pump is failing or failed - it will run without pumping fuel when the one-way valve(s) fail.

A faint purring or vibration when touching the lp body indicates it is running, but the internal valve-shuttle armature is stuck due to mechanical failure.

So what kind of problems will it cause if the lift pump isn’t working? Many. It will cause all sorts of fuel related issues*. It can be some of the following:
°Stumbling
°Hard Starting
°Lack of Power
°DTC (Diagnostic Trouble Codes) on the Computer
°Stalling

There are be other* issues too, but these seem to be a very (un)popular list.

Where do I get one of these? Some of our site vendors, or any of the popular auto parts stores, or the dealership. More often than not, the auto parts store is a good bet for Ineeditrightnow.

Diesel Place tip: Ask for the version for the '93 6.5TD truck - it is a direct-fit HD replacement that will supply increased fuel pressure and volume to the Inj Pump, which is a good improvement for the '94-up EFI trucks.
FYI: don't mention that you have a '94-up truck, or you will just confuse the parts guy\gal - just describe your truck and drivetrain as a '93.
The ACDelco or Delphi lift pumps are the best replacement, which you can get from O'Reilley's or NAPA , and some of the site vendors, such as Heath Diesel.
Also FYI: the parts-guy\gal may call it a fuel pump.

Are they difficult to change? No - loosen the tubing-fitting nuts on either end, remove and reinstall. Now, if it were only really that easy. Let’s get real here, no camera out-takes…. You’ll end up getting a Diesel bath. You see, depending upon how much fuel is in your tank, there will be fuel that wants to come out of the fuel line. Be ready, because your chances of a date after changing that lift pump dwindle a bunch, unless they are really into the smell of a Diesel cologne. J

After the successful fuel lift system repair you will need to refill the fuel filter cannister - after opening the air-bleed valve, which is the plastic nut on the very top of the metal cap, power up the lift pump, then close the air bleed when the bubbles stop and fuel begins spurting out - if you still get no fuel, you likely have a stopped-up fuel filter - no need to tell you what to do in that event, right?
FYI: after successful fuel flow out the air-bleed, the engine may take a while to restart and run as the Inj Pump begins to draw fuel from the filter, fills internally, then starts pump-up to 1900psi injection pressures.

This is at least enough to get you started on your lift pump journey. If you have any specific question, feel free to post them by starting another thread. We’ll be glad to help you out.
 

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Oil Pressure Sender

Here are a couple of photos as attachments of the location of the Oil Pressure Sender from a 1995 6.5 turbo Diesel Suburban.
 

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Lift pump operation

Based on your troubleshooting chart for the lift pump I have a couple questions.
1. How does the pump run when cranking when there is no oil pressure, or for example with the engine shut off and the tranny in D with the igntion key to start?
2. How does it run after the engine is shut off , with no oil pressure?

3.Is it possible for the pump to run when cranking, but not when the engine is running?

I am trying to troubleshoot a problem on my 95 burban, that maybe the lift pump.

Thanks
 
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Discussion Starter #4 (Edited)
1. Depending upon your year of your truck, the ECM maybe powering the relay to have the lift pump run during the Wait to Start period.

2. As you crank, you will develop oil pressure still, even if the engine is not running.

3. When you shut down, the oil pressure will slowly fail, it doesn't just "stop". Depending upon how cold/thick your engine oil is, will depend upon how soon your lift pump may stop running. In cold weather, if I just move the bus from one side of the driveway to the other, after shut down, the lift pump may pump for as much as 30 seconds. After it's warm, it may take a few seconds, to shutting of instantly once the engine has stopped.

4. Yes, if your OPS is bad, it could be powered during the wait to start, and then fail to work after that when it relys upon the OPS to do it's thing. this was an incorrect statement.

5. Independent study by Turbine Doc and gmctd have found that the OPS and the lift pump relay will BOTH power the lift pump on the OBD-II systems (post 96 model year). If the OPS fails, the lift pump relay will still power the lift pump will the engine is running. As long as the ECM is seeing that the engine is rotating, it will continue to power the lift pump. OBD-I systems (pre 96) did not. Lift pump relay was only on during the cranking/starting only. This is why some will prime by placing the automatic transmission in gear and turn to "start." This will actually cause the lift pump to run with the lift pump relay, but the neutral safety switch will not allow the starter to turn the engine over.
 

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Here's a couple pictures and Part number for the OPS, I don't know if it's the same part number for all the 6.5TD out there, but this is for the 6.5TD in my sig. Hope it helps.
P/N GM: 12555492 or ACD: D1808A
 

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Test gauge

To check lift pressure use a test gauge on sale often at Harbor Freight or at most auto part stores if you can't wait;
connected to the filter manager drain tee, connect a long hose to the gage & run to inside of the cab, take care to not pinch off the hose and dead head the reading. After careful routing close the hood and go for a drive.

At idle healthy lift pump should supply 5+ psi, when at cruise speed it should maintain 3-4 psi, on hard acceleration pressure may dip to 1-2 psi but should recover back to 3-4 cruise pressure, if pressure drops below 1 psi pump is weak or OPS is acting up and one or other needs replacing.
 

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Shown are jumpers on the 98 Burb & truck makes it real easy to purge the filter after a replacement, also for force run to test lift pumps 12v to connector forces the run.
 

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Found more on the OBD-I

DLC is the test point for lift pump OBD-I as per the 95 manual I was gifted by a former 6.5 owner here, I don't spend much time in it since I have (2) 98 OBD-IIs, anyway in it is test procedure to jump pin G with +12v and lift pump is supposed to run.

If pic looks familiar; it is, I did some picture edit of the one Teroma25 provided

Thanks T25
 

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To update this a little , while troubleshooting snot in stockings Snot in my Stocking GMCTD & myself did some experimenting; OBD-II lift pumps will run with PCM or OPS voltage, both are active and back each other up, you can lose 1 and lift pump still runs.

Lose both and lift pump quits the PCM looks for crank rotation or absesnce of it to energize/deenergize, so it gives the 1st voltage to lift pump for crank/glow cycle, no rpm it stops energizing the pump, and OPS works same as OPS powering of the OBD-I trucks no oil pressure/no lift pump voltage
 

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OPS Exposed

Okay I had one go bad not to long ago on the truck I saved it for show & tell, it broke on it's own at the base of it where it joins the metal portion, was more or less intact until I cut it open.

Heat over time had embrittled it, any it was cracked and just barely holding one, when I was doing some lower intake modifcation and since it was exposed as a preventative I replaced it, good thing I did it was almost toast. Another common fail is the orange diaphragm inside it they get tired from cyclic fatigue and don't allow proper pressure reading or soft contact closure of OPS and not power then for the OBD-I lift pump, or no back up power for the OBD-II pump.

GMCTD & I during the earlier linked trouble shooting surmise that dual power on OBD-II truck was a modification to attempt to keep PMDs alive longer, as low fuel flow to the IP is bad for IP itself, but also bad for PMD as there is lowered internal & return fuel flow, as fuel returns to tank it is also suppoded to be carrying away fuel driver generated heat, low flow low transfer. (this is conjecture on our part) But like in a neither proven or disproven experiment on the myth busters show, we deemed it as is plauseable

Replacement OPS was a GP Sorenson p/n OPS132
 

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OBD-1 power supply to pin "G"

Here is a picture of a ign. power source per TD's request. Above where it says DRK GRN are three terminals. The one on the right where you see the black wire, is an ign. power source. Leo
 

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knkreb

First as a new member, Im happy to have found this site. Seems like alot of knowledgable people to exchange info with. I bought my 95chevy 6.5 2 months ago, and the first thing I noted was the hesitation/miss at high throttle. More to be exact, at the 3rd to 4th gear change. I could "feather" the throttle to get past the hesitation, but that got old, and I wanted to find out why it did this. FF to the finding of this forum. I spent almost a month studing this and other forums as well as a local truck mechanic. I also had a ses light, that he fixed by removing a inline interference resistor, the code was 88, and relearning the pmd. He didnt have the time to fix the hesitation, so I, with your help, did. I ran the test that you outlined, opening the "T" valve. It took a few more seconds than the 30 you mentioned, but it did die. I changed the OPS, changed the fuel filter, purged the canister, and the engine started. I git under the truck, and the Lp 's heart was beating,. Thanks for helping.
 
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Is there a link for the OPS mod for his '95? If not, I have a link I can post where I list the procedure and parts I used when I did mine, using two relays and didn't have to cut anything on the factory wire harness. I'm not sure if I'm allowed to post a link to another 6.5 forum, though.

Please advise.
 

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Is there a link for the OPS mod for his '95? If not, I have a link I can post where I list the procedure and parts I used when I did mine, using two relays and didn't have to cut anything on the factory wire harness. I'm not sure if I'm allowed to post a link to another 6.5 forum, though.

Please advise.
Why not re post your procedure here with no link! I dont know the "rules" but I say keep it in house! Dieselplace is hands down the best 6.5 information source on the internet:clap: Not to mention the great group of helpful members:thumb:
 

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I have 12 posts in my other write up and about 15-20 photos. If it's not proper to cross post from another forum, then someone can send me a PM if they want to see it and I can send back a link.
 

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Is there a link for the OPS mod for his '95? If not, I have a link I can post where I list the procedure and parts I used when I did mine, using two relays and didn't have to cut anything on the factory wire harness. I'm not sure if I'm allowed to post a link to another 6.5 forum, though.

Please advise.
Did it all the time.
 

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From our 6.5L FAQs, DIY page: OPS Relay Mod



Links from other forums are OK if they are technical and help someone out.

:shake: :shake: on non technical links to other 6.5 forums.
I had no interest in this modification if it affected the factory wire harness and it looks like the link, above, suggests this. Here is the link to the write-up I did when I made my harness. Inside, are links to other threads for schematics and such. I put it together this way, in the thread, so that the information was all in one place.

Enjoy! Lift Pump Relay Upgrade - '94-'95 | The Truck Stop
 
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