Chevy/GMC 6.2L and 6.5LDiscussion of Chevy and GMC Trucks with 6.2L and 6.5L Diesel Engines
i have a 1995 chevy 6.5. It was always a good running truck. Started having problems with it so fuel system was redone. And ran fine for a while. Then it go tto where it wouldnt start. the wait light would just flashe real fast ... JOIN NOW TO REMOVE TRACER
i have a 1995 chevy 6.5. It was always a good running truck. Started having problems with it so fuel system was redone. And ran fine for a while. Then it go tto where it wouldnt start. the wait light would just flashe real fast then off. Didnt matter if hot or cold out wouldnt start. So i replaced the glow plugs. Same thing sometimes would start sometimes wouldnt. Then it would start but sometimes it would shut off at random and wouldnt start again. Soi got a glow plug relay and it started right up and ran fine for a day and a half then wont start again. Someone said to clean the grounds so i did and didnt help. Someone else said to jump the relay for 90 seconds and it should start didnt help. When i turn the key the relay just clicks and thats it. I tested the plug going to the relay with the key on and have power to the passenger side wire and ground to the drivers side and nothing to the middle wire.I figure the middle must be some kind os switched or something. I dont want to go buy another relay if there is something that will keep making them go bad or if the relay didnt fixe it and there is something that runs the relay making it do this. If someone could help it would be great thanks
Hmm.. I belive you might have software issues or ECM issues but there could be somthing more simple then that. For better information you might want to contact
They might direct you to somebody near you if it needs to go to a shop. they also write custome tunes for your truck
I replaced both batterys and the glow plug relay and the glow plug and about a year ago i had the fuel pump replaced. Someone said about the PMD (Pump Mounted Driver). I guess it makes since because i dont get any smoke or anything like it isnt getting fuel. I dont know what that would have to do with the glow plug relay. But i might have a bad PMD (Pump Mounted Driver). I just replace the glow plug relay and it ran for a day and a half. But i jumped the relay it should have start i think but no smoke or anything
i know you said redid fuel system one ? come mine did you put acdelco OPS in if not it could have gone bad and did you check see if you are getting fuel out T-Valve with key on and in drive the only reason asking is that have seen new OPS go bad just like that,that way you to acdelco only
maybe PMD did go bad on you i never seen one just go out with out some kind warning most time you get stalling
Sorry this is all new to me. I can redo a gas motor in a day but this is my first diesel So i have no ideal what acdelco OPS are. I had a injector pump put on it ididnt do that i had a shop do it. I didnt and know about checking to see if i m getting fuel out of the T-valve. I will have to look it up to see where it is and how to do it. Thanks for the help
EGR= Exhaust gas regulation valve on emission engines center rear of inlet
L56= "emissions engine" usually at 1/2 T truck or some LD frame 3/4Ts (S engine 8th VIN digit code)
L65= non emission HD truck engine (F engine 8th VIN digit code)
LD/HD= light or heavy duty frame, HD frame and suspension a little "beefier" weigh more, heavier tow/load capacity. Check RPO codes HD is usually L65 code
RPO= regular production options ??? list inbyour glove box of options your truck was delivered with
CAT= catalytic converter slang generally L56 trucks have this
WG= waste gate, flapper in turbos outlet that aids in spool up of turbo to reduce lag time until turbo camn make efficient boost
PCM= powertrain control module brain/computer that runs it all.
OPS= oil pressure switch
WIF= water in fuel
SES= service engine soon/check engine in GM manual this is MIL malfunction indicator lite
APP= accellerator pedal position, electronic module on go pedal that talks to the PCM for more or less fuel from the IP
OBD I or II= on board diagnostic generation I or II, in 1996 Congress mandated all US mfg use common diagnostic port and naming conventions OBDII trucks have electronic IP and require scan tool to read codes and perform diagnostic tests.
MAP= manifold absolute pressure mounted on firewall in OBD II L56 trucks provides EGR feedback and baro conditions to PCM, boost sensor mounted on both L56 & 65 upper intakes sends boost pressure signal to PCM,
In L65 OBD II it also serves as baro sensor at start up, OBD I L65 uses a firewall mounted baro, both look the same but are keyed to only accept correct harness connection.
MAF= mass air flow flow sensor used in L56 to tell PCM how much flow in turbo inlet used to help calculate proper EGR flow
VSS= vehicle speed sensor
IAT= inlet air temp gives input to PCM how hot post turbo air is
ECT= engine coolant temp
IC= intercooler or aftercooler air to air heat exchanger that drops post turbo compressed air temp
WMI= water mist injection another way to cool turbo air using fine water mist
DTC= diagnostic trouble code
Part two come
Reference Material: Lift Pump/OPS
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.
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.
Part 3 to come
Last edited by Crazy; 12-23-2010 at 07:24 PM.
Reason: Automerged Doublepost
The Following User Says Thank You to Crazy For This Useful Post:
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:
°Lack of Power
°DTC (Diagnostic Trouble Codes) on the Computer
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.
My lift pump is workinh fine. I m going to have to print the last reply out just for all the info. I already had the lift pump and the IP (Injection Pump). And the the glow plugs. And it ran great for like a month. then it started not to start again. The wait light on the dash would just flash real fast when you turned the key on. But sometimes you would go to start it and the light would stay on for 90 seconds or so and it would start right up. Then next once you got it to start it would run fine for a while then just shut off and not start again sometimes not for days then it would start and do the same over and over. So then i replaced the Glow Plug relay. And it started right up and ran great for a day and a half then back to not starting. And the wait light is back to just flashin for a split second when you turn the key. So i dont know if when i put the GPR on it was just one of those times it desided to start and run or if it fixed it and went again. Then someone said about the Pump Mounted Driver. so i researched it and found out that it is a common problem for the chevy 6.5. So i dont know if they are relaed or not or if i have 2 problems.
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