• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

Lift Pump

rockfoot

rockfoot

New Member
Messages
21
Reaction score
1
Location
Moncks Corner, SC
In the schematic for a 1995 6.5 Sierra K1500, there is a wire coming from the Fuel Pump Relay to the "Fuel Pump Prime Connector". I am assuming that this wire is the wire that energizes the LP when the switch is placed in "Run". Where is the "Fuel Pump Prime Connector"? The problem is: the pump used to come on when I turned on the ignition... I realize that there is an OPS that maintains power to the LP while there is Oil Pressure present. But what gives the power (1995) to the LP prior to the oil pressure being present. (I am thinking it is the "Fuel Pump Prime Connector"). Is it the "Wait to Start" circuit? My fuel pump works when I jump out the relay. The relay works in the "Start" position. The relay is good. The truck is very hard to start and I really believe this is my problem. No problems once it is started. Thanks in advance, R. Barrett
 
For all you '95 6.5 Sierra owners out there... this is what I have found so far. The lead goint to the "Fuel Pump Prime Connector" is goint to the Data Link Connetor under the dash (F). The pump does not run until the switch is in the "crank" position. Then the OPS (Oil Pressure Sensing) switch gives the relay power and the pump continues to run. Trick: place the transmission in any position except "park" or "neutral" and turn the ignition switch to "start" and hold it there... the pump will run and prime the fuel bowl or pump the tank dry... or whatever.
 
Yeah... I read all that stuff in the technical reference section. Then I posted my question that was not addressed in the technical reference section and continued to research on line and found the info that I needed for a 1995. if you had the answer you could have saved me a lot of time and trouble by responding to my queary.
 
Sorry your true question was buried in the questions about the lift pump.

So you have a "hard to start" issue.

Note: The lift pump, raptor, lots of money spent on trying to prime the fuel system with a lift pump isn't going to help you start the engine. Diesels either have fuel in the IP ready to go or they are a bitch to start getting the air out. You can't run a lift pump and push the air out like a gas engine can because the fuel won't get past the rotary vane pump in the IP.

Step 1
Do you have 100 RPM while cranking cold and 150 RPM hot? These are minimum numbers.
No: Check connections, test batteries separately, starter replacement, cable replacement. Note a bad starter can still crank the engine, but, not fast enough to start.

Step2
Check the glow plug system. Even in 100+ degree weather the glow plugs help get it started.
Do you have voltage at each plug with the WTS light on? May take several key cycles with help to verify. Second each plug needs to have around 1 ohm resistance - higher (double digit) or open resistance is a bad glow plug. (Of course tested with the spade connector off the glow plug.)

Step 3
Get the air out! (This is actually step 1 in the GM manual.)
Air leaks will cause starting problems because air compresses and no fuel gets delivered.
Put a 1/4" clear return line on IP and watch for air bubbles. Primed the system and you are NOT ready to go the next morning if you have an air leak. Any air leak allows the fuel to drain back into the tank overnight. Then you grind the starter getting the air back out of the system. So check the return line when first cranking the engine to see if a bunch of air comes out.

Fuel quality: (Does it smell like gasoline?)

Injector age? They are good for 100K. China nozzles are good for 30K. They can allow air back in the injector line and cause extreme hard starts if they are bad and sticking open.
 
Step 3
Get the air out! (This is actually step 1 in the GM manual.)
Air leaks will cause starting problems because air compresses and no fuel gets delivered.
Put a 1/4" clear return line on IP and watch for air bubbles. Primed the system and you are NOT ready to go the next morning if you have an air leak. Any air leak allows the fuel to drain back into the tank overnight. Then you grind the starter getting the air back out of the system. So check the return line when first cranking the engine to see if a bunch of air comes out.

Thanks. I think the above (step 3) is spot on! I found the hose that goes up to the bleed valve, near the bottom was wet... upon inspection, it was bleeding fuel... the hose was old, dry rotted or whatever.... I replaced it and hopefully have fixed the problem. I was a little confused at first because my '97 suburban lift pump starts every time the ignition is turned on.... the '95 ..doesn't. Thanks again, I do appreciate the help. R. Barrett
 
Some confusion over that. The lift pump on ignition I guess is year dependent as 1992, 1993, and 1995 for sure don't turn the lift pump on during glow plugs or ignition on. OPS turns on or during crank only. Some have wired the lift pump to come on with the ignition as the OPS dies often. Other newer years are 100% ECM control and rumor is the ECM turns the lift pump on during WTS and maybe key on for a few seconds...

Check other fuel hose condition as well and be careful of kinks. The Fuel Filter manager also is known to leak at o rings, heater, and WIF sensor including through the wires. The Pickup assy in the fuel tank also gets pinhole air leaks.
 
The Fuel Filter Manager is going to be next... I've already decided that. I see there is an excellent explanation and photos in the Technical Reference Library. That is going to be a very handy reference. I am not sure about mine. When I went to get a filter for it, I could not find one to fit. I finally found one and it is a Wix. Is this normal? It's not like the one on my '97 Suburban.... I will have to take pictures.
 
Two filters out now using the same FFM:
Obsolete that require re-use of a ring and specific rotation of the filter to line up the slots.
New style that isn't rotation specific on install and has the entire top cover plastic included as well as a separate gasket.
 
The new style WIX is having issue with the bleeder valve.
Basically, it does not have a hole to the bleeder valve from the filter.
Not sure if they have fixed them.

I sent WIX a message that it is not just a decoration and it should have a hole to the valve.
They may not like it, though.

As for the LP, the LP is never energize even when crank at all.
That is why there is a mod to add relay in the LP/OPS circuit and you can make it so it energize during WTS.

IIRC, Leroy may have sell the harness that can do that also.

This is a weak point in the 94-95 model year.

If you are fixing the hoses, I suggest that you fix all the hoses that goes under the intake.
If 1 is bad, chances are the one goes to the IP is also bad and it is not visible.
 
It's best to avoid Wix fuel filters. Their quality and failures that make you walk with Duramax fuel filters are well known.
 
Okay... More symptoms.. thought I had found the problem a couple of times but it is still being hard to start.
>#1. Starts like a gas engine in the morning when cold... very prompt response to the starter.. starts right up!
>#2. After it warms up to right at 205*, it is hard to start. The weather here is very hot...90's+, this symptom came on slowly... doing it every now and then and now it is routine.
>#3. I changed out the PMD. The connectors were very wet and corroded. I thought "aaha, got it now!"... not so. Still hard to start after warming up.
>#4. Took out thermostat for test purposes... engine ran at 180-185*. Still hard to start after warming up.
>#5. Went back after all said and done and realized that the little "resistor" thingy is missing from PMD. Old PMD did not have one and it just came to me that the resistor is supposed to be there. Next step is to get one and install.
Does any of this help? I fully intend to do a removal of the manifold and check and replace all associated fuel lines. ]
R. Barrett
 
With a hot start problem that starts easy cold, get it hot then pour cold water on the ip too see if that gets it to start easy.
 
Will, ak diesel driver.... I dumped cold water on the IP... .while it was hot and would not start. It started. Now.... is that water supposed to cool off the PMD or the IP?
 
Supposed to cool the IP where the injector lines to the injectors come out. The Head and rotor get worn and with hot thin fuel just won't give you enough fuel/pressure to start. Foot to floor may help. Glow plug override and hot glow plugs do help start with a worn head/rotor.

Nothing to do with the PMD or electronics.

Note good IP's are hard to find that have the head and rotor replaced and are NOT on ebay.
 
You must log in or register to reply here.
Back
Top