Check Valve?

[CedarGrove]

Is there a serviceable check valve at the injection pump? Is there a check valve in the lift pump? My '95 is losing prime overnight if I park facing uphill. Does not seem to be much of a problem if I am facing downhill. Just replaced the fuel filter manager and this helped my problem considerable. Before I replaced that I would lose prime even facing downhill. It is making me crazy.

[DmaxMaverick]

Yes, it has check valves there, and the return line. If you're losing prime, you have leaks, and/or the fuel filler cap is failed. If it doesn't vent properly, fuel contraction will pull air into what may otherwise not be a leak, like QD's and weak seals and connections. This can also cause it to pull air back through the injectors. The only positive pressure in the system should be between the lift pump and injectors, and the only suction should be between the tank and lift pump. The remainder of the system should be neutral pressure, with only gravity playing on it. Leaks lets gravity move fuel where it shouldn't.

[CedarGrove]

Thank you. I suspect the line from the FFM to the injection pump. Going to take the intake off and inspect. I am going to put about 5 psi on the system at the tank fill and look for leaks from there. Any other tips or tricks to find the culprit? When I loosen the fuel tank I do hear a whoosh but it is not clear to me if it is pressure or vacuum. Is this a clue?

[Yukon6.2]

You probably don't need 5 psi.
First try leaving your fuel cap loose overnight in the position where you loose prime.
If that fix's it, Awesome problem solved.
Start with the easy stuffs, a whoosh from the cap is a definite sign to look there

[DmaxMaverick]

The Diesel fuel filler cap vents in both directions, and should not retain noticeable pressure or vacuum. If it does, it's failed, or it's a gasser cap. A bad/wrong cap won't cause leaks, but can exploit weaknesses in the system that may not leak, otherwise. Low air pressure on the system is good for finding leaks, but leaks that occur during vacuum (air intrusion) won't always leak under pressure. Any leak between the lift pump and injection pump should be evident under normal operation, being pressurized, the same as it would if you apply 5 PSI air to the tank. The lift pump should provide 4-10 PSI at engine idle. Applying low air pressure to the tank may show a new leak that is outside that part of the system, such as a return line, which should not normally leak at low/no pressure (during operation), but may if higher pressure is applied. Age, deterioration, and wear can also add to the possibilities.

[CedarGrove]

Did not see any leaking hoses with 5 psi on the system. Found an injector line that was not as snug as it should be at firewall on drivers side so snugged that up. Also drilled a small hole in gas cap to prevent pressure/vac issues. Parked uphill yesterday and it started this morning so making progress. Going to let it sit for 3 days facing uphill. If it passes that test I will call it solved.

[CedarGrove]

Still acting up and making me crazy. Left it parked up hill for 2 days and would not start today with out priming. I took the lift pump off and checked it out. Check valve there seemed to be working. Got everything back together and primed it and got it started. Once I got it fired up it sounded like two skeletons having rough sex and had a service engine soon light on. Shut it off and tried to immediately restart but no love. After a very long crank time it did fire back up but still loud and still SES light on. Drove it around the block to see if it would clear up. No love. Plugged in to get some data while SES light was on and it was loud:
Had 3 error codes: Crank position error, Injection pulse width error, and Accelerator pedal position.
Desired RPM was 600 and measured was 750.
Desired timing was 0 degrees and measured was 0 degrees. TDC offset was -1.76. Actual pump timing was 47 degrees.

Turned it off and immediately fired it back up, no problem. SES light is off and it is running normally, not noisy. Desired timing was 8.7 degrees, measured was 8.8 degrees, Actual was 31 degrees. TDC offset was 1.76. Desired idle was 600 and measured was 600.

Any clues to be found in this data?

[Yukon6.2]

My 98 3500 6.5 5 speed,tow truck always had electrical issues till i replaced the ignition switch.
Not saying that is the issue, but sometimes the issue is not where you would look.
My 2004 Topkick had a bunch of electrical issues to. The main issue was the heater didn't work.Changed the heater control with no luck.Had a lead hooked to the blower fan that i would clip to a power source for heat. When i did that other things would work while getting back feed through the harness. A new ignition switch cured it's electrical issues

[JohnC]

The trans in my '95 would go into limp mode. Dealer rebuilt the trans on warranty. Finally, a new ignition switch cured it...

[DmaxMaverick]

I think we're chasing either more than one issue, or misreading symptoms. You have a 1995 model, which is unique in a few respects. One is, you can engage the lift pump from the driver seat with only the key, and not crank/start the engine. The LP engages while the key is turned to START (crank), then switches to the OPS (Oil Pressure Switch) power when the key is let off to the RUN position. The Lift Pump, while the engine is running, is dependent ONLY on power from the OPS. Add together a weak Injection Pump and weak/absent Lift Pump operation, and you have a recipe for a no-start that may appear like an air-lock or fuel system drain-back, or all of the above. Repeated start attempts will force the Lift Pump to run during cranking sessions, so it will get fuel moving during that time. This is easy to test. Unfortunately, it will not account for an intermittent condition.

Test Lift Pump operation (this is also handy for purging after a fuel filter change or other fuel system service).

Step 1:
Park on a level (or nearly level) surface, set the E-brake and/or chock the wheels, and open the hood. Route the FFM water drain line (Tee valve hose outlet near the thermostat housing) into a clear container you can see from the driver seat, and secure it so vibration (running engine) won't allow it to fall or tip. Open the Tee valve a few turns. Turn the ign. key to RUN, but do not crank for a start. Move the shift lever to a driving position (anything but P or N). Turn the key to START/crank. The lift pump should run and fuel should flow into the container. Once verified, turn the key to OFF, return the shift lever to P, and close the Tee valve. Leave the fuel catch container in place. This only confirms the Lift Pump is able to move fuel. Simple, but very important.

Step 2:
(Truck still in the previous condition, Tee valve closed, catch container in place, key OFF, etc.)
Start the engine normally. Let it run a bit and warm, but not necessarily up to full temp. Open the FFM water drain Tee valve for several seconds. Fuel should flow into the catch container at a good rate, and the engine should not stall. Repeat the Tee valve opening every few minutes to ensure the LP is continuing to run (you can return the catch container fuel to the tank between steps as you go). If the fuel flows during engine idle and the engine does not stall, it indicates the Lift Pump is capable of supplying sufficient fuel volume to start and run the engine. It does not confirm the overall health of the fuel system, Injection Pump, or Lift Pump. Repeat at 1500, 2000, and 3000 RPMs. If during this step, fuel does not flow, flow is intermittent, or the engine stalls, the Lift Pump is not pumping when it should, or is weak or restricted (suspect the OPS first, Ign Switch next, or some other electrical problem). This may not prevent an engine start or run, but can if the conditions are right (wrong).

Step 3:
(Continue if steps 1 and 2 show successful fuel flow when it should)
Install a fuel pressure gauge (any fuel gauge, 15 PSI or more) at the FFM water drain line, position it where you can see it from the driver seat (preferably with the hood closed so it can be observed while driving), and open the Tee valve. Start the engine normally. Fuel pressure at idle should be between 4 and 10 PSI, but 12-15 PSI is OK. Less than 4 PSI is a weak, failing, or restricted Lift Pump. More than 15 PSI is a wrong Lift Pump. More fuel pressure is not necessarily better, and higher pressure (such as a gasser pump) can cause issues or damage. The Injection Pump only needs any pressure above zero (it has an internal transfer pump). If you are able to see the gauge while the hood is closed, go for a drive and watch the pressure under various conditions, including heavy acceleration. It should remain above zero under any condition. If pressure falls to zero or below at any time, suspect a weak Lift Pump, leak, restriction, or some intermittent condition (may be caused by engine torque, vibration, or some other gremlin).

The 6.2/6.5L engines can run with a failed or weak Lift Pump, if the remaining fuel system is generally healthy. It may produce less maximum power, but may not be noticeable during normal, moderate conditions. The IP's internal transfer pump is capable of drawing fuel from the fuel tank, but it is weak for that demand, and may cause premature failure, eventually.

Grounds are always suspect, any time electrical gremlins appear. The OPS is always suspect, any time you have fuel delivery issues. If you use anything but an OEM/ACDelco fuel Lift Pump, all bets are off. There are some good aftermarket fuel pumps, but quality is less consistent.

[CedarGrove]

Thank you for the response! The situation continues to develop. I pulled the intake and determined that the lines and FFM are all good. I am quite certain that there are no leaks from the tank to the IP. However I am still getting fuel in the valley so I am also quite sure that the fuel is coming from the weep hole at the bottom of the IP. If I run my lift pump for 20 seconds (ebrake on, shifter in D, key to start position) before attempting to cold start the engine it will start. If I do not do that I will have to crack an injector and grind on it for 3 sets of about 10 or 15 seconds each. If I shut it off before it has warmed up good I will probably have to run the lift pump again to start. It loses prime immediately. Once it is warm I am good for the day. Pretty sure this pump is ready to be replaced. Once we are up and running she runs great. I have fuel pressure gauge on the A pillar. I have 4 or 5 at idle and 1 under heavy throttle and never zero, which is how it has been for the last year or 2 since I installed the gauge. Probably could use a new pump but that is not the biggest problem at the moment in my opinion. Any other lines to pursue before replacing the DS4? Is there a preferred source of rebuilt pumps? Thank you again for the very good response!

[a5150nut]

Years ago I talked to John Kennedy about ordering a fuel gauge. Those pressures you are seeing are what he told me to expect from a 6.5

[DmaxMaverick]

If you're leaking fuel into the valley under pressure, it certainly could be the IP weep hole (inner input shaft seal). It can allow the return circuit to pull fuel through the pump and back to the tank, losing prime. If the pump is healthy otherwise, the seals can be replaced w/o a complete overhaul. Anyone who services Stanadyne IP's can do it. There may be a DIY hack, but it's been many years since a discussion (maybe try searching the forum for it).

Your pressure numbers look perfectly fine, and no indication you need a new lift pump.