Lift Pumps & OPS operation and troubleshooting

[Turbine Doc]

Okay fuel delivery basics for the 6.5

On the 6.5 you have a fairly simple fuel delivery system.

Starts at the tank, a suction strainer, to lift pump, to filter manager, to IP shut off solenoid, to IP, flow is controlled to IP via PCM demand to PMD/FSD.

Depending on vintage power to the lift is either from OPS (OBD-I/92-95) or OPS/PCM (OBD-II 96+ years)

Healthy IP is capable of pulling it's own fuel past a dead or weak lift provided it is GM flow on fail type lift pump, performance can suffer at higher rpm "fish biting" is common sign of dead lift or bad ops in OBD-I truck, "f-b" same feel one gets when fish 1st hits your line, jerking/tugging you feel in truck sort of similar to "miss" in gasser truck. In OBD-IIs lift power is from 2 places PCM & OPS you have to lose both for total no power to lift.

Lift is supposed to supply 15 gph, a check is capture & measure output from filter mgr drain 1/2 pt in 15 sec should be good lift delivery at idle, yet I've seen "healthy" pumps purr perfectly, and pump meeting idle demand still be bad, so my preference is to load test the lift.

At the filter manager drain snake into your cab on long hose a test gauge, make positive to not pinch off the hose and dead/static head the reading, at idle with good stock pump 5-6 psi is normal, go for drive, at hiway cruise speed 70 mph 3-4 psi is strong lift pump, 2-3 psi is good lift pump but has some weakness, if lift will not sustain 1.5 psi or better, get a new one.

Then do an acceleration test on hard acceleration it is preferable to maintain > 1 psi, it may dip below that but once at desired settled out speed, lift needs to recover quickly to 1.5 psi or better, if it remains less than 1.5 or takes more than about 5 second to recover to cruise psi observed before accelerating, either lift is bad or tank sock is dirty.

Pic of simple vac/press gauge about $20 at part store or less on sale from Harbor Freight a must have piece of 6.5 test gear :

FYI post will close periodically until I finish with it

 

[Turbine Doc]

Tank sock is 70 micron strainer, has bypass check valve in end of it, at bottom of fuel sender, in emergency and you suspect blockage from your road test results remove the lift pump inlet, fuel cap and blow back with 3-5 psi air, (lung pressure if dead on side of road-don't inhale with lips still on tube), and blow back, that should unplug you long enough so you can get back home, some pics of tank sock:

 

[Turbine Doc]

Lift pump located on frame rail drivers side, looks like gasser style fuel filter, but is actually a pump, the black wire is gnd, lift pump has it's own gnd on top of frame on drivers side just below fill neck on extended cab trucks not sure where it is on burbs and dual tank truck but it has an independentg gnd you just need to trace from pump to end to find it.

 

[Turbine Doc]

OPS sends power to lift has dual function, sends info to dash and carries power to pump solo on OBD-I, and backup on OBD-II. Contacts are weak not really capable over long haul to power up lift, relay back up (another post to come later) is best way for long term powering lift.

OPS is located on back of block at bellhousing joint, or under the intake on later models, will fail mechanically also as seen in 1st pic I just touched it and it broke, IMO a 50K mile item only about $30 when I bought this one Replacement OPS was a GP Sorenson p/n OPS132

 

[chrisk1500]

You can delete my post - but please also mention that in OBD I trucks, the LP is powered up when cranking as well....

 

[Bullseye240]

Thanks Tim this will be great info for any new guys and most of us "old" guys and gals here. This is one of the many reasons why I love this site.

 

[bralos]

I noticed on my truck that when I turn off the truck the lift pump will run for another 10-15 seconds? Normal or advanced warrning?

Thanks

 

[packratt]

Normal runs until oil pressure drops.

 

[Turbine Doc]

You can delete my post - but please also mention that in OBD I trucks, the LP is powered up when cranking as well....

Okay power portion, Lift pump gets power differently depending on vintage, OBD-I's get constant power during crank as mentioned above, but if PCM does not see oil pressure at OPS power, it shuts off, this is why OBD-I trucks hear lift run until oil pressure bleeds off OPS contacts.

OBD-IIs get power from 2 sources as commanded by PCM at key on (pump runs briefly with key on, PCM is looking for rotation signal from CPS and will shut down without it), and backup from OPS like OBD-I, (I have not seen confirmation but this feature came in 96 I think to help combat failed PMDs which are passively cooled by fuel flow in IP, dead OPD & delivery flow to IP suffers and IP temps go up which would mean hot driver & early PMD death); back to the lift pump power for OBD-II truck you have to lose both the PCM & OPS to lose power to lift.

Now also common to both OBD-I & II is the gnd and the pump relay located in the underhood fuse center, issues to these cause power problems

To test pump for running with engine shut down/also expedient way to prime a fuel filter manager after filter change turn ignition to run, then on OBD-I pin G to +12v will make pump run, on OBD-II +12v to underhood test jumper will make lift run indefinately until power is removed, also a good way to empty last of the fuel from fuel tank, (not recommended for fully emptying tank, remember only 15 gph)

 

[Turbine Doc]

More pump dissection photos max is 10 pics per post

 

[Turbine Doc]

Fuel filter manager, 5 micron nominal rating, a manager because in addition filtering solids above 5 micron, it has heating element in center, and a water in fuel sensor that turns on WIF light, at which point you should open the filter manager drain valve and hopefully can get rid of excess water.

Filter element is treated with water block but does have a saturation point, also often forgotten is "last ditch" screen which sometimes becomes lost inside the old filter, check to see if yours is still there when replacing it. Stanadyne part not GM Fuel manager filter screen # 29244, any Stanadyne dealer should be able to order it for you.

 

[Turbine Doc]

Filter manager exposed, some pics borrowed from my buddy Canadian Rigger

 

[Turbine Doc]

IP parts location breakdown

 

[chrisk1500]

Now also common to both OBD-I & II is the gnd and the pump relay located in the underhood fuse center, issues to these cause power problems

Except for 94 vintage. In that case, the pump relay is under the black plastic cover on the passenger side of the firewall. That is also where the LP fuse is located IIRC....

 

[fastjohnny]

Great info, can you post how you did your racor vac switches and leds?

 

[Turbine Doc]

Great info, can you post how you did your racor vac switches and leds?

Yes next installment, when I get a breather takes time to do post worthy of making into reference resource

 

[ghost183]

Yes next installment, when I get a breather takes time to do post worthy of making into reference resource

thanks doc..

that pic of the LP run jumper is great, wish me luck with my WIF issue..

 

[Matt Bachand]

I was giving my filter mgr a thourough cleaning yesterday. I have 2 questions.

1. My last-ditch-screen not only doesn't accidently get stuck in filters, but doesn't want to come out by hand. Maybe thats a good thing? I didn't get at it with tool, just left it alone.

2. Where inside the filter manager does the dirty fuel enter?

 

[Turbine Doc]

I was giving my filter mgr a thourough cleaning yesterday. I have 2 questions.

1. My last-ditch-screen not only doesn't accidently get stuck in filters, but doesn't want to come out by hand. Maybe thats a good thing? I didn't get at it with tool, just left it alone.

2. Where inside the filter manager does the dirty fuel enter?

Good thing, and here is inside a filter mgr

 

[jrsavoie]

Except for 94 vintage. In that case, the pump relay is under the black plastic cover on the passenger side of the firewall. That is also where the LP fuse is located IIRC....

You wouldn't happen to have a pic of that would you?
If the fuse would be blown on a 94 would jumping the DLC still run the lift pump?