Lift Pump Problem - OPS or relay?

[ShawnR]

I think I have narrowed down my hard starting / smoking problem.

Truck will start, but starts hard and smokes (white / blue) a lot for 2 - 3 minutes. All GP's are new and I have confirmed voltage to all plugs. The truck will finally start, run for a few seconds, then die. you can restart, it will run longer (8 seconds) then die again. After it is running, there is still a steady haze of bluish/white smoke from the exhaust for a couple of minutes.

Thoughts were a stuck injector (all fairly new) or air in the fuel lines. How does air in the fuel lines cause white smoke? How can it get in the lines if I don't have a leak somewhere? So I started snooping.

Earlier, I had tested my lift pump by hotwiring it to the battery, and of course, it pumped fuel out just fine. I tested today while the truck was running and nothing, no flow at all. If I jumper a hotwire to the fuse, fuel starts flowing.

Is there a way to test to see if it is the OPS or the relay? I have already replaced both since I have owned the truck (4 years). The relay is a LOT easier to replace than the OPS, so I may just do it first.

Thoughts?

 

[Coghlin]

OPS is cheap. I'd just replace it. On the newer trucks the lift pump relay is activated by the computer on startup, not sure on yours though.

 

[ak diesel driver]

This is why I did an opsu relay setup opsu won't handle amps long term.

 

[mattthebrat]

If you can get to the relay, just test for power coming in with the truck running, if you have power, bad relay or wiring; no power, bad OPS.

 

[JMJNet]

It is probably the OPS.

Make sure you changed it with ACDelco.

4 years is already good life for OPS.

 

[ShawnR]

I checked for voltage to the relay and these 2 contacts had power.

I opened up the T- valve and fuel came squirting out while it was running. Huh? It didn't do that earlier in the day.

So anyway, I have stuck a pressure gauge on it (reads 8psi at idle) and I am going to let it sit until morning and see what happens. Should it hold the 8 psi of pressure in the line overnight?

I am also hoping to find out if the LP comes on with the glow plugs. I am fairly confident that it doesn't since I can never hear it. I will probably just replace the OPS with a new one to eliminate that as a variable. It is just a pain to change now since I could have done it with the engine out just a few weeks ago.

 

[buddy]

I have heard the LP in 93 is supposed to come on during GP but no one was willing to post a schematic that showed that.

The pins on the relay that you circled are the voltage from battery + (orange wire), which goes to relay contact A, and the other one, contact E goes to the lift pump (grey wire) So you should get 12V across them with the truck off (LP acts as ground). But when the truck is running and the OPS is functioning then you will get 12V on contact E because the OPS power output and the relay output to the LP are just spliced together. During cranking you might get current being fed by both, but when running and the IGN is in the ON position instead of START the contact in the relay moves from A to C, which in my 94 sends the OPS output voltage to pin G on the OBD connector under the dash, and you can monitor it there to make sure the OPS is providing 12-14V to the LP. I dont know if they sent LP relay contact C (red wire) to the connector in 1993 though because I havent seen the manual or schematic.

 

[ShawnR]

I just went out and checked. PSI was back down to 0, no big shock there since I didn't have hose clamps on the gauge.

I turn on the key, GP light comes on, no lift pump.

I start cranking after GP light is out, no lift pump, fuel pressure pulling slight vacuum.

Keep cranking (5-6 seconds) until oil pressure comes up, LP kicks on & truck starts.

It started MUCH better that it has in the past, dunno why. I am going to leave the fuel pressure gauge on (taped to the windshield) fo a while to troubleshoot. Since a relay is only $11, I am thinking of picking one up just for giggles.

 

[Turbine Doc]

This is real safe & simple fix, let the OPS control (OPS will last longer, though it is still a fail item as mylar diphragm over time can fail also) the control coil side of a high current relay, then wire the lift direct to the high current side of the relay. For priming run a spring loaded momentary switch also to the control coil leg so when you push the button the pump runs, mount it beside your fuel filter so you can let go and stop pump when air stops and you get solid fuel. OBD-II guys have a ready made test port leg under hood just below the master cylinder (pics in the OPS FAQ) that can serve same function for testing lift/venting filter housing if you want to install a switch there.

 

[daustin]

I have heard the LP in 93 is supposed to come on during GP but no one was willing to post a schematic that showed that.

I agree with Doc, let the OPS run it like it's supposed to. Sorry about not posting the schematic/diagnostic page sooner for the '93. It's not that i'm "unwilling to post" the schematic, i loaned my FSM to my brother-inlaw to fix his truck and have not gotten it back. I will get it back this weekend and scan/post the desired info. (It's fixed now) On my '93 when i turn the key on (like first thing in the am when it's a little cool outside) the lift pump purrs as long as the wait to start light is on. When the WTS goes out, it stops. When the engine fires off and runs, it resumes it's operation via the OPS. Mine didn't work when i got my truck, so i had to chase that issue. Turned out the OPS was bad and the CTS didn't work so it never ran so my ruck ran like cr*p. :rolleyes5: I replaced both and it's been all good ever since. (Easy fix for less than $60 for both, got those at my favorite Pontiac/Hummer dealer as the hummer H1 previously had the 6.5 in it so they carry some 6.5 parts)
Don

 

[JMJNet]

I just went out and checked. PSI was back down to 0, no big shock there since I didn't have hose clamps on the gauge.

I turn on the key, GP light comes on, no lift pump.

I start cranking after GP light is out, no lift pump, fuel pressure pulling slight vacuum.

Keep cranking (5-6 seconds) until oil pressure comes up, LP kicks on & truck starts.

It started MUCH better that it has in the past, dunno why. I am going to leave the fuel pressure gauge on (taped to the windshield) fo a while to troubleshoot. Since a relay is only $11, I am thinking of picking one up just for giggles.

This looks like the relay works, LP run during WTS.

OPS is bad, LP does not run after WTS.

Remember, ACDelco OPS!!!!

 

[Chevylover]

I agree with Doc, let the OPS run it like it's supposed to. Sorry about not posting the schematic/diagnostic page sooner for the '93. It's not that i'm "unwilling to post" the schematic, i loaned my FSM to my brother-inlaw to fix his truck and have not gotten it back. I will get it back this weekend and scan/post the desired info. (It's fixed now) On my '93 when i turn the key on (like first thing in the am when it's a little cool outside) the lift pump purrs as long as the wait to start light is on. When the WTS goes out, it stops. When the engine fires off and runs, it resumes it's operation via the OPS. Mine didn't work when i got my truck, so i had to chase that issue. Turned out the OPS was bad and the CTS didn't work so it never ran so my ruck ran like cr*p. :rolleyes5: I replaced both and it's been all good ever since. (Easy fix for less than $60 for both, got those at my favorite Pontiac/Hummer dealer as the hummer H1 previously had the 6.5 in it so they carry some 6.5 parts)
Don

So does mine on my 1995, too !

Cu,
Sven

 

[buddy]

This looks like the relay works, LP run during WTS.

OPS is bad, LP does not run after WTS.

Remember, ACDelco OPS!!!!

I think you meant the opposite.

His relay isnt working during GP/WTS or cranking, but OPS is intermittent maybe so the LP is running when engine running.

 

[ShawnR]

I think you meant the opposite.

His relay isnt working during GP/WTS or cranking, but OPS is intermittent maybe so the LP is running when engine running.

I put in a new relay and the LP still doesn't kick on with the glow plugs. So whatever is supposed to turn the relay on with the glow plugs isn't working.

Anybody know where to start checking? Somewhere between the GP controller and the relay?

I am going to replace the OPS too (with an AC Delco) just to eliminate it as a variable. The LP wasn't working when I first tested it, but was the other times. Just a feeling that the OPS might be dying.

 

[tracy]

Shawn I'm in the same situation, not getting power from I assume glow plug controller to the fuel pump relay. I'm tempted to run jumper wire from the output wire (goes to the glow plugs) on the glow plug controller to the solinoid on the fuel pump relay. I better wait till Don can post the schematics. The funny thing is the truck has been in my family since new and no one can remember the lift pump firing up with the glow plugs.

 

[ShawnR]

I don't ever remember it running with the glow plugs either, but mine doesn't even kick on when I am cranking until the oil pressure comes up.

Tomorrow morning I am going to hotwire the LP on before I even try to start. I want to find out if it starts any better (or smokes less) if there is fuel pressure right off of the bat. Something ain't right.

 

[JMJNet]

It is not suppose to run with GP. There is no connection between GP and LP.

Try to jump it and see if the LP runs. Open the t-valve to see if it has fuel. If not, LP is bad. Also measure the voltage at from the OPS to the LP at the connector down there with ign. on. See if there is voltage. If not, OPS is bad.

The best solution is to add a relay to override the OPS portion that switch the LP. You can search or it is probably in the Technical Information on the sticky.

 

[buddy]

It is not suppose to run with GP. There is no connection between GP and LP.

The best solution is to add a relay to override the OPS portion that switch the LP. You can search or it is probably in the Technical Information on the sticky.

That is the stated difference between 93 and OBDI 94-95. 93 they run with the WTS/GP, but no one has stated how it accomplishes this, where the switching comes from and power is provided. Should be the much anticipated schematic this weekend though that explains it.

The extra relay to add is not really an override, but just triggered/switched by the OPS, so you have the amps/current needed to operate the LP going through a real relay, to take the load off of the OPS.

 

[Classified]

Hopefully this may help?

This is for my a 93

 

[buddy]

Thanks Doug, is there more to that wire diagram? that shows where the purple/white wire goes to, because from that diagram it doesnt show how the LP gets power during WTS/GP.

Although it does show that you can monitor OPS operation in the cab via the diagnostic port, circuit 490 via the red wire. While driving you can put a voltmeter on that contact to ground and see that the OPS is allowing voltage to the LP.