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Lift Pump Problem

Thanks for the schematic.. here's my problem. According to that diagram, if the OPS went out, it would have no effect on the operation of the LP.

Why then, does a dead OPS kill these things?

Another question - from above - When I say 'went out', I mean either burned contacts - the usual problem - or if the oil pressure dropped low enough to disengage the switch ... if the OPS has no impact in the OBD-II trucks, as a safety mechanism it is useless.

Strange.

Thanks guys.
 
Boys boys stop bickering. When my buddy came over we were able to check power going to the LP and it was determined the ground was bad so we just ran a jumper directly back to the neg post of battery for now. Pump works fine now so the debate continues.
I'm still having a problem with the truck out of the blue sputtering and losing power. This morning on way to work which is 15 mins away the truck started to sputter ]nd lost power almost stalling. Do u think its the new PMD ? I t was late last night and I couldn't get the plug back on to the old one to check. Any tricks at getting in ther ? Thanks Stev. Sorry I'm typing on blackberry
 
Logic did prevail, good find on the broken ground.

burned out OPS wont stop an OBDII truck's LP, unless the relay is also bad. Thats why its important to understand how and when the LP is supposed to be powered. I think people jump to the OPS conclusion so often it is always the first thing recommended and other things are fixed in the process. They so often fail and are the problem in OBDI trucks that it just has a bad reputation.

The LP would not stay powered in an accident if engine died (oil pressure drop), because the PCM only powers the LP in response to a WTS (intial ignition power) or in operation (PCM knows if engine is running). So OPS is not needed as safety item.
 
I'm getting the feeling that 97dually's problem is a clogged tank sock or other restriction, so the lift pump is struggling to provide fuel.
 
guybb3 said:
I'm getting the feeling that 97dually's problem is a clogged tank sock or other restriction, so the lift pump is struggling to provide fuel.
Click to expand...

Definately a possibility. A fuel pump gauge and some fuel hose strapped to the mirror would help diagnose that.

We are assuming there is no SES light or DTCs.
 
Buddy said:
burned out OPS wont stop an OBDII truck's LP, unless the relay is also bad. Thats why its important to understand how and when the LP is supposed to be powered. I think people jump to the OPS conclusion so often it is always the first thing recommended and other things are fixed in the process. They so often fail and are the problem in OBDI trucks that it just has a bad reputation.
Click to expand...

Almost right... jumping to OPS as an issue is only one step in the chain of troubleshooting that starts with fuel. If there is no fuel at the injectors, or at the IP, or at the FM drain/bleed, then is the LP running? If yes, change LP or find blockage. If no, check electrical supply to LP - and OPS is part of that.

In this case, Steve supplied most of the necessary info in his first post - power was an immediate suspect, as was prior replacement with aftermarket OPS . Ground is always a suspect on these damn things. First task: get the LP running and get fuel to the IP.

Looks like that was accomplished, so the next step is to monitor fuel flow as you and guybb3 suggest... there may well be other issues to find and correct.

-- Questions I still have - for another thread, on another day - According to the diagram you supplied and the rationale given by you and TD, there should never be a need to change the OPS on an OBD-II truck, something I still struggle with. Why is the darn thing even there, if the PCM does all the work? redundancy dosn't seem important, as if the PCM dies, so does the truck (even with a functioning OPS). :( I hate being confused...

Glad your truck is up and moving, at least, Steve! Keep us posted!
 
Well, Ive seen plenty of posts that jump to the OPS conclusion, or at least suggest it as the failure mode. This thread is a good example where make sure power is there is suggested and then jumped to OPS which started this discussion. Point is OPS doesnt matter, so even if voltage wasnt there it wouldnt be the OPS.

It is redundant in case the relay fails, not the PCM. The PCM controls the relay trigger. The original relay was massive, and maybe they were afraid a regular one might crap out more often. Another nice benefit of having the OPS in the loop is the priming it performs post shutdown. Mine always runs after shutdown, would expect most OBDIIs will too.

The ground made the most sense, since PCM wasnt throwing a fuel pump DTC (unique to OBDII), so there was available voltage. Otherwise LP was broke or not connected.
 
buddy said:
Definately a possibility. A fuel pump gauge and some fuel hose strapped to the mirror would help diagnose that.

We are assuming there is no SES light or DTCs.
Click to expand...

No trouble lights come on just the problem. I just replaced the fuel sending unit 3 months ago along with the line from there to the lift pump as well as the fuel manager. One issue I falled rto mention is when I fuel up the filler neck leaks and the mechanic who did the work said the tank was alittle rusted on top.
 
vacuum in the tank is supposed to help keep the fuel from bubbling, more stable. but Ive not heard of someone removing the fuel cap and having a problem, so not sure if a hole in the tank would cause a problem. Rusty lines are a problem of course, but sounds like youve gotten those taken care of. And with new sending unit, would hope its not plugged up.

Might not be a lift pump issue, although a weak one could be the cause, its good to know what kind of fuel pressure you have at WOT. What kind of boost are you getting? Running on the factory vacuum actuator system? Do you know if it has an aftermaket PCM or just stock?
 
JiFaire said:
-- Questions I still have - for another thread, on another day - According to the diagram you supplied and the rationale given by you and TD, there should never be a need to change the OPS on an OBD-II truck, something I still struggle with. Why is the darn thing even there, if the PCM does all the work? redundancy dosn't seem important, as if the PCM dies, so does the truck (even with a functioning OPS). :( I hate being confused...
Click to expand...

I think Jim the $$$ to redesign system without it were more than to leave it alone and just add in the redundant PCM power path, we'll never know the reason to keep both circuits as entire setup is obsolete as far as GM is concerned and I'll bet notes on discussion of it have long since disposed of.
 
buddy said:
vacuum in the tank is supposed to help keep the fuel from bubbling, more stable. but Ive not heard of someone removing the fuel cap and having a problem, so not sure if a hole in the tank would cause a problem. Rusty lines are a problem of course, but sounds like youve gotten those taken care of. And with new sending unit, would hope its not plugged up.

Might not be a lift pump issue, although a weak one could be the cause, its good to know what kind of fuel pressure you have at WOT. What kind of boost are you getting? Running on the factory vacuum actuator system? Do you know if it has an aftermaket PCM or just stock?
Click to expand...

The boost goes to around 8psi and I'm running on a new oem vaccum pump and system. I also replaced the turbo relays and such. The PCM is stock.
 
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