Help! Another OPS?

[Black95TD]

Just when I was really enjoying my truck, these symptoms are back AGAIN! 4th time. Starts fine, does not stall, runs OK at a steady pace. Once hot, if I get stuck at a toll booth for a few minutes, it stumbles on hard acceleration. This is the 4th time I've had this exact symptom.

At 50K the dealer replaced the lift pump. That worked for 3K. At 53K dealer replaced lift pump and OPS. (I had extended warranty). That lasted about 30K. At 80K I replaced OPS and that cured it till now - about 20K.

I presume the OPS does not send a good signal to the lift pump and not enough fuel is getting to the injection pump for hard acceleration. I'm going down to the dealer to buy an OPS this am and I bet it will fix the problem (I sure hope so!)

Why do these things go bad so often? Do they get heat baked too? When they go bad, does it damage the lift pump? Should I replace the lift pump to be sure? When I turn the key to start with the shift lever in Drive, I can hear the lift pump clicking. However, after 10 seconds or so, it seems to slow down and get quieter. If the lift pump is slow/not working. I can see that there would be additional heat on the FSD. I need to get a remote located FSD too. I have agout 60K on my pump/FSD.

Any help from you guys that know more than I do about the OPS would be greatly appreciated. I noticed in another post that a member said he went through a few OPSs last year. Thanks.

[FarmerDale]

Check your fuel cap after you have driven the truck. There should be no vacuum when you start to unscrew it. Vacuum makes the LP work hareder, shortening its life. Fuel cap is a $8.00 (GM) part (ask me how I know!).

Also, I would make sure that your fuel and air filters are clean. Just standard maintenance, but a good thing to check.

[rjwest]

Install a fuel pressure gage in Cab,
Makes trouble shooting much easier...

[piersbc]

I have the exact same problem. With truck in "drive" and key turned the lift pump clicks. I'm assuming that means the OPS is good because it's sending juice to the lift pump...someone correct me if I'm wrong. I'm going to get a new lift pump this week. Also with the motor running I unscrewed the T connector and no fuel came out but the truck died. I was told that would prove it was the lift pump...any advice??

[Black95TD]

Thanks all.
I have my new OPS but its not on yet. I'll report back ASAP. piersbc - I looked at the wiring diagram today down at the dealers. Looks like the key in the start position activates the lift pump so the engine will start and then the OPS keeps it going after it starts. I think the clicking you hear is the lift pump but I don't think it proves the OPS is good. Have you ever replaced the OPS? According to the posts I've seen, lots of us seem to have this problem.

[DickWells]

Hi there: Just a couple of notes, for WIW. It's been a long time now since I have had to work on my OPS, or lift pump. I installed a cheap little relay, right on the firewall, behind the fuel filter. Did it right after I put in my last burned out OPS. If memory serves me right, the hot wire to the lift pump was white with black stripe. Regardless, it's a good little bit of insurrance against switch burn-out. I tore my last one appart, and found the contacts thoroughly cooked! The relay should put an end to that.
All the other notes here are good advise for you, too.
If you take off the power dist. cover under the hood on driver's side, you can take out the lifter pump relay and jumper no's 85 and 86 together to work the pump without even turning the switch on. That way, you can drain all the fuel you want at the petcock up front to purge the filter can. This, after you have let air out the top, of course.I always do this when I change fuel filters. The Sub. starts right up and runs fine.
If you want to guard against water even getting to the lift pump, you can install a pre-filter back behind the cross-member. I put in a RaCor 645 a year and a half ago. I drain it about every two months or so. Should be more often, as I found some rust up inside the filter element when I finally changed it after more than 30K miles!
You probably are all set by now, and don't need my two cents, but just in case, these are some things that have helped me.
Good luck.
Dick Wells (Gabby Wells at the SASS shoots) (Wonder why???)

[Black95TD]

Thanks all again. I made a list of things to do. Last night I changed the OPS AGAIN and its fixed. Noticed significant resistance between middle prong and base whereas the new one has no resistance. I guess its burned up inside. So my 1st one went 50K, 2nd 30K, 3rd 20K - this looks like a 9th grade math problem. How far will this one go? 10K? Does anyone know if there is a better aftermarket OPS or something else causing this?

Thanks again for all your help. I would have sold the thing by now if I didn't know about the Desel Page.

Now lets see ..., 60K on my injection pump and FSD. Better start thinking about remote located FSD.

[DieselDavy]

Black95TD,
Dick did outline a bit of relief for your OPS. It was just included in with a few other things and probably got past you! His cure (and I'm sure it is a good one) was to take the switched line from the OPS and have it activate a small relay. This small relay will take less current to drive than what the OPS was driving before, thus extending it's life. Just make sure the relay is automotive grade or protect it from the elements in some way. Very harsh environment under the hood! He can probably outline the exact steps for this install but essentially you are buffering the output of the OPS. It is good practice.
Good Luck!
Dave

[Kennedy]

Just FYI, I'm working on a universal lift pump control harness designed for the Dmax which runs w/o a lift pump. This will be well suited for the 6.5 as well, and will incorporate a prime as well as manual bleed functions. It will be a few weeks yet, but the T's will be crossed and the i's will be dotted...

[Phil Holmen]

I added a momentary on switch to my OPS relay to aid with priming. I works real slick, don't have to leave the engine compartment, easy to watch for spills and the works. I also added a small LED to show when the relay is sending power to the fuel pump.

[damork]

Anyone with OPS problems should seriously consider adding a permanent relay between it and the lift pump. The OPS contacts seem to be marginal at best and it is a crap shoot as to when they will fail.

The relay is not that difficult to install and once done it even provides a place to let you run your fuel pump for priming from under the hood. The most difficult part of the job I found was finding the feed wire to install it in.

BTW - the fuel pump is louder when pumping but once pressure builds it quiets down.

[Polar Diesel]

my lift pump is wired direct to my fuel shutoff solenoid. when the ignitions on, the pumps on.

[norm]

Do you guys using relays have a source/part #? I have some 12v lighting + horn relays sitting around that I could use, but would not mind something a little more HD looking.

[CareyWeber]

Originally posted by damork:
Anyone with OPS problems should seriously consider adding a permanent relay between it and the lift pump. The OPS contacts seem to be marginal at best and it is a crap shoot as to when they will fail.

The relay is not that difficult to install and once done it even provides a place to let you run your fuel pump for priming from under the hood. The most difficult part of the job I found was finding the feed wire to install it in.

BTW - the fuel pump is louder when pumping but once pressure builds it quiets down. I aways thought the 1997 and up used a relay to power the lift pump am I wrong in thinking that?

Carey

[JohnC]

Originally posted by Polar Diesel:
my lift pump is wired direct to my fuel shutoff solenoid. when the ignitions on, the pumps on. It's illegal to design a vehicle this way, and you might want to consider a different way too. If you get in an wreck and bust a fuel line, the pump will feed the fire at about 30 gallons an hour which is about 45 times more than my home heating unit.

[damork]

CareyWeber,
I think you are right with respect to newer trucks and relays for the lift pump. I have a 1995 and it didn't have one. I'm not familiar where the cutoff point was.

The way I wired mine retained the safety so as soon as you lose oil pressure, the pump goes off (unless somehow the relay contacts get smashed together and the pump continues to run)

[rjwest]

CareyWeber, your are correct....On 1996>
PCM Controls Lift pump through relay,

Oil Press sw is in parrellel with closed side of relay, If relay fails closed. Oil Pressure will
operate lift pump.

I have added a second fuel filter and 2nd in line lift pump controlled by a switch on dash.

Use 2nd lift pump for bleeding system and backup.

NO I don't drive with it on...


Odd note. With truck camper on ( less than half tank) and on a very
wavy cement interstate, the type where the
truck starts to gallop, I notice fuel pressure
fluctuate, sometime close to zero...

[DBlake]

For all who wish to add a relay; RadioShack has a nice 12VDC, 200mA automotive grade relay for about $4. It is a SPDT with a nice mounting tab. It fit very nicely under the power distribution cover on the passenger side firewall. Has worked fine for almost 3 years now.

[norm]

According to the wiring diagram that I have, it appears that the '94 and '95 trucks have the lift pump relay as well. This does not solve the problem at hand though because the OPS still handles the voltage for the lift pump when the relay is not energized. When the relay pulls in, this is the hot-in-start mode, and pulls power from that ECM circuit; when the relay is static, there is hot at all times through the OPS for control and safety. The additional relay will use that (OPS) circuit only for switching as has been explained, with a new power circuit to run the pump when the engine is running.

[tom.mcinerney]

Norm-

In another thread, GMCTD explained that all the relays installed in a harness with electronic computer engine-controls should have diodes to subdue ('clamp') the reverse voltage spike ('back EMF') caused by the collapsing electric field in the relay magnet coil {when the relay turns off}.
This pulse could damage some of the logic junctions in the digital circuitry.

Having all ready installed a bunch of relays, i wired some diodes in my harness extensions. The more suitable solution would have been to install good quality relays with the integral clamping diodes . They might cost near $9 from Bosch, Magnecraft, P&B, etc.

I used $5 from Bosch (30A) via Waytek. I also bought a $5 automotive relay from a local auto aftermarket supplier (no-brand, offshore). I opened and compared the innards of the Bosch & offshore-brandless. The points of the Bosch were nearly twice the diameter of the brandless, and some other interior conductors were significantly larger as well....

[DickWells]

Carey: Did you read Norm's and GMCTD's notes a couple of notches back? They explain the relay thing much better than I can. The relay under the cover of the power dist. center won't help the OPS switch as they explained, so you have to wire up a seperate one for that. Cheap insurance! I've been running mine for 60K miles and four years now.
Good luck.
Dick