Oil pump replacement

[coxfmly]

Hello,
I'm looking into replacing my oil pump in my 94 sub. Should I look for a high pressure or high volume pump? Any words of wisdom before I jump into this? Its a 4x4 so I'm sure it will be tight..

Thanks..

[DmaxMaverick]

You need a STANDARD volume pump. The high volume pump is a step backwards. More is not better, in this case. A high volume pump is necessary for piston spray engines only.

Why are you replacing the pump? They very rarely fail, and are almost always followed by a catastrophic engine failure, or an indicator of another serious failure looming. The regulator and bypass valves do get week over time/use, located under the oil filter/adapter in the block.

[coxfmly]

Hi DMax--

Within the last week I changed the PMD, OPS and fuel filter. I changed the PMD because after a fill up, I pulled away from the pump and it died. I installed my back-up PMD, no change. It just cranked. I checked the lift pump and it was working fine. After 20 minutes, I tried again and it fired right up. I assumed my original PMD had failed, so I installed a new one. Only significant change was my oil pressure, which dropped to 40 psi. The truck runs a constant 60 day in/out. After a 2 hour haul through the desert this weekend it died when I pulled over to refuel. I noticed the oil pressure gauge was bouncing a bit, then the RPMs dropped and it stalled. Went through the same routine, checked everything, and after half an hour it fired up again.

I'm trying to figure out why it's been cutting off when the RPMs drop. I assume that the oil pump was going, which was telling the OPS that the engine had lost pressure, which was shutting off the truck.

I just came in from a 300 mile jaunt across the dessert. It ran fine as long as the RPMs were above idle. I didn't have any cutoff episodes on the way home.

Can you purchase regulator and bypass valves for replacement? I'm not sure if I'm going down the right road, but it seems to be related to the oil pressure.

The truck has 215K. New heads, lift pump and above-mentioned parts within the last year.

Thanks.

[a5150nut]

Have you cleaned all the ground connections?

[DmaxMaverick]

If the oil pump (or pressure regulator) is allowing oil pressure to drop off enough to cut off the OPS, and causing a stall, you have WAY bigger problems than an oil pump (overhaul time, big time). It is MUCH more likely the OPS is bad (even though it isn't that old), or you have an electrical issue. If the oil pressure is "bouncing", you can just about bet it's electrical. The grounds are a good place to start, as 5150 suggested. Next, get a real close look at the OPS. If it's more flaky at idle than higher RPM, it or its electrical is suspect. Even if the lift pump cuts out, it shouldn't cause a stall. The IP will draw enough fuel to maintain idle and provide low to mid range power. With the multiple electrical issues, grounds become more and more suspect.

Robyn posted some pics, R/R info and part numbers for the regulator and bypass some time ago during her last (or previous) rebuild. Try a search or ask her.

[JohnC]

Stalling combined with erratic oil pressure readings points to the oil pressure sender/switch. Probably time for a new fuel filter, too.

[coxfmly]

Hi John,
Since I replaced the fuel filter a few weeks ago, I'm going to assume the switch is going. I have no idea how old the oil pressure sender/switch is, so Its going.

Dmax suggested that the regulator and bypass valves could be going south as well. I ordered the bypass valves today but couldn't locate the regulator? Mr Goodwrench wasn't familiar with the regulator so I'm still searching.

Do you think a larger than factory oil filter would have any effect on the bypass valve/regulator? I installed a filter that is about 2" longer than what the book calls out.

Ive always run 15w40 but I noticed the manual calls out for 30w when its hot. Is this the way to go or stay with the multi-grade?

Thanks for the help....

[DmaxMaverick]

Sorry if there was some confusion, but I listed POSSIBILITIES, not probabilities. As John said, it's likely going to be the OPS, or another electrical problem. With a flaky gage, and a corresponding stall, you can nearly rule out instrumentation (which is a "dumb" circuit, meaning it only shows what its told to show), and focus on the source. A failed OPS shouldn't cause a stall, but can if the IP is tired. If you have other electrical issues at the same time, look to the circuits and senders. One failed/failing battery can cause the same thing, as well.

[coxfmly]

No worries...
I always appreciate the help and direction. Unfortunately, my local diesel shop charges $200 just to pop the hood !!! At this rate I don't mind trying a few things before taking it to the shop $$$. I always come out ahead of the game with the help of the on line team...

I only know of 2 shops locally that actually know anything about these things. Would be nice to have a 6.5 club locally, I think they are outlawed in So Cal!!

Thanks again.....

[JohnC]

A failed OPS shouldn't cause a stall,

True, but the fact is, it often does. whether it's a partially clogged filter, or just "fuel hysteresis," it happens frequently.

[racer55]

With a 94 a failed OPS will most certainly cause a stall if there is any pinholes in the suction side of the fuel system anywhere between the fuel level in the tank and the IP.

Since the OPS powers the LP any loss of power to the LP requires the IP to pull fuel the entire length of the system-1 pinhole and the IP will loose it's prime and act like you just ran out of fuel.

When restarting the LP relay powers the LP during cranking and may re-prime the engine- eventually.

Any 6.5 pre 96 would benefit a lot from the "OPS relay mod" that may be posted here or found on google.

[coxfmly]

yep, I just ordered a relay and dropped off my alternator for a rebuild. After running for a few minutes my alternator would get smoking hot. The gauges would jump when the blinker was on and it would dump below "12" if i ran all the systems and lights. working on the grounds as well....