What good is the OPS to the lift pump?

[markelectric]

I am in the process of replacing my 5th OPS on the 95 sub in 5 years of ownership. Always the same symptoms, sluggish acceleration and some missing. With all the pain this has caused I have created a gage tool to confirm the diagnosis. Hook the hose to the pitcock, lay the gage on the windshield, turn key on, put in gear, turn key to start, watch needle go up to about 10psi, start engine, watch needle go down to 4in of vacuum, step on pedel, watch needle go to 10in vacuum.
I realize GM wanted to kill the engine with a loss of oil pressure, but we all know that does not happen. Why not just wire it to the key? It sure would take the mystery uot of the system. Not to mention save a few injector pumps. My 2000 Dodge is that way and all seem to work well.
What am I missing?

[Turbine Doc]

The OPS isn't there to stop on loss of oil pressure, it's for fire protection in event of a crash, you don't want an energized fuel pump squirting Diesel out feeding the fire which could happen if key on is what energized the pump or you hard wire the pump to a manual switch.

You crash pass out and can't think to turn off the ignition or manual switch, OPS control is a good thing IMO but like the PMD they didn't "thunk it" all the way through, & went for cheap rather than best.

The fix is to use the OPS to control a heavy duty relay that powers the fuel pump, buy & install your own, or if you want plug & play simplicity get Kennedy's OPS kit, which is all wired up & ready to go.

[HowieE]

There is a diagram of the relay fix on my site noted below.

Not sure I can see a link between poor acceleration and the OPS since the switch is just a normaly open contact that closes with 4 lbs oil pressure. Unless you have wildly fluctuating oil pressure the switch should remain closed. The OPS fails open due to the fact it was designed to fail and sell parts. It was designed to carry 1 amp of current and the fuel pump draws 4 amps. If the OPS fails that should be it, it should not produce intermittence.

I would look at the fuel pump of fuel filter for your poor acceleration first and then the injection pump. But do the relay mode because it is cheap and will save the OPS in the long run.

[arveetek]

Originally posted by HowieE:


Not sure I can see a link between poor acceleration and the OPS The reason you can have poor acceleration and/or misfire is because the lift pump is not working due to a failed OPS. I had the same symptoms on my '95 when I bought it. Started and ran just fine, but hesitated, stumbled, and misfired when accelerating, but not all the time. I opened the fuel filter drain valve with the engine running and the engine immediately died and was hard to restart. The injection pump was able to pull enough fuel to stay running, but barely enough. After fixing the OPS, all those symptoms went away.

Mark,

Either do the relay mod or get Kennedy's OPS harness and you'll be done with that problem. The one great advantage that Kennedy's OPS harness has over the relay mod is that it will prime the fuel system every time the key is turned on, but will still shut off the fuel pump with the loss of oil pressure. Of course, the relay mod is cheaper, and is still better than the factory setup.

Casey

[JohnC]

I put 130,000 miles on an oil pressure switch connected to a relay as in the dieselpage writeup. It was still running when I traded the truck last month.

The OPS cutoff is required by law. Any non-engine driven fuel pump must shut off automatically if the engine quits running.