Oil Pressure Gage - Newbie Owner - Ford Powerstroke Diesel Forum
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Old 07-05-2012, 07:24 PM
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Oil Pressure Gage - Newbie Owner

As all Newbies, I need a little help. I have a 97 F350 that I bought back in December. Haven't driven it much yet and just now getting it ready for a trip. Changed the oil and noticed that the oil gauge is stuck.

From reading online the oil gauge is just a needle version of an idiot light - if the low oil pressure pump puts out greater than 6 psi the gauge registers good. The engine is running fine so I am guessing I do not have a problem in my oil system (other than a decent leak which I am also trying to track down).

This e-mail is for two items - to fix the oil gauge or install one that is actually worth something, to get a little bit of an education on the engine oil system. I have not been able to find a good manual yet.

Where is the oil pressure sender at? I have found a few threads discussing it but either I am finding different engine models or inaccurate info since they say it is in different places. One says on the HPOP (I don't have a clue where it is) another on the front of the engine. Are there any aftermarket gauges that will replace the dash gauge and be more than an idiot switch? Or do I have to go with an under the dash unit?

As to the education, from what I can tell there is a Low Pressure pump and a High Pressure Pump.

If the engine is running well is there any need to worry about oil pressure at all?

Does the LPOP only feed the HPOP? Or does it feed other parts of the engine also?

Sounds like high oil pressure is used to run the fuel injectors. Does this drain into the rest of the engine to lubricate it?

What is normal operating pressure for the LPOP and the HPOP? Is there any need to put a good gauge on the HPOP?

I hope someone is willing to educate me!

Thanks...

ExTexan66
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Old 07-05-2012, 07:50 PM
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"LPOP" is just a buzzword (buzz-acronym?) that applies to HEUI engines like ours. In any other engine, it's simply called the "oil pump". And yes, like the oil pump in any other engine, it's primary purpuse is to provide lubrication oil pressure to the rest of the engine. The lubrication system on our engines is pretty-much the same as on any other engine.

But our oil does double duty. It's not just lubrication oil, it's hydraulic oil. The lube oil pump, or LPOP, or whatever you like to call it, also pumps oil up to the HPOP. The HPOP provides high hydraulic pressure to the injectors. Thus the acronym HEUI:
Hydraulically powered
Electronically controlled
Unit
Injection
The oil used by the injectors does drain back down into the pan, but as said, the engine lubrication is by its own set of passages, separate from the HPOP / HEUI operation. This is also why you MUST use diesel engine oil, and why good oil quality, oil level maintenance and prompt oil changes are important.

The sender for the dash "gauge" is on the HPOP reservoir (since it's a reservoir, i.e. the input to the HPOP, it's kept at the pressure provided by the LPOP). There's actually a write-up on one of the diesel forum sites on how to make the gauge into a real gauge. You either solder in a resistor, or de-solder one that's there, on the back of the gauge head, and I believe it requires a different sender as well. Somebody here might have it bookmarked; I always forget what the policy is here on the org about posting links to threads on the other forums.

The best way to gauge the HPOP pressure is digitally. The engine has an ICP sensor (injection control pressure), which reads the HP oil pressure and informs the PCM, which uses it to control the injection pressure (HP) via the IPR - injection pressure regulator. You can plug a gauge or real-time data reader into the OBD-II port to spy on that reading.

Oh, just noticed your first sentence - so the gauge needle is always centered, even when the engine's off? What about when the key is out? Try disconnecting the sender wire and see what it does then. If that doesn't do it, you could always just take the cluster bezel off and tap on the needle and see if it's not just physically stuck, or dragging on the gauge face. If that doesn't do it, you'll probably have to replace that "segment" of the cluster; it contains that gauge and the temp gauge. Look for someone parting out a truck on one of the forums.

Was the gauge stuck before the oil change as well?

BTW, one cool coincidental "safeguard" about the HEUI design - if you're really low on oil, there won't be enough oil provided to the HPOP to fire the injectors, so it's extremely unlikely that you'll do any damage to the engine related to low oil level, like you could on a more "conventional" engine.
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