Compression Ignition Addict
Join Date: Aug 2008
Location: Madison, WI
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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:
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.