Oil Pressure Light and Gauge?

[CobraSix]

I've lost my manual and couldn't find any information from searching here on the forums. 4 years or so ago, I made a post about oil pressure problems. To make a long story a little shorter, here I go:

2001: I noticed my oil pressure gauge dropping at idle speeds. I went for 10w30 to 10w40 to solve the problem.

2005: Problem had gotten progressively worse to the point that I'm using 20w50 year round just to keep oil pressure at RPMs above 1200. I replaced the sending unit which didn't solve the problem. I tested with a mechanical gauge and found that at 2000 RPM I was at 35psi. At idle I would have 0 psi, infact the oil was sucked out of the capillary tube for the gauge.

Present:
Things I've noticed:
1) If the oil is new, the oil pressure drops off at a lower RPM. As the oil ages the pressure drop comes at a higher and higher RPM.

2) When the engine is cold, oil pressure after start up is fine. The pressure doesn't drop off until the engine has warmed up.

3) Hot days make the oil pressure drop off at a higher RPM.

4) There is NO lifter ticking, engine still purrs like a kitten.

5) The engine does not burn any oil.

All of this has been verified with a mechanical gauge. I've put approximately 80k miles on it since the problem first started.

So, it's gotten me wondering...is it just a blockage around the sensor port? The reason I ask is my oil light has NEVER come on (and yes, the light bulb is working). But I wasn't sure if the oil light is based on level or pressure, or both.

The bottom line is I'm at a turning point on my 91. If indeed it's an oil pump problem, then likely my main bearings are toast and the engine isn't long for this world. Of course I've been saying that for awhile. I've only kept the truck under the condition of "I'll keep driving it until the engine explodes". However, currently the truck won't pass inspection and needs about $600 worth of parts if I do the work myself. The transmission is in decent shape, but not great. I know I've lost some compression on the engine. I've found some decent 1994 explorers for the $500-1000 range with a manual transmission (which is what I want anyways) and don't want to sink $600 into my 91 if the engine is on it's way out. I'd rather get a 93 or 94 with 4 wheel ABS and a manual tranny for that amount.

However, if it's possible the engine isn't going to grenade, I may keep the 91 since I know all of it's problems and a known evil is sometimes better then an unknown one.

Thanks.

 

[Maniak]

1st thing.. on a 1991 the oil pressure gauge is just an analog idiot light UNLESS your or the previous owner modified the gauge (short/remove a single resistor) and changed the sending unit to the older style.

The factory gauge shows normal pressure when there is 5psi of oil pressure. Under 5psi and the gauge reads 0. From what I've seen over the years is that the gauge will move slightly if the instrument cluster is having voltage issues (or if the truck is), especially as the rpms change.

Everything you ever wanted to know about the oil pressure gauge.. http://www.explorerforum.com/forums/showthread.php?t=14894&highlight=fake+oil+pressure

Also, the yellow oil light near the bottom right of the gauge goes on when you are 2+ quarts low on oil.. its not related to pressure.

~Mark

EDIT: for reference. Our modified stock gauge reads around the A-L when its cool at idle.. and when hot its around N-O and when hot at rpm its around the M. That gives you an idea of how much swing you will get when the gauge is a real gauge.

 

[CobraSix]

Mark,

I know not to trust the gauge for direct measurement of pressure, that's why all my measurements were done with a mechanical gauge.

but thank you for guidance on the oil light...that was one of the questions I didn't know for sure. My 65's light is based on pressure as is my volvo's light. But then again, both of those cars have no read out of pressure.

 

[Maniak]

I just reread your message and you did say mechanical.. whole different ball game..

I really don't like the 0 at idle..

The sending unit goes into a removable adapter on the side of the block.. It possible it is blocked, but I don't see how if it was blocked that it would suck the fluid back out of the capilary tube. I've never heard of one doing that..

~Mark

 

[CobraSix]

I know...weirdest thing I've ever seen. I was like "WTF?"

I've rebuilt my share of engines and never saw anything like that...marine or auto.

I don't like 0 pressure either...but I also can't argue with 80k miles of working that way. I really wish it would just die one day so I could move on. I've had this truck since 1995. I swear it keeps running just to spite me since I said I'd keep it until the engine grenaded. I've also abused the crap out of it the last 2-3 years, barely fixing anything unless it directly kept the truck from running. I'm even down to changing the oil once a year, in May.