slowly decreasing oil pressure?? please help!

[diesel_junkie]

i have an 80 caddy with a 1.6d n/a out of an 84 rabbit. I recently did lots of work to it, including a new vdo oil pressure gauge and sender installed in the head. I drive it 134 miles a day back and forth to work. She runs like a champ but this past week i have noticed my oil pressure slowly decreasing on my drive. it usually sat at about 15-20 psi and now my gauge is showing 6-8 psi and im not exactly sure what would cause it to slowly decrease like that. Any thoughts or solutions would be greatly appreciated.

[ORCoaster]

Time to take the Oil pan down and start looking at that gear pump wear.  If you need one let me know or just give a shout I think most of us have a spare or two.  You can upgrade if you have the smaller geared one.  Look for topics here with search function.  Maybe Oil Pump or gear pump would do it.

[vanbcguy]

Or let's start simple and make sure the gauge is accurate! Wouldn't be the first time a gauge or sender has gone bad.

[ORCoaster]

yeah, yanking the pump is more involved than some of the options. 

Here is another thought, the sender on the head itself.  Is it right?  I am assuming this is an electric oil gauge not a mechanical one.  The resistance of the sender may be going wonky on you.  Yes I know you mentioned new gauge but frankly I don't trust electrical stuff as much as mechanical.  So maybe borrow a mech gauge and plug it into the side of the head and see what it reads. 

Could the pump be starved for oil after a period of time?  Maybe a blockage on the return side is pooling the oil and it is low and not getting a full draw of fluid.  Sucking air? 

[diesel_junkie]

yeah my next step was to verify the oil pressure with a mechanical gauge. One other thing that makes me question the integrity of the engine, it almost sounds like i have a rod knocking and im getting a fair amount of blowby. I planned to pull the oil pan this weekend and visually inspect whats going on down there. If it makes it that far, haha but if it does blow that gives me the excuse to throw a 1.9 in it or try to find 1.6td block (easier said than done) thanks for the advice guys, really appreciate it.

[Dakotakid]

You want "thoughts?"
http://www.vwdiesel.net/forum/index.php/topic,34730.msg329063.html#msg329063
If that engine is truly out of an '84, that is probably a "JK" code engine and those are very good engines and deserve to not become Aztec sacrifices to ineptness.

[air-cooled or diesel]

electric senders to guages can be more inaccurate than mechanical ones, draw back for mechanical gauges to work it needs to hook up from motor to gauge, ie like a oil pressure gauge you have oil pressure running thru the firewall to the dash. an electrical sender can also have grounding problems, this will effect your reading. my fairly refreshed motor with 5w30 synthetic has 12-15psi oil pressure @head, hot. on the one hand you state you think you hear rod knock, and may have excessive blow-by, perhaps when you got it together and running something in there gave, you should check this out before taking on another long daily trip. if you have something coming apart in there, you dont want to hurt anything more,,

[Toby]

You guys do get wrapped around the axle!

1) Is that 6-9 psi at idle or at 60 mph? If it is at speed you have serious trouble. Never trust a cheap gauge. Try another before you panic.

2) Jason's Rule #1: Its NEVER the oil pump, unless its been FODed. FWIW I have disassembled about 1000 engines in my time and not one had a bad oil pump as the cause of failure. Often they are a casualty of some kind of catastrophic failure. A "bad oil pump" is the lie people tell when when they run a car out of oil. Its always BS. The oil pump is the best lubricated part in the motor and the last to lose lubrication. That means that the rest of the motor will be junk due to lack of oil long before the pump gets damaged. Now you can damage a pump by running trash through it, but the trash usually comes from ground up bits of motor so it was broken already, you just had not figured that out yet. There are a tiny number of exceptions, like SBFs that have the oil seals get hard and bits of this end up in the pan and get through the "bypass" grommet in the pick up screen and lock up the gearoter pump and twist the oil pump drive rod like a barber pole, but that is .005% of the bottom end failure and it is FOD problem.

3) Low and falling oil pressure as the temps climb, (like a long drive) is almost always caused by too much bearing clearance. Maybe lots of typical wear, but more often its about 10 minutes before you turn a rod bearing and your life gets much more complicated. Pull the pan NOW. You might still save it. Do a quick "Babbit Beater" rod bearing clearance test: grab a rod big end and wiggle it. It should move back and forth as it slides (parallel to the centerline of the crank) on the crank journal. It should slide freely. Now wiggle it radially on the crank. It should not have perceptible motion. It definitely not click. I am betting yours will. I am betting you will have sparkly oil and a pile of shavings in the bottom of the pan.

(Believe or not, this was the factory process for checking and setting the rod clearance on pre 1955 Chevy six cylinder with poured babbit rods.)

4) Pull the rod cap furthest from the oil pump and inspect the bearing, if its still a bearing. It will be the worst in almost all cases. Post the pics.

[air-cooled or diesel]

would obviously seem its 6-8psi hot@idle in this case, when im cold my oil pressure at the head is prob over 60psi(@idle), my reading of 12-15psi(fluctuates)is with a 5w-30 oil(syn), a 5w-40 may read higher, and a 15w-40 will be even higher. 6-8 is dangerously low. if it was 6-8psi @60mph that engine would be gone, im easily pushing around 25-30psi up to speed. so if he is using a thicker oil like a 15w-40 this reading is even more dangerous.
ive never had a 'bad oil pump', but the only way to know is to measure it, even a new one just to check, and check oil pump for scoring, im shaft and bearings and vac pump drive are directly related to oil pump. i had a few well used oil pumps that were getting near the spec, just swapped them out,,
although a 'bad oil pump' doesnt seem to explain the recent loss of oil pressure, unless the oil pump went 'bad' in that time, doubtful, seems something in motor is coming apart, you have to catch it before theres more failure.