my 97 3.9 runs great considering it has 250k miles on it but it does have a few issues. the main problem being that once the engine starts to get warm, the oil pressure gauge will drop. i can shut the truck off, start it right back up, and the gauge will go back up near the middle. i plan to replace some stuff on it anyways but don't wanna leave anything out that could be causing the issue.

my parts list so far:
new oil pump
new oil pick up tube
oil pan gasket
fresh oil change
new oil pressure switch/ sending unit
new lifters ( one has collapsed. i'm assuming from a lack of oil pressure)
new pushrods

anything else i'm forgetting? i have most stuff already just need a day off from work that isn't raining

 

Maybe do the plenum fix why your are their. Get new manifold bolts the factory ones are torqued to yield and can not be reused.

 

Check the oil pressure with a mechanical gauge FIRST, before you go tearing into the engine. The electrical gauges are not accurate.

From your description, It sounds like some of it may be a gauge issue.

 

I agree. If it slowly falls down then spikes back up on restart, I'd verify the gauge first.

 

i plan to put a mechanical gauge in, but i need to pull the motor down anyways sine i have a lifter that's collapsed. i'm assuming it's either from oil pressure loss, or just because it has 250k miles on it. with high miles like it has, i'm going to replace as much as possible before i start driving it. i'd like to have as many trouble free miles as possible. during the winter, i'll probably throw in a new timing chain and some other good just to make sure it doesn't leave me stranded.


magnethead- it doesn't fall slowly. it sits right near the middle( i think it's 40psi) and drops quickly like it would if the truck was turned off. i'll be running some engine sludge remover through the motor before i pull the pan off. hopefully clean out any trash that might cause a problem later on

 

If you have a bad lifter, you can change it without pulling the engine. Do the plenum at the same time.

 

If the plenum gasket is toast and has been for awhile, expect to see large amounts of sludge all over your engine. A bad plenum will create sludge, and that sludge can cause the other problems you are seeing: a collapsed lifter and low oil pressure. When I pulled my intake apart to replace the plemun gasket, I had stalagmites ans stalagatites of oil sludge all throught the eninge, with a good 2" of sludge collected at the bottom of the pan. The oil pickup screen was half plugged, and no amount of solvent got all the sludge out of the screen. I had to pull the oil pickup out of the motor and clean out each hole in the screen with a sewing needle.

If you have this problem, don't disturb the sludge stuck to the engine if you can help it. I had the bright idea to try to scrape/vaccume as much of it out of there as I could, only to be rewarded a month later with a collapsed lifter and an engine rebuild since it also had a cracked head and was leaking coolant into one of the cylinders.