Oil pressure drops at hot idle

[Devin LeBlanc]

I was going through some things and was just doing some overall checks on my car, Might as well start off with some specs if you don't know.. Its a 145,000 mile old car, Original engine (7MGE), Other than the head gasket being replaced 15k ago nothing has ever been rebuilt. I have been running it NA-T now on a stock CT26 making over 300HP and 352ft lbs of torque on a low 12lbs on 91. The car runs great and is a properly done setup. I am getting ready for the drag season and just doing general checks.. I have a oil pressure gauge and watch it all the time however I have been noticing large drops once hot. So after work today at the shop I busted out the mechanical oil pressure gauge to check and verify that my other in car gauge is right..

At cold start up I am getting 70PSI of oil pressure, as it warms up it starts dropping. If I go and drive the car and rail on it 3 or 4 hard WOT pulls then drive back to my house the oil pressure is about 5 PSI. Now I know that in spec a minimum of 4.3 PSI at idle is normal, and 37PSI or higher at 3000rpms. So technically everything still is in "spec" However I used to have 12-15 PSI hot idle oil pressure so its alarming to me. As far as the motor I don't think Im having any clearance issues, mind its a 145k old motor yes it has some ware, but as much as I know 5 PSi at idle is "normal" I still don't like the fact that it is what it is.

So what in an oil pump wares out, I know there is a spec on the oil pump gear back lash and the relief spring etc, but what would cause so much of a drastic drop in pressure once fully warm?

I am running 10W40 and is what I have always run, Here in vegas I tend to run a thicker oil with it being so hot durning the summer 117F+ some days.

Anyways, I am open to all suggestions , I really am the type of supra owner that does no short cuts so right now my car is not being run due to this issue. Im ready to pull the motor and figure out what's causing it at any time.

I took 4 years of automotive repair and know advanced engine building etc so know that if my bearing and journal clearances were a large amount out of spec that would affect my oil pressure everywhere. I know that pressure is restriction of flow so that all makes since to me, Im just thinking components of my oil pump are worn out causing these symptoms, or could be as simple as a piece of dirt holding the pressure relif spring slightly open causing oil pressure to be bled back off into the pan.

Thanks in advance,
Hope to hear from the professionals in engine assembly and oiling

 

[hvyman]

Oil gets thinner when it gets hot. It doesnt drop in viscosity but it gets thinner. Perfectly normal and every car will do that.

Kinda like how cooking greese is like water when hot and when it cools down it solidify s except oil doesnt turn into a solid.

 

[Devin LeBlanc]

Exactly understand what your saying, But what could be the cause that it only used to drop to 12-15 PSi hot before and now to 5 with the same brand and weight oil. Im wondering if something in the pump is starting wear out, or possibly could be a internal leak that when the oil gets thiner it gets worse, main feed line to block?

 

[Poodles]

Weather is warming up, oil is breaking down, etc...

 

[hvyman]

0w or 5w would be better for cold starts also.

 

[Devin LeBlanc]

Gotcha, we'll see how things go with budgeting, Ill probably have the motor out to do a once over and just throw in a new oil pump with shimmed relief spring, a new pump wont hurt anything especially having 145k on this one

 

[hvyman]

I would use the turbo pump since your on the n/a one right now. Turbo pump flows more.

 

[Devin LeBlanc]

yeah thats what I would be doing, GTE pump and shimmed relief spring should be great for this motor. The fact that no piston oil squirters help bump my oil pressure at idle, Im just trying to keep my 7M in one piece, Others fail to do

 

[Poodles]

If you're going to go that far in, why not just budget for a rebuild?

 

[Flateric]

When I had this exact same set of symptoms it turned out to be a failing oil pump and bad bearings. As the oil gets hot it won't hold the pressure well with bad bearings. Judging by the mileage of your motor and I'll make the assumption that you actually "drive" your car not just putter around and baby it.....I think you should really do a rings/bearings/seals rebuild with an oil pump, refresh the whole works.

I honestly hope it turns out to be something simple for ya. Also try swapping out the oil pressure sender.

 

[Devin LeBlanc]

Yeah, I would assume that could be it, however I don't want to tear into it that deep seeing I just did the head gasket 15k ago and have 150psi of compression when new spec is 156 and minimum is 128, as far as the rings and condition overall I think its pretty damn good for that many miles! Im just trying to go over and see what else could cause it, I have seen some stupid things as easy as a fitting being loose bleeding off pressure internally. Or a piece of dirt could have gotten lodged in between the relief valve and the clearance in the case causing it to not close fully at idle.

I mean I have to do what I have to do, Im not going to cheat my way out of anything, so I guess as it stands ill pull the motor and go over everything internally If I don't see anything loose or what not Ill pull the rod caps off and check the bearings. I'd really hate doing that, and wouldn't feel comfortable putting it back together without replacing the rod bolts. But thats just me.

What are the chances it could just be the oil pump? slim to none huh?

 

[CyFi6]

If you are ripping the whole thing apart anyways the oil pump is easy to check. Just a couple clearance measurements and make sure the relief valve is working right. The spring on the relief valve will lose tension over time but that wouldn't explain your sudden drop in oil pressure. Something to take a look at is your cam journals. I have heard of the rear journals of the cam getting starved for oil and taking out the cam bore in the head (since oil has to flow all the way through the cam to reach the rear journal). Even good running engines seem to have way too much clearance on the cams because from the factory, the cams have a babbit material basically coating the cam journal, so if/when that coating wears off now you have a ton of clearance between the journal and the head. This is actually relatively easy to fix too without getting the whole head align bored.

If I were you and I had the engine out I would probably throw in a shimmed gte pump like you said you are doing(EDIT: make sure you take it apart and CLEAN it out well, then pack it with grease for easy priming. New Toyota pumps are known for having metal filings in them strait from the factory), plastigauge the bearings, plastigauge the cams (if you put the plastigauge under the cam instead of under the cap, the tension of the valve springs wont mess up your readings). Also make sure there's no holes or cracks or leaks anywhere on the oil pump pickup tube that would cause it to suck air in. Interested to see what you find (if anything). Also don't forget to do any other measurements while you're in there like rod side clearance and crankshaft end play etc.

Not sure what you mean by replacing the rod bolts considering you would have to do a full tear down at that point to get new ones pressed in and have the rod big ends resized.

 

[Flateric]

I think the easiest thing is to either check your cam bearings and see how healthy they look. Or from the bottom pop off a rod cap or main cap.

The cam bearings towards the back of the block will be the most hurting since they are furthest from being "freshly oiled" and I would make it on of them as far as cam bearing check goes.

The original 2JZ I had gotten for my swap had a modified union bolt with a drilled out pressure valve and this caused massive engine failure real quick. So if yours is misbehaving that could be it.

I'm really curious to know what it turns out to be for you.

 

[jetjock]

..It doesnt drop in viscosity but it gets thinner...

Say what?

Devin: Quit worrying...

 

[Devin LeBlanc]

I know the cam journals are not in the best of shape, when I had the cams out to do the headgasket and lash adjustment they were scored pretty bad.. It's probably nothing, but i'd still like the throw In a new gte pump seeing that the ge pump flows less and having the turbo on there is pretty much a regulated pressure leak.

Thanks for chiming in jj