Bill N

So I had an engine rebuild on my 1986 951 last winter, but am finding that oil pressure is dropping on the track. Seems (4+) fine around town and on the highway, but at sustained high revs, it will drop to around 3 with 20-50 Amsoil and an extra oil cooler.

I sure hope it's nothing major....can the oil pressure relief valve cause this? Oil pump?

Thanks for your help.

420WHP944

4+ around town is quite normal and the pressure will always drop that bit when on the track with sustained high revs.....the oil gets much hotter and thinner... therefore the lower oil pressure. I have heard of tiny particles stuck in the oprv causing the oil pessure to drop a bit...you could try checking that out???

Bill N

Thanks, will start there.

reno808

Extra oil cooler?

Bill N

Yeah, it has two 951 oil coolers, one in the stock position, the other on the other side.

User 41221

How long are th elines running to the extra oil cooler? Additional length can cause a drop in pressure, altho I would have thought you'd see that right away after installing it and not a gradual loss over time.

Regards,

DanG

Do you run them in series or parallel? Dual coolers seems like a cheaper, easier upgrade than a Mocal or other aftermarket cooler.

Sami951

I've got the same thing going on. Before the rebuild the oil pressure used to drop down to 3.5 bars at ~5k rpm after several fast laps on the track, 4+ when driving around town.... now it drops to 3 bars at the track... not good

What's weird is that after the track session, when I drove back home (like 70 miles of highway) I noticed the oil pressure would stay at 3 bars at idle, where as just yesterday it used to dip down to 2 bars at idle... no clue what's up with that ...

Got a KISS (mocal) additional oil cooler waiting to go in. We'll see if that helps any, probably not

Ski

We also put dual stock coolers on our track car when we built it. The driver side of the radiator bracket already has the holes as the passenger side does, you just have to add the nuts to the back of the bolts. You INVERT the second cooler and we run them in series; the oil comes out of the housing to the stock cooler, then OVER to the second cooler, and then back to the oil filter housing.

As these engine were rebuilt, did everyone use the old sender or get new ones?

Gator_86_951

I have similar behavior, but I have not taken my car to Sebring with the additional oil cooler. I suspect it will be making a big difference, as the oil pressure is always high around town. At least higher then it was before the other oil cooler.

Sami951

I used the old one ... do they wear out?

Oddjob

First, 3 bar oil pressure on track is too low. Its an indication of something wrong or something not working right.

I would check the cheap, easy to swap, diagnose or replaceable items first.

First thing is to check the oil temperature when it hot (when you are seeing only 3 bar). For the oil to lose enough viscosity to drop down to 3 bar, especially with a top of the line synthetic like amsoil, I would expect it would have to be upwards of 280 deg F. The normal operating temp for oil is a little over 200 deg up to maybe 240-250 max. If the oil temp is too high, make sure your oil thermostat is opening. After driving the car very hard, are the oil lines and coolers warm/hot to the touch? The oil doesnt get that hot during street driving, so you may not notice any heat in the coolers after a short drive - so you will have to drive it hard on a hot day to check this.

If the oil is too hot (in the pan), and the coolers are cold, its the oil thermostat. If the oil is within normal temp range, and oil appears to be flowing through the coolers, I would think about replacing the oil pressure sender, or at least check the signal from it (I dont know what the resistance values should be, but you may be able to find out). I have had a faulty oil pressure sender cause similar low oil pressure readings when the oil was at operating temp, but indicated normal/high pressure when the car was cold.

If the oil temp, thermostat, and pressure sender are good, then you could think about the pressure relief valve (though Im not sure it would fail in this way - where it functions properly when cold/warm, but bleeds off too much pressure when hot).

Chris Prack

Jim is correct in his oil temp suggestion. I also didn't see where "at the track" you were seeing 3 bar of pressure. 3 bar coming off the track at idle or at 6k rpm on the track?

If it is 3 bar in the paddock after a session then I would tell you not to worry about it. 3 bar at idle hot is not an issue. I put over a 100k miles on my 89 between the street and track in 7 years and in the summer coming off the track I would see <2 bar indicated at idle. I have never had a bearing failure and the rod bearing were replaced two years ago and looked perfect.

I think your find provided the coolers are getting hot.

Bill N

Thanks, everyone, for your input. In response to earlier questions, the two coolers are in series, as described by Bret.

Nope, didn't replace the oil pressure sender, but the readings on the pressure guage are too consistent to lead me to suspect that's the problem. It reads fine until I'm on the track.
I am currently having an oil temp guage installed, but don't know right now how the temps have been running. It does seem like an important question, though.

Actually, oil temps seems like a likely culprit, and I guess I could determine pretty quickly if oil is getting to the coolers appropriately by just checking their temperatures as the car warms up, but I don't have the car right now. Will do. One reason it makes a lot of sense to suspect the temps is that the pressure drops when the car is working really hard at lower gears (eg, turns 3 through 5 at Mosport). It's not related to the turns themselves (ie concerns about the pump sucking air on hard turns). The pressure returns to 4 or close to 4 when revs drop for a while.

So, yeah, thanks, Jim, good thought.

Bill

Oddjob

Definitely check the temps first, but like I mentioned, I had a pressure sender fail in with similar symptoms. Car ran high pressure when cool, and when it heated up and got hot, the oil pressure dropped. I replaced the oil thermostat, but still had the problem. I then stuck a temp probe down in the oil pan (through the dip stick tube) and found that the engine oil was around 205 deg F, which is actually on the cool end. So swapped out the sender, and bingo, high oil pressure hot or cold.

I also have dual external in series oil coolers on both my race cars. If the coolers are not plugged and the oil thermostat is working properly, I doubt that you have an oil overheating problem. Im assuming that the engine/head water temp is fine?