QuestionQUESTION: My Spitfire's oil light came on the other day. I installed an oil pressure gauge so that I could start diagnosing the problem. To my surprise, the car was making nearly 80lbs of pressure at idle cold. After warm up, the pressure went down to 50psi at idle, but was still at about 80psi at 3500 rpm. I removed the oil pressure bypass valve hoping to find carbon build-up on the valve, but there was none. I also shot a little compressed air into the hole that the valve came from in hopes of knocking loose any deposits, but none of this has changed this high pressure symptom. I have learned that 75 psi cold is normal, but 75 psi at cruising rpm warm seems wrong. Is this normal?
Oh, and I'm planning to replace the oil pressure switch for the light.
ANSWER: Brian,
75psi at cruising speed is a bit high... but it could be valid.
First off, what oil wt are you running? Oil cooler installed? Ambient temperatures?
All of those can have an effect.
Cheers,
Jim
---------- FOLLOW-UP ----------
QUESTION: Ok, I am using Rotella 15-40 diesel oil. I use this oil because flat tapped motors, like the one in the Triumph Spitfire, require an oil with sulfates, phosphates and other additives that current api approved oils no longer have. The manual actually specifies 50 weight oil, but I think that's a little high. I do not have an oil cooler installed. I don't know if the Spitfire has an internal oil cooler. It was about 65 degrees in the garage when I observed all of these oil pressure readings.
ANSWER: Brian,
Was the runnning obersvation taken on the road? Or just in the garage?
65 is still on the cool side, so that will help keep the o/p up.
Objectively, it's not a "bad" reading, if it's correct. It implies that the bearings are still in good shape and the pump is also still well within tolerance.
Without having had a pressure gauge before this all started there's no way of knowing what pressure you were running previously.
Cheers,
Jim
---------- FOLLOW-UP ----------
QUESTION: Yes, all of the readings were taken in the garage. I agree that 65 degrees is coolish, and that could lead to a higher pressure reading. I just don't want to damage the motor when I suspect something is wrong. You're also correct that I have no idea what the pressure was before the gauge installation. Perhaps the pressure had been this high all along. That doesn't mean that this is an acceptable oil pressure though.
I am considering adding a brass washer or two between the bolt that holds the oil bypass valve in and the block. Perhaps that will decrease the high rpm pressure and maintain the decent low rpm pressure.
AnswerBrian,
The main drawback to slightly higher oil pressure is that it will rob some engine power in order to run the pump that hard.
If it's really excessive (120+) then you can potentially have the oil filter itself fail.
Personally I'd give it a reasonable test run in the local area, checking pressure after everything is up to temp.
Also, how good is the pressure gauge you're using? It might be reading a bit high.
Low oil pressue does basically kill the engine.
Cheers,
Jim
