wonky oil pressure issue on rebuild

jaybird840 · 11-22-2010, 05:17 PM

Hi All,
Please see the thread in my signature for details of my build. In short, 2.7RS spec, built from the crank up. I have about an hour total run time on my rebuild (I'm sorting lots of issues) and my oil pressure has me concerned. I installed a Supertec-modifield (supposedly never run) carrera oil pump from a fellow pelicanite. I have a VDO oil pressure gauge marked in PSI (not bar), and I'm pretty sure I matched it to the oil pressure sender I have (single pole '75). At startup, I have about 20-25 psi at 1200rpm idle which appears to be about normal. The idiot light extinguishes immediately on startup, and never so much as flickers. However, as the car warms and the RPM's go up, the pressure seems to "sag" a bit, and doesn't get much over 40 psi. Occasionally, the pressure will respond in a manner that appears more normal, and will head over toward the 70psi mark as the RPM's climb. Being scared ****less to grenade my new motor, I need to know if i should shim the pressure relief spring or check my sending unit and gauge. Is it possible to attach a manual pressure gauge that doesn't rely on a sending unit to verify that my gauge is right? Thanks all!!!!

--jaybird

jpnovak · 11-24-2010, 10:56 AM

Jay, I would start with a mechanical gauge to verify your dash reading. If you find a difference then look to the sender or receiver as the cause. If there is no difference then we need to investigate why you are seeing strange pressures.

I hope this is not related to the front mounted oil tank.

Ken911 · 11-24-2010, 04:02 PM

I have to replace oil pressure senders about once a year. but my car is a turbo so there is a lot of heat back there and also i drive it every day so i put a lot of miles on it.

jaybird840 · 11-24-2010, 04:29 PM

I can only hope that is my issue. I'm still trying to figure out how to plumb in a manual oil pressure gauge to verify....

turbo nut · 11-24-2010, 07:30 PM

Can you access the fitting by the thermostat.

jaybird840 · 11-25-2010, 03:30 AM

Yes, in a 914/6, both the idiot light sender by the thermostat and the sending unit by the chain box are very accessible with lots of room around them to work--moreso with the idiot light switch. I just need to know what type of manual gauge and where to pick it up. Also, does oil pressure vary with load on the motor, or is it directly proportional to the RPM's? It would seem that since it is gear driven off the crank that load would not matter. thanks!

BURN-BROS · 11-25-2010, 08:24 AM

Pressure only varies with temp and rpm. Did you update to the later spring and pistons? Bypass mod done? Also, the pressure relief pistons may not totally seat(or seat intermittently) if the passages were not cleaned properly.

jaybird840 · 11-25-2010, 08:51 AM

Aaron,

Yes, the bypass mod was done by Craig Garrett when the rest of the machine work was done. Craig installed the correct spring and pistons. I removed them prior to final assembly and checked (the pistons were the later style anyway--I can't remember if Wayne's book talks about how to check the springs.. My first plan is to manually verify the oil pressure, and if it looks like it's actually a problem--I can then attack the problem. Thanks for the input!

--jaybird

lucittm · lucittm's Garage

What brand and viscosity are you using in your engine? Aaron is correct about temp and rpm affecting pressure but the way it acts on different viscosity oils, is... different.

Mark

jaybird840 · 11:23 AM

Brad Penn 20/50. Also, it looks like HF carries an oil pressure tester. I think it should be accurate enough for comparison purposes with the electrical gauge. Off to HF on black friday!

jaybird840 · 11-27-2010, 06:52 AM

Well, I'm somewhat encouraged. I purchased the HF oil pressure tester. The kit didn't include the correct adapter to fit the idiot light switch, but did have a tapered thread that was the same pitch. I threaded it in just until it snugged up, and figured that if it leaked a bit, it would be OK. I started the car, and at 40F with BP 20-50, the car showed 5 bar of oil pressure. It also responded quickly and proportionally with a couple of quick blips of the throttle. I will go to the local hydraulic supplier on monday to see about sourcing the correct adapter so that I can actually do some road testing with the gauge and the oil warm. I have my fingers crossed! A bad sender or mismatched gauge would sure be a great problem to have!!

lucittm · lucittm's Garage

Jay,
Your oil pressure sender is at least 35 years old, it owes you nothing at this point. Order and install the correct pressure sender for your gauge and you will be a happy camper.

Mark