Lower oil pressure and compression

[kilrwail]

Last week I made a 1200 km round trip over two days in my ’78 Spider. After the first 400 km at a steady 4000 rpm (110 km/h), I noticed the oil pressure drop by about 1/3 during a 5-10 minute interval. I had an early Spider oil pressure gauge installed when I bought the car in 1978. It has two scales marked in white and yellow numerals. The normal reading is about 40-45 on the white scale. The engine temperature had been about 180-185 F when I started out, but crept up to 190 after about 80 km and stayed there. At the next service centre, I checked the oil level and added about ½ litre. For the remaining 150 km of my trip I kept the speed around 100 km/h and monitored the pressure, which stayed about the same, at 25-30 on the white scale. For the return trip I kept the speed at 100 km/h or less, the car didn’t use any more oil and the pressure remained the same. I am not seeing any smoke in the exhaust.

After getting home, I did a compression test, twice. The compression in all cylinders was 120 psi, whereas a couple of years ago it had been 150 psi in all cylinders (with the same tester, at 104,000 km). I did a leak down test and could detect a low level of leakage only through the dipstick tube, suggesting piston ring degradation. There is no evidence of oil in the coolant.

The engine has never been rebuilt; the head has never been off; the car has 116,000 km on the odo. I have used Castrol GTX 20W-50 for the last 25 years or so. At about 115,000 km I had a very brief (seconds) overheating situation, which led to a re-coring of the radiator and a new fan switch (the thermostat is fine). The engine runs as well as it ever has.

I am wondering if the oil pump is beginning to fail, resulting in the lower pressure. Could it be coincidental that the expected wear in the piston rings has resulted in the reduced compression? If I decide to replace the oil pump, should I go the distance and replace rings/recondition the head? So I’m wondering how long I should wait before doing a complete refresh. Any thoughts?

[So Cal Mark]

before jumping into anything, I'd connect a mechanical oil pressure gauge and get an accurate reading

[kilrwail]

I knew you'd say that! There are a couple of engine builders in the neighbourhood where I can probably borrow one. Do you know the thread size and pitch of the fitting?

[htchevyii]

I just did the same thing and the gauge or sender was the problem. THe parts store sold a metric gauge adapter kit that included several adapters, one of which was correct.

[kilrwail]

After a long break for a big race weekend and visiting brother, plus issues with the other cars, I finally got a mechanical oil pressure gauge and the necessary adapter (by the way it's M14 x 1.5 thread). The oil pressure shows 45-65 psi across the rev range at all temperatures, so it must be the electrical gauge or sender that's slipped. We are good to go to FFO!

[racydave]

Peter, have a good time for us too!

[kilrwail]

Dave - Thanks, we'll try. Too bad if you can't make it this year - we'll hoist one for you. There's a convoy of at least 10 cars coming from Ottawa, through the Adirondak back roads. Should be a blast just getting there.