One test I did not perform yesterday, but have done many times before, is to let the car idle and watch the oil temp. It will rise steadily, even with full oil flow through the cooler. I have to use a fan or drive the vehicle to force air flow through the cooler or it will overheat just idling. Maybe you can perform a similar test to to baseline what is happening with yours. Note the temperature at which there is flow through the oooler.
I did a similar test on my 1978 a few months ago to check operation of the cooler. That car has a stock an on-engine cooler and a Carrera front fender cooler (no fan). All temps except "Gauge" are measured with an IR gun. "Sensor" refers to the temp sensor screwed into the R chain case. All others measured at the external thermostat.
Time |
Gauge |
Sensor |
Scavenge |
To Tank |
Cooler |
Cooler |
|---|
| 1 | <120 | | 78 | 80 | 78 | 75 |
| 5 | <120 | 117 | 95 | 93 | 73 | 73 |
| 10 | | 119 | 119 | 112 | 75 | 75 |
| 15 | | 148 | 140 | 140 | 75 | 75 |
| 20 | 175 | 164 | 143 | 140 | 75 | 76 |
| 27 | 180 | 174 | 179 | 181 | 179 | 181 |
| 35 | 180 | 182 | 175 | 175 | 187 | 189 |
| 40 | 181 | 189 | 197 | 182 | 192 | 195 |
comments/observations:
- External thermostat appears to open around 180F (as expected).
- Fender cooler doesn't do much when car not moving.
- Don't take the IR gun too literally - the surface properties can affect the reading.
This test and my general experience idling the car that has no on-engine cooler make me suspect you have an issue with the thermostat.
I mentioned the plumbing before because the on-engine filter interferes with some generations of scavenge lines (there are at least 3) and racers often replace it with flexible lines which could be routed different than stock lines.
Happy hunting,
Steve