[T3] Thoughts on overheating

[Phil Hof]

As many of you know, I have seen overheating (and apparent overheating) of my ’71 Fastback since it was rebuilt  The Invasion journey was trying, but we made it.  This is with a recently new/rebuilt long block from a trusted mechanic.

On that trip I had loose points which messed with timing, which caused some of the overheating and pinging.  But with the ISPwest gauge I had installed in the full low line indicated I was seeing temps as high as 280-300F.

The points are tight now, and the timing is stable.  It no longer pings.  But the temp gauge still scares me.  I think I have the root cause, though this may be a bit of wishful thinking.  Here’s what I found.

The sending unit is in the line between the galley (right after the pump) and the filter.  Seeking to protect it from debris, I located it inboard of the left HE.  But this puts it in the flow of exiting cooling air from the 3-4 head and cylinders.  I hadn’t really worried about it, but I think this is the problem.

I took out my laser/IR thermometer, and found the sump was 220, the oil cooler was about the same (the bit I could see on top was slightly cooler but it’s hard to be precise and hit the fins, since they are of course hidden by the tin).  But the sender and nearby fittings were all higher, and all over the place.

The sender was 240 or so, but the base of it (where the element is located) was more like 160.  The fitting it screwed into was 280, and the nearby fitting was 300!.  The filter housing was also about 220 or so.

I now believe these fittings are absorbing the heat from the cooling air, and that is what I am seeing with my gauge, not oil temperature.  I also think I need a longer insulating sock on the lines so they do not pick up heat, too.  I have one on the section near the HE itself, but one in the stream of cooling air will probably help a little, too.

To move the sender outboard, I need to order more lines and fittings (and figure some sort of debris shield), so it will take a bit to set it right (or at least test my theory), but I am breathing again with the likelihood that there is little or no actual overheating.

220 is a bit high but not unreasonable on a hot day, and with the slightly larger displacement and higher compression.  It should be manageable with a good quality oil (now 30wt Valvoline VR1 plus a zinc additive).

Two flies in the ointment.  First, when it is really hot out, and I have been driving on the freeway, and then I let it sit at idle, I see my pressure is 10 or even a bit less, triggering my oil light.  This may not bother some, but it bugs me.  I am thinking of going to a multi-grade VR1 oil so that it doesn’t thin out quite so much when hot, maybe a 20-40.  This may lower initial cold pressure (now at 50-60psi) and increase it when hot.  Second, I am told there is an oil smell when following the car.  I haven’t been able to locate a source, though the bottom of the sump is slightly wet, as are my six acorn nuts on the plate.  Something to track down, though I see no apparent leaks to deal with yet.

So my fingers are crossed while I try to find the time to measure for hoses and fittings from Gene Berg.  But I am breathing a little easier with this thermometer data.

Any thoughts?
Thanks,
-Phil