Ok, well I just replaced one of the tranny cooler lines today because it leaked at the transmission flare nut, and noticed something funny. I had the car just idling in my driveway for about 5-10 minutes to make sure I had fixed the leak. When I shut the car off, the temp was at nearly 260 and I heard what sounded like the fans still spinning and hitting something. I popped the hood and it was coolant rushing into the reservoir; no leaks that I saw, just my reservoir level climbing. It rose almost 1" above the "HOT" level. Normally my car will idle at just under 220 when warm. Any ideas why my coolant was so rapidly filling the reservoir?

 

It filled the reservoir because the pressure at the cap exceeded it's capacity to keep it from venting. This could be completely normal.

The cap could be de-rating and venting before it's rated pressure or your system could be exceeding the rated pressure due to temp increase. OR everything could be working fine. A new cap is around $3-5 or you could conduct a pressure test of the system.

Your fans should, of course, be on if the temp was that high.

 

I'll look into a new cap; thanks for the tip. Regarding the fans, the previous owner wired them for constant operation - which I know is more detrimental than anything at high speeds, so I plan to rewire them to a switch or stock setup. Would the cap failure result in a high temp reading?

 

Cap failure could cause high temps, but the cap would be venting BEFORE it overheats, not the other way around. Don't know which happened in your case.

Go back to stock for your fans. The pieces/parts can be had all over the JYs.

 

Ok, do you mean IF the cap caused the overheating, it would vent first? I don't know which happened first, but do you think a faulty thermostat could cause the high temp? And what psi rated cap should I get - stock replacement, or something else? I've heard lower psi (~13) rated caps on non-stock motors can decrease operating temps.

 

with all cooling problems/leaks a pressure test is a good idea . most parts stores like advance , napa , autozone etc. have testers that you can check out or rent . some will press. check your system for free . after all you are going to buy the stuff to fix it from them . remember some of the hoses may be 20 + yrs. old . anyway not a bad idea to replace old hoses and clamps , may take care of future problems . good luck .

 

I know the hoses are less than 5 yrs old, because they were replaced by the previous owner with the engine rebuild. I'm gonna replace the old cap anyways, and if the hoses are good, what else could cause it to drop in pressure?