overheating: itís not just for beginners
Guys, I’ve finally been bit by the overheat bug that I have, myself, offered advice to others on over the years. My collection of 289 mustangs is now up to five, and I’ve owned one since I was 14 years old. There is literally no part of those cars I have not rebuilt and restored, and I feel I know them as well as anything else. I’ve had some short-term cooling issues that I have traced back to some minor issue, and I’ve always been able to remedy the situation. I’ve also been reading and contributing to threads on the topic of overheating for over a decade on this forum, and I feel like I know what to look for. This time I’m at a loss. Please help.
So the car in question is actually the car I’ve owned since I was a kid. It’s a 66 C-code coupe that I re-restored over the past couple years. The engine is a newly-rebuilt 66 289 with stock cam and heads. Bored .030” over. Since this car is not rare or special, I thought it would be a good candidate for a little modernizing. I’m using a Holley Sniper 2-barrel TBI unit (not controlling timing at this point). Base timing is set to 12* BTDC with 12* additional manifold vac. advance. I’ve been driving the car most of the summer trying to tweak my tune on the efi. The car is running well overall, but here’s the cooling situation: the car takes a long time at idle speed to reach operating temperature (20-25 min), but once it gets there, it just keeps rising all the way up to 220 (and probably beyond, but I’ve never let it go that long). Once I get on the highway, the temps cool down very quickly and stay down until I slow down or stop, then the temps come up, and much more quickly. I’d guess that an in-town trip would see temps above 200* consistently. I do have AC on the car, though it is currently not charged, and not using it (small leak somewhere I still need to find—for another day). The other modification is I’m using an AOD transmission.
As for my cooling system, I have a new aluminum ACP two-row radiator with large tubes (using the built-in transmission cooler). I’m using the original 7-blade clutch fan with a new clutch (I’ve changed the clutch twice now thinking it could be the culprit-never feels any different to my turning it by hand when hot or cold, but that hasn’t changed with the swap). I have changed from the thin metal stock fan shroud to a larger plastic one (3+ inches). This also had no effect. I had a high-flow 180* thermostat (Robertshaw style) installed originally, and changed twice to see if sticking may be the problem. I tried a 195* of similar design that was older, but tested in hot water to confirm its opening. This made no difference. I now have an OEM type 180* thermostat from NAPA installed. The one interesting difference is that the car now maintains the minimum temp better than before when in the highway. It was getting down to the 150s I presume because of the continual flow of the high-flow-type thermostats. Still the same overheating behavior. My water pump is a high flow cast iron (Tuff Stuff if memory serves). It appears to be circulating coolant very well by looking in the radiator when running. Lower radiator hose has the spring installed. Incidentally, I’m using 20w50 VR1 Valvoline racing oil with excellent oil pressure.
I’m checking engine temperature in three ways, the stock gauge is actually now attached to the thermostat housing, as the EFI CTS sensor is in the stock intake spot. This, of course, is unreliable, but still gives me a relative idea of coolant temps. The CTS sensor displays on the handheld unit in the car to corroborate, and I’m also using an IR temp gun on the upper radiator hose when checking temps. Although I wouldn’t trust any of these to be completely accurate, they do all agree with each other quite well, and I’m convinced that I’m in the ballpark.
When I assembled my engine, I took the time (as I alway do) to thoroughly clean out the water passages. I’m always amazed by how much sand, rust, etc remains in there even after hot tanking the castings. I use brushes, pressure washer, and rust solvents/degreasers until the water comes out clear. I rotate the engine on the stand and repeat that process to make sure everything gets out. As an additional safeguard, I did put on a Gano filter and did a Thermocure coolant flush. I was then using 50/50 (yellow) coolant, but added additional distilled water and Waterwetter. This seems to have made no difference.
Coincidentally, right after changing my fan shroud, I suffered a failure of my Pertronix ignition module, and took the opportunity to go back to points and condenser. I rechecked my timing after installing, and found that it had become retarded to around 6-8* BTDC. I’m not sure how that had happened, and wondered if the Pertronix was somehow stopping the breaker plate from returning to its normal position, artificially increasing the base timing. I thought I might be on to something, so, I reset that to 12*, checked by dwell and vac. advance using the advance function of my timing light...car still overheats the same way.
I’m idling at around 700 RPM, and have experimented with different AFR settings on the Sniper, with seemingly no effect on the cooling issue. I wanted to be sure I was not running a lean mixture though to contribute to the problem. I am arguably too rich at idle at 12.8, but that seems to be where the car idles the smoothest.
I really thought I was going for overkill on cooling on this car, so this truly has me stumped. My personal thought at this point is that, because the temps cool down so well at highway speed, that this is primarily an airflow issue. I just would think that my OE clutch fan would be getting the job done, especially with the new shroud. I’m considering trying a direct-drive fan, or even an electric one. I just don’t know what’s my best bet, and I hate to keep tearing this thing up without direction, and without knowing it will make a difference.
Thanks for reading the long-winded story. I wanted to provide as much background as possible, so we can bypass the usual rank-and-file cooling solutions. I appreciate any help and advice.