Engine Temp Rising During Normal Driving


May 13, 2006
Hello all,

It's been a very busy summer for me so I have recently just begun to dig into this issue. Let me begin by saying that a few years ago I took some guys out to lunch in my car at work and the serpentine belt flew off right in the middle of a construction zone. Needless to say the cause was the alternator pulley was off by about 1/8" from the rest of the pulleys and just happened to chew its way through and when it broke it wrapped itself around the water pump about 942580342583 times. It boiled over and the temp gauge went well beyond the 280 degree mark on my mechanical gauge with me limping it and shutting the engine off on downhill slopes all the way back to work. Anyways, on my way home, I filled it up with water and all seemed well. It seems ever since then I have had issues with cooling to some degree or another. Come to find out after that my thermostat was stuck open and I think that was causing a lot of those issues after driving for long periods of time.

Fast forward a few years I put a new top end on the engine and I cant seem to get the engine to run cool enough for my own comfort. I put a mark 8 electric fan, a 180 degree thermostat, a brand new aluminum radiator. The car runs like a top and has all the power in the world; launches like a rocket, no missing, no smoking, or anything indicating a problem. Head gaskets are installed correctly, I done a compression check some time ago, and I checked the coolant for combustion gasses today. All turned out well.

I have recently gotten into the world of tuning and purchased a quarterhorse and have been working on my own tune. Working on my laptop at idle, the car will sit and run forever with the ECT reading 178 - 180 degrees. The minute I take it out and start driving it, the temp begins to climb under acceleration into the 200's. It was fairly warm today, about 90 degrees and humid but I sort of gave it the benefit of the doubt and drove it anyways. The temp hung around 180 for a little while and quickly began to climb into the 200's under acceleration. During deceleration, the temp will come down to around 188 or 190. When I pulled into the driveway, I shut it off at 204. Last time I datalogged, I was up to I believe 210. I feel that if I keep going then it will keep climbing, so I usually just find the shortest route home to cool it off. I am stumped. The only thing I can think of is possibly a radiator cap not relieving pressure when its supposed to? Or still air in the system. Or maybe my timing is off. I've searched in hopes with others having the same issue. I am hoping some of you can throw out suggestions. Any help is appreciated.

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This is a big reason why I pulled it out
Mod Dude
Feb 18, 2001
200 degrees is not that hot for a modern efi engine. Take any car off a dealer lot and chances it runs right around 200-210 degrees. This is where they run efficiently.

With that said, I can understand how uncomfortable it is to watch the temp Guage creep up to 210 and beyond. Most fox guys target 180-195 as an ideal range to be in for most driving

Airflow at speed would be my first suspect. How is the defector under the radiator and ibside the engine bay down by the sway bar mounts? Are they there and in good condition? Is the ac condenser still in front of the radiator? Is it clogged with crap and bent fins?

How is the engine running? Any codes? Specifically lean codes?

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SN Certified Technician
Aug 27, 2012
In the garage
Couple of questions

Any vacuum leaks
What is your fuel pressure set at
What is your timing set at
do you have u/D pulleys
do you have air in the cooling system

If you are experiencing 90+ degree temperatures like I am in the South. expect to see 195-200 temps, especially with alum heads and /or a a power adder. The heat and humidity arent helping any. IF anything, it sounds like an airflow or clogged radiator possibly.


May 13, 2006
Thanks for the replies/suggestions! Turns out that I do not have the air dam beneath the front of my car. I will definitely get one and put it on. I removed the A/C condenser over the winter since the whole system leaked terribly anyways and did not have much desire to fix it. I may decide to put it back on in the future..we will see. But for now I enjoy driving with the windows down anyways. I don't have any codes and the engine runs great. I may be a little bit lean but I need to look into my last datalog and go from there. I checked for vacuum leaks and my fuel pressure is set at 42 lbs with the vac line disconnected. My timing is set at 12 degrees, but I may back that off to 10 in the near future. Factory size pulleys and I feel like I have gotten all the air out of the cooling system.

I took the car out for a long ride yesterday since it was a much cooler day (mid 70's) and it ran between 185 - 195 the entire time. I kept the RPM's way down though. I kept them below 2k as much as possible. As I accelerated up hills though it got closer to 200. But I will definitely start by putting on the air dam beneath the car and flushing the radiator. Although it is brand new, a piece of gasket or other debris could be causing issues. I am very meticulous when I have anything apart exposing internals, but anything is possible. I thought maybe my lower hose was collapsing while accelerating but I haven't been able to witness that. When I put the new top end on, I did put a new lower hose on since the old one was looking a little beat, and the new hose did not have a spring.

So I got a few things to try.

Thanks again!


Sep 4, 2016
Usually a soft/bad lower hose will only collapse under throttle, so if the temps are climbing while driving, it's certainly a possible culprit. I'd swap the lower hose and see if anything changes. The lower air deflector does make a difference under normal driving, but it doesn't sound like it would cure the problem of rising temps under acceleration like you're experiencing. Good luck!