My engine use to run cold according to the temp gauge but it turned out to be a failed sensor, so I replaced it with a newer stock temp sensor from a Mustang which looked identical, at first it ran fine then the temp gauge started reading VERY HOT when in stop and go traffic, I hooked up an autometer temp gauge to get some actual readings and its not as bad as it seems. When on the full H mark on the stock truck gauge my autometer gauge read 230F, which isn't as hot as I thought, when I am running on the highway the autometer gauge reads 195 right on, so thats good, but its the stop and go traffic that worries me. When moving the temp gauge will read 210-220 and when stopped it goes up to 230 and might go higher, didn't sit for long.

What could cause this? I am going to flush the engine and rad, but did a mini flush when I changed the sensor as coolant poured out the manifold. Coolant looks green but with a slight rusty color, but not much, more of a puke green. I herd a fan shroud really helps, could this be the problem since I am not running one? I never had this problem before without the shroud so I am not sure if its the cause or not, what are your thoughts?

 

Sounds like you answered your own question. You got nice temps when running down the freeway shoving air through and it climbs when you are reying on the fan. You need more air at low speed. A shroud is the first thing to have. Look at the clutch also, if you have one. Dirty (iron filled) coolant doesn't help, but it has to be pretty thick to affect efficiency. Anitfreeze raises the boiling point (a very good thing), but actually has poorer heat transfer than plain water; thus the 50/50 mix. GL

 

yeah I figured the fan shroud would help but was just confused why it ran fine until just lately, it never used to overheat. I was suspecting maybe the clutch but how do you check that? The fan spins when the engine is running, it always spins actually, even whe you start it on a cold morning it will be spinning right away. I am new to clutch fans, all my cars have electric fans.

 

use a 13lb rad cap and 50/50 mix of coolant to bring the temp down some.

 

whats the difference with the rad cap, how does that change things?

 

the higher pressure keeps the temp down sort of like a pressure cooker it raises the boiling point.

 

The clutch should offer resistance when the fan is turned by hand. A free wheeling fan is not turning enough.
A rough air-flow test is stick a red rag to the radiator, it should stick completely, at idle and operating temp.
The static pressure (the cap) raises the boiling point.
Seeing how you got by without shroud before, and nothing has gone haywire with the engine, you are down to a few things: pump, clutch, radiator. (Also it is unseasonably warm there?, may just be mother nature stretching your limits for you.) GL

 

its been hot all summer but less humid the last two weeks so I don't think its thats. My coolant doesn't look too bad so I don't think the rad is overly plugged, unless its just not effecient anymore. As for the pump, would't the weep hole be leaking if the pump were bad? I don't think it is. Thats why I am guessing my clutch fan maybe. Then again the rad cap is very old on my rad so could that possibly be a factor? Once in a while a bit of coolant will come out the overflow tube, I have no catch can so it goes on the ground, coudl this be a cap problem?

 

Radiator caps prevent boilover, they do not change the heat transfer characteristics of the coolant.
For example: you run water and no cap at all, the water boils at 212 (disregarding the for simplicity the higher pressure it is under inside the engine where the water pump creates back pressure against the thermostat restriction). Steam does not act as a coolant. When the operating pressure is raised, steam is prevented from forming. Plain water at 15 PSI goes to about 250. 50/50 boils at only 227 at 0 psi and 265 F at 15 PSI. So yes, if your system is not maintaing pressure, your coolant starts boiling and it will not be moving heat anymore. Most shops can pressure test your cooling system and your cap, or you can make your own guage with coupling for compressed air and charge the system yourself and see if it holds. (Ever seen plumbers check the water lines on a new home for the building inspector?)

Water pump impellers can and do wear away before the seal is gone, resulting in not enough flow. When I am in doubt, I temporarily remove the T-stat and observe the flow through the open cap. You should see strong flow at idle and a torrent with 2000 RPM. This can be checked with the T-stat in, but I like looking at cold water and not having to guess what the T-stat is doing.

How did it do on the "red rag test"? What happens when you try to spin the fan by hand, does it free-wheel?

 

I didn't get a chacne to do the rag test not check the fan today, the last 2 days I have spent installing my new kitchen cupboards, been doing a ton of renovations on the house lately and its been taking up a lot of my time. I'll see if I can get to it tomorrow though, as I want to try adn figure this thing out sooner then later