‘91 302, coolant loss. Any other suggestions?

[Cfehrenb]

Wondering if you can offer some suggestions or strategies for addressing an engine coolant loss? I have ’91 fox body, 302, stock.

Over the weekend, my WATER gauge suddenly registered a temp of 250 (pegged the highest). Appropriate range is 180-190. It was appropriate within 2 minutes of the sudden spike.

I allowed it to completely cool down. Then completed a 2:09 hr backroads cruise. Throughout trip, temp appropriately registered as 180-190.

On my trip back to LaCrosse, cruised interstate at speeds approximately 80-85 (Wisconsin has HIGHER speed limits J), I noticed that once again, the WATER gauge temp registered 250. I pulled over and let it cool down. I checked the radiator & it needed coolant. Purchased and dumped in almost an entire container.

Back on the road, within 1 hour of driving 80-85 mph, again noticed WATER gauge registering 250. Seems that I was out of coolant again. I pulled over to assess where coolant might be leaking. No visible leak on/around the engine. Only leaking was from the overflow on the radiator recovery unit.

I searched suggestions online. One individual suggested: “The first thought that comes to mind is that the coolant is getting pushed out the reservoir overflow tube when the engine warms up because of a bad radiator cap.” While this sounds suspicious to me, visually that is what I observed at my last stop. Could it be that high temps were reached when driving on Interstate & not backroads. If radiator cap failed, it was not able to relieve pressure, thus pushed it to the recovery unit and out the overflow. ?? What are your thoughts?


My plan is to pressure test radiator cap (I’ve already bought a new one – I’ll replace the rad cap if I can’t determine any other cause). I plan to pressure test the coolant system this evening.
When I parked last Saturday, the oil dipstick did not seem to have any foaming substances within the mixture. I’ll check for that again tonight.

Any other suggestions on what might be going on?

Again, I run a ’91 fox body, 302, stock.

Thank you for any and all suggestions!!
CraigF

[Railroad]

I think your thermostat is stuck and you are over heating. This will push out the coolant.
Do the simple things first.

[JMan]

I’d check the thermostat and radiator cap. Both can cause this independent of each other.

[steno]

Craig, when it was dumping coolant, what temp did you have with your temp gun?

[Cfehrenb]

Sten - with temp gun, on upper radiator hose, temp was around 190. Radiator metal cap registered less than that.

[Jeff Kleiner]

My first thought was a faulty thermostat also.

Jeff

[steno]

Craig, the stat may well be your culprit! Make sure the bleeding/ purge vent on the stat is on top.

[cv2065]

Remove your t-stat and place in a pot of boiling water and see if it opens and closes around the correct temp. I'm assuming its a 180. If that checks out, replace your radiator cap and ensure that it is the for the correct pressure. Check for blockages in your radiator and ensure when it is running that the upper hose is hot, and the return hose is a little less than hot. Squeeze the return hose itself and check its integrity, ensuring that it is not collapsing when at higher RPMs. Keep your cap off on warm up and ensure that the water is circulating in the radiator, especially when the t-stat opens.

If all that checks out then I'd be looking at my water pump and leaks around it. Check your oil as well and ensure that it does not have coolant in it.

[Redlinejoev]

Craig,
in addition the the stat and cap, check your lower hose. Where the upper hose tends to expand, the lower hose does just the opposite and collapses, hence the coil spring inside the lower hose of most vehicles. Squeeze the hose to see if a coil spring is present.

Joe

[scottiec]

Also check your oil. Make sure it’s not milky.

[Itchief]

Criag

Have you installed a radiator cowl cover without the cowl cover air maybe going around the radiator at higher speeds cause the motor to overheat

Good luck

Rick

[Cfehrenb]

Great! Thank you for all the suggestions. Here is an update: oil is not foamy - r/o head gasket. I was not able to pressure Check the radiator cap or system - the unit I picked up from Advanced Automotive did not have adapters large enough to pressure check rad cap/rad system. I replaced the thermostat. Visually, comparing the old vs new, it appears the old was stuck open. (I’ll attach pictures - old on the left, new on rt.). I went for a quick drive - allowed the system to warm up. If it slows up raining this week, i’ll Keep testing.
A positive sign is that my fan came on automatically. That hasn’t worked in over a year. Makes me believe that thermostat is the contributing factor.
I’ll keep everyone updated. Thanks for the suggestions!
CraigF

[Cfehrenb]

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[Cfehrenb]

Can anyone see those attachments? They uploaded awkwardly

[cv2065]

Visually, comparing the old vs new, it appears the old was stuck open. If it was stuck open, and not closed, then you would not get an overheating situation. The car would just take longer to warm up and my not run efficiently as its hard to get to the correct operating temp. Heater might not work properly either. Not sure about the fan, but it seems like your over heating issues are beyond the fan itself as you are getting it when running at highway speeds, which pulls the fan out of the equation.

[Ian G]

I see you have a 91 mustang (27 years old). The rad could be restricted from old age. I had a car that would stay at normal temp in town but at highway speed after a while it would over heat. I got home by turning the heater on full heat and the heater fan on full. The heater core acted as a radiator. I went about 50 miles like that to get home. Replaced the rad and all was good.
Hope this helps.
Ian G.

[Cfehrenb]

Cv2065 - good point. I just now completed a boil test on the old thermostat. It contracted/separated from its bace. In your opinion does that then eliminate the old thermostat as a primary culprit?

[Cfehrenb]

Criag

Have you installed a radiator cowl cover without the cowl cover air maybe going around the radiator at higher speeds cause the motor to overheat

Good luck

Rick

Hi Rick. Yes I have a cowl cover.

[AC Bill]

A weak spring in a rad cap, could be the culprit.
Maybe it's only holding back 5-10 lbs pressure. As coolant is lost out the overflow, the higher the temps, thus more pressure. So then even more coolant is lost, till it finally becomes evident on the gauge. Most shops have a tester, so that would be my first step.

[cv2065]

Cv2065 - good point. I just now completed a boil test on the old thermostat. It contracted/separated from its bace. In your opinion does that then eliminate the old thermostat as a primary culprit?

Sounds like it, but I’d probably just test the new one and put it in while you have it apart. Replace your radiator cap as well. These are cheap parts so you’re not out much by swapping and eliminating them as a possibility.

[Cfehrenb]

Quick update. I was able to warm up the engine yesterday. The previous thermostat was rated at 180 - the new one I installed is OEM at 195. During the 15 minute run, the water did not climb any higher than approx 195. That was good to see. I was, however worried when oil temperature started to climb - leveled out around 205. It’s never reached those levels, yet due to the 195 thermostat, that makes sense. Anyone have worries about my 302 fox body oil temp warming up to that temp?

[cv2065]

I would stick with whatever t-stat that engine calls for. If it was a 180, I’d stick with a 180. I wouldn’t worry about the oil temps. Pressure yes, but not temps for a garden variety street engine.

[Big Blocker]

If it's a stock block 302 (5.0) with EFI, stick with the 195° T-Stat. EFI engines require the higher temp stat for the O2 sensors and the EEC, so it can switch between open and closed loop operation. If it's carb'd, 180° is OK.

FWIW, a bad rad cap will allow more then normal overflow. If you don't have a properly sized overflow/catch tank, excess water could be pushed out and when the engine cools, there isn't enough to pull back to keep the engine full. That condition leads to air pockets in the head/upper manifold area and hot spots - causing overheating.

205° for oil while driving isn't bad at all . . .

Doc

[Cfehrenb]

If it's a stock block 302 (5.0) with EFI, stick with the 195° T-Stat. EFI engines require the higher temp stat for the O2 sensors and the EEC, so it can switch between open and closed loop operation. If it's carb'd, 180° is OK.

205° for oil while driving isn't bad at all . . .

Doc

Awesome! Thanks!

[steno]

I hope you have it licked!

[Jeff Kleiner]

...Anyone have worries about my 302 fox body oil temp warming up to that temp?

No. You're good to 260ish, a little more if it's a pure synthetic like Royal Purple.

Jeff

[Cfehrenb]

Okay. I’ll run with this set up & see how it shakes out. As always, thank you for all the advice you guys offer! I appreciate it!
Craig f