coolant system pressure

It could be that the new cap is bad, or the it is not the right fit for the radiator. Meaning the the seal doesn't make good contact with the top of the spout.

You could rent or buy a coolant pressure tester and test the system. the would help eliminate the cap from the system. I think there is a different test for the cap as well, but I am no familar with it.

Just my 2 cents

How did you ascertain that you have no pressure? The above thoughts about using a tester mirror what I'd have recommended too.

Also, there's a good chance the fan set-up is your issue. I don't know about San Antonio right now but it's in the 90's out here already (AZ). The fan's gotta be up to par.

Good luck.

need a better fan, like a MKVIII fan-your issues will be gone forever.

dude- you dont have enough air crossing the RAD, MKVIII fan and be done with it. I had the same exact prob's, Now-I can sit with A/C on idling for an hour in bumper to bumper traffic and not even have to look at the temp gauge.

If the AC is on though, it starts a slow steady march (i would say 5-10 mins of sitting) to 210-220 if you're sitting still. As far as no pressure, i can squeeze the top radiator hose and tell there is no pressure, and I can open the radiator cap practically right after turning off the engine.

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Water boils at 212* at atmospheric temp. so it cannot be at 210-220 when you open the cap with any pressure. It would be boiling and steaming with no pressure and gushing out with pressure. Get an aftermarket gauge and verify temp.

I live in AZ and my 89 GT is doing the exact same thing. My setup includes a fluidyne aluminum radiator, brand new stock fan, new heavy-duty fan clutch, silicon hoses, and 180* thermostat. I took off the underdrive pulleys and bought some stock ones. I just bought a new radiator cap that I am putting on in a few minutes, but I have not pressure tested the cooling system. My oil is not milky...I have an autometer temp guage and it consistently measures about 190* on the freeway with a/c on in 105-110* AZ weather. However, if I idle for about 5-10mins with the a/c on it will climb to 220*. Originally, before I bought a new stock fan/fan clutch, I bought a Mark VIII fan, but I didn't want to do a 3rd gen alternator to run it (too much $). My question is:

Is a 93 cobra grill insert + some type of junkyard pusher fan from an a/c condenser a feasible option? Or will it obstruct flow at 40+mph? And yes, I realize that all of the money I have spent could have given me the 3rd gen alt. upgrade already, but I already have wasted enough money on this setup!
Also, does anyone know where to get a nice reliable 180* T-stat that opens all the way or should I go down to 160*?

mhultstrand said:

I live in AZ and my 89 GT is doing the exact same thing. My setup includes a fluidyne aluminum radiator, brand new stock fan, new heavy-duty fan clutch, silicon hoses, and 180* thermostat. I took off the underdrive pulleys and bought some stock ones. I just bought a new radiator cap that I am putting on in a few minutes, but I have not pressure tested the cooling system. My oil is not milky...I have an autometer temp guage and it consistently measures about 190* on the freeway with a/c on in 105-110* AZ weather. However, if I idle for about 5-10mins with the a/c on it will climb to 220*. Originally, before I bought a new stock fan/fan clutch, I bought a Mark VIII fan, but I didn't want to do a 3rd gen alternator to run it (too much $). My question is:

Is a 93 cobra grill insert + some type of junkyard pusher fan from an a/c condenser a feasible option? Or will it obstruct flow at 40+mph? And yes, I realize that all of the money I have spent could have given me the 3rd gen alt. upgrade already, but I already have wasted enough money on this setup!
Also, does anyone know where to get a nice reliable 180* T-stat that opens all the way or should I go down to 160*?

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Alright, a fellow AZ'er (Tucson here. Sounds like Phx area for you, from the temps you noted).

For starters, your temps sound fine (not out of the ordinary) given the conditions.

I'd not do a pusher fan myself. As you noted, they can be an impediment to air flow at speed.


For your stat, get a Mr Gasket 180* unit. They're balanced, open fully sooner and have a jiggle valve. I dropped ~8* just from adding one of these (from a new parts store 180*F stat).

I know you dont want to hear it, but I think an issue you're seeing is that at idle, your alt output is low enough that it's slowing down an E-fan (if you've tried running the Mark VIII. I wasn't sure).

Otherwise, what did you use for a mech fan blade and clutch? On my 88, I had good results with a FAL metal replacement blade and Imperial HD clutch. It's not as good as a Mark VIII but works decently. I dont think this combo would cool you down hardly at all from where you are now - certainly don't spend any more money on mech fan components (HD clutches are pricey, ain't they).

I hate to say it (and you'll wanna slap me) but cooling is so important down here (Southern AZ), get a 3G (junkyard units work) and get that Mark VIII going. In the end, it will be cheaper than hosing with other things and is a known and proven combo.
If you hit the Lotto, get a Dc Controller for the fan.

If you're not doing it already and want to, drop the coolant/water stoichiometry down (more water, less coolant in the summer). Water rejects heat ~2.6 times more efficiently than E/G coolant.

Random thoughts.

Good luck bud.

Thank you very much for all the help! I will get that Mr. Gasket stat and I will be receiving the cobra grill insert in the mail shortly. I was told to stay away from metal flex fans as they tend to fly apart. I bought the plastic OEM style fan blade. I never actually put in the Mark VIII, it is hanging out in the garage. I thought it would be too hard to fit because it looked like the clearance between the fluidyne and the water pump is worse than stock. Also, I was hesitant with the wiring...

Summitracing- Mr. gakset 180 T-stat

And forget the cobra grille insert- Just be done with it and get the MKVIII fan hooked up-Say goodbye to running hot issues. Dude-a 130 amp PA perfromance alt is only $200... Your problem is your not getting enough air flow through the rad, The fan will correct that.

Later Ant

Go with the electric fan. I did, now no more heating up issues, stays at 182* in traffic with 90+ Maryland heat and humidity. Very happy with the e-fan.

Good luck....

1bad86GT said:

I have an autometer sportcomp water temp gauge, and i know what temp. water boils at. Now I've never got out of the car and tore off the cap after shutting it down at 220. Normally if i've been going down the road its at 180 or so, and when I check, there is little to no pressure in the system. If it heats up over 200, like you said, it probably has some pressure, or it would be boiling off. I'm just saying under normal driving conditions, without getting hot, there is no pressure like there used to be when everything was stock.

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Your gauge is probably correct, it may be around 220F without boiling. Remember pure water boils at 212F, but a 50/50 mix of water and coolant raises the boiling point to 223F.


The fact that it becomes a problem at idle indicates that you need better airflow across the radiator or a larger capacity radiator.


I would do as others suggested. You can gain some cooling efficiency by decreasing your ratio of coolant and water. A 70/30 would be good. You still want some coolant because it has inhibitors that fight corrosion and lubricate the water pump. Water transfers heat much better than ethylene glycol coolant so this will help. Also, a Lincoln Mark VIII electric fan and a Mr. gaket 180 themostat will help too.