'78 v6 overheating Help!!!

KGeeks1978 · Jul 14, 2019

Hey everyone,
My 78 with the v6 is overheating after about 25 minutes. Mechanic is baffled we've been through a whole list of things to try on it and nothing seems to take. She fires up and sounds/goes great, until she gets too hot. Does anyone have any suggestions to look at?

ALSO, because we are cant figure it out, we are starting to look to other options, if we were to do a swap, v6 or v8, what other vehicles/year/make could we be safe working with??

LILCBRA · Jul 14, 2019

As mentioned in another thread, as I understand it there may be 2 thermostats for whatever reason. It sounds weird to me, but I guess it's been done. Otherwise we're making a lot of somewhat educated guesses....

KGeeks1978 · Jul 14, 2019

LILCBRA said:
As mentioned in another thread, as I understand it there may be 2 thermostats for whatever reason. It sounds weird to me, but I guess it's been done. Otherwise we're making a lot of somewhat educated guesses....

We will check it out, thank you!!!!

jozsefsz · Jul 16, 2019

It's typically just usual stuff - clogged radiator, bad water pump, bad thermostat, worn out fan clutch if you use one, a missing fan shroud. Worst case a blown head-gasket which would have other symptoms.

If you could list the things you've tried, that would help us give better suggestions. An engine swap for overheating sounds a bit dramatic.

KGeeks1978 · Jul 17, 2019

jozsefsz said:
It's typically just usual stuff - clogged radiator, bad water pump, bad thermostat, worn out fan clutch if you use one, a missing fan shroud. Worst case a blown head-gasket which would have other symptoms.

If you could list the things you've tried, that would help us give better suggestions. An engine swap for overheating sounds a bit dramatic.

Thank you!
We have a new thermostat, top and bottom radiator hoses, flushed radiator.
What other symptoms would we be looking for? It smokes like crazy when it finally gets hot!

LILCBRA · Jul 17, 2019

KGeeks1978 said:
It smokes like crazy when it finally gets hot!

Sounds like maybe you're burning antifreeze. Could be a cracked head or a bad head gasket? Are your levels staying consistent or are you losing antifreeze? And have you checked your spark plugs to see if there are any signs of contamination? Here is a site to help you with that.

Diagnosing Spark Plugs | Know Your Parts

Misfire issues may not be caused by a bad spark plug. Learn how to diagnose this and more with Know Your Parts.

www.knowyourparts.com

The specific section:

Coolant Problems
Internal coolant leaks can foul a spark plug and cause a misfire. The problem could be a leaking intake manifold or a head gasket, and the fouled plug might be localized to one or two adjacent cylinders. The burned coolant leaves deposits on the electrodes and insulator, creating hot spots that could cause pre-ignition and a misfire code being set.

When the plug is pulled, it might have a chalky appearance on the ground strap and center electrode. Modern coolants do not cause this type of buildup quickly due to the reduction of phosphate, zinc and other additives that can contaminate the catalytic converters. Unfortunately, it also means that drivers will run a vehicle with a coolant leak for several thousand miles while the plug becomes slowly fouled. In the past, the converter would become clogged and stop the engine before significant damaged was caused.

apple99Mustangdude · Jul 20, 2019

Could the radiator still be bad? Maybe fins inside actually collapsed - ? Keep in mind a brand new part can be bad also - like a new thermostat that does not open - seems like usually we "exercised" a new thermostat before installation, and would sometimes test them in a pan of water on the stove to make sure they were opening at the correct temperature - and closing again when cool -

jozsefsz · Jul 25, 2019

Another member just posted that a new radiator was the key to the overheating problems with their 2.8 as well. After 40 years, a radiator thoroughly clogged by corrosion is not unusual. They're still available for the 2.8 II's.

gregorh · Aug 10, 2022

not sure if this helps but I recently completely rebuilt a 78 mustang ii with a 302 (5L V8). Engine, tranny, diff, body work, and paint etc. a full restoration and im struggling with it running hot. the issue seems to be more prominent when outside temp is at/above 25C (77F), the needle climbs to 3/4 and holds steady when running at 90-110Km/h (56-68mph) the killer is when you get into town and you hit a red light the gage climbs to the tippy top of normal. With my ass clenched i make my way home, suffice to say highway driving something i avoid doing. After talking to older guys who ran them all say that they run hot, however im looking for way to keep this beast cool.

LILCBRA · Aug 11, 2022

Let's work our way through the "easy" stuff first:
How is your radiator - new, re-cored, stock or aluminum?
What radiator cap?
Overflow system?
Engine was rebuilt to what specs?
What water pump are you running?
What is your thermostat opening temperature and did you by chance verify that it opens at or near that temperature?
What kind of cooling fan - clutch, flex, electric?
Is there a shroud?
It's been verified that there is no (or very little) air in the system?
Coolant mixture is good?

Moving on to a problem that I've been guilty of that had my car acting very similarly - have you verified timing? I had my car timed directly at TDC for a while and didn't realize it. It ran well, but it would have cooling issues in heavy stop and go traffic, especially on warmer days. That might be worth checking into if you've not verified it with a light since this has begun.

LILCBRA · Aug 11, 2022

Oh, one other thing - do you happen to have a mechanical gauge to verify that your stock gauge is working well enough? One option you can look into using if you don't want to add a gauge inside the car is to pick up a radiator cap with a thermometer built in, like this:

Amazon product ASIN B000OF9LROView: https://www.amazon.com/Mr-Gasket-2471SDomestic-ThermoCap-PSI-SLVR/dp/B000OF9LRO/ref=asc_df_B000OF9LRO/?tag=hyprod-20&linkCode=df0&hvadid=241973068692&hvpos=&hvnetw=g&hvrand=15768141049803244305&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9010941&hvtargid=pla-523287798873&psc=1

LILCBRA · Aug 11, 2022

And another thing! How about your A/F ratio? Have you at least pulled your spark plugs and checked to see if you're running it lean? That can also cause overheating.....

gregorh · Aug 11, 2022

-radiator is recored (thinking of throwing in a aluminum one)
-rad cap is new (oem replacement i believe)
-overflow tank is good (when hot tank isnt fill the tits and when cold isnt bone dry)
-when the motor was rebuilt the cylinders had to be machined slightly and such there are bigger pistons however if this where the issue wouldn't it overheat no matter the situation? not just when i hit stop and go traffic
-water pump again is new cant remember the brand but is a replacment for D6ZZ-8501-B
-for the thermostat i did verify that i opens at the correct temp
-have fan shroud and fan is stock(bolds on the water pump and is driven by a belt)
-however we have not tried burping the system
-and the mixture is premix 50/50 (didnt have distiled water at the time)
-timing was checked with timing light
ill start with pulling the plugs and burping the system, then look into the gage and aluminum rad, thanks for the tips

'78 v6 overheating Help!!!

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