Probably an easy one, but thought I'd throw this to the boys to confirm:

'95 EB Bronco, 351, 150k miles.

My GF has been driving the Bronco this winter, but we just switched. Very cold here in Michigan now, and so when I went to drive it yesterday, I noticed it didn't give out any heat in the first 5-7 minutes. (its usually unbearable by this time).

I then noticed the guage rising to the far right (overheat). Started to worry, then all of a sudden it dropped way back to the left -- then heat blows. Moments later, back to the right, and no heat.

Did this the whole way home (20 miles). I asked the GF and she said this has happened all winter. (why did she not complain about no heat? i dunno. oh well).

Haven't even opened the hood to check anything, but isn't this a Thermostat 101 issue?

 

That's where I'd start. Sounds like it's hanging up. I'd swap it out and if you find rust and crap, flush the system good.

 

It sounds like it could be the thermostat. I'd change it out without thinking twice just to be sure, especially since it's a pretty easy procedure and not very expensive. I had a thermostat fully seize in the open position and it would take a good 30 minutes for the engine to fully warm up in the winter, and when it did, the heater was never very warm. New thermostat and it was fully warmed up in 5 minutes with a toasty heater.

IF that doesn't solve it, check for a plugged heater core (for the heater at least) by getting the engine hot and then feeling the in and out hoses on the heater core. If ones hot and ones cold, it's plugged.

Lastly, stock gauges are known for this problem. They fluctuate on my '81 as well. Next time it goes up, see if your fuel gauge and oil gauge read a little higher than they did before, and a little lower after the heat drops back down. If so, your voltage regulator inside the dash is going finicky. Not necessarily important to change, but you'll know why things act the way they do.

 

A good t-stat that is installed backwards will also give you this same symptom.

 

Not necessarily a thermostat issue at all... other than it is working properly but the cooling system has far too little coolant in it. The heater core is the highest point in the system. Therefore, if the coolant level is too low nothing will ever flow to the heater core. However, as the engine is running and the small amount of coolant in the system OVERHEATS (because there isn't enough in there to do the job effectively even in a Michigan winter) it expands and boils up high enough in the system to push some if it up to the t-stat which allows it through to the heater core. The run through the frigid heater core and equally cold air trapped in the hoses and core, cools the fluid far enough that the thermostat snaps shut. When the low coolant level cycles through this again, the situation repeats itself. Hence the intermittent and brief periods of strong heat.

Check the coolant level with the truck cold at the radiator cap NOT the overflow reservoir. The reservoir can show everything as being fine simply because most people check the coolant before they go anywhere so the t-stat is closed and there is enough coolant for the system this way but not when the thermostat opens.

I'm not saying you don't have a dead/dying thermostat by now anyway. I merely suggest that the symptoms are indicative of this unique issue. I would be interested to know for certain. So if you check things out, let me know what you find.

BTW, the fix for this problem requires filling the system at the radiator cap with the engine cold but running. Once the radiator is full to about an inch below the surface that the cap seals against, let the truck run until the thermostat opens. If my theory is correct, you will need to add coolant again after it does. Once you have topped off the system at the radiator cap, replace the cap and top off the reservoir to the "full hot" mark. Keep any leftover coolant in the truck with you... that much coolant has to have seeped out somewhere... keep an eye on it.

 

thermostat then the little guage sensor for safe measure. together is maybe $15 (thermostat, gasket and the sensor) all located within 4 inches of eachother

 

And the winner is... Unsure!

I thought GreyStreak was onto something when I read his post, but thought I'd just change out the Thermostat for extra measure.

I finally got to digging into it, but its pretty dang cold here and have no garage heater... anyway, when I tried to tackle it this morning, I couldn't get the dang hoses off the block, off the water pump, etc. Just too cold to mess w/.

At this point, I decided to actually check the coolant level w/ a flashlight. It was down. Way down, and reservior empty too. I reread GreyStreak's post and thought that was it, so I didn't mess w/ replacing the Thermostat.

First, I started engine. Cold. I filled the radiator up to about an inch below the rad cap. I let the vehicle run for a while (5-10 mins idling). No heat yet in the car, and I kept checking temp of coolant in rad w/ my finger w/ the cap still off. Nice and green w/ the new fluid, and it remained cold. Temp guage in car stayed far left. (cold).

After 10+ minutes, things began to happen. Guage in car started to goto the right (hot). Then I have to assume the thermostat opened (or began to open) cuz the water turned an ugly color (dark, gray color). Then the water level dropped down, way down... I poured the rest of my 50/50 mix in there, and checked guage -- it was back to the left (cool), and heater was blowing hot. Water level was still very low, so I went into house to mix another batch of 50/50 from a fresh gallon of coolant. Car was still running. I returned and immediately filled the rad back to to top. (I had not put the cap back on yet).

At this time I noticed the reservior was half full of ugly colored coolant and also noticed coolant spray over passenger side of engine. I looked at ground and noticed a huge puddle of water as well. HHHmmmm... I filled the radiator up and put cap back on.

Now is when I came in here to type this.

The car is running now, the temp guage reads normal (to the left-ish), and the heater is blowing hot. Gonna let it run a while (while i type this) then go back out and see if I can see where it all blew out from (it is possible that it burped while I wasn't around).

Maybe I overfilled it w/ cap off? I dunno.

So far, I've probably added 1.5 bottles of 50/50 mix to it.

If I can't figure it out, then its going to the garage down the road -- the dreaded step of admitting failure. Haha.

Thanks, everyone.

 

when you saw greyish in the radiator i hope thats not oil in the case of a head gasket or intake gaskets leaking.

 

It was more brownish really, and now that I've looked at it more, it mostly green w/ w brown hue to it. Am probably due for a flush, eh? :-P Engine oil looks clean too.

Cannot find any leaks anywhere -- car has been running for ~20 mins w/ no issues. It must've just burped/overflowed when I wasn't around.

 

If it was that low then it probably has a lot more burping to do. It could take maybe even another gallon over the next few drive cycles to get it full as it pushes the air out and pulls coolant in from the reservoir. Of course that's assuming that there's nothing actually wrong with the truck.

 

Well, you are on the right track.. the burst of coolant was most likely an air bubble forcing its way out and it took some of the coolant with it as it came out the cap.

mprice is right, having been that low, she's gonna burp and bellow air for quite a while... The suggestion would be to have a couple of gallons of coolant with you and check the reservoir each time you reach your destination. Periodically, check the radiator level at the cap with the engine cold to be certain you maintain enough coolant in the system to actually make the syphon work between it and the reservoir.

 

Thanks guys. Will do.

I checked it after running around in the car today, and the reservior was empty, so I half filled it. Will keep coolant on-board as recommended.

I honestly don't know how long its been since coolant had been checked or serviced. I could've be years and years -- just got the Bronco from my mom this past May.

Thanks so much again.

Andrew.