Gents -
Got one for the books here, & hoping someone has a fix.


95 Bronco (Eddie Bauer Ed & has factory oil cooler on rad feed hose), w/ 275K total miles.


A factory long block was installed about 7 years ago - at 210K. New rad, & water pump inst at that time. Daily driver, regular maint, & runs good.


Here in S. TX, it's warm most of the year, so the heater is rarely used. Usually engine temp is normal, but lately I've noticed that at highway speed, when the heater - or defrost is turned on, within 4 - 5 miles, the temp gauge spikes (fairly quickly) ALL THE WAY to the top. It will drop off to normal 195 deg, just as fast - when the heater is turned off.


I saw this happen twice before since the motor install, but it never re-occurred w/ any frequency, & I never connected the heater operation - to the problem. It was extremely intermittent before, but is now consistent.


No leaks / not going through coolant / no overflow noted.


I'm on the 3rd thermostat in 6 weeks, w/ no change. Installed a mechanical temp gauge yesterday - to rule out false gauge reading.


With the heater off, I can drive all day on the highway (so far), & the temp remains normal - without fluctuation.


My mechanic suspects some kind of internal coolant channel blockage, but I'm starting to suspect some kind of issue w/ the heater itself. Possibly heater hose (or core) routing issue. Reversed maybe?? When at operating temp, the heater seems to blow normally warm.


Water pump maybe??


For whatever reason, the thermostat seems to be opening too slowly, when the engine is at normal operating temp - & the heater is turned on.


Really can't make sense of it, but it is starting to get... old.


Be happy to answer any questions, & any one's ideas would be greatly appreciated...


Thanks in advance.


GCF

 

Sounds to me like your coolant level is too low and you've got air trapped in the system. As for the flow being reversed through the heater, that just ain't gonna happen, it works the same either way it's routed. The temps are rise slower with the heater in operation cause in effect, the heater is a mini radiator that dissipates heat from the coolant.

 

Thanks for clearing up the heater flow thing. Makes sense. Interestingly though, it's only when the heater is turned on - that the temp spikes.


Figure it's because when the H/C valve opens, the core / hoses fill, & pull a volume of coolant out of the system. It would make sense then, that if as you suggest, there is air trapped in the system - the temp would spike.


If that is the case, I'm wondering how it got that way - in the first place, & how to correct the situation. The coolant level in the radiator has not gotten low recently.


I've noticed there are quite a few comments on this forum about needing to "burp" the coolant system, but have not seen an explanation of how to bleed off a problem air bubble...


One other detail that I forgot to mention earlier, would seem to re-enforce the air bubble theory. At one point when the temp was spiking, I was able to pull over & get the hood up quick enough, to see the radiator return (top) hose completely collapsed. The stat was obviously closed, & the pump pulling a vacuum on the radiator. Within about 45 seconds the thermostat opened - accompanied by a bunch of gurgling, the hose opened back up, & the temp dropped back off to normal.


Never seen a return hose collapsed like that. Usually buy Gates, but this one is Brand X...

 

Find the highest point in the system and open it up at that point, then start and run the engine till the T'stat opens up and the trapped air releases. Also look at the lower hose, if theres no spring inside to keep the hose from collapsing, that too will cause problems.

 

Thanks for your reply. Highest point in the system, would be the heater core input / output. Would that work?


I've heard a bit of gurgling from the core, when turning on the heater. Sounds like coolant displacing air. Never thought much of it, but in light of the current problem - I'm beginning to wonder. Does that seem normal?


Noticed today that the temp spike will occur even without using the heater. Did it to me several times. Only at highway speeds - so far. Runs down the road 20 - 30 miles, at a stable 195 deg, then out of nowhere - BAM. All the way to 240+ in very little time. Pulled over, & noted the return hose collapsed - until the stat opens. Let it run at idle for a few minutes, & its back to 195.


Need to get this fixed before it does some damage.

 

You may need to replace the T'stat too, it could be sticking. Don't forget to over look that little 3/8" coolant hose that passes through the upper intake behind the EGR valve when looking for the highest point in the system. This is the one that returns directly to the radiator from the EGR

 

I'm on the 3rd 195Deg TS since this fiasco started. Gonna' check out the 3/8 hose for a bleed point. Will update.


Oh yeah, 67 Fastback? My ALL TIME FAVORITE!


Thanks - GCF

 

Some comments, take them for what they're worth...

Stop calling the Upper Radiator Hose a "radiator return hose", it's the upper radiator hose, don't confuse people

I haven't seen a spring in a lower radiator hose for many many years. They were used when lower hoses were made of thin rubber, to prevent them collapsing under water pump suction at high RPM at highway speeds. But lower hoses have been made of thicker rubber for many years now, no spring.
Unless there is still some brand X cheapie thing that is thin and uses a spring, in that case, don't buy it!

There is ALWAYS coolant flow through the heater core, whether you are using the heater or not. There is no inline heater control valve to shut off coolant flow through the heater core, unless someone put one in after-market.

Your upper radiator hose may be a worn-thin piece of crap. Or, you may have a flow problem, read on...

There is a Coolant Bypass Hose, it is a short hose with a preformed 90 degree bend in it. It clamps onto a fitting at the front of the intake manifold, does the 90 degree turn downwards, and then clamps onto an auxiliary inlet of the water pump. The purpose of this hose is to continue to circulate some coolant from before the thermostat, down into the water pump. When the thermostat is closed, this a source of coolant going into the pump (the flow through the heater core is another loop that bypasses the thermostat, too. Back when it was common to have a heater control shut-off valve, the Bypass Hose then would be the ONLY source of coolant to the pump when the heater control valve was shut).
And if the thermostat is wide-open, there will still be flow via the Bypass Hose.

I wonder if you have a bypass flow problem, like little to none.

 

OK, point taken about the upper hose / return hose thing. To clarify, the hose problem noted, is a collapsed upper radiator hose.


It happens when the engine is an overheated (20 - 40 minute period at highway speed) state. It inflates quickly - when the T/S opens.


Originally thought the thermostat was sticking, but it's been changed twice now.


The 90 degree bypass hose does not appear to be constricted, & has been replaced since this problem was first noted.


I should point out that this truck does have an inline heater control shut off valve. I bought the rig in 2001, & it's always been there. Figured it was factory.


So if I'm understanding this correctly, w/ the H/C valve closed, "normal" bypass flow is effectively reduced by 50%. Is it possible that this could cause system temperature spikes, that thermostat operation could not keep up with?

 

Just me but I was never confused about which hose you were referring to. And yes, if the heater is shut off, the bypass volume is reduced by half. Sounds to me like your T-Stat is still malfunctioning, or the bypass hose is kinked (or both) That hose comes in one length to fit both a 351W and a 302, it must be trimmed to the correct length or the 302, and usually so for the 351W too. I would also verify that the T-stat is installed in the correct orientation (direction), there's a right way and a wrong way it can be installed

 

What brand of t-stat? I only use Motorcraft for this reason.

 

Well, at the risk of throwing a hex on myself, it looks like we got her fixed. That said, after fighting this problem for a couple of months, got to admit - I'm still a little spooked.


Although it looked for all the world like an "intermittent" stuck T/S, multiple thermostats made no difference. 10 - 20 miles down the highway be OK - then BAM. Explored the air pocket idea, but the 3/8 bypass hose from the throttle body, was bleeding off any trapped air effectively.


Even drove around w/ no thermostat for a while. No temp spikes, but that was obviously not a solution.


Finally decided to do a good chemical flush of the cooling system, then install a new water pump - & try it again.


The flush was as expected, uneventful. Took my time w/ the pump - looking for ANYTHING that could be causing the problem. Nothing noted out of the ordinary.


The last thing to get buttoned up was the T/S. Scraped the old gasket off the block & the housing, & then - before applying gasket sealer, I set the new T/S in the milled housing recess.


It was then that I realized, the T/S was NOT sitting flush in the milled recess - of the housing. The edge of the T/S, was actually sticking out on one side - just a bit. It appears that what had been happening, was that when the housing was snugged up to the block, it was putting a good bind on the T/S, forcing it up against the opposite side of the housing recess - & causing intermittent sticking. This was probably made worse, when the surrounding iron got good & hot.


I tried several thermostats in in this aftermarket (made in China?) housing. Some fit right, & some didn't. It got replaced, regardless, & an American made 195 thermostat installed.


Been testing it for a few days now (around town & extended highway), & so far, looks rock steady. Pretty sure we've got it licked, but time will tell. As mentioned, I'm still a little spooked...


Guess the lesson here, is ALWAYS consider BOTH HALVES of the equation, & don't overlook the small (who'd have guessed it) details.


As this rig is my daily driver, this has been a major stress event. Just want to say thanks to all you gents - for the input!

 

GCF: I just want to say thank you for coming back and posting what you found. Too often I read threads with comments, suggestions and advice addressing one or more of the concerns I may also be having on my truck only to be left hanging at the end of the thread wondering what specific advice, if any, fixed the problem.

As to my two peanut gallery suggestions below, I know you said that a factory long block was installed 7 years ago. But that doesn't mean they didn't re-use your sepentine pulleys and your AC pump. So:

1. Just for fun, try disconnecting the two pin electrical connection to your Air Conditioning pump and see if it lessens a load on your engine and causes an increase in performance. The AC system in my '95 Bronco has problems which when the pump is connected causes an idle surge from 750 rpm to 1000 rpm and was causing an unnecessary load on the engine. The AC runs with the heater and defrost. Hopefully, I will be rebuilding the AC system this Spring.

2. Also if you haven't done it already, replace the serpentine tensioner pulley and the serpentine idle pulley. It isn't fun on a daily driver when the tensioner grenades, which mine did last month.

 

Now THERE are a couple of good thoughts - THANKS!


Here's a pic of Ol' Reliable - in better days: