(1991 351W) All common sense fixes have already been attempted...need someone who has already gone through this and knows where the problem lies. I AM AN EXPERIENCED HOME MECHANIC AND AT MY WITTS END.

Problem: engine is overheating and overcooling. Struggling with prob since purchase 2 years ago.

Engine fully warmed only reads very cool on temp gauge...verified by luke warm heat blowing from heater. Able to hold radiator hoses in hand indefinatley (just warm to the touch). Flushed and reverse flushed engine and heater core...no change.

Out of the blue one day it overheats after a 5 minute drive and shoots coolant from overflow.

Next day its fine, again running on the coldest side of normal on the gauge.

Not wanting to take any chances after overheating episode, I repaired with NEW waterpump, & NEW 195 stat (pre tested in boiling water). Checked old pump and impellor intact and turning fine.

After full warm up, temp gauge is again steady at COLD. NOTHING CHANGED. AS ABOVE , heater output only warm...able to HOLD rad hoses...just warm. HOW CAN THIS BE?

THIS WEEKEND ...round trip from Ohio to North Carolina and back to Ohio....25 hours of driving.

12 hours out there towing a car, runs entire way at cold. Coming back today/tonight, towing an empty trailer, it goes nuts. First half of trip runs at cold/cool. Then out of nowhere temp gauge begins to climb until reading nearly full hot....pull over and coolant blows out of overflow. Hoses are hot as hell and can't even touch them.

Waited 15 minutes and get back on the road....gauge is now reading right in the middle of the normal range (right where a normal vehicle would read at all times.) After 2 years of the gauge aways running at cold...now like crazy its in the normal range. Drove for a few hours and gauge fluctuates slightly but all in the normal range...not cold and not overheating...just right. This is NUTS? Rad hose is still way too hot to the touch...just like it should be.

Starts to rain and getting cold in Ohio, so I turn the heat on. ONLY BLOWS COLD AIR NOW (it always blew at least decent warm air), now even with engine running at a hot normal range.

Should I be looking at a head gasket? But that wouldn't keep hot coolant from reaching the heater core.

Isn't there somekind of coolant bypass in the system?

2 years of fighting this and I'm done trying.....anyone with the right answer?

MATT

 

almost sounds like you have something floating around in your cooling system.mabe someone before you used too much blue glue installing the water pump, and they're big chunks floating around in there. see if your heater hoses are hot to the touch when warmed up. if not, remove them at either end, to see if there's anything in them. might be sedement from someone not mixing the antifreeze with clean water. could be one of your hoses has disintigrated inside, and restricting your flow only on a temporary basis. I had a linclon one time, turned out to be big chunks of rust through out the whole cooling system, and it was acting similar to your truck. good luck

 

agreed...maybe try replacing your coolant hoses?

 

Sounds like you need new hoses and a good coolant flush my friend!
Please list all the "common fixes" you attempted as well.
Sounds like a sticky thermostat to me.

 

I had a 351M that did similar scenareo as yours is doing. I had to loosen the heater hose clamp while running and bleed the system and add coolant to radiator at the same time ,finally got the air out and everything ran normal again.

 

I also think air in system.

 

my guess is you have a coolant leak, or maybe a bad thermostat... I'd bet on the leak though. like previously posted It's important to make sure you are filling the cooling system properly... making sure to get all of the air out of the system... sounds like you know what your doing though.

low coolant will give you cold air out of the heater and fluctuating readings on your temp gauge... keep in mind that the coolant can be low but not cause overheating... My truck does that exact thing when its low... temp gauge gives funny readings and heater blows cold air.

your 12 hr trip sounds like a perfect example of how a coolant leak would act... low coolant, probably slowly leaking until it gets to the point where there is not enough coolant to cool the engine... engine overheats.

I'm a 4th year automotive mechanic and turned wrenches for a number of years... when a customer had an overheating problem the first thing I'd check is the operation of the fan and the condition of the rad including the coolant, making sure there was proper coolant (not just water) and that it was clean. If the fan wasn't workin... fix it. If the coolant was dirty or there was only water in it... I would flush it and replace with proper coolant.

Then I would pressure test the cooling system. If there was a pressure loss or an apparent leak... I would fix it.

If all that checked out then I would replace the thermostat... now you might say that's just throwing parts at it but you cant proceed any further without knowing the thermostat is good, you can check to see if it's working once it's out... but for the 6 dollars...I'd put a new one back in. I never had a vehicle that went any farther than that cause that covers all you need to cool an engine.

Now if you have done that and you still have problems then it's time to get into the weird and odd stuff... plugged radiators (most common of the weird thing...lol), water pumps that for some reason don't pump water, and other things that are odd like hoses that intermittently or erratically flow coolant, never seen or heard of that though. I have to say that 99.999% of the time it's the basic stuff or a combination of them that cause problems.

one other thing I can think of is to make sure you have a thermostat in there. I've heard form people who race cars that running with out a thermostat actually causes overheating on some vehicles... just something I've heard.

any how I hope this helps a bit...

 

It sounds like you've delt with this for awhile, know the system and have checked or repaired all the common stuff. that said there are some good suggestions here already.

I need to press one thing hard, and I'm suprised nobody has said this yet. YOU NEED A REAL TEMP GAGE. You CAN NOT know for sure that it is cold until you get a real temp gauge on it. It is normal for these trucks to run rather cool on the stock gauge generally the needle sits right on 'O' in NORMAL.

But in looking for a diagnoses my experianced brain goes right to head gasket, I'll explain. It's all about the gasses, your cooling system is not complicated, but it does really on water. You can have a minor head gasket leak and it will release small amounts gasses into the cooling system. Like the feller above you told you to get the air out. It is impossible to get the air out when the engine keeps pushing more in. This air will effect the heater, thermostat, and with enough the temp of the upper hose. All of thse things full of air will be too cold. The heater will blow cold and the thermostat my not open.

This must sound bad, and if you don't know for sure what do you do? Well there's an easy test that will tell you for sure. The only problem is finding one to buy, you need an auto tool guy, like a truck, maybe NAPA can order one. They sell a tester in which you draw the gasses out of the radiator and through a fluid that turns from blue to yellow in the pressence of combustion gasses, which is what will be in there if you have a bad head gasket. This tester isn't expensive just hard to find, but well worth it to know for sure.

 

The thermostats they sell these days are junk imo. The ones they sell at autozone are crap, i had 4 do as u described then i got a better one from summit and my problem similar to urs went away.

 

Pulse what everybody said above it also sounds like someone changed out the bottom radiator hose and left the spring out. Also try swapping on end of your heater hoses and see if that helps.

 

I second what opossum said about the heater gauge... you cant get a proper idea of what the engine temp is from feeling the hoses or the gauge it's self. Fix the overheating problem and don't worry about the temp gauge or the hose temp... it'll just confuse things.

And I agree with opossum on the head gasket as well... although a pressure test and or a visual inspection of the exhaust will more often give telling signs of a head gasket leak, there are times when a head gasket leaks only exhaust gasses into the cooling system and not colant into the combustion chamber. most of the time it's obvious as there will be bubbles in the cooling system or the coolant will be forced out by the gases while the engine is running. I would imagine that a very small leak would complicate it further by making the diagnosis even harder, but my experience is that when a head gasket leaks... you'll be able to tell its the head gasket. Testing the coolant for the presence of exhaust gas is a good idea... although I've never done it.

 

I second that and I think this is what he is talking about.
Universal Test Tools (TTIBT-500) (Or BT-500) Combustion Leak Test Kit.
Sold by a lot of sites and Amazone,com for $29.76.


http://i54.photobucket.com/albums/g9...500_AA200_.jpg
http://i54.photobucket.com/albums/g9...tem/BT-600.jpg


/

 

Yep that's it, ya need that thing.

 

UPDATE:

Flushed heater core (again) for the second time heater core is plugged with black desbris...flushed it again and black **** came out. Heater core flushed solving heat output. Something is plugging heater core? What is it?

Backtracking...opening up the rad cap show full coolant after long road trip..ITS FULL!... NO LEAKS

 

MattP51-

I once saw a Chevy pickup do EXACTLY the same thing you are describing. In the end we found that someone who owned the truck before had put some kind of "quick fix" sealant in the radiator for a prior leak somewhere in the cooling system.

We finally unsoldered the tanks on the radiator and it was just FULL of what looked like oatmeal that had turned into cement. We ended up putting a new radiator and heater core in the truck and doing a VERY thorough flushing of the block.

This fixed the problem and we could only later surmise that there must have been a coolant leak somewhere and somebody had dumped multiple cans of "Stop Leak" in there. Who knows, but is sure sounds like what you have going on right now.

Long story short, it really sounds like a blockage in the cooling system that is alternately keeping the thermostat open, and then later inhibiting coolant flow.

Dig into that cooling system deeper, your answer in there. You just need to find it.