I cut my teeth curing overheating on a 1967 Jaguar XKE. I thought I could diagnose pretty much ANY overheating problem within an hour or so but a recent one took me awhile. The afflicted vehicle was a Toyota 20R equipped mini-motorhome, but...

The lessons should help any Z owners battling overheating.

Problems started when the water pump began making a racket. It wasn't leaking and the vehicle wasn't overheating at this point. The pulley was visibly moving in and out 1/4" or more meaning the water pump bearing had failed. Frankly it sounded like loose metal in the bottom end of the engine rattling around!

I replaced the waterpump in a mini-market parking lot, filled the radiator, warmed the engine up to try and "burp" it, put the radiator cap back on and took off. Ran fine for several hundred miles.

Then she started getting hot, going right up to the edge of the red zone on the gauge, then back down again, almost AS IF it had an air bubble or intermittent pressure leak. I noticed some slight seeping from the radiator cap, and sure enough, the rubber seal on the bottom of it was cracked. A loss of pressure could easily cause it to run hot; at the next O'Reilly's I bought and installed a new radiator cap thinking I was done.

(I'm so proud of myself --- how did I get a radiator cap off an already overheating radiator without scalding myself? I pulled into a car wash and used the magic wand to spray down the radiator until it was cool...O'Reillys just happened to be right next door --- sometimes we get lucky)

But it was still running hot...

At a truck stop I tightened all of the hose clamps, all 16 or whatever of them, suspecting a pressure leak. Even a single dripping hose / hose clamp will cause a loss of pressure and the temperature will shoot up 20 degrees or so.

On down the road I'm still having problems running hot, and I'm battling some humongous headwinds. If I pull over and stop for 5- 10 minutes then when I take off again it will stay in the normal region for 10-15 minutes before heading to the hot zone again. It's crazy!

At this point I'm running the heater in the truck on FULL BLAST to try and dump excess heat out of the engine and make it to the next town. 90 degrees out and I've got the heat on...misery! As long as it's just on the EDGE of the hot zone I keep going...

Do I have a head gasket starting to go out, one that is "injecting" air into the cooling system? Shutting it down causing it to heat soak and "burp" out a bubble, and then head out again? I'm at wits end.

I finally call a better mechanic than myself and he sez, "If it's intermittent, it's probably either the fan clutch or the thermostat." If it's not running bad, it's probably not the head gasket. He asks if the fan clutch is freewheeling, or offers resistance. I tell him it's closer to freewheeling and not offering any REAL resistance.

I replace the two items, warm the engine up without the cap off again to burp it, and....problem is cured. So was it the fan clutch, or was it a sticking t-stat?

When a vehicle is going down the highway at 50mph+ it's generally getting enough airflow through the radiator not to NEED the fan, especially bucking a headwind. Overheating at highway speeds SUGGESTS that there isn't enough coolant flow.

On the other hand, if a vehicle overheats mostly in town, in traffic, it suggests there isn't enough airflow. Yet this vehicle did just fine in town, it was when I was really pushing out on the highway it was getting hot.

What I have found, as a mechanic, over years of time, is that often a thermostat will stick a few hours or a few days AFTER any new part is installed in the cooling system --- a new radiator, a new water pump, any fresh new metal that can cause some electrolysis to occur.

Yet another mechanic I spoke with said "Oh, you just had an air bubble and got it out when you replaced the t-stat." He ascribes to the air bubble theory causing so many overheats AFTER work on a cooling system is done.

One thing I did see was that I had too much antifreeze, and not enough water in my cooling system. It should be a 60/40, or 50/50 mix. Water can absorb and transport more heat than antifreeze can.

It is weird, when a cooling system is working properly, it seems bulletproof. When it is not working, it seems so fragile. There's a simple explanation.

When a cooling system can get rid of even 1% more heat than the engine is generating, then it can keep up. If it can get rid of 1% less heat, then heat is going to accumulate --- and the engine will rapidly overheat. Now consider this. If an engine is putting out 100 horsepower, it is putting out nearly 200 "horsepower" worth of waste heat. Internal combustion engines are very inefficient --- less than 20% of the energy in the gasoline is converted to actual WORK --- usable power at the driveshaft. The rest gets converted to heat.

So which was it that was causing my overheating --- an air bubble, a sticking t-stat, or a failed fan clutch. (or too rich an anti-freeze blend) I still can't say for sure. Or perhaps it was a combination of all four. Remember, I only had to have 1% less cooling CAPACITY then I needed to suffer serious overheating!

Here's another thing; a lot of do-it-yourselfers, when battling overheating, go into the auto parts store and buy a 170 or 180 degree thermostat instead of the 190 t-stat called for. In olden days they used to refer to these as "summer" and "winter" thermostats.

The truth is putting a lower temp thermostat in your vehicle will almost NEVER make an overheating problem go away. What it WILL do is damage your fuel mileage. My engine had a 180 t-stat in it, after installing the factory spec'd 190, I notice I got 1-2mpg better fuel mileage even if the gauge sat a little higher. Engines ARE designed to run at a certain temperature, and below that they'lll use more fuel and create more pollution.

Back in the Jaguar days I proclaimed that pressure testing the cooling system was ALWAYS the first step in diagnosing overheating. I'm gonna stick by that; its simply too easy to have a loose radiator or heater hose and lose pressure. Lose pressure, you overheat, period.

At one point in this ordeal I was fairly certain there was some debris in the cooling system, perhaps blocking a passage or the thermostat itself. The blades off the old (oem) water pump were pretty chewed up, perhaps a piece of one was floating throughthe system. When I pulled over, perhaps it sank tothe bottom again. or perhaps shavings from the blades grinding against the housing had caused some electrolysis.

I'll never know for sure.

I'm sure some of you can add some stories, some lessons learned battling overheating to this.

Summer's here so it's a good time for this thread.

About six years ago, not long after buying my 73, took a trip to Tucson in July. On the up the mountain east of Palm Springs it overheated and lost some coolant. Let it cool off, topped it up and nursed it along to Blythe. Couldn't go over 60 mph without overheating. Stopped at Albertson's for some lunch, car wouldn't start - vaporlock. It was about 115F that day. Went to a motel, about half the cars there were having overheating problems. Got up before dawn and hit the road at 4:30 AM. Time and temp sign at the bank said it was 99F. Got across the bridge when it overheated, so I headed back to Blythe and found a little independent garage, they showed me the fan clutch had no resistance, replaced it and the water pump, not expensive.

Made it to Tucson but had to keep the speed down a it would still overheat. Checked for exhaust in the coolant - negative. Ordered a oil cooler from MSA and hooked it up. Got a 180 F Nissan thermostat. A kid at a local garage told me to crack the hood, with that and the oil cooler it ran cool enough up to about 75 mph, gauge would read about 200F.

Back on the coast a week later, flushed the radiator several times, installed a shroud and a "spook", bypassed the leaking heater. No big problem, but had to add coolant now and then, and there was some "smoke" from the exhaust. A year later the "smoke" got worse, then way worse. Saw water in the oil and have it towed to the garage. They pulled the head and found a corroded area by #6 cylinder and a hole in the gasket next to it. They put a new (81 JDM) head and metal gasket.

No more problems except near Riverside, on a hot day, hit a "parking lot" on the freeway after a long fast run and saw the needle go up to 200F. Put an aluminum 3-row, also removed the plastic elbows for the interior vents to get more airflow.

Now I can go up the mountain as fast as I want or hit "parking lots" on hot days and the gauge stays in the middle no matter what. Guess it was all worth it.

Ah.. the joys of owning and driving OLD cars.

First - most of our engines have cast iron blocks - the internal coolant passages on which are not coated with any corrosion protectant. What protects them is the use of anti-corrosives contained in what is commonly called Anti-Freeze & Summer Coolant. We used to call it Permanent Anti-Freeze, but that term was actually misleading to many people - they thought you left it in the car Permanently. Nothing could be farther from the truth - to remain an effective anti-corrosive protecting that cast iron - you have to change that coolant every year or two at the most. {depending on mileage/conditions}. The anti-corrosive chemicals do get diluted and are no longer effective past that point.

Not too many people change their coolant every year or two - so most engines build up a lot of corrosion on top of the cast iron coolant passages. That acts somewhat like an insulation - and reduces the efficiency of the cooling system. In effect, the coolant is not making as direct a contact with the cast iron as is needed to extract the maximum amount of heat and carry it away to the radiator.

This is not something that can be easily cured when you have an over-heating problem. It is something that needs to be addressed - usually over time - to prevent a problem. First step is to flush the cooling system, getting rid of all the old coolant and as much curd as it can carry away with it.

After a through FLUSH - you need to add some type of Coolant Flush and run that though. Usually that requires leaving it in for a few days - so that it can dissolve and absorb as much of the built up corrosion as possible, then carry it out on the next Flush...

Cleaning these old cooling systems out in the above manor - can reveal other "issues". Some say that these Flushings can "cause" other issues. In some heater cores for example - it is the built up corrosion that is the only part of the metal left - remove that and the heater core starts to leak. The only question is - do you want to be revealed while you are at home or when your on a long trip?

Corroded Radiators are easy to deal with - you just pull them and have a trusted Radiator Shop flush and pressure test them - then flow test them.

After you have flushed that old block a few times - you need to return with fresh Coolants that contain the proper anticorrosion chemicals and water pump lubricants. Then keep up with it - clean it out every year or two.

FWIW,

Carl B.

Some say ...Flushings can "cause" other issues..

That's the camp I'm in, Carl. I used to believe in flushing with caustic chemicals, trying to dissolve out all the calcium & other minerals. What I experienced was LOTS of sticking thermostats days or weeks later.

By the time I got to Nissan I became a "If it ain't broke, don't flush it" believer. Yes, perhaps the additives in antifreeze DO wear out, get consumed, whatever. The absolute MOST I would do would be to drain ONE gallon of old, and replace it with one gallon of fresh. No garden hose fresh water flushes, no chemical flushes, distilled water, nada.

Flushing is usually a last, desperation step by people battling overheating, and indeed, it can reveal or expose pinholes in the heater core and elsewhere that can be VERY costly to fix. More importantly, I NEVER ONCE saw flushing CURE an overheating vehicle in my entire 30 years of messing with overheating cars, from the XKE's onward.

The solution to curing overheating is, as with MOST auto problems, PROPERLY DIAGNOSING what the cause of the problem is.

With overheating, the "universe" of possible causes includes; in ROUGH order of frequency:

Loss of system pressure -- loose hose clamps, radiator Cap not holding pressure, weeping around water pump, ...

Failing head gaskets

Sticking thermostats

Inoperative fans / fan clutches, missing fan shrouds, bugs covering the front of the radiator...(airflow related)

Low coolant level

Failing water pumps (blades eaten away by corrosion, etc)

Incorrect temp gauge readings!

and probably LAST on the list: old or plugged radiators, which are often the FIRST and COSTLIEST thing people replace when battling overheating.

What are the tools of diagnosis:

A radiator pressure tester is #1

A chemical tester to look for exhaust gases in the coolant #2

Test strips to check the coolant/water mix #3

Some items, like the radiator cap & thermostat you "check" simply by replacing them with a new one.

Sight & sound --- a foggy windshield and cockpit smell --- leaking heater core

Determining WHEN AND WHERE it overheats - high speed, in traffic, continuously, etc. If it doesn't do it ALL the time then why would you condemn a part whose failure WOULD cause it to occur all the time -- like a water pump?

I'm ashamed to say "most" shops "diagnose" overheating simply by throwing parts at the vehicle until it quits overheating. For instance, the chemical you use to look for exhaust gas in the coolant --- quickly goes stale. A tech must "test" it for freshness by exhaling (C02 in your breath will trigger the color change) and throw out "old" chemical on the order of once a month. How many shops are willing to incur that expense --- or have that knowledge? Not many, in my experience.

A quick side note: How do radiators / cooling systems GET plugged up? It's when they start losing coolant and the owner continually tops up with city water, laden with minerals. The whole overflow bottle was supposed to put an end to that practice, letting a system suck coolant back in after the engine cooled down.

A head gasket just beginning to fail is probably the trickiest to diagnose. One way is to inspect the spark plugs. If coolant is getting sucked into the cylinder, that plug will be immaculately clean. But head gasket leaks can be two-way or just one-way, blowing exhaust gas out INTO the coolant but not sucking any in. I've gotten suckered by any # of Subaru's with early stages of head gasket failure, replacing every part of the cooling system before finally breaking down and doing the head gaskets. Before catalytic converters came along you could often SMELL coolant getting burned by the engine.

Lastly, it's probably beneficial to DEFINE overheating. The temp gauge is in the red zone AND the radiator is spitting steam and/or coolant out the overflow. I've seen more than a few vehicles with temperature gauges that SUGGESTED they were overheating, and...the gauge was wrong! Check the actual temp with an IR thermometer pointed at the thermostat housing / water outlet.

Summary: There are flushers and non-flushers, and I'm one of the latter. The solution to curing overheating is PROPERLY diagnosing it instead of throwing one after another part at it, especially the more expensive ones like radiators.