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Getting it wrong: Misdiagnosing a cooling system problem

I recently helped a friend with a cooling system issue in his 1974 BMW 2002. It was one of those experiences where everything seemed straightforward, yet we came to exactly the wrong conclusion. It’s worth understanding why.


This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2019/11/04/misdiagnosing-a-cooling-system-problem
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I wouldn’t call that getting it wrong, Even old cars can be difficult to diagnose and your issue was an odd ball. Certainly not simple to diagnose. Sometimes you go around a few times before you understand all the inputs.

I told the “mechanic” that the radiator was plugged up. He acted like I am stupid because I’m a girl. I put a new rad in her and she never heated again.

I had a mid '80’s Ford pickup that I purchased used with a few thousand miles on it. It was a GREAT truck but always seemed to run hotter at night than during the day. I suspected a gauge issue so put an aftermarket electric gauge under the dash as a backup to the factory gauge. To my chagrin the aftermarket gauge showed the same temperature rise at night.

One evening in preparing to leave I just happened to glance at the auxiliary temp gauge at the same time I turned on the headlights. The temp jumped 40 degrees when I turned on the headlights! While watching the factory gauge the temp slowly climbed to it’s “usual” night time temperature. It turns out that someone had replaced the valve cover gaskets on it and failed to reinstall a factory ground strap between the block and the firewall and the headlights were for some reason grounding back thru the temp gauges causing the “increased” temperature. Quick addition of a couple of ground wires between the block and frame took care of the “overheating” problem!

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Ahh cooling issues, the bane of old car ownership. My humble advise is, before chucking new parts at on old car, go back to zero. Some cars had inadequate cooling systems but most cars were not designed to overheat. Go over the stock system from water pump to T-stat to gauges and flush the system before throwing that big aluminum radiator and C-130 sized fans at it. I’ve seen many old cars, muscle cars especially, with big shiny new parts and owners that say they still run “a little hot”.
My daughter drives a 1991 Mercury Capri XR2 in college and those cars are notorious for having oil pressure gauges that read too low and temp gauges that read too high. I tested the resistance values of several temp sensors before I found one with the correct specifications. Now, even in hot weather with the AC on, the little turbo car reads normal.
A final note; rust anywhere in the system, just like icy silence from the wife, means something, don’t ignore it.
Now, back to my 1983 Saab I’m restoring with the temp gauge a little north of normal, hmm…

About a year ago on my Jaguar XK8, I blew the top radiator hose. The gauge read normal with no indication of overheating. I later learned from forums that these gauges are not reliable and many have replaced them with after market gauges. Instead, I purchased a “heads-up display” that connects to the OBD2 port and gives accurate readings for both water temp and voltage.

Edison said it best when he was trying various filaments for the lightbulb: “I have not failed. I’ve just found 10,000 ways that don’t work.”

Thank you for sharing this info. I have a 1976 530i (E12) and I’ll go through and clean the contacts as preventative maintenance.

I’ve had similar issues on everything from SBC to Geo Metro with a similar problem.
My culprit ended up being an aftermarket Red Lower Rad Hose Heater. It limited flow similar to corrosion on an impeller or closed Tstat.

When I was a young lad, I performed a valve job on my dad’s '65 Rambler. After the job was done, he complained that the engine ran hot. As with femurphy77’s experience, I found that I had left the ground strap off the valve cover.

I have a '97 Miata which has an oil pressure gauge, but it really only shows mid-scale when the engine has oil pressure. I swapped the gauge and sensor from an earlier model for a true reading. Some months later, I noticed that the reading was low. I checked it with a mechanical gauge, it read normal. So I searched the internet to see what ohm values would produce which readings. The sender seemed to be okay. It turned out that the screws holding the gauge to the dash and printed circuit board had loosened, making a poor connection.

Here’s a truly rare guage problem: circa 1968 I had a 62 Thunderbird that suddenly started overheating. After trying to diagnose the problem I realized that the engine wasn’t overheating, but the guage indicated that it was. At the same time the car developed another odd problem: it would run out of gas when the guage said it was still half full. For some reason my father and I never suspected that the problems were related. The problem was finally solved when a mechanic friend told us that when Ford converted to 12 volt ignition they had a lot of leftover 6 volt guages and rather than throw them away they reused them, by installing a resistor on the instrument cluster to drop the voltage to 6 volts. We replaced the faulty resistor; problem solved.

Interesting test case. It would have really sucked if you didn’t have a correctly running car right next door! My classics are all English, so my process here would be to replace the stat, if that didn’t work, assume the gauge. LOL.

Think you have a bad thermostat that won’t open? Before wasting time and money on a new one - test it. Put the thermostat in a pot of water and bring it to the boil. Depending on application thermostats open between 140 F [60 C] (marine engines) and 190 F [88 C] (typical post 1974 auto engine). As the water passes the simmer a good thermostat will be fully open.
Note: Be sure the thermostat is FULLY open. A bad thermostat may only partially open and still cause the engine to run hot.

Rob - you didn’t mention if you ever squeezed the upper (or either) hose. We all learned (many of us the hard way) decades ago never to touch the upper hose of an overheated motor but if I can lay an unprotected hand on it the motor’s not hot. If I can crush it either the coolant is cold or the system’s leaking. The heater can be another clue: most of us (New Englanders anyway) kinda know how our heaters feel at full blast; when the coolant is over heating it is frighteningly hot. These clues can offer a lot without even opening the system. Of course this is all Monday morning post analysis but we road warriors like to pride ourselves on our resourcefulness…During the heat wave this past July my wife and I took a train trip to Florida to drive back a car she saw on Craigslist. Aftermarket guage under the dash got to 220F so we drove the 1100 miles back to Massachusetts, in the heat, with no A/C and the heat on full blast to keep it from boiling over. Think we grow out of this stuff? Think again. We’re in our 50’s and would do it again if we needed to!

I don’t know about BMW, but with most cars, the upper hose becomes warm first when the thermostat opens. The lower hose won’t be warm until the radiator warms up, as water is drawn into the engine from the lower hose and exits the engine out the upper hose.

What was not discussed is the full flow recirculation of coolant during warm up concept that the 2002 and even the most recent BMW engines have.
The thermostat housing is located on the engine inlet side of the coolant circuit. The housing has 3 nozzles, the outlet to the engine water pump, the inlet from the radiator that is on the cold side ot the actual thermostat in the housing and a third that enters above the thermostat element and receives coolant flow through a full size hose from the water divider at the head outlet. This concept ensures full circulation through the engine during warm up to avoid hot spot build up in that aluminum head.
So the temperature of the recirculation hose even with a cold radiator outlet hose will tell if the pump is flowing water and rules out a bad pump. When the thermostat begins to open the coolant from the head is mixed with cold coolant from the radiator to provide a constant coolant temperature to the engine pump suction.

These comments are how we all learn to keep our vintage, and some modern, cars productively on the road. In my case I spent literally years trying to figure out why my 1947 Cadillac sedan would indicate hot during highway driving, but would never boil over. I had done it all: thermostats, pump, radiator, hoses. I was lamenting my overheating at dinner with some fellow Cadillac Club board members when an old timer (who had driven his LaSalle all over the country) had the solution. He informed me that at highway speeds the 6 volt generator can overcharge the system, hence my gauge would read higher. A refresh of the generator and voltage regulator solved it.

This article was very helpful in solving my over-temp reading. I bought a cheap infrared heat scanner, determined my temp was normal. I then found a ground problem, now I have a properly functioning gauge. Anyway, Thanks!

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No one has mentioned a problem I had with my trusty '65 Valiant overheating. It began overheating at freeway speeds, but was fine at slower speeds. Investigation showed a bad water pump, which we replaced to solve the problem. About a month later, the same thing began to happen, overheating at higher speeds. After going through everything from thermostat to leaky radiator cap to collapsing lower rad hose, out of desperation I took off the new water pump to find the pressed on impeller freewheeling on the shaft. A third water pump from a different mfg finally solved the issue and the little slant six gave many more years of trouble free service.

My elderly neighbor’s ten year old vehicle was overheating so he had the local mechanic replace the thermostat. The overheating problem persisted so it was back to the shop for more fixes. Upper and lower hoses were replaced but the overheating continued. Back to the shop again and the radiator cap was replaced, still overheated. Next the radiator was flushed, still overheated. The next step was to be the replacement of the water pump. But before replacing the water pump the owner of the vehicle was pretty frustrated as you could imagine so he drove his vehicle to another shop. This second mechanic after hearing the complete story of replacing parts and repairs asked what was the very first thing the other mechanic had done on the vehicle. The thermostat he said was replaced. This second mechanic discovered that the first mechanic had inadvertently installed that thermostat upside down! So what is the moral to this story? Pay attention to what your doing I guess! We all make mistakes and when working on projects we need to pay attention to what we are doing as we are all human.

A story that I thought you would enjoy.

Bad or high resistance ground connections. The primary bad actor of automotive electrics.
They become more numerous with a vehicles age (oxidation) and almost no one performs preventive maintenance on them, especially under the dash
(out of sight/out of mind).
I have found a good cleaning and OX-GARD application to be an excellent preventive cure for poor ground connections, especially trailer lights/even lamp, pin or base connections. Just don’t be too generous or a ground can be established where it is not desired.