Gauges: Can They be Trusted?
I was backing my '95 Taurus out of the driveway one Saturday morning several years ago, when a loud thump sounded from under the hood. It was accompanied by a gaggle of illuminated warning indicators and an immediate loss of power steering assist. A peek under the hood revealed a broken belt tensioner, but the car had 180,000 miles on the clock, so it didn't owe me anything.
Around the corner at the local mass-franchise parts house, I paid $80 for a replacement belt tensioner. It was a brand I recognized and looked OK, but the price was, I thought, a bit high. I paid the extra money because I needed the car that weekend.A few thousand miles later, the tensioner failed in such a way that the pulley was out of line with the other belt wheels and the 3.0L started tossing its serpentine belt every few days. I threw the $80 aftermarket tensioner in the trash and paid $55 for an OEM replacement at the Ford dealership.
This experience hasn't been unique. I've seen it many times before.How much do inferior parts cost? Well, they cost a lot in time, inconvenience and lost labor, but that isn't what this story is about — at least, not entirely.
On more than a few occasions, when I've replaced a thermostat that was opening too cold, the replacement part opened even colder — and on today's emission-friendly engines, 210° F is a good temperature, but 170° F isn't. Not only does a perpetually cool engine prevent the emissions system from doing its job correctly, it promotes engine wear: Water is created in the combustion chamber, and as some of that vapor blows by the rings on a cooler-running engine, it tends to combine with sulphur in the lube oil to form sulfuric acid, which isn't good for the innards of any powerplant.In those cases where the first new thermostat didn't warm things up, I'd finally get a good one after a couple of tries. It's more than just annoying if the thermostat replacement is a tough job.
Uncontrolled Coolant Flow
The 2004 Sebring originally came to the school shop with a P0125 code, and we replaced the thermostat to correct it. The thermostat on this one is kind of buried; it's not under the upper radiator hose neck where it is on most other cars. The bleeder is there, to be sure, but the 'stat is on the forward bank of the engine behind the alternator, and it controls flow through the lower radiator hose.In spite of the odd and somewhat aggravating location, everything went smoothly and by the numbers. The code was gone, the engine was warm again. What else could we ask for?
Fast-forward 3,000 miles and a few months. The car is back with the customer reporting cold-engine symptoms — no MIL this time, but poor cabin heat. The temperature needle was near the center of the gauge. Prior learning pointed to the thermostat (again), so I passed that ticket to Andrew. He went to work replacing it, which turned out to be a great adventure.Andrew and I tested the thermostat he removed with a torch, a thermometer and a can of water, and it began to open at about 190° F. No problem there.
A few weeks ago, we had tested a thermostat this way to verify the problem on an overheating vehicle (different car) and that 'stat wouldn't open at all, not even with the water at a full 212° F. And while this one didn't seem to be opening too soon as I had originally believed, we figured to replace it on general principle.
Because I had seen so many replacement part failures, I had erroneously jumped to the conclusion that the thermostat was letting the water flow too early again. It was my error, at the student's expense, but at least it was a grand teaching exercise.
When Andrew was reinstalling the thermostat housing, he was fingering the bolts into their holes and didn't like the way they felt going in. Subsequently, he decided to chase the threads with a 6mm thread tap. He somehow managed to break the tap off in the second hole.
This engine block is aluminum. Can anybody say "disaster?"
Solve the Problem and Move On
Removing the A/C compressor for access (after first identifying and recovering the refrigerant), then drilling past the flutes in the tap with a 1/8–in. drill bit on all four sides, Andrew found a way out for the broken tap, then followed it with a 1/4-in. drill bit and a 6mm helicoil. Problem solved.
Installing the replacement thermostat and refilling the cooling system should have been the end of the job, but it wasn't. That's when we discovered what we should have checked before plowing into the thermostat replacement.
With the EASE Diagnostic wireless interface connected to the Chrysler and the Engine Coolant Temperature (ECT) numbers running bigger than life on the flat-panel monitor, we watched the coolant temperature climb into the danger zone (more than 240° F), and there was no fan operation at all.
Interestingly enough, the instrument cluster temperature gauge (which gets its marching orders from the PCM by way of the Body Computer) remained near the halfway mark. I didn't want to tempt fate and let the coolant reach 260° F just to see what the gauge would show, but it was this peculiarity that fooled us into replacing the thermostat the second time. After all, it would seem to me that the gauge should have been more reliable than that!
I had seen a similar situation on a 1995 New Yorker with a 3.5L engine. The panel gauge never showed hot, but the engine started making air and percolating within a minute or two after a cold start, and it was obvious that it was hotter than the comfortable temperature. In that case, the owner claims to have fixed the New Yorker with a thermostat, and I never understood why the gauge didn't show an overheating condition. Furthermore, it was amazing how rapidly it started making air.
Perhaps the gauge error was due to a cool air bubble trapped at the sensor bulb? Who knows? Chrysler says if you experience cool heater hoses and an overheating engine with a full cooling system, then the water pump is probably at fault. But the 2004 Sebring had a clean cooling system and less than 70k on it, so I mentally discarded the idea of a bad water pump.
Ordinarily, I like to engage the A/C in a no-fan situation like this to see if it forces fan operation, but we had drawn the refrigerant out of the A/C to remove the compressor. Turning the A/C on wouldn't engage the fan anyway, thus it wasn't an option unless we recharged the A/C (that would come later).
The fan on this vehicle is driven by the PCM, which uses the ECT sensor to provide engine coolant information.
Low Fan at 220° F
The instrument cluster uses the same sensor to operate the gauge. Interestingly, the transmission fluid temperature sensor can also affect fan operation — not to mention high refrigerant pressure readings.
Chasing Sparks — and Heat
Flipping the cover off the underhood fuse box, Andrew found the high and low fan relays. It only took a moment for him to determine that there was no power available at the common terminal on either relay.
Further research led him to check the fuse that fed the relays, and it had died the way so many other fuses and connectors do — a little resistance at the spade generates voltage drop and heat, which creates oxidation on the terminals, which creates more resistance and more heat until a meltdown occurs and current flow stops. Would the junction box need replacing? Maybe, but we hoped not.
A replacement fuse normalized the fans, and while the PCM engaged both fans at the proper reported temperature, they would never cycle off. The coolant kept getting hotter, while the gauge (once again) didn't reflect the dangerous heat.
A temperature gun reading on the upper hose showed nearly 200° F, but the lower hose (with the thermostat) was steady at 85° F. That ding-dang Chinese-made thermostat was sticking when it was fresh out of its little white box!
Another 'New' Part
I told the parts counter man to "bring me a thermostat that's not made in China." He did. It looked and felt different. When attempting to open the Chinese part with my thumbs, I could actually feel it stick, while the American-made unit had a more substantial spring and a better-looking wax pellet, for what it was worth. And when installed in the vehicle, it corrected the overheating problem: The fans cycled on and off normally, but I'd never be happy with that temperature gauge if I was the owner.
Trusting the Gauges
Let's get serious here: Who among us can legitimately claim we always watch our gauges close enough to prevent an oil pressure or temperature disaster? Nobody, obviously — that's why some manufacturers include warning lights and alarms that tell us to check our gauges when a needle drifts into the danger zone. But even a serious gauge-watcher can be fooled by a cheap thermostat and a less-than-accurate gauge.
Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. Richard is now an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, AL. E-mail Richard at [email protected].
