Temperature control systems can get out of control

A/C work fills the bays during the summer months.

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Since our world is driven and ruled by the three unbreakable Laws of Thermodynamics, it is foregone that extreme ambient temperatures will just about always bring customers and their vehicles in for unscheduled maintenance — work that isn't listed in those neat little owner's manual maintenance tables. Even though we still had frost-covered cars on some April mornings in Alabama and it snowed in North Dakota in June, the summer has been plenty warm enough to fill our service bays with temperature-related problems on both sides of the engine compartment bulkhead.

Warm weather notwithstanding, not all of the temperature-related problems we've dealt with are "too hot" problems; we've replaced faulty thermostats on four different vehicles that were running too cold over the past couple of weeks. The too hot problems tended to be HVAC concerns.

Too Cold, Too Hot, Too Smelly

The Dodge Stratus came to us with three primary concerns, and Ethan tackled the Malfunction Indicator Lamp (MIL) concern first. This was one of the two vehicles that week that threw us a P0128 code (coolant temperature below thermostat regulating temperature), and because the thermostat is so incredibly accessible on this Stratus (right under the radiator cap), we went there first and found that it had literally broken. A nice hot replacement thermostat warmed the engine up and neutralized the MIL in short order.

Two of the four cold-running vehicles we did during the past two weeks illuminated their MILs and tossed P0128 codes that took us directly to the thermostat, because the PCM can't control emissions worth a darn unless the engine is running at about 200 degrees Fahrenheit.

Besides the emissions factor, a cold-running engine will eventually ruin itself because of the resulting sludge buildup and sulfuric acid production in the crankcase oil, not to mention the lost fuel economy from running in Open Loop (where the PCM ignores the O2 sensors). In the early part of a career that has spanned more than three decades now, I can remember a time when the cooling system thermostat was a lot more prone to be the cause of overheating problems rather than cold-running problems. But that has flip-flopped. When overheating does take place, the thermostat is still an item in question, and we've seen stuck closed thermostats in my department that wouldn't open even in a pan of boiling water – and we're pretty near sea level. It's really surprising how long a thermostat can last in the hostile environment where it lives.

The other two cold-running vehicles (a 2004 Trailblazer and my 2001 Cherokee) were noticed by the respective persons behind the wheel, but for whatever reason, the PCM didn't pick up on the problem on either vehicle. The Jeep was running only about 120 degrees Fahrenheit, and in spite of the fact that my thermostat seemed to work right in the hot water can, a new thermostat put my needle back on 200 degrees. Go figure.

The next order of business on the 2004 Dodge Stratus was to diagnose the HVAC system — the unit seemed to cool tolerably well, but on the morning we were checking it, the ambient temperature was running about 70 degrees Fahrenheit. We identified the refrigerant as 100 percent R-134a and then connected a set of gauges to find that the pressures were ridiculously low across the board, actually pulling into the negative on the low side and running about 130 psi on the high side.

Was the refrigerant low? Static readings aren't at all reliable, so we fired up the engine and the heater to make the A/C system toasty warm from stem to stern, then yanked the gas out with our recycler (heating the A/C system helps get all the gas out on cool days), cycling through twice to be sure we had it all. The refrigerant charge for this car is exactly one pound, and that's what it had in it.

Just for grins, we re-charged it only to see the same low readings on the gauges, which led us to a stuck closed expansion valve.

We removed the valve and replaced it with a new one, vacuumed the system and recharged it. We were rewarded with cold air and normal pressures.

Next, we checked the exhaust smell concern, and it didn't take long to find. While the exhaust system from the cat-back was sound and tight, the flexible coupling that handles torque angle changes between the engine and exhaust on this Stratus is right at the exhaust manifold outlet on the header pipe. All front wheel drive vehicles have some kind of flexible joint to allow the engine to torque without putting stress on the exhaust system. The joint on the Stratus is actually a part of the catalytic converter, and it was all but coming apart, a condition that wasn't visibly apparent. But putting a hand on the pipe and rocking it a bit revealed a ragged and worn out flex coupling that was releasing some untreated pre-cat exhaust to the atmosphere, some of it making its way to the nostrils of the driver, who had been cruising around on hot days with the windows down because of the A/C concern.

An aftermarket bolt-on catalyst was just under $300, and with a little torch work it was quite easy to replace. The Stratus passed its test drive with an A-plus.

A Fat 'Stat Gasket

Like the Stratus, one Pontiac Montana had an illuminated MIL, and beyond that, an HVAC blower that would sometimes just stop blowing, which is a revolting development in the middle of a summer day. The owner found out somehow that she could slap the silencer panel beneath the glove box under the right side of the dash and the blower would wake up and blow for a while.

Isn't it interesting what people will do just to get through their day with a malfunctioning automobile? I heard of one woman who found that when her car wouldn't respond to her turn of the key, she could pour water in the ignition switch on her vehicle (I think it was a Volvo) and it would work. So she kept some water on hand for months and poured a little in the lock cylinder whenever it wouldn't crank. People are adaptable by default, and we'll do what we have to in order to make it from point A to point B.

Well, with the scan tool plugged in and online, the Montana threw a P0128 code, the second one of those we had seen in as many days. I assigned that thermostat replacement to Jesse, who was confused by the shop manual's instructions that the exhaust had to be removed to replace the thermostat on the 3.4L. Indeed, that appeared to be the case, but those of us familiar with this ubiquitous family of V-6 engines know better. I shared with him the fact that the manifold crossover pipe can be left in place while removing the thermostat, visual evidence notwithstanding. Further, the lower housing bolt is a slot rather than a hole, which is quite a boon.

The aftermarket thermostat and its rubber slotted ring gasket, however, turned out to be something of a bother. The manifold's nest for the thermostat is vertically oriented and it has a rather familiar machined inset for the thermostat/gasket assembly like the old paper gasketed thermostats. This inset, however, is machined deeper and larger to accommodate the thermostat and its rubber gasket.

This arrangement isn't unusual – a lot of cars manufactured around the turn of the century are set up this way.

Well, the aftermarket gasket turned out to be a little too large to fit in the machined inset when installed on the thermostat. Thinking the gasket might be shifting, Jesse applied some weather strip adhesive to hold the gasket more firmly to the thermostat plate, but to no avail. I ordered a thermostat from the GM dealer, which was a totally different design and was absolutely painless to install.

The engine was warm again, and the MIL code was gone, but the blower was still an issue, and in that case, we found that the blower connector had suffered heat damage due to oxidation and resistance on one of its cavities. A new pigtail took care of the blower problem.

Rapidfire A/C Clutch

Blower and cooling fan circuits have always been current intensive – with the exception of the starting/charging circuits, no circuit on a vehicle is more prone to resistance/heat damage than fans. On late 1980s Ford vans we had to clip the fuse panel out of the blower motor loop and install a heavy standalone circuit breaker because the fuses were melting in the panel. They wouldn't blow, mind you — the plastic would simply melt. Some Motor Age readers might remember a similar situation we ran into on a 2004 Sebring that had melted its cooling fan fuse in the PDC.

Well, vehicle No. 3 in our repertoire is a 1997 Honda Civic with lackluster A/C performance.

First we found a blend of R-134a and R-12, which had to be pumped out and into a gray and yellow junk gas bottle. We did that.

Next, we refilled the system with a good clean charge, only to discover that the head pressure was shoving the high pressure switch open (400 psi), causing the A/C clutch to machine gun in and out. The condenser fan had melted its fuse (somebody had installed a 30 amp in place of the required 20) before it died – we found the fan totally open electrically and inoperative. What's just awful is that Honda put those little mini-fuses in the panel pulling that blower. It's surprising that more of them don't melt.

And the heat-related problems (A/C and engine cooling) are still coming, making for a good summer for the guys in my shop.

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, Ala. E-mail Richard at [email protected]