Diagnosing cooling system woes: Do it now

Motor,  Aug 2002  by Weissler, Paul

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A neglected cooling system doesn't read the calendar, so there's no `right time of the year' when it comes to cooling system service. Warn your customers: Postpone service now and you'll regret it when the seasons change.

You've got a vehicle in your bay whose engine is running hot, maybe even has overheated. The motorist wants a diagnosis, but perhaps only to see if he has to get the repair now or can safely postpone it. In his mind, the approach of fall with lower ambient temperatures may lead him to believe his problems are over until next summer. However, some of the root causes for hot engine temperatures also can be responsible for lack of heat in the cold weather. In addition, some of the basic reasons for unsatisfactory passenger compartment a/c cooling in summer can lead to inadequate warmth in winter.

So the problem you fix now also

can deliver benefits in the cold months to come. Anything that reduces coolant flow in summer can raise coolant temperatures and possibly cause overheating. That's straightforward. In winter, low coolant flow combines with some reduction in coolant temperature (from low ambients and heat loss from the radiator and engine block) to produce a heater problem, even when coolant temperature itself isn't low. Here's why:

You can't just look at actual temperatures. A half-gallon of coolant at 220 deg F contains a lot less heat energy (in BTUs) than two gallons of coolant at 160 deg F. If some problem keeps coolant temperature relatively high but significantly reduces coolant flow through the heater, the heater can't transfer as much heat to the passenger cabin in the same amount of time. Approximate numbers, if we assume a half-gallon of coolant per minute is flowing through the heater at 220 deg F vs. two gallons at 160 deg F:

*850 BTUs every minute for the half-gallon of 2200F coolant.

* 2500 BTUs every minute for the two gallons of 160 deg F coolant-about three times as much heat energy available to warm the cabin.

Here are the service problems that may have two or more causes:

Restricted Cooling System

What can restrict a vehicle's cooling system? There are many causes, but the two we see most often are rust and corrosion and sealer in the system. Electrolysis is also a big contributor to restricted coolant flow.

Rust and corrosion. In case you thought that everything is made of aluminum and won't rust, take note of the fact that trucks and SUVs are half the market, and those engines usually have cast-iron blocks at least-even the Toyota VS. With poor cooling system maintenance, you can believe the reddish-brown stuff forms.

Sure, the extended-life coolants usually carry a 5/150 or 5/100 change interval. However, poor maintenance is not just failure to change the coolant on a time-based schedule. Even worse is allowing the cooling system to run with a low coolant level, which allows air into the system. That creates a great breeding ground for rust, and there's no more severe example than GM's 4.3L V6. The photo on pages 28-29 is not a one-ina-million case. This is a frequent problem, at a fraction of the 5year/150,000-mile recommended service interval for DexCool.

The OAT (organic acid technology) coolants are in wide use and gaining in OE usage. However, I can't say I'm a big fan of wholly-OAT antifreezes such as DexCool, which seem to be overly sensitive to low coolant level (among other things). However, in the GM 4.3 V6, the root cause for low coolant level at low mileage, without a leak, is a weighted vacuum valve in the radiator pressure cap. A factory fix to forestall the problem is a cap with a spring-loaded valve.

The longtime OE weighted valve is open until closed by buildup of cooling system pressure. Before that occurs, some debris in the system can be deposited on the upper side of the valve. Over time, the buildup is enough to keep the valve from sealing closed when the pressure builds. Coolant flows out of the radiator into the overflow reservoir, which itself eventually overflows, causing loss of coolant.

The replacement cap deals with that problem, but cleaning out the rust powder buildup in the system is something else. There's a GM bulletin on the possible use of a "heavy-duty" cooling system cleaner, which is a specific two-step product-oxalic acid and neutralizer. It's not easy to use properly, and if you make a mistake, don't say we didn't warn you.

Of course, you can't just install the right cap on a plugged system and expect good results. One possible approach is to backflush with a full-feature flush & fill machine-first the heater core and then the engine block (thermostat removed because of the debris issue). Replace the radiator, then really fill the cooling system (a good flush & fill machine also is key to success in this area) and warn the motorist to keep the system topped up and to have the cooling system checked once a year.

GM continues to endorse DexCool for this engine (and all others except its new Pontiac Vibe, which is Toyota-engineered). However, some shops point out they never saw these problems with conventional American green/gold (silicated) antifreeze. So if you agree with that line of thinking, you may be more comfortable with installing a green/gold antifreeze with silicates in cast-iron block engines. Green/gold may require a shorter service internal, but the motorist who encounters the rusting problem should be ready to shorten maintenance/inspection intervals anyway.