Trouble Shooter

Motor,  Dec 2003  by Seyfert, Karl

• E-mail
• Print
• Link

Many makers emphasize the low maintenance requirements of their vehicles. But neglect of something as simple as engine fluid levels still can have expensive consequences.

Hot Under the Collar

I am working on an overheating problem on a 1999 Olds Alero with the DOHC 2.4L Quad Four engine. The car was bought at an auction and has 26,000 miles on it. I believe someone else has spent time on this problem, evidenced by a thermostat change (I have since installed, another new one). The water pump also looks new because there is fresh sealer visible around the mounting flange. We replaced the water outlet on the left end of head because it was leaking coolant at the O-ring groove.

The car was monitored for several miles after warmup. It seemed okay, then the coolant temperature jumped to more than 250°F on the dash gauge. My scanner agreed with this temperature reading. I was not using the heater at this time; however, there was no heat from the heater vents when I tried to switch it on.

The engine was shut off for a few minutes, then restarted. The coolant temperature dropped to 230°F, then to 210°. Heat was available from the vents, and everything seemed okay during a test drive of several more miles. But the overheating symptoms will repeat if the engine is allowed to cool completely, then restarted.

I have some questions about this engine. With the thermostat located near the water pump, it seems the coolant would have to flow backwards through the cooling system. The sensor side of the thermostat faces toward the water pump, so the heater hoses and the bypass have to work properly in order to warm the thermostat sensing bulb. They seem to be okay.

After checking the cooling system with an infrared thermometer, I'm convinced the engine has a circulation problem, but don't know where. Is there something I'm overlooking that is a known problem with Quad Four engines? I'm chasing my tail.

Carlton Wrench

Whitney Point, NY

First of all, Carlton, the Quad Four's coolant flow isn't backwards. Granted, the thermostat is in an unconventional location, but the coolant still flows in a conventional manner. The water pump draws coolant from the bottom of the radiator and circulates it through the coolant jackets in the engine cylinder block and cylinder head. The now hot coolant is then directed back to the radiator. The system directs some coolant through the hoses to the heater core to provide for defrost and passenger compartment heat. The radiator has no filler neck, so a surge tank is included. Its pressurized cap is at the highest point in the cooling system, and pressurized coolant continuously flows through the surge tank.

The overheating problem your customer's Olds is experiencing could have several possible causes. You mentioned that the vehicle was bought at auction, so its service history is probably unknown. Your detective work has determined that someone else has worked on the cooling system previously, indicating that a problem may have existed before the vehicle went under the auctioneer's hammer. In fact, the cooling system problem might have been the very reason why this relatively low-mileage vehicle was auctioned off in the first place. Some folks operate under the philosophy that if you can't fix a vehicle quickly and cheaply, it's best to dump it fast and cut your losses. Now it's your job to sort it out.

What's the quality of the coolant in the cooling system? Quad Fours of this vintage came from the factory with DEX-COOL in the cooling system, which should not be mixed with conventional coolant. DEX-COOL is designed to be an extended-life coolant (150,000 miles or five years), but it doesn't perform well if the cooling system is allowed to run low. Perhaps the original water pump had a slow leak that went unnoticed by the previous owner. Excessive air in the cooling system has been known to cause DEX-COOL to break down to a granular material. This material could be clogging the radiator or other small passages in the cooling system. The clogging could be causing the intermittent over-heating you describe, which seems to occur when the system reaches full operating temperature and pressure. It may be necessary to flush the cooling system (including the radiator) to remove all traces of the broken down coolant residue.

Air in the cooling system could have caused another problem: engine over-heating. If the engine was allowed to run very low on coolant, it may have overheated once too often. This could cause permanent damage to the cylinder head or head gasket. You didn't mention whether excessive pressure was building up in the cooling system at the same time the coolant temperature was rising. If it was, it's also possible that combustion gases are entering the cooling system. You can carefully check for the presence of CO in the cooling system using an engine exhaust analyzer probe strategically placed near the pressure cap opening in the surge tank (don't dunk it). Dedicated test equipment is also available for this purpose.