Auto Service World
Feature   April 1, 2007   by Jim Anderton, Technical Editor

White Box, Cold Car

An unusual GM cooling issue shows how to avoid a poor outcome for a simple service procedure

In cooling system service, there are a few procedures that are so simple that even the greenest apprentice can “dive in” without fear. Flush-and-fill is the first, with thermostat replacement close behind. While thermostat change-out used to be a seasonal spring and fall ritual, it’s much less common today, and only if the engine has been well maintained. Here’s an example where it’s possible to do a simple service to deal with a typical thermostat complaint: slow warm up … and avoid a comeback.

The engine is an Olds-designed Quad OHC, installed in millions of mid-size GM vehicles in the 90s. While prone to cylinder head cracking, they also have an unusual water pump design that puts the unit at the back of the block (and under the exhaust manifold), where it’s internally driven, without a belt. It’s a very high efficiency unit, capable of surprising pressure, which can cause an unusual issue with the thermostat: poor warm up. While a stuck thermostat is the normal diagnosis for slow warm up, changing the unit might not solve the problem. While the tech might suspect a bad part, I suspect that some lower quality thermostats are “universal” designs that allow some bypass of cold antifreeze, possibly because of a weak spring. Air lock might also be an issue, as the thermostat is higher than the pressure cap in many units, and there’s no bleed valve. There’s an obvious solution: use a quality part, plus a small modification that I’ve seen help many times when air is an issue.

The quality thermostat cured the problem, while the additional bleed was extra insurance against a problem that’s very difficult to diagnose. And remember to keep the customer away from DIY cooling system sealers; overuse of leak stoppers is a sure way to ruin your good work. OEM manufacturers have been known to add sealants at the factory, but that should ideally be the first and last time for a good engine.

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