Tire dealers who offer automotive services know that underhood fluids are often overlooked by the best of technicians.
Checking and correcting the coolant level could lead to the quickest driveability diagnosis your techs ever performed! Here's why.
Some things never change, and overlooking fluid levels is one of them. When I was a manager, I sometimes checked fluid levels on vehicles that were supposed to be finished and were waiting on the lot for customer pickup. The number of vehicles I found with low oil, coolant etc. startled me.
This was one of my first experiences with good techs missing very basic things, including fluid levels. Techs can become so consumed with things they perceive to be big issues that they overlook very fundamental matters.
Back then, missing a low coolant level was understandable, because you had to wait for the engine to cool down before you could open the typical radiator cap safely. But there are no excuses today, readers. Every vehicle has a transparent coolant-recovery reservoir. Open the hood and, at a glance, you can tell if the coolant is low—not to mention how low it is.
Now let me restate something I've emphasized before: The engine's coolant sensor exerts a tremendous influence on overall engine operation. It also affects the operation of many electronically controlled automatic transmissions.
Coolant level and coolant condition affect the accuracy of the signal the coolant sensor produces. That's why checking—and where needed, correcting—coolant level is an absolutely vital diagnostic step.
Some real-world examples illustrate my point. One came from a colleague who's one of the sharpest driveability and emissions diagnosticians I know. This tech was diagnosing an unstable idle-speed complaint on a General Motors Corp. vehicle. At first, the symptom stymied him. But graphing the test data on the shop's personal computer flagged the problem.
Instead of trying to watch a myriad of raw numbers, some savvy techs download their test findings into a personal computer. Then they use automotive-specific software programs to convert vital readings (including the coolant sensor) into easy-to-follow line graphs.
Lo and behold, the graph showed the coolant sensor signal—which should be relatively flat and stable after the engine reached operating temperature—was dancing up and down erratically. Then and only then did this experienced tech realize that the coolant recovery tank was empty. Further diagnosis revealed a slow leak in the cooling system.
The tech retested after carefully topping off the cooling system. Both the coolant sensor signal and the idle speed were now very stable. Problem solved. Then the vehicle received some much-needed cooling system work.
You don't have to be a worldly technician to realize that a cold engine requires a higher idle speed than a warm engine does. This requirement is often called "cold fast idle." A low coolant level can cause inaccurate, inconsistent engine temperature readings that trick some engine-control computers into erroneously adjusting idle speed.
"If I had checked the coolant level first and topped off the coolant the way I was taught to, I would have solved the unstable idle complaint right then and there!" the tech admitted.
The GM vehicle in this example has a little electric air valve. On command from the engine-control computer, it raises idle speed by leaking a calibrated amount of extra air into the engine.
On the other hand, many popular, service-age Japanese engines use a coolant-heated fast-idle valve to leak that extra air into the engine when it's cold. After the engine warms up, this valve should close, allowing the engine to settle down to its normal idle speed.
It's fairly common for a low coolant level to trick these coolant-heated valves into holding fast idle speed after the engine is hot. Ultimately, this erroneous fast idle fools the engine computer into thinking the engine's decelerating. The computer then activates something called "decel fuel cut," which causes idle speed to surge rhythmically after the engine warms up.
Ultimately, a hapless tech assigned to diagnose this car can't believe a low coolant level was the root cause of this mysterious surging condition. It just seems too basic to be true.