TechTips
The Case Of The Hot Toyota
by Brad Davis* What Can Go Wrong With A Simple Head Gasket Replacement
Nothing is more frustrating than doing a big job on a vehicle, only to have new problems appear once it’s back together.We get a lot of calls from techs who have replaced head gaskets on Toyota trucks with the V-6 3VZ engine. They’ve either performed a valve job or had the heads off to replace a failed head gasket.
After putting the heads back on, a most disheartening scenario develops during the road test. The "check engine" light that was not on when the customer brought the truck in, is now on. Your heart sinks. To make matters worse, the engine is overheating.
Overheating? No Problem, Just A New ...
You figure you got a bum thermostat. But replacing it doesn’t help. Neither does a new water pump or a new radiator.Now you’re not so sure you’re on the right track. You retrace your steps. All you did was replace the head gaskets and they were identical on both sides -- weren’t they? You have this terrible feeling you’re not going to make it home in time for dinner.
The truth is, the gaskets are not interchangeable. They look almost identical. But if you look closely, you’ll notice a small hole in the driver’s side head gasket that feeds coolant to the thermostat. With the gaskets on the wrong sides, there is no coolant flow to the thermostat so it never opens. With the heads still on, look at the rear of the passenger side head. If you can feel the head gasket sticking out, it’s on the wrong side.
Now, on to that check engine light. You connect a jumper wire between terminals TE1 and E1 in the diagnosis box. The check engine light reads a knock sensor code 52. Bad sensor? Why now?
Knock, Knock -- Forget The Hammer Trick
To test the knock sensor, some techs grab a hammer and tap on the block. While tapping, they look for spark retard. But most techs don’t know how smart this sensor really is. The knock sensor in this engine is tuned to 7,000Hz, the frequency range of a spark knock condition. With that narrow range of sensitivity, the computer can’t hear a noisy idler, worn cam lobes, rod bearings or a frustrated tech banging on the block with a hammer!Getting no retard out of the hammer test, many techs pull off the intake manifold and replace the $130 knock sensor. They’re shocked to find the same code 52 on their next road test. Now what?
The cause of the code 52 problem is usually not the knock sensor itself, but the signal wire between the knock sensor and the external connector by the intake manifold. This wire gets brittle if it is overheated. Removing the connector during disassembly can break the signal wire inside the shield. Obviously, that prevents the signal from getting to the PCM.
We advise techs to replace this short length of knock sensor lead wire any time the intake manifold is removed. Especially if it’s been overheated.
Knock, Knock--It Can’t Hear You
The Resonance-style Knock Sensor Will Not Respond To Your Hammer Blows
Back in engine performance class you probably learned that there are two common types of knock sensors -- piezoelectric and piezoresistive. The piezoelectric sensor creates a voltage when it is exposed to vibration, while the piezoresistive type changes resistance during vibration.That information stills holds. However, in addition to the two types of sensors, there are two different styles of piezoelectric sensors -- mass and resonance. The mass sensor has its "ears" up to all vibrations. It produces a voltage at all vibration frequencies, creating its highest voltage at around 7,000Hz. This sensor forces the PCM to filter out the voltages above and below those generated at the 7,000Hz level.
The resonance sensor is much smarter. It sends a significant voltage to the PCM only when it detects vibration in the 7,000Hz range. At all other times it is quiet as a mouse. This self-monitoring takes a load off the PCM.
While Toyota uses both styles of piezoelectric sensors on its engines, the engine on the reverse side of this page uses only the resonance style. Unlike knock sensors from other manufacturers, striking near this sensor with a hammer will most likely get you nowhere, unless you are just looking to vent some frustration.
About the only way to test this sensor is to over-advance the timing, attach your digital meter (AC scale), and take the truck for a test drive. If the sensor is working, you should see AC readings that correspond to the pinging you hear.
* Brad Davis is the most recent addition to the Import Department of AutoLine Telediagnosis. He has five years of formal vocational automotive training and eight years experience as a fully trained Toyota dealer tech. Brad specializes in driveability and drivetrain mechanics. He also serves on the Advisory Committee for North Hennepin Technical College in White Bear Lake, Minn.
Charging The Customer For Your Call To AutoLine
Fact No. 1: You can’t know everything about every car make and model on the road.
Fact No. 2: You can’t afford to build the data base of information you need to fix every car make and model on the road.
Fact No. 3: Maybe not today, but sometime in the future, you will be using a technical assistance center. And, you will have to charge the customer for the call.Here’s What To Tell Your Customer
Many shops bill the support call as an information access fee, data access charge or expert consult fee. It’s smart to tell your customer up front that you use a support line.If they question the need for such a call or its cost, ask them this: "Would you think less of your doctor if she called in a specialist to help with your diagnosis? Or, would you rather that she just conduct hours and hours of exploratory surgery searching for a clue?" You know they would feel more at ease knowing that their doctor put ego aside and called in an expert. It’s the same with auto repair.
Recite facts No. 1 and No. 2 above to your customers and explain to them that, with a maze of sensors, wiring and computers, you want to make the diagnosis without doing exploratory surgery or unnecessary tests.
Contrary to what you might think, customers want you to call in a specialist, rather than rack up their bill with your guesses.
Call AutoLine customer service to get a flyer you can hand out to your customers.
TechTips is a monthly feature in AutoInc. Each month, technicians from AutoLine Telediagnosis share valuable TechTips with our readers. As an ASA member, you’re already signed up to use the AutoLine service at a discounted rate.
AutoLine Telediagnosis : 1-800-288-6220 Monday-Friday 7 a.m. to 7 p.m. CST
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AutoInc. Magazine ®, Vol. XLIV No. 7, July 1996