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Old 05-03-2021, 08:42 AM   #1
SteveMD
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Default Compression Test Procedures in the Age of E-Throttles

Back in the day, the procedure was to prop the throttle valve open before commencing w comp test. However on modern cars the damn e-throttle is often buried under the intake where the sun don't shine.

So, what's the procedure these days?
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Old 05-03-2021, 08:45 AM   #2
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i recently did a compression test in my 240 and opening the throttle vs leaving it closed made zero difference in the reading
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Old 05-03-2021, 09:05 AM   #3
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Quote:
Originally Posted by esmth View Post
i recently did a compression test in my 240 and opening the throttle vs leaving it closed made zero difference in the reading
Thanks. I'm starting to see it now: the cylinder being tested will pull in air from the intake manifold into which air can enter from the other cylinders as their valves open and close.
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Old 05-03-2021, 01:32 PM   #4
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The difference in the test results in a car with a conventional throttle will likely be in how quickly the cylinder reaches its final test pressure. The throttle plate creates a restriction in the intake which drops the intake manifold pressure. However, on older EFI cars with conventional throttles, they typically have an external idle air control valve and some of those valves are open when they are unpowered (Bosch 2 wire valves) and will admit enough air to permit a compression test. DBW (E throttle) throttles typically do not have an external idle air valve and rely on the DBW system to hold the throttle open to control idle speed. If the DBW throttle is completely closed during the test you will end up with significantly lower pressures in the intake manifold during the test which will screw up the results - like doing a compression test at the top of Pikes Peak. I have an Acura with a DBW throttle and the service manual is explicit that the throttle must be propped open for the compression test.

The theory of back flow through the other cylinders will not work. In order to get back flow through the other cylinders you would need less than atmospheric pressure in the intake manifold which corrupts your test results. Compression test results are always based upon having atmospheric pressure in the intake manifold. Factory test specs are based upon sea level so you also need to correct for test results at higher altitude.

Check your service manual to find out the recommendations for the compression test procedure. The system may have an auxiliary air by-pass that is sufficient to allow the compression test or it may have a special procedure for electrically holding the throttle plate open for the test - something more special than my Acura's pull the rubber intake boot off and jam a stick in there.
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Old 05-03-2021, 02:46 PM   #5
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Use a bidirectional scan tool.
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Old 05-03-2021, 06:04 PM   #6
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As a further note, my previous comments were based upon using a starter by-pass switch to operate the starter motor during cranking with the ignition switch off. If you disable the fuel pump and the ignition system, you can crank the engine using the ignition switch in which case the ECU is energized during the test. Under those circumstances you should be able to hold the throttle open with your foot during the compression test.

I think most DBW ECUs will allow the throttle to be held completely open during cranking as that is the common flood clear mode. However, you can find comments on the Internet (source of all that is correct) that some ECUs will prevent this (I would be surprised) or may generate a TPS error code because it is expecting to see a low % open during cranking (that is a possibility). I have never tried because I have always done the test with the ECU off, stick up the throttle body and a remote starter switch.
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Old 05-03-2021, 08:34 PM   #7
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Clear-flood mode is great suggestion. On some cars you press the brake and the gas pedal (to the floor) and it disables the injectors and opens the throttle.
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Old 05-03-2021, 08:38 PM   #8
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The idle air valve on this non-volvo engine is a 3 wire valve. I guess it is stick in the throttle body if I want accurate results though I will check the service manual as suggested.
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Old 05-04-2021, 11:52 AM   #9
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If it is a Bosch 3 wire valve, it might also be open when un powered. I just mentioned the Bosh 2 wire valve because I know that it is open when it is not powered up. Checking the service manual would be the sure fire answer.
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Old 05-04-2021, 11:59 AM   #10
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Sticking something in the throttle and turning the key on doesn't sound like the best idea to me. I've never broken a throttle, but I have heard rumors of the teeth on the gears breaking.

You may wanna just pinch the starter relay closed with your fingers or use a jumper wire.
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Old 05-04-2021, 12:10 PM   #11
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Motors with a large intake plenum and modern PCV systems don't need the throttle open for a decent compression test.
You might have to crank it a touch longer to get the peak reading, but it still gets there.

More importantly I would be concerned about spraying fuel in a motor you are testing, disable the injectors while testing so you don't contaminate the oil.
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Old 05-04-2021, 12:51 PM   #12
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why not just take off the vacuum hoses & PCV hoses?
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Old 05-06-2021, 04:43 PM   #13
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You can do the test by holding the throttle pedal down. I do it all the time. But they're right, the test can still be done with the throttle closed it just takes a couple cranks longer to get the top number.

I unplug the injectors.

These cars do have flood clear mode though. So holding the throttle to the floor will hold the throttle wide open.

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