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The mkiv Supra Owners Club

How to do a compression test?


Tom
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Dunno about the Supra, but for the Pinto in my kitcar you disconnect all the plugs (remember the order) test one cylinder at a time, remove sparkplug, screw in your compression tester, make sure on zero, open throttle (most people forget this) and crank over once through 3/4 cycles.

That's about it. Write down the results. Any low results can be diagnosed by squirting a small amount of oil down the plug hole - if that helps the result then maybe piston rings are worn. If you have two adjacent cylinders down then possible gasket blown between the two cylinders.

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I honestly didn't know it was gonna be this easy!, I just remembed seeing some guide somwhere and there were gauges and things all over the place! maybe it was something different!

 

anyway i'll be back later to post results.

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it's the fuel you want to turn off, you don't want to be squirting fuel into the cyl while trying to measure compression. it's not goint to be doing much sparking unless you plug in the coils and then put the plugs in the coils.

 

pull efi1 & 2 fuses.

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won't the battery recharge? compression testing usually shows up a poorly battery.

 

did you do the same amount of crank revolutions for each cyl?

 

throttle full open for every cyl?

 

because you haven't stopped the injectors spraying you are washing the oil seal off with petrol. so while you where testing no1 cyls 2-3-4-5-6 where receiving petrol (reducing your compression), then when you where testing no2 cyls 1-3-4-5-6 where still getting more petrol. so on and so forth until you come round to test no1 again which has seen quite alot of fuel by now since the first test.

 

hot and cold engines will provide different results also.

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also to add,that I searched on here about compressions tests too and someone mentioned that all the plugs should be pulled? is that correct?

Don't think so. I've always done one at a time. Thinking about it I'm not sure it'd make much difference. You could argue that the other pistons will have less resistance/compression making the crank easier to turn over maybe?

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Hmm interesting . Can i ask why there is a reason why all the plugs should stay in and only be done one at a time . I have ALWAYS done it with all the plugs removed.

 

Here is a little linky walkthrough. http://www.pumaracing.co.uk/comp.htm

Also has a guide for leak down testing also ...far easier than me typing it in my bad english.

 

Oh and this has been covered before :tongue: http://www.mkivsupra.net/vbb/showthread.php?t=45597&highlight=compression+test

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Not only that Termy ..but with all the plugs removed the cyls that are being tested can pull the max amount of air from the plenumb while the others are venting from the spark plugs hole ...therefore making sure that the maximum volume of air is going into the tested cyl and not being shared making for a quicker test, which means less stress on the battery and less time for dry cranking to cause damage.

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if you crank with the plug out it will blow and suck from the plug hole, chance of some nasty foreign object to be sucked into the chamber.

 

it takes me about 20 seconds to remove a plug, so after the extra 2 mins taken between doing the no1 and no6 i dont think my engine has changed temp enough to warrant any real concern over the effects that would have on the final figures.

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if you crank with the plug out it will blow and suck from the plug hole, chance of some nasty foreign object to be sucked into the chamber..

ah, but don't you always blow compressed air around the plugs before getting them out?

Basic precaution, isn't it?

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