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Old 02-27-2003, 07:17 PM
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Got new compression and vacuum gauges ... now what?

What kinds of things can you diagnose with a compression tester and a vacuum gauge? And exactly how do you use them to perform these tests?

I'm interested in checking the engine in my 450, and hopefully pinponting things such as worn rings, blown head gaskets, and so on.

Hopefully, I'd like to test this Mercedes' vacuum door lock system as well.

I've hearn terms such as "leak-down test," "wet test/dry test," and so on, but at this point, it's all Greek to me!

Any and all help appreciated

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Old 02-28-2003, 03:16 AM
afmcorp's Avatar
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hello bungee

your cylinder compression gauge is used to check the compression in each cylinder. simply put you removed all your spark plugs and thread in your adapter fitting at the end of the hose. i hope that is the style you have because they are a lot easier to use. it should have come with a small fitting for the 5/8 plugs and the larger fitting for the 13/16 plugs (size of socket)

so now you're ready to take your tests. write down your cylinder numbers 1 thru 4,6 or 8. then go thru the proceedure i'm sure you got with the gauge. crank the car over 3,4 or 5 times. watch your gauge and see when the needle quits moving. ok now record that number. continue on the rest of the cyl's. now you have your data to make some comparisons. Note: save those numbers permanently for future reference.

now you can read them. there is a rule of thumb that if they are say within 10 - 15 (this maybe a bit wide of spread but 10 lbs is maybe closer to being correct) from your hi and lo everything is fairly balanced and there are no great issues to deal with. there will be an absolute loss of power but because everything is in tolerance you won't be able to preceive the difference.

now the book will say each cylinder should have 150 lbs of compression( or some number to that effect). but that is for a new engine. remember that an engine can not heal itself regardless of what people say. from moment one when you first start that motor it is going downhill to it's grave. so what you really want is for all the cylinders to be close which will keep the cylinders performing equally.

a dry test is what you did in the beginning. now regarding wet test etc:. now what you can do to determine which way the leak is going is by squirting motor oil into the spark plug hole. what this does is wet the cyl walls and thereby placing a film on the piston rings aiding in sealing the cyl walls from blow by. so what you're looking for is an increase from your first set of recorded numbers. do this again to all cyl's recording them again. now compare them. if you don't see any increase in the compression the leak or blow off is considered to be thru the valves. therefore your fix would be a valve job. so if you do record an increase it would be considered as ring failure thereby needing new rings. this operation could be as simple as removing the head (s) breaking off the ridge of the cylinder with a ridge reamer and then using a drill powered hone and scuffing up the walls so the new rings you're installing will have something to seal against. but..... you won't know the extent of the problem until you get the head (s) off to look. to make it right you may need a bore out meaning new pistons to make up the difference in the id of the bore. in worse case situations you may need a whole new liner because you can't bore it out far enough to cut the gouge in the cylinder wall. but in that worst case situation you would probably have an engine that runs (if it runs at all ) really really bad.

so if your engine is running smooth about all you'll have from your compression test is a baseline from which you can compare against in the future. remember i said keep it forever. get yourself a 5 x 8 spiral bound note book and start recording your work by date and a detail listing of what was done. even if it was done by a professional shop. trust me it will come in handy at some time in the future.

ok vacuum gauge. this is used at various points in the manifold, smog and distributor functions. you should get the correct maintenance manual for your vehicle. in the tune up section and perhaps the troubleshooting section too. you will be advised to take vacuum readings. these readings will be an indicator of a potential problem. your book will tell you where and under what circumstances your gauge will be reading.

on your dash (instrument cluster) you might have a small gauge that is noting how your engine is performing. in the economy mode or in the red area. that is nothing more than a vacuum reading of the manifold pressures. if you hook up the gauge to a vacuum port on the intake manifold you will get a vacuum reading on the gauge. it might say anything from 12 to 17 inches of vacuum. it doesn't matter right now for this purpose. either by hand increase the throttle engine speed or have someone you trust to do it from the inside on the gas peddle. what you will see is as the engine rpm rises ( going faster) your vacuum will begin to drop. what is happening here is you're opening the throttle body letting more air into the engine. there is a formula that can be used in judging the potential cfm of your engine. well rpm affects this cfm because cfm = cubic feet /min. that is the air your engine is moving at certain rpms. (just a side note: way back when it was kinda the joke of the day when you saw some with a 900 cfm Carburetor on top of a 260 cubic in motor because there is no possible way that 260 ci engine could use all the air that 900 cfm carb could deliver)

so... what i am saying is there is a draw of vacuum caused by the cylinders trying to pull the air into them. so as you make that hole in the middle of your manifold bigger the vacuum in inches starts to decrease or as your dash gauge says you're no longer in an economy mode so take your foot off the gas and right back down the needle goes.

as for bad head gaskets the one real sure piece of evidence that there is a faulty head gasket is oil in your coolant water. i went thru a time where i popped 3 head gaskets one right after the other in my hemi and all three times i had oil in my radiator. another way you might discover a bad gasket is if you are low on your cylinder compression test. it's possible that you might decide to do a valve job because you couldn't get any additional pressure on a wet test only to find out it's blown between cylinders. but i'm sure the problem will usually lay in the valve job.

you might consider adding a good induction timing light and a VOM meter to your tool box.

well good luck in your up coming adventure.

and above all remember the backyard mechanics motto. "POUND IT TO FIT AND PAINT IT TO MATCH"
Thanks Much!

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Old 02-28-2003, 08:57 PM
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Wow! Great info. Thanks!

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Old 03-05-2003, 09:48 PM
pesuazo's Avatar
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About how much oil should be "squirted" into the cylinder for a "wet" compression test?

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