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Compression Testing; How Close & How Many Pounds
Topic Started: Jun 9 2010, 10:38 PM (856 Views)
snowfish
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Basic GearHead

I finally got around to doing a compression test on my 91 Metro project. I did 5 dry tests and averaged the results. 120-122-118. I did not do a wet test. Not sure what the point is to the wet test. Run it? or tear it down & build?
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JellyBeanDriver


wet test is to tell you if the valves or rings are worn. Adding oil will help seal the rings and if your pressure improves a lot, your rings are to blame for your low compression.

Did you:
- fully warm up the engine?
- pull all the plugs?
- keep the throttle wide open during cranking?
- have a fully charged battery?


Is the car exhibiting any problems that you're trying to solve?
Edited by JellyBeanDriver, Jun 9 2010, 10:42 PM.
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snowfish
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Basic GearHead

Engine was fully warmed up, but I forgot about the WOT! :smackface It actually runs really nice, but can't road test due to a rusted K Frame. The engine is going into my wifes Metro that runs really rough. Pretty sure the valves are shot. Thus the engine swap.

Plan is to give her my engine, if it's up to par, and rebuild hers for my project.

I'll test again, but this time at wot. Now I understand how the wet test should tell the whole story. I guess the best/worse case scenario is I rebuild two. :gamerz

What is an acceptable compression & variance? Thanks.
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Rooy


Factory manual wants 156psi minimum. New should be 190-200ish I believe. Mine is around 150 across the board and runs fairly well and still returns good mpg. Variance between cylinders should be less than 10%. If you forgot to hold the throttle open, you can expect the numbers to be low by roughly 30 psi.
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snowfish
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Basic GearHead

Since yesterday was a knucklehead :smackface and didn't test WOT, I did the whole drill again. Also did a wet test to observe the change. (JellyBean explained in a language that I can understand. Thanks)

Here's the results of a 3 Test Average........

Dry
165, 167, 165

Wet
203, 221, 210

What are these numbers telling me? This car has not been down the road in over 7 months since the frame relaxed.

Maybe plop it into the Wifes car, SeaFoam the daylights out of it, and let her run it for a year. That way I can fix my frame, 3Tech her engine to put in my Geo, and see how the story progresses. :news
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Rooy


Your numbers are nice and even and the car will run fine with that compression.

As JellyBean already said, if the numbers go up a lot on the wet test, that means the rings are to blame.

How much oil did you add for the wet test? If you overdo it, that can make the numbers a little high. About a tablespoon per cylinder is all you need.
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Coche Blanco
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Troll Certified

Probably want to be saving for a rebuild.
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snowfish
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Basic GearHead

I squirted 4-5 good pumps from the oil can. Sounds like that's a bit much. This one will be built at a later date, since it appears to be in spec. At $1k per build, I'd rather just do one at a time. :thumb
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starscream5000
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Got 70 MPG?

You may want to pour 1/3 can of seafoam into the motor and run the engine (parked) for a while (a couple of hours at least) to see if any of the rings are stuck. Seafoam may unstick any rings, but they could just be worn out and need to be replaced.
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