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PostPosted: Thu Nov 26, 2009 8:19 am 
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Location: Ashtabula, Ohio
In order to determine if a rough running engine has internal issues, then a compression test is highly recommended. This is a very simple test and you can usually rent or borrow a compression tester to get your readings. You could also purchase a tester fairly cheap if you shop around. Here is what a typical compression tester looks like..........

Image

To do this test, you will need an assistant to help you. It is best to perform this test with the engine warm, but if it will not start, then you have no choice but to test it cold. First off, you need to remove the coil wire and then remove all the spark plugs. Be sure to mark your wires so you do not get confused. Find the appropriate adapter in your compression test kit to match the spark plug thread and insert it into the #1 cylinder. You just thread it right into the cylinder by hand until it's snug.

After you have the tester inserted, make sure your car is out of gear or in Park (Auto trans) and the parking brake is set. Have your assistant hold the gas pedal all the way to the floor (WOT - Wide Open Throttle) and begin cranking the engine over for a count of 5 to 7 seconds. Record your reading from the gauge and repeat this on all cylinders..............

Now add one teaspoon of engine oil into the cylinders and repeat the compression test one cylinder at a time. This will give you the "wet" readings and determine if you have worn rings.

The compression readings should be no less than 156 PSI on all cylinders and they should not vary much between the others. Here is an example of a low compression reading.........

Image

A healthy reading would look like this............

Image

A really healthy engine will have around 200 PSI of compression. Please note that these engines will run with lower compression numbers, but will not run correctly or get the expected fuel mileage.

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PostPosted: Sat Jan 16, 2010 8:30 pm 
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I did a compression test on my 89 GTi the other day and got 225 psi in all cylinders, i was quite surprised as its got 250,000 kms!

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PostPosted: Sun Jan 17, 2010 12:25 am 
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Doing a compression test is like a Doctor listening to your heart and lungs with a stethoscope. It's something which is quick and easy to do, and gives a great deal of information about the overall 'health' of your engine.

If you have purchased a used car, and it isn't running that great, or is getting poor mileage...then you should start with a compression test.

The guide above is excellent, and is a great way to do it, however...there are a few 'shortcuts' that a seasoned mechanic will take and rarely mentions.

Consider these two cases:

Image

Here, we have a 4 cylinder engine with good compression in all cylinders. You can see the tips of the spark plugs are not fouled. The readings are with a 'dry' test (the first part of the compression test above).
Shortcut #1: No need to do a 'wet' test if you have great numbers on the dry test...

Next, consider this:

Here's one of those $100 cars which has a hard time starting, runs...but you have to floor it to keep it running, and it dies at the stop lights:

Image

Once again, you've done a dry test, but one of the cylinders is WAY BELOW Johnny's 156 limit (above) and two cylinders are right at Johnny's lower limit. This engine is tired...and how.

Shortcut #2: No need to do a 'wet' test if one of your cylinders is 0 or thereabouts and the others are near 150...you'll be pulling that engine apart anyways for a complete refresh/rebuild.
:wink: :wink: :wink: :wink: :wink:

This is something which most people know, but I haven't seen anyone post.

Finally, once you do a compression test:

You will get 'more expert' advice if you post your results as a picture like you can see above.

Bring all your tools to the table to solve your engine problems...in the 21st century proper use of a digital camera is an essential tool for a competent mechanic wishing to help others...don't get left behind............. :stupid: :stupid: :stupid: :stupid: :stupid: simply because you were born in the 1900s.

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Once you get the cars dialed-in (compression, leaks, bearings, alignment, brakes) swap in new rubber and glass, you've got something which should last for years!


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PostPosted: Sun Jan 17, 2010 12:47 am 
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Phil N Ed, Excellent post....................

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PostPosted: Tue Feb 09, 2010 10:50 am 
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Location: Vallejo, CA
I am mystified by the readings I am getting on my freshly Ring and Valve'd G10. New SS exhaust valves and guides, lapped in the valves, +10 degree cam sprocket, filled it up with Castrol 10-30 for break in, and put ~200 miles on it, including three early sessions alternating acceleration and engine breaking, said by some to insure ring seating.

154,156,155 dry, 165 168 164 wet. This is warmed up. I am due for a smog test soon, and would like to pass without a new CAT or whatever. This is not sealing up much better than it did before the teardown! It sure holds oil better though!

Is the cam advance costing compression?

The gauge on the compression tester agrees with a couple of others on the 115 lb shop air compressor.
The cam advance is the only non stock thing I have here, and I expected 190 or so compression.

Am I just being a nervous Nellie? The car drives fine. From a driving standpoint the most important part of the whole rebuild was the smaller O-ring on the fuel injector, and that's fixed.

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PostPosted: Tue Feb 09, 2010 12:21 pm 
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drbobw wrote:
I am mystified by the readings I am getting on my freshly Ring and Valve'd G10. New SS exhaust valves and guides, lapped in the valves, +10 degree cam sprocket, filled it up with Castrol 10-30 for break in, and put ~200 miles on it, including three early sessions alternating acceleration and engine breaking, said by some to insure ring seating.

154,156,155 dry, 165 168 164 wet. This is warmed up. I am due for a smog test soon, and would like to pass without a new CAT or whatever. This is not sealing up much better than it did before the teardown! It sure holds oil better though!

Is the cam advance costing compression?

The gauge on the compression tester agrees with a couple of others on the 115 lb shop air compressor.
The cam advance is the only non stock thing I have here, and I expected 190 or so compression.

Am I just being a nervous Nellie? The car drives fine. From a driving standpoint the most important part of the whole rebuild was the smaller O-ring on the fuel injector, and that's fixed.


Don't mean to hijack a nice thread, but:


Most people don't do a compression test on a new rebuild, as the rings are still seating, so maybe you are being a little nervous at 200 miles.
5W-30 might help.
It might be better to put the old cam gear on until you pass your smog.
Flathead Ford era rule:
"The last thing you did to the engine is what screwed it up..."

Sounds like the only difference is that gear; maybe it's not quite right.
Did you get it from 3 Tech?

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DIY Shocks: viewtopic.php?f=10&t=45483
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Once you get the cars dialed-in (compression, leaks, bearings, alignment, brakes) swap in new rubber and glass, you've got something which should last for years!


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PostPosted: Tue Feb 09, 2010 12:42 pm 
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Location: Vallejo, CA
A local machinist set up his dividing head on his mill for me, more to save the Canadian customs issue than anything else.
I wouldn't have checked so soon, but the deadline for a smog test is coming up, and I was hoping for better sealing by now. Old flathead ford racers used to feed in Bon Ami to seat rings quickly, but I hope that's not needed.
Considering it made it through its last test worn down to lower numbers, 140 to 150 IIRC, I suppose I will just take it down and give it a try.

I still hope to see those 190 psi readings. That was a lot of work for 15 - 20 psi !!

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PostPosted: Tue Feb 09, 2010 7:41 pm 
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No worries, the blue car made it by smog with just barely low enough NOx to pass.That's with the +10 degree cam drive, 120K miles old CAT, and not really broken in!
posting.php?mode=reply&f=2&t=45994&sid=56e2c833e94cbb6936007f985cf63dc1#

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PostPosted: Fri Jun 04, 2010 7:27 pm 
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Location: Dunnellon FL.
Not to hijack,but Ive got a question. Does anybody know how it would effect a copression test if you forgot to do the WOT ? Just curious because I have forgoten to press the throttle wide open before while doing the test and acted on those numbers that I recorded. :o

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PostPosted: Sat Jun 05, 2010 3:16 am 
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It adds a restriction, as such, it will lower the compression readings, this is why you are supposed to do it at WOT.

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PostPosted: Tue Feb 23, 2016 6:01 pm 
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To stop the belt form squeeking...
Remove belt use a piece of 80grit...
Sandpaper suff up the crankshaft,alt,waterpump,
Pullys......to get the undid of the crank pull put the car in 3rd and push/pull the car to access the underside of the pully reinstall the belt and your good to go.....jv&s
.

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