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Compression Testing.................Something You Need To Know

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I went to the coast and back yesterday (about 390 miles total) for the sole purpose of checking my fuel mileage on a long trip at mostly freeway speeds. As I had been getting 14 mpg on my local trips I was sure I would get at least 16 mpg or more on my coast trip. I was very disappointed when I only averaged 13.5 mpg and wondered why it had gotten LESS mileage. I had noticed my exhaust sounded a bit different under a heavy throttle and I had also noticed my engine seemed to "lope" more than usual when I was idling thru the big parking lot in front of the Pismo Beach pier. But other than those two little things it ran perfectly smooth and trouble free.

This morning I used my IR gun to measure the exhaust port temperatures and found the #2 cylinder was showing about 250 degrees less than the others. I did a compression test and it BARELY made 80 psi after about 10 compression strokes. I tried it again and it BARELY made 80 psi after about 10 more compression strokes. I tried it a third time and again it BARELY made 80 psi after about 10 compression strokes. I wondered if I could have a broken valve spring so I pulled the valve cover and found nothing broken or out of adjustment. Hmm. I did a fourth compression test and this time it showed 165 psi after about 3 compression strokes (???). As that #2 cylinder hadn't been firing carbon had gotten stuck on the valves or seats and was the cause of the very low compression and failure for that cylinder to fire.

I checked that #2 spark plug wire and found it had about twice the ohms resistance than the others so that wire was no good. I just ordered a set of MSD wires from Summit Racing and they'll get here early next week. The whole point of this thread is to tell you compression tests CAN be faulty if carbon or other debris has gotten stuck on a valve or seat. Had I been "Joe Average" and taken my big block '71 to a shop they would have almost certainly told me I had a burned valve or damaged valve seat and tore my engine apart for nothing.

If you ever do a compression check and find a low cylinder check it again and again and again and again. If it continues to show a low pressure try pressurizing the cylinder with about 150 psi of air pressure (using a spark plug adapter and air compressor) and spin the engine over again. You MAY just have a little piece of crud stuck on the valve or seat and the pressure will dislodge it.
 
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Joined
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The "average joe" would have checked things like leads and spark plugs before going to a compression test.


The "Average Joe" relies on his dealer or mechanic to check his car. When I found my plug was fouled I checked the compression in that #2 cylinder since I already had the plug out.
 

kpic

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As you had a plug(s) fouled due to your ram air mod and now only one (1) plug fouled; you lost me. Is only one (1) plug fouled or are more? How did you fix douling all the plugs caused by your ram air?

I don't understand your MPG.

I've always considered myself an average Joe and I don't rely on a dealer except for inspections.

As this:
100_1494_zps61f75c43.jpg

Gets nicely over 30 MPG on flat terrain at 70-75 in 6th. You are getting terrible MPG. It might be the under inflated tires you mentioned??
 

Mac

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As you had a plug(s) fouled due to your ram air mod and now only one (1) plug fouled; you lost me. Is only one (1) plug fouled or are more? How did you fix douling all the plugs caused by your ram air?

I don't understand your MPG.

I've always considered myself an average Joe and I don't rely on a dealer except for inspections.

As this:
100_1494_zps61f75c43.jpg

Gets nicely over 30 MPG on flat terrain at 70-75 in 6th. You are getting terrible MPG. It might be the under inflated tires you mentioned??
Such a nice clean installation...

Mac
 
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In the 50+ years I have been twisting wrenches that was the very first time I ran into a problem like this. I took at least 10 compression tests before it suddenly read normal. As it had been running perfectly up until a few days ago I just couldn't imagine it suddenly having a burned valve. So I'm glad I kept on testing it and didn't rip it apart after the first test.

In the past I have several times found a tiny piece of carbon bridging the spark plug gap and causing the plug to not fire. And I found a RJ-12Y Champion plug with it's resistor missing back in the early 1970's when resistor plugs first started being used.
 

Vettehead Mikey

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At least 10 tests before you got it right?

Only from you would that be believable.
:D

More than a little questionable that the supposed chunk of carbon wasn't dislodged with the the engine running yet somehow doing a compression test blew it free.

Yet another 'miracle', they're coming fast and furious of late.
 
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kpic

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More than a little questionable that the supposed chunk of carbon wasn't dislodged with the the engine running yet somehow doing a compression test blew it free.

Yest another 'miracle', they're coming fast and furious of late.

Toobroke... will be patenting his compression test with the plug still installed..

It's closer to broadcasting than a miracle.
In farming, broadcasting is spreading fertilizers uniformly all over the field. Toobroke is broadcasting a site and as you mentioned it is coming fast and furious..



 

Antz81

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At least 10 tests before you got it right?

Only from you would that be believable.
:D

Plus A normal person wouldn't have bothered with the first test. They would have seen the fouled plug and checked all of the other things that could have caused it first.
 

SVO

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I read these posts and think "Don't feed the trolls, don't feed the trolls".

And yet, here I am......
 
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That would be the one of the last things on the list. Definitely after the ignition system anyway.


As I have always craved knowledge please tell me some things I don't know. Throughout my life I have always loved to be proven wrong because that's a good way to learn the truth about things. Most guys think they know it all whereas I have always known I don't know it all. That's what separates me from the rest of the world and why I continue to learn when others don't.
 

kpic

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As I have always craved knowledge please tell me some things I don't know. Throughout my life I have always loved to be proven wrong because that's a good way to learn the truth about things. Most guys think they know it all whereas I have always known I don't know it all. That's what separates me from the rest of the world and why I continue to learn when others don't.

Is that why you continue to argue?
 
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The whole point of this thread is to inform you of the REAL possibility a compression test with poor results doesn't necessarily indicate a bad valve or seat. If there is any carbon stuck to the valve face or seat the compression test will yield a low reading. In my case my compression tests didn't show normal pressures until about the 10th or 11th test.
 

dougelam

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That would be the one of the last things on the list. Definitely after the ignition system anyway.
YES a compression test is valid but as Antz stated it is last on the list of proper diagnostics!

BUT it us on the list

AND for some reason shade tree mechanics like to start at the bottom of it

Sent from my SM-T530NU using Tapatalk
 

kpic

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In my case my compression tests didn't show normal pressures until about the 10th or 11th test.

That is really odd. Are you using a CR tester with a rubber cone or threaded end?

Doug is a professional and although I don't know what Antz does from 8 to 5; however he is damn sharp also. I'd listen to what they say.



 

dougelam

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That is really odd. Are you using a CR tester with a rubber cone or threaded end?

Doug is a professional and although I don't know what Antz does from 8 to 5; however he is damn sharp also. I'd listen to what they say.



Thanks kpic, I have been thinking about his scenario and I am not sure what happened yet to explain the change in compression.

Right now I'm thinking operator error or very old equipment.

Sent from my SM-N920P using Tapatalk
 

kpic

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Thanks kpic, I have been thinking about his scenario and I am not sure what happened yet to explain the change in compression.

Right now I'm thinking operator error or very old equipment.


You're welcome. However, after 45 years in mechanical design; it fits or it don't fit. It's kind of simple, no matter what the text books say.

Neither can I and so far the explanations don't make sense. I could say as usual; however, Mac said be nice. ;) Believing carbon would hold a valve off its seat; only to be cured with multiple CR tests, is a bit too far for me to buy.

Your second sentence is why I mentioned the rubber cone type CR tester. Years ago, I had one and they can be a challenge to get a good seal. It was one of the very few tools I ever threw away.
 
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