Compression test advice needed

[crzyfun]

Hi all!

So over the weekend I did a compression test on my '72 vert. 1-4 were all pretty much exactly at 120 PSI. 6-8 were at 110 or 115 PSI, but 5 was at 90 PSI. I even did it twice just to be sure. It held steady at 90 PSI.

I'm a total newbie to all this but I recall reading that the next thing to do is squirt a bit of oil in it and do the test again, which will help determine why the compression is so low. But it was pretty late at that point and dinner was getting cold, so I stopped.

My question is: it's now been a couple of days... should I just do a compression test on that one cylinder cold (with a squirt of oil)? Or maybe do a pre-squirt test again (while cold) then a post-squirt test?

Seems like a lot of work to put all the spark plugs back in just to warm it up to then take them all out just to test one?

I welcome advice!

Also, why are the spark plugs so hard to get to? Especially 5-8 were really tricky to get the socket wrench in, and then really really tricky to get the threaded compression nozzle (head? whatever you call it). Maybe there's some trick to it that I don't know. And while I understand the importance of doing it warm, that means I'm reaching in & around the engine block while it's warm (read: hot!). Oy.   :-/ :shootself:

-Dan

CrazyFun!

 

[Carolina_Mountain_Mustangs]

This model much easier than some to get to plugs. Some of the FE engine mustangs you had to take motor mounts loose and jack the engine up to get them out.

Warm engine has the cylinders to the diameter they run at, same with the pistons. Might have a ring issue on that one cylinder causing the lower reading or valve not sealing. Oil helps to close the rings off but not much if a valve leaking. Would be better if valve then just pull head. If ring have to get piston out.

Has car sat a while? If so squirt some penetrating oil in that cylinder and let it set a while can be a stuck ring.

 

[Don C]

Yes, I would try it again, warmed engine is better, but you can get information from the cold engine.

Then, if the compression is still really low I would put some air pressure in that cylinder, with piston at TDC with the valves closed, to find out where the leak is, listen to the crankcase, exhaust, and intake manifold.

 

[delawarebill]

bringing a cyl to tdc and doing the air test will tell u what is leaking... hear with carb or exh and pull the valve cover vent to listen there also. even a cold engine will give u info with oil into cyl. compression goes up= rings most likely. no increase = valves

a couple of good squirts and spin the engine to lube the rings then test.

 

[boilermaster]

drsharp,

You did not state in your thread why you decided to do a compression test in the first place.

Was the engine misfiring ? smoking ? rough idle?

One thing I would do BEFORE taking anything apart would be a proper top end cleaning.

This is where one gradually pours a portion of the cleaner down the throat of a warm engine and then the remaining portion down the carb to stall the engine out.

One then lets the engine soak for 30- 45 minutes and then re start the engine.

If you still get a low reading, then it is time for an air hold test or leakdown test (two different tests)

What you are trying to do is isolate where the compression loss is coming from.

Boilermaster

 

[crzyfun]

drsharp,

You did not state in your thread why you decided to do a compression test in the first place.

Was the engine misfiring ? smoking ? rough idle?

No, actually the engine seems to run pretty well (at least from my not-an-expert perspective).  The reason to do the compression test was two-fold: 1) because I've never done one before, and mostly 2) because the plan is to strip the car down and rebuild it and I wanted to know the state of the engine before. I'm not sure (yet) how deep a rebuild it will be at this point and it seemed like a good data point to gather.

Anyway, it was good to do... fun to include the wife and kids in it... and now I have some data points. What I do next about them, I'm not sure. ::thumb:: Hehe.

-Dan

Crazy! Fun!

 

[tony-muscle]

drsharp,

You did not state in your thread why you decided to do a compression test in the first place.

Was the engine misfiring ? smoking ? rough idle?

No, actually the engine seems to run pretty well (at least from my not-an-expert perspective).  The reason to do the compression test was two-fold: 1) because I've never done one before, and mostly 2) because the plan is to strip the car down and rebuild it and I wanted to know the state of the engine before. I'm not sure (yet) how deep a rebuild it will be at this point and it seemed like a good data point to gather.

Anyway, it was good to do... fun to include the wife and kids in it... and now I have some data points. What I do next about them, I'm not sure. ::thumb:: Hehe.

-Dan

Crazy! Fun!

I see why doing the compression test in your case. I did the same and even though all was good I decided to rebulid the whole engine and stroke it to 408. If budget allows it is the best way to know exactly was in it.
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[crzyfun]

I see why doing the compression test in your case. I did the same and even though all was good I decided to rebulid the whole engine and stroke it to 408. If budget allows it is the best way to know exactly was in it.

Yeah, that's the same place I'm in. I think I'll at least do a partial rebuild just because I've never done it and want to learn. The engine doesn't have any obvious trouble signs (other than the low compression on cylinder 5), but if I read correctly, a top-end rebuild is mostly just cleaning and oiling and new seals and such. I may be totally wrong (and I'm open to correction as this is a huge learning curve for me... crazy, fun!)

Also, I've never done a compression test before and I wanted to to do it. If nothing else it was good to have done it, and not screwed anything up.

-Dan

Crazy! Fun!

 

[crzyfun]

Yes, I would try it again, warmed engine is better, but you can get information from the cold engine.

Then, if the compression is still really low I would put some air pressure in that cylinder, with piston at TDC with the valves closed, to find out where the leak is, listen to the crankcase, exhaust, and intake manifold.

Hi Don!

Yeah, it would be better to do it warmed, but I went ahead and did the "wet" compression test cold on cyl 5. It had been sitting since Sunday (so 6 days). I put in a little bit of oil. Hard to tell how much because (a) I didn't measure it per-se... just poured it... probably 2 teaspoons I'd guess, and (b) some of it missed. I was trying to do a make-shift funnel into a straw to get it in. Hehe.

Anyway, the result was 110 PSI on 5 wet. So 90 "dry" and 110 "wet".

The full results are:

1: 120

2: 120

3: 120

4: 120

5: 90 (110 "wet")

6: 115

7: 110

8: 120

::thumb:: 

-Dan

Crazy! Fun!

 

[73pony]

If your heads still have the original ford multipiece valves you should take to machine shop for a valve job and have hardened seats and good one piece valves installed at the bare min.

 

[73pony]

Plus new guides and seals.

 

[Don C]

I agree with Jason. When you have the heads off you'll be able to inspect the cylinder walls, too.

It sounds like your problem is rings. If the rings are stuck, after sitting with the oil on them for a few days they may free up.

The spreadsheet in this post will give you something to play with

https://www.7173mustangs.com/thread-compression-altitude-calculations?pid=115411#pid115411

 

[tony-muscle]

You are geeting great suggestions. Thats why at the end of the day "while at it" you end up rebuilding the whole thing.

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[crzyfun]

You are geeting great suggestions. Thats why at the end of the day "while at it" you end up rebuilding the whole thing.

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That's ok. I kinda want to rebuild it anyway, because I never have and want the experience.

-Dan

Crazy! Fun!

 

[Don C]

I'm sure you are going to get your chance. It seems like you can start out on something relatively easy and wind up with either a rebuild or new engine. Like they say, ask me how I know.