Question about compression check

[Ltngjim]

Recently purchased a 1977 280Z 2+2 with a 5-speed and 77K miles (I'm told that its 77K actual and the title says the same). 

 

Been doing routine maintenance, and decided to do a compression check.   With the engine warm, all spark plugs out, I get an even 110 in each cylinder (+/- 3 PSI), so its balanced across all cylinders.  This seemed low to me based on the FSM, so I added a tablespoon of oil to the #1 cylinder and tried again and it went up to 133.  The FSM says that its likely the rings and that an overhaul is required. 

 

My only concern is that the car runs great and hardly burns any oil.  This is my first Z, so I'm not sure if the performance is lower than normal or not.  It generally lacks power below 2K RPM, which may be an indication, but not sure.

 

Will changing the cam timing (accounting for a streatched chain) affect the compression check?  Any ideas on next step?

 

 

[Ltngjim]

Forgot to mention that the pressure in each cylinder will hold steady for 5 minutes without any leakdown.  Is it possible I did the compression check wrong?

[johnc]

Did you block the throttle wide open?

[Domzs]

_{Like John said . I always have the throttle wide open and battery charger hooked up when I do compression test .}

[BLOZ UP]

Forgot to mention that the pressure in each cylinder will hold steady for 5 minutes without any leakdown.  Is it possible I did the compression check wrong?

 

Leakdown and compression tests use different tools. The compression tester will show you the maximum pressure a cylinder makes, not any leak amount.

[SleeperZ]

Your compression doesn't sound that low, perhaps you have some wear in your timing chain.  Check that timing and make sure your cam is not retarded, if it is, you may be able to advance it by removing the cam gear and putting it on the next highest setting.

[Ltngjim]

I didnt specifically block the trottle wide open, but I had the gas pedal pushed to the floor.  Does that accomplish the same thing?

 

So if I understand this right, the cam timing being retarded can affect the compression in each cylinder.  Guess the next step for me is to check the cam timing and adjust if necessary.

 

How do you explain the oil in the cylinder causing the compression to jump 30 PSI, if the possible cause is a streatched timing chain?

Edited October 1, 2013 by Ltngjim

[NewZed]

Sometimes people put late model heads, like the P79, on dished pistons - like here:http://www.classiczcars.com/forums/engine-drivetrain-s30/49184-1978-280z-2-2-cylinder-head-removal.html  Or swap in low compression turbo engines, but run them NA.  That would give a low pressure reading.

 

Or you might not have the pressure check test right.  If you use the adapter tube that comes with many pressure testers, you'll get a low reading.  The adapter is a big hollow tube, that lowers the compression ratio for the test.  If you are using a typical pressure tester it will have a valve on it to hold pressure inside so you can see the gauge reading.  If you don't release the pressure between each test you'll get a bad reading.  Your readings might be going up when oil is added because you're not spinning the engine quickly enough.  You should re-run the test, with all plugs out, a fully charged battery, throttle blocked open, no adapter, and the gauge reset before each cylinder's reading.

 

You might be worrying about nothing.  You're planning to take apart an engine that runs great.  The engines don't have much below 2000 RPM in general, so that's not abnormal.  Edit - forgot to add the usual advice about adjusting valves.  Valve lash can affect pressure.  And it's always good to check valve timing on an unknown engine anyway.  It's easy to do, and it will also verify that your ignition timing mark is correct (damper rubber good), since the notch, groove and zero mark should all line up if everything is right.

My only concern is that the car runs great and hardly burns any oil.  This is my first Z, so I'm not sure if the performance is lower than normal or not.  It generally lacks power below 2K RPM, which may be an indication, but not sure.

Edited October 1, 2013 by NewZed

[Ltngjim]

I have been reading up on how to check and adjust the cam timing chain.  What are people using to lock the chain in place for the adjustment?  Seems like some people have a special tool that keeps the chain and crank from moving during the procedure.

 

I will start with checking the valve adjustment and ignition timing, then will check the cam timing.  As NewZed said, at least it will give me an idea where all these settings are on a "new to me" engine.

[SleeperZ]

All you have to do is turn your crank to TDC for number 1, and look at the cam gear notch in relation to the cam tower.  It will be obvious if the notches do not line up.  If you need to advance it, wedge the chain, remove the cam gear noting which number is pinned, rotate it to the next numbered hole and re-install it after rotating the cam slightly.

 

Here is the proper tool for securing the chain:

 

 

http://www.thezstore.com/page/TZS/PROD/classic07/60-9921

[Ltngjim]

Where do you place said "chain restraint" device?

[Domzs]

Read post # 8 . Check valve clearance . If it runs good then leave it alone . Don't   over nothing .

[djwarner]

A few basics are called for. I bought a Harbor Freight compression test set that had a 12 inch hose on the gauge and another 12 inches of hose for each plug adapter. At the bottom of the plug adapter there should be a Schraeder valve (like what you find on the tire inflation valve). Mine had an open line going all the way to the gauge with no valve.

 

This did two things, first, it drastically increased the cylinder volume reducing the effective compression ratio. Second, once the piston retracted, compressed air flowed out of the gauge and hose. I was reading the peak number and ignoring the fall off. A correct tester will give increasing readings with each compression stroke and stable values between each compression stroke. When the gauge stops increasing, you have your final value.

 

A new compression tester from Sears demonstrated my compression numbers were not low, but in fact, higher than spec indicating serious carbon build-up in the combustion chamber.

 

Just to minimize the load on your starter, I would remove all spark plugs while testing.

[Ltngjim]

The specific compression tester (from autozone) has an 8" hose on it.  It has a quick disconnect just below the guage that includes the release valve.  There is no way to use the guage without the 8" hose as it doesnt have the correct connectors.  I'll keep you updated after I do the valve adjustment this weekend.

 

The FSM calls for a special tool for the valve adjustment.  Is that too a necessity?

[Domzs]

Would be nice to have , get one if you are plan on keeping the Z for awhile . Feeler gauge and wrench will do just fine .

[Ltngjim]

Digging in now and in the process of adjusting the valves.  This is a photo of my timing chain.  Appears that its already at #3 (perhaps you can confirm) and the marks are just a bit off.  Are they off on the less streatched side or the more streatched side?  Thoughts on whether it needs adjusted?

 

Other posts say the line should be just to the left of the V, but it doesnt say how far left is acceptable.

 

As chains streatch does the V move left or right?

[NewZed]

Is the tight side of the chain tight, and the timing mark on zero?  The tight side is the driver's side (left).  Put the timing mark on zero and use a 17 mm wrench to turn the cam sprocket back to make the tight side tight.  There is a lot of slack in an old chain and the v notch can off even though the timing mark is at zero,

 

Also, I think the number 3 that you're looking at might be for alignment with a bright link, which is only used when installing a chain from scratch.  The hole, with number, that you want to see is under the big washer on the end of the camshaft.  If you look closely you might the edge of a stamped number peeking out from under the washer.

 

Could be wrong on the hole number, I'm going from examination of my own random parts.

 

The description with pictures that you want to follow is in the Engine Mechanical chapter of the FSM.

[Ltngjim]

Pushed it through again and lined up the 0 degree on the ignition timing scale.  Apperanly I was a fraction past TDC.  They now line up just like the FSM, so the cam timing is good.  I adjusted the vaves and most were just a bit tight. 

 

I ran a post adjustment compression check and these are the numbers:

 

               1      2       3       4      5      6 

Dry       120   113   113   110  109   112

Wet      137   156   136   132  131  134

 

These numbers point to worn rings, which requires an overhaul.  My gas mileage hasnt been too bad (~20 MPGs on a commute at 50 MPH) and the power seems to be sufficient.  Is there any danger to running the engine in the condition and doing an overhaul at a later date?

[BLOZ UP]

Not really. Maybe diluting and consuming oil faster than normal.

 

I've been running a VG30ET with really low compression for a couple years now. I have to add oil every 500 miles or so, it fouls plugs, it gets about 13 mpg, and I run 20 psi on it at some points.

 

But then again, I don't care about it since it's just the test mule.

Edited October 7, 2013 by BLOZ UP

[Tony D]

"It generally lacks power below 2K RPM, which may be an indication, but not sure."

 

OK, step 1: it's not a Small Block Chevy, the intended operational range is 3,000-5,000 in daily driving. For spirited driving, 3,500 and on up. Drop a gear, drive it right, and be amazed!

 

"These numbers point to worn rings, which requires an overhaul."

 

2: An engine that does not consume oil and is performing well is NOT a candidate for overhaul.

 

You crack this one open, you ruin it and will end up worse off in 10,000 miles than you are currently.

 

Run it another 100,000 and then see where it's at...overhauling this engine is pissing $3,500 down the drain for WHAT quantifiable improvement?

 

You quantify what you gain by the overhaul other than theoretical numbers on a piece of paper.

 

I will also like to point out something that apparently EVERYBODY else is missing:

 

YOU ARE AT ALTITUDE, THE COMPRESSION NUMBERS YOU GET WILL BE CONSIDERABLY LOWER THAN AT SEA LEVEL (which seems to be the only numbers anybody is familiar with!)

 

Leave it alone and drive it. There is nothing wrong with it, and you won't gain anything "overhauling" it!

Edited October 8, 2013 by Tony D