Please educate me about the Compression Test.

[maketracks]

The van is getting a compression test done to it tomorrow. For free..

What would normal results indicate? Too high? Too low?

The mechanic told me that the "lifters don't sound happy." I did not mention that I put in 5w30 conventional, from whatever blackness was in there. I also did not get the chance to mention about the Rite Aid soap.. though he told me the water in there would be ok. It got to about 25 or so this weekend. I have been contemplating a two parts 0w30 super exotic and three parts 5w30 normal oil for her.

I get information tomorrow. What could the readings mean, in advance? Its a RWD Van, 1994. Thanks!

 

[GMBoy]

You need a service manual to tell you what the spec should be. Then all cylinders should be about the same. Any differences past a certain spec indicate a bad cylinder(s).

That's the short version. Basically - a compression test tells the condition the engine is in. Worn=poor compression.

 

[Falken]

It also checks to see if the intake and exhaust valves are sealing.

If the rings and cylinders are worn and scuffed, you'll fail the spec of the engine, and they should be relatively the same for power balance.

A compression test is a good quick check to see if the engine is in good shape, if it is then great.

A leak-down test is better and can identify where the leaks are occurring. But more costly of a test.

If you are buying a beater a compression test is great. If it fails horribly just walk away, no leak-down test needed.

A compression tester is just a gauge that screws into where the spark plugs go, and you pull the injector fuse and the fuel pump relay and run the engine until it dies before taking the plugs out.

They should be pretty cheap to purchase. If you car shop a lot for beaters it would be a great tool to use as it is very simple.

 

[maketracks]

Do you start to use heavier oil if the compression readings are low? I presently plan/planned on keeping a 5w30 range oil mix in there.

If the compression test comes back low, should I just use a 0W-40 weight oil and be done with it? Or GC 0w30 mayhaps?

 

[chubbs1]

As far as your oil goes, no need for "exotic" 0w30 mix. Just use M1 0w30 (if you are worried about cold) due to its reasonable price, or just use a good 5w30. That van would probably do well with M1 High Mileage 5w30. Any major brand conventional 5w30 would be fine though.

 

[cchase]

If compression is low, you're beyond "a thick oil might help".

 

[Donald]

You can dump a small amount of oil down any cylinder with a bad reading and retest. If the compression improves, its rings, if it does not valves.

Good compression does not rule out a bad or stuck ring with respect to oil burning.

 

[mechtech2]

Evenness is important .
And HOW the test is done is crucial.Many don't open the throttle, or note HOW the gauge climbs up for each cyl.

 

[TFB1]

Originally Posted By: cchase
If compression is low, you're beyond "a thick oil might help".


Agreed...

Basically you want consistency between cylinders, 15% variation is usually the accepted maximum but I've seen engines that run fine with 25% variation... Don't worry unless one or more cylinders are under 100psi, which generally indicates excessive cylinder wear, or leaking valves...

 

[rslifkin]

Originally Posted By: TFB1
Don't worry unless one or more cylinders are under 100psi, which generally indicates excessive cylinder wear, or leaking valves...


That depends on what the factory spec is. If the spec is 180 psi, then testing at 110 would indicate that the engine is pretty well pooched, IMO. If it's tested with the engine warm and the throttle held open, I wouldn't want to see numbers more than 20 - 30 psi below spec at most.

 

[maketracks]

Thanks, all.

I am now worried again, as I am almost CERTAIN that this compression test will be done when cold. Also, I gave the mechanic the new AC Delco plugs, in the hope that he will reinsert the new plugs, after testing each cylinder...

Not only that, but I have heard a couple of different ways to DO a compression test. Wet vs dry, anyone? I once saw someone test a car, not sure if this was the right way, but.. The car was stone cold, and a 4-cyl. He:

1. Unscrewed the spark plug.
2. Screwed in the compression gauge.
3. Turned the engine over, never started it..
4. Looked to see what the highest reading on the gauge was.
He did one or two cylinders that way, and decided he didn't want the car. It never started. ?

Also, Donald: You would pour motor oil down the cylinder, or transmission fluid? I have heard of both, not sure if that was good advice or not.

If they were easier to get to (I have seen cars with them right out on top! This is not one of them) I would maybe piston soak it, BUT this mechanic seems concerned about my valves/lifters. Again, I did not tell him I put 5w30 in the 181k engine. Not that that should matter, but this engine appears to have been abused, and produced smoke. Almost looked like it was running too rich, not a sooty black smoke, but, something. Ominous grey looking, darker sometimes. Not blue or white. I felt and hoped a sensor is off..

I was given a checklist of plugs, wires (don't cross 'em), cap, rotor, fuel filter, intake air filter, PCV valve, clean EGR system / check vacuum function of valve, oil change, check all sensors, connections, and vacuum lines. Lastly check for any exhaust or intake manifold leaks. if none of the above correct your issue an internal fault of the engine should be suspected. from a Safari forum, for these. I delivered to the mechanic a new distributor cap and rotor, 6 AC Delco plugs OEM for that engine, and a new air filter with no hole in it, parted from scrap yard.

I do wonder if he is doing the compression test wrong, or will let it run first. I just can't know.

 

[maketracks]

Cyl. 1 = 25psi.
Cyl. 2 = 55psi.

Cyl. 1 = He said the spark plug fell apart when he took it out.

Seriously, though.. What will some thick oil, I'm thinking 20W-50, do for me? And he put new spark plugs in those two.. thanks.

 

[cchase]

Thick oil won't do anything for low compression, as was mentioned earlier in this thread.

 

[maketracks]

Had Bosch Platinum+2 plugs in it. (?)

This is what the one looked like. He gave it to me.

 

[PandaBear]

Where did the rest of it go?

 

[maketracks]

Originally Posted By: PandaBear
Where did the rest of it go?


That is a good question, and I am unsure. I asked "Is there something missing" in a joking tone, he says "there sure is." Electrode..

I am unsure where the rest went. if the cylinder swallowed up that electrode, then hopefully it is long since obliterated. It seems I was running on 5 cylinders. And we don't know how the rest look; He only did the two.

Since receiving the van back at 4PM, I put on the air intake with no vacuum leak, connected the sensor, filled it back up with water, and added 5 gallons of gas. It now does NOT stall at lights, smokes much less, and is still running on the battery.. I made an appointment with different shop for tomorrow.

I have yet to check the oil, and the temp gauge pegged on the way home and steam came out. It seems it was low.. No more soapy water, and I still need a new upper radiator hose.

And I still have 4 of the other plugs, it is the CR43TS. Only two were changed.. and it runs better..

 

[maketracks]

 

[TFB1]

If you ran it hot enough, the cylinder detonated and shattered the tip of the plug...

With those compression readings, you likely need a new engine or at least the heads rebuilt, but with the looks of that plug it's almost a sure bet there is piston and/or ring damage as well...

 

[PandaBear]

Originally Posted By: TFB1
If you ran it hot enough, the cylinder detonated and shattered the tip of the plug...

With those compression readings, you likely need a new engine or at least the heads rebuilt, but with the looks of that plug it's almost a sure bet there is piston and/or ring damage as well...


and O2 sensor and CAT if it wasn't still in the engine.

 

[SteveSRT8]

I have had engines that actually ran pretty good with the top rings broken into many pieces in every cylinder!

If it has been detonated regularly and at larger throttle openings they may be history.