compression test

[Premierexplorer]

I did a compression test on my 92 4.0 with 240,000 miles. I had between 170 and 177 but one cylinder had 155. I searched and did not find info on how close they should be.

 

[NHale623]

Try squirting a little oil in the cylinder through the plug hole and repeat the test on that cylinder. That'll tell if if the problem is due to rings or not.

Typically on an otherwise well running engine, I don't necessarily look for a value. I look at the pressures in the cylinders compared to each other.

Have you put a vacuum gauge on it? How did the spark plugs look, did the plug from that cylinder look different from the others?

 

[Heavymentill]

Do a cylinder leak down test. A compression test is simply that. Although it will indicate if a problem exists, it does not pin point its source. A leak down works by putting compressed air in the cylinder and if you hear air through the exhaust its a exhaust valve, air through the intake its a intake valve, remove the oil fill cap and listen through there or the dipstick and if you hear air there its rings related.

 

[shamaal]

I did a compression test on my 92 4.0 with 240,000 miles. I had between 170 and 177 but one cylinder had 155. I searched and did not find info on how close they should be.

The general rule of thumb is the readings be within 10% of each other. The 155 reading is out of family. Make a cylinder diagram like this and add the readings.

REAR
3 6
2 5
1 4
Front

The two additional methods identified in posts 2 & 3, are to determine whether the problem is head related (valves, gaskets, etc) or ring related. This is important because if ring related, the engine is pulled to replace the rings. Otherwise it's a head job that can be done with the engine in place. Both tests work real well and require squirting some oil to seal the rings. 1996Limited's test while a little bit more complicated (injecting air through the spark plug hole) has the added benefit of narrowing it down to which valve.

Do searches, ask questions.

 

[Premierexplorer]

The plug that i pulled out of the low cylinder looked like it had a little oil fouling the tip nothing major though

 

[NHale623]

I would perform the two additional tests that have been described to try and determine where the problem is. In addition, I would put a vacuum gauge on the engine to see if I could confirm the results of the compression and leak down tests.

If you're not familar with the use of vacuum gauges, this link to will provide some very good information. Click on the different scenarios and you'll see how the gauge will respond due to various conditions.

http://www.secondchancegarage.com/public/186.cfm