How to do a compression test..

[F Bill]

I forgot I never fleshed out this post, so here's what I wrote on teh 2A page last year...


spent some spare time writing this up.. Feel free to add any questions or suggestions for editing. Or if you want to make the Geico version, which this sure isn't, go right ahead!
 
Compression Testing the Willys engine....
 
It seems often folks find old jeeps and have trouble getting them to run. Like all four cycle internal combustion engines, the WIllys requires several things to run correctly, or at all.

1: Compression
 
2: Fuel in the proper amount to create a burnable air fuel mixture
 
3: Spark
 
4: Air which provides oxygen for burning the "sparked" fuel.

5: Correct timing of all of the above.

This post will refer to the most basic of the above, the compression. We will assume in later discussions that the compression has tested good and that also will take care of most of the "air" issues as well.

Your engine , if we look at just one cylinder, is a very simple machine consisting of a piston going up and down in a cylinder, with valves controlled by the camshaft that let in the fuel/air mix, and the piston is connected by the connecting rod to the crankshaft which creates rotation.

The piston starts at the top of the cylinder, and as it goes down, an intake valve opens, letting fuel and air in.

Stroke two is the piston rising, compressing the air fuel mix. This is the stroke we are measuring. At the very end of this stroke, the spark plug fires.

The third stroke is downwards again, with the ignited fuel air mix expanding and providing all that horsepower WIllys owners have boasted of for years..
 
The final stroke, upwards again, releases the spent charge out of the cylinder through an opened exhaust valve. This discharges all that greenhouse gas the Global Warning crowd talks about, along with a throaty roar, but we aren't going there today.

That's all there is to the four stroke cycle, which gets repeated thousands of times in your flathead on a short trip to the corner store. Each of the four cylinders goes through the same four strokes, all at a different time so the engine runs smoothly. At any one time one piston is in stroke one, one is in stroke two, etc. (your spark plug wires must be assembled on your distributor cap in the particualr firing order the engine uses, but that is beyond the scope of this post) .
 
Ok, all that scientific mumbo jumbo aside, your jeep doesn't run and you want to do a compression test. What will you need?
 
1: A Willys engine, all assembled and rotating nicely. (If it has seized up you are out of luck. See posting "How to spend a lot of money on your motor", or similar..)
 
2: A well charged battery and good starter to spin the motor over. If you have already burned up your starter jumping it with 12 volts, and cranking till the cows come home, go buy another starter, and this time be gentle on the poor thing, would ya?
 
3: A spark plug wrench that fits your plugs. A nice long 1/2 drive breaker bar to fit the plug socket will help if the plugs haven't been removed for a while.

4: A known GOOD compression gage. You can rent one, or buy one. There are screw in gages, and ones with a rubber end you just hold in place while a buddy cranks the motor. If you have no friends, get a screw in one. The other ones are a pain anyway. While we are on the subject of rental gages, sometimes the guys who used them before you have let dirt get in the Schrader valve in the inlet, and the gage will not hold presssure. If this is the case, do NOT just put the valve out of your spare tire in it, the spring is too strong and it won't work. Just go back to the rental store ans tell them the gage won't hold, can ya get another one.
 
5: Optional ....Air hose and blow gun to clean around your plug holes, engine oil for wet testing. Tape and Sharpie for marking plug wires. Notebook and pen to record results.

Ok, got all your supplies? We are almost ready. If your jeep actually runs, start it and warm it up a little for a better reading of actual compression. In any case ,block the wheels , apply the parking brake, and place the gearshift in Nuetral.

Open the hood if you haven't already, and take a look at the spark plug wires. the one on your front spark plug is number one, the next is number two, etc. Use a piece of masking tape and a Sharpie, mark the wires where they go on the plugs, and carefully twist and pull the labeled wires off the spark plugs. If any of them pull off and leave a metal piece still attached to the spark plug, you will need to repair the wire or replace it when you are done. Normally jeep wires don't do this but it seems every late model engine I have is apt to break wire ends off even when I use dielectric grease on the boots.....
 
Clean all the dirt, dust, dried bugs, mud, and rust from around each plug before you remove them. You don't want all that stuff going into your cylinders to scratch the cylinder walls. If you have a source of compressed air, use it with a blow gun to blow off all the loose stuff. Watch your eyes, Safety glasses are always a good idea.
 
Use a spark plug socket and ratchet, remove the spark plugs. If any are REALLLY tight, use a little soaking with PB Blaster to loosen the rust bond holding them in. Try to not break them off, if you do refer to a post like "spending even more money at the machine shop" to see how to pull a head and get a broken plug out. TO BE CONTINUED.....

[F Bill]

continued....


Keep the plugs in order so you return each one to the same hole it came from. You might want to make a note of the plug condition, and if any are really oily and black or rusty... (See "replacing piston rings and spending yet even more money at the machine shop" posts. )
 
Note: A lot of guys prefer to remove one plug, test, replace, then go on to the next cylinder. I prefer to have ALL the plugs out so the starter doesn't have to work so hard, and my battery is only pushing ONE cylinder to compression at a time. Also, if you have blockages in your carb or air filter, the engine will still be able to suck air thru the other cylinders and give you a true reading.

Install the compression gage in the first spark plug hole. Tighten it by hand if it is the screw type, until it seals on its O ring seal. You don't need to use a wrench on it. If it is the hold in place type, get the rubber end of the compression tester standing upright in the plug hole and press it firmly in place. Make sure either gage is at zero before you start, normally there is a small button to push somewhere near the dial end of the gage.

Now you will need a way to crank the motor at starting speed. Fortunately Willys decided to install electric starters on our jeeps, unlike Henry Ford and his Model T. It will be a lot easier to use a buddy to operate the starter, than for you to do it and try to see the gage at the same time. If you have a solenoid operated starter, you can use a remote starter switch, but a stock 2A has a floor mounted foot operated switch that is not something that can be operated remotely. If you got the screw in gage you can just go inside the jeep and crank away , then jump out to see the final reading. MAKE SURE you leave the key off, you do not want to burn up your points, condensor, and coil.

Crank the engine until you hear the compression stroke happen four times.You will hear the starter speed slow down each time the cylinder is compressing the air in it. If you can, watch the dial while you are cranking, It should go up each time the compression stroke occurs, less each time. If the gage goes to zero between strokes, refer to the above paragraph about bad rental gages.....Write down your reading and remove the gage from the cylinder.
 
Repeat the above for each cylinder. Write down your results. On a Willys the readings should be around 90-130 lbs for each cylinder. Some gages read high, others are really pessimistic and may read as much as 20 lbs low. What you are looking for is uniformity of the readings across all four cylinders. If one cylinder is significantly (20 or more lbs) lower than the rest it has a problem. If any cylinders have no reading or are in the 5-10 lb area, they have a serious problem. You can do a wet compression test to further diagnose poor compression if you like.
 
A wet compression test puts oil in the cylinders to seal up the piston rings. One cylinder at a time, put about a tablespoon of ordinary engine oil in each cylinder. (You may have to use a small funnel and hose to direct the oil into the cylinder instead of over the valves which are directly under the spark plug hole. I have used PB blaster and the little straw they include for this, altho PB is not really heavy enough to do much sealing) ) Retest that cylinder. A significant gain in compression to normal readings would indicate that the ring seal was inadequate.(see posts about "replacing piston rings, boring cylinders, and installing new pistons"..) The oil is only a temporary fix, and it won't fix the cylinder for more than a few strokes. (Sometimes a marginal engine can be made to start with oil in the cylinders, but most of us are not that lucky...)If there is no improvement with oil, that usually indicated the compression loss is through the valves or head gasket.

Once you have finished, go ahead and put the spark plugs back in, and reconnect the plug wires. Even if the readings were poor the engine should be buttoned up to prevent further damage. I could tell you storys about frozen and rusty cylinders after compression testing but that would be too depressing. We don't want that, do we??

If all your cylinders are within 10-15 lbs of each other, and near 100-120lbs, you have adequate compression for the engine to run well. If the engine isn't running, check out the other factors I mentioned above. If I had to guess, it will probably be an ignition or timing issue.( It has often been said most carb problems can be cured by fixing the ignition system..)
 
If all cylinders are similar in compression, yet under 100 lbs, your engine is probably getting long in the tooth and I would bet it also uses oil and smokes. Power will be down, and fuel economy will suffer. Eventually you will need to rebuild. There is one exception to this, that is an engine which has set for a while. It is possible a microscopic layer of rust has formed where rings and valves seal, and running it hard for a while may bring back the engine to good performance. Hopefully your local homeowners association doesn't kick you out during the smoky run in phase.....
 
If only one cylinder or two non adjacent cylinders have low readings, there is something mechanical causing the problem. If the wet test reveals no change, look at valve action and adjustment. Engines that sit, especially in damp climates, are likely to have rusty valve stems which stick in the valve guides when starting is tried. ............................

[F Bill]

If they aren't stuck too badly, a good soaking with PB Blaster and some gentle persuasion from below may free them up. If not, see posts involving "valve train repair and more machine shop spending"....Be aware also that valves do burn, especially if an engine is run hard while set too lean, and compression leaks past the burned face of the valve or seat.

When you have two adjacent cylinders with zero compression, suspect a failed head gasket. The seal between the two cylinders may have blown out, and neither cylinder can build compression as one always has an open valve, or the pistons may be going in opposite directions. See posts on "cylinder head removal and spending a little money at the machine shop having the head checked"....It is also possible for the outside seal of the gasket to break and let compression leak to the atmosphere. Normally in this case you will hear the leak or see a spot where oil and fuel is leaking out between the head and block. There is also a coolant passageway around most of each cylinder, so you could also have overpressurization of the radiator and overheating. There are commercial test kits that can check your antifreeze for combustion leakage using chemicals if you want to pin down the problem more.

Hopefully all your cylinders were at 120 plus, and you only have a set of points to install to get your Willys running again. My condolences if this isn't the case. At least there isn't a connecting rod sticking out of the side of your block. (Is there? Search for posts on "good engines for sale")..............
 
Hope this helps someone!
 
For more information on testing compression, check out older Motors manuals or automotive textbooks... I'm sure I left out a few things, but this sure ended up being longer than I initially thought!


Edited by F Bill - 01 Aug. 2011 at 5:41pm