Compression test.

The standard is three revolutions for an I6 258 while doing a compression test, would that be standard on all gas engines?

After warming up the engine I preformed a test with three revolutions (115, 115, 130, 125, 150, 130) for kicks I preformed the test again using 4 revolutions (160, 140, 155, 150,160, 150).

Compression Pressure for a 258 is 150 psi with a 20 psi maximum variation between cylinders.

After the compression test my son and I pulled the engine on his 79CJ5, plan on installing a Ford 260.

 

Thanks for the reply Patrick. I’m just trying to make sense of it all. The shop manual states three revolutions yet I see it stated at time up to 5 or 6 on the net, and then others like you turn the motor over until max compression.

There has to be a reason for the 3 revolutions mentioned in the shop manual, maybe it’s the thought that anything more would at times eventually get the reading up to where you want or maybe the three revolutions are for optimum performance.

I have two readings, three revolutions say I have engine issues, four revolutions say I’m within range.

Thoughts on this would be appreciated.

 

I'm with Patrick.

 

Mmm... me too. I've never counted turns, but cranked till the reading was stable.

260s aren't very common ... 258s are almost free. Only 2 cid difference.

 

Why does the factory recommend only 3 revolutions while performing a compression test on an I6 258?

Many if not all perform a compression test without counting revolutions I also believe now it’s the best way especially with older engines. I did some research and believe I have the answer on why the factory recommends 3 revolutions.

If you crank the engine too long, there is a measurable decrease in readings due to reduced oil on the cylinder walls. The factory recommendation is in part due to that consideration.

On the other hand three revolutions can be "iffy" with a used starter motor, drawn battery and other variables. Also, gauges vary, and with a compression test, you're filling the hose, gauge and so forth. It’s better to give the engine a chance to show its stuff.

I do get caught up on the why sometimes, but I do have a greater understanding now.

 

I just performed a leak down test on the engine in question. As you know the engine was pulled so the test was performed cold. According to the FSM I used 80psi with the spark plugs out the oil filler cap, air cleaner off and blocked the throttle open.

The readings, (10%, 10%, 30%, 2%, 40% 26%) most of the leaking came from the valve covers small amount from exhaust, all had carbon buildup except the number one piston. There should be no more then 25% leakage.

Worn rings, sticky valves?....would you still use this engine without a rebuild?

Edit: Correction, I just drop off the leak down tester to the machinist that offered it to me. We discussed the proper way of reading the dial it was my thinking that high meant high compression when he said it meant high leakage. He said that he would double check the instructions but I’m sure he was right though, maybe someone here knows.

Anyways as you can see this doesn’t make the reading any better. (70%, 70%, 50%, 78%, 42%, 54%) leakage. He also said that I should check the one clean piston for problems. The engine has been sitting for some time before I ran it before testing, carbon can be the cause of a lot of the leakage that’s present.

 

I think your leakage test figures are suspect Chuck, for whatever the reason might be.
I have seen leakage figures on some 258 engines showing 30-40%, that ended up being a burnt valve or 2. Those engines ran poorly, missing at idle, shaking badly.
I can't imagine an engine having those high leakage figures, and still run/be driveable.
Not saying it can't be done, but highly unlikely in my opinion.
if all the plugs are removed, finding TDC on #1 cyl is easy; the others take a little more care; maybe even remove the valve cover to watch the valves and make sure they are seated properly before testing. Even at that, I have watched as air was introduced into the cylinder, air pressure would rotate/turn the engine; several attempts sometimes have to be made on TDC to keep the air from turning the engine over.
If you heard air coming thru both carb and exhaust, then either not at TDC or major internal problem.
I have tested new 258s & 304s & 360s (under 12,000 miles) and had readings as high as 10-12% (6-8% preferred) which factory said was normal; I have also had engines under warranty use as much as 1qt per 1000 miles and the factory (Jeep) say that was normal oil consumption (BS)
have also seen the Bonami thing down the carb trick work to solve oil consumption problems when rings won't seat (don't recommend that)
re-test the engine again

 

Jim, I appreciate your response. I too believe the reading may be suspect, but not by the leak down procedure performed. The only thing that wasn’t done prier to testing was warming the engine. I made sure every piston was TDC by turning the engine while feeling for the compression stroke with my finger over the spark plug hole. When I was at TDC I double checked it by feeling the top of the piston with a screw driver while moving the crank slightly back and forth so I would get the piston exactly where I wanted it. I only had to redo this procedure on cylinder 3 and 5 after it had move when introducing air. After I finished testing and writing down the results I retested all 6 cylinders again coming up with the same exact results.

The engine ran, but was shaking badly and smoked. One of the things I should have done was take off the valve cover and try to free the valves and springs of carbon with a hammer and performed the test a third time, maybe that would of improved the readings.

But why the good compression readings while turning the engine over until the reading stabilizes?

The things I see that’s consistent are the three revolutions readings, the leak down readings and an engine that shakes and smokes.

Let me know if there’s something I’m missing or can do better.

 

I've always let the engine turn over 4 times on comp readings, moot point there I believe.
By chance did you hook up a vac gauge at idle and what was the reading?
Sounds like you were very thorough on TDC with each cylinder.
Warming of the engine is moot also; some do some don't.
Engine smoking means oil burning; thru valve guides or piston rings.
Engine shaking = vac leak(s), cylinder imbalance, misfire, timing issues, etc.
I suppose the only way to know what's wrong for sure would be to tear it down for inspection.
A very tired, worn engine can have lots of issues.
armchair mechanics isn't that easy, is it?

 

No, but I’m having a great time getting myself deeply involved with this fantastic hobby. I just wished I had more of a back round in it.

Sorry, I didn’t hook up a vacuum gauge as I don’t own one (on the list).

I know I can get another 258 for next to nothing, but what will I learn? I will break this one down and do a thorough inspection when I have the opportunity.

Thanks Jim

Edit: the timing is off on this engine, the rotor did not point to the number one cylinder at TDC.

 

There lies the question, 4 compression strokes (160, 140, 155, 150,160, 150)?

 

I edited my post above last night concerning the timing not being correct then went to bed without thinking about what I wrote (read focus on compression test). If an engine isn’t timed correctly wouldn’t the valves be slightly off at TDC giving a poor reading on a leak down test?

 

Nope. Valves sealing at TDC depends on the cam timing, driven by the timing chain off of the crank. Ignition timing won't affect the cam timing wrt the crank.

<edit> If this is a high mileage engine, the timing chain could have "jumped time" - I don't think this is a big problem with AMC I6s in particular, but it's a universal problem for tired engines. The distributor shaft is driven off of the cam shaft. Try turning the crank back and forth and checking the slop in the timing chain by how far the crank must turn to move the distributor rotor.

 

I did marked the alternator belt with white out, then turned the crank left to take all the tension off the chain until the rotor turned. I then turned the crank right until the rotor began to move. The alternator belt traveled one inch before the rotor moved. I also measured the travel at the handle tip of a Craftsmen ½ inch drive at 3 3/4”. The belt measurement is more accurate as the wrench measurement can be misleading.

 

If your wrench is about a foot long, that's about 18 degrees - seems like a lot. You may be able to see the chain deflection through the fuel pump hole in the timing cover. The spec says max deflection 1/2 inch. I know the Jeep V6s had plastic cam timing gears that would lose teeth after a while - not sure if the I6s suffer the same problem. With the V6s, you'd find the teeth in the oil pan. Pretty much guaranteed to jump time if the gear loses teeth.

 

The wrench is about 9 1/2" from the middle of the socket to the end of the handle.
When I get home I will take a look at the chain. Thanks Tim.