Compression Test Results

[willincalgary]

I did a compression test on my engine (2.25L with 7:1 head) this evening and found the following:

#1 107psi
#2 60psi
#3 80psi
#4 105psi

This strikes me as #2 and #3 are communicating somehow. Wet compression test gave similar, if not slightly improved results. What would you folks suggest to do next? Remove the head and check the condition of the valves and head gasket? I suspect the gasket is blown between #3 and #4.

Any help would be appreciated.

[rwollschlager]

I'd say your on the right spot with the head gasket. Replaced mine this summer... pretty straight forward and simple job. Directions, tightening sequence and torque settings can be found in the green bible and the haynes manual.

http://www.youtube.com/watch?v=gZL4c8dF3e8

-Rob

[Nium]

willincalgary,

Unfortunately, yes you are going to need to pull the head and see what's going on with cylinders #2 and #3 probably a burnt valve in each but may be a blown head gasket.

With the wet test yielding higher compression the corresponding cylinders (yielding higher compression) either have bad piston rings or the cylinder is worn.



If you have an air compressor a final test you can perform to narrow down the culprits.

1. Set piston #1 to TDC (ie. piston at top of power stroke with both valves closed)
2. Insert compression tester tube into spark plug hole for #1 cylinder with gauge removed (if gauge can't be removed then you can't perform this test)
3. Attach air compressor to compression tester tube. Set air compressor to deliver very low pressure (5 psi - 10psi).
4. Have the carb fully open (floored) with air filter removed from top of carb
5. Listen to top of carb, exhaust pipe, and oil filler tube.

If you hear rushing air at...
carb = bad intake valve
exhaust pipe = bad exhaust valve
oil filler pipe = bad piston rings or worn cylinder

Relive air pressure and repeat for other cylinders

[willincalgary]

Updates

I've dug into the engine and removed the head. Evidence of communication between cylinders 2 and 3 was not super obvious but I think it's convincing enough. The first picture below is the copper gasket which is blackened between 2 and 3. On the head and the block there was carbon build-ups between 2 and 3 corresponding to the blackened area on the gasket. This is shown as some lumpiness in the second photograph.

The wet compression results on the good cylinders (1 and 4) was not different from the dry test so I'm not really worried about the piston rings on all the cylinders. Is this a reasonable assumption? The cylinder bores looked perfect. Unexpectedly large ridge at the limit of the piston rings travel inside the bore. I chock that up to a lack of experience on my part.

I'm fairly uneducated when it comes to what the valves/combustion chambers should look like and wouldn't know bad valves if I was looking at them. There is nothing missing from the valves but I haven't taken them out of the head to look at them. There seems to be a lot more carbon than I expected. Especially above the intake valves. Also some oily look to the number 4 exhaust valve which surprised me. The oil was on the shaft of the valve when looking through the exhaust flow path. That, I think, must be coming though the valve seal recently because it would burn off pretty quick in the exhaust stream.

Back to my lack of education. I'm thinking I'll just upgrade to the 8:1 head with the hardened guides/valves. Any thoughts on the relative merits of having this head worked over versus getting one of the pre-packaged heads?

[czenkov]

Higher compression head might cause other weaknesses to show their ugly head? Like rings?

[leafsprung]

those are very low numbers even for the 1 and 4 cylinders. 7:1 factory spec is 145psi

[Nium]

Keeping track

As you continue the disassembly it is extremely important not to mix up where stuff goes and where it came from. Make up a board that you can keep track of everything. For example use a 1" x 6" about the length of the head, or something similar, label one end "front". Label spaces for cylinders 1, 2, 3, and 4. Then drill holes in it where you can drop in the intake valves, exhaust valves, spark plugs, push rods, and cylinder head bolts from the corresponding cylinders. When you take apart the rocker arm assembly keep track of where each bolt goes, the lay of the rocker arm assembly, etc. It is important to reassembly old stuff where it came from. Plus it makes it easier to be able to go back and look at things and know exactly where it came out of.

Has this engine ever been rebuilt? How many miles are on it aproximately?

[gudjeon]

The lower end of the engine is probably a bit tired/worn is you are nearing 100psi. A ridge means you probably need to go to the next oversize. While your this far.....

[Nium]

Yeah, it's probably time to stop and think about what you want to do. A complete engine rebuild or a new engine. A cylinder is suppose to be completely smooth and straight from top to bottom; no ridge at the top of the cylinder. If you are unfamiliar with what you are looking at it doesn't mean it isn't worn out and I mean that with all due respect.

Consider if you have the time to commit to this project or if it would be better to find a shop to take over. If you have the time and necessary tools and space it can be quite a learning project to rebuild an engine. If you are inexperienced and can't work at it everyday it will probably take a few months, at least, to complete an engine rebuild. The cost will, probably, be relatively comparable to a new engine. It's not impossible if you've never done it before but it isn't easy either. You will also need the services of a good automotive machine shop. You can always put on a new or reworked head but it's not going to fix the worn out cylinders.

With my Best Regards,
Walker

[willincalgary]

those are very low numbers even for the 1 and 4 cylinders. 7:1 factory spec is 145psi

I agree it seems low. The wet compression test on the good cylinders was nearly identical to the dry test (#1 104psi, #4 105psi, within the limitations of the test equipment and procedures) suggesting the rings were not the issue.

The engine has 54k miles on it and has never been opened so it's not technically that old (albeit it's still 50 years old). It was just parked for about 20 years.

I should check to see whether the ridge was just carbon build-up the whole rest of the combustion chamber was well coated (>1mm if carbon).

[willincalgary]

Another question. What's the safest way to get the carbon off the pistons/head? Words of wisdom?

[kevkon]

Another question. What's the safest way to get the carbon off the pistons/head? Words of wisdom?

Yes, DON'T!
That carbon isn't hurting anything, in fact it actually helps raise the compression ratio and insulate ( keep heat in the combustion chamber). It's why we would lave it on the racing engines.
On the other hand,attempting to remove it will result in carbon debris getting in places it does not belong and there will certainly be some risk of engine damage as a result.

[gudjeon]

Carb or brake cleaner can soften the carbon and removal may be easier. We used to use the old tetrachloride extinguishers to clean up this kind of stuff (large electrical contacts, electric motors etc of carbon and gunk), but it would probably rearrange your DNA.

[Sputnicker]

I don't think there is enough evidence yet to say that you need rings and a re-bore. The presence of an "unexpectedly large ridge" is an indicator, but without an actual measurement it's inconclusive. Rings are much harder than cylinder bores and there is always some wear and a noticeable ridge on any engine with that many miles. Carbon buildup can also exaggerate the depth of the ridge. Rings are capable of compensating for wear in a cylinder - to a point - and it's not clear that you have reached that point. The fact that the wet compression test didn't raise the pressure argues against the rings being the problem.

The quickest way to gain more evidence is to see if the valves are leaking. You can re-install the spark plugs and fill the combustion chambers with fluid and see how fast it leaks out and where. You can also pull out the valves and take a look. Once you get a look at the mating surfaces between the seat and valve it will probably be obvious. My hope is that you find four burned exhaust valves with 2 and 3 being the worst. Running too lean for too long could easily do that kind of damage.

Finally, the pictures of the head gasket aren't conclusive to my eye. If there was leakage between 2 and 3 you caught it early.

You can remove the carbon on the head with a wire brush on a drill motor (don't try this on the pistons). I'd wait until you were through diagnosing things.

Before I tore into the lower end, I'd probably do a valve job (new seats, new/re-ground valves, new stem seals, magnaflux, and resurface) and reinstall things to the point you can do another compression check using the starter motor (i.e. torqued with rockers working and valves adjusted). It's not that much work to pull it back off at that point and all you've invested is a head gasket. If the compression test looks good, you've saved yourself the trouble and expense of doing the lower end.

[willincalgary]

I don't think there is enough evidence yet to say that you need rings and a re-bore. The presence of an "unexpectedly large ridge" is an indicator, but without an actual measurement it's inconclusive. Rings are much harder than cylinder bores and there is always some wear and a noticeable ridge on any engine with that many miles. Carbon buildup can also exaggerate the depth of the ridge. Rings are capable of compensating for wear in a cylinder - to a point - and it's not clear that you have reached that point. The fact that the wet compression test didn't raise the pressure argues against the rings being the problem.

The quickest way to gain more evidence is to see if the valves are leaking. You can re-install the spark plugs and fill the combustion chambers with fluid and see how fast it leaks out and where. You can also pull out the valves and take a look. Once you get a look at the mating surfaces between the seat and valve it will probably be obvious. My hope is that you find four burned exhaust valves with 2 and 3 being the worst. Running too lean for too long could easily do that kind of damage.

Finally, the pictures of the head gasket aren't conclusive to my eye. If there was leakage between 2 and 3 you caught it early.

You can remove the carbon on the head with a wire brush on a drill motor (don't try this on the pistons). I'd wait until you were through diagnosing things.

Before I tore into the lower end, I'd probably do a valve job (new seats, new/re-ground valves, new stem seals, magnaflux, and resurface) and reinstall things to the point you can do another compression check using the starter motor (i.e. torqued with rockers working and valves adjusted). It's not that much work to pull it back off at that point and all you've invested is a head gasket. If the compression test looks good, you've saved yourself the trouble and expense of doing the lower end.

This is the direction I've elected to go. I'll get the rebuilt head, install it and see what the compression looks like at that point. I'm only 3 1/2 hours invested at this point (and I farted around figuring out what I needed to do with the green bible). It is likely less than that before my next information gathering session with the compression tester and new head.

You're quite right, it seems to early to right off the rings. Particularly since it is low mileage and the results of the wet compression test were not particularly startling. It had no evidence of ring problems.

The head gasket failure wasn't the big obvious chasm I was hoping (sort of) for. I've seen pictures of failed head gaskets and there's a 1/4" gap connecting the cylinders. The carbon on the head and block was quite obvious but difficult to photograph.

I'll keep you posted when the head's back on and I've got some new information.