Another Compression Test Question

Another Compression Test Question

MDJEEPER

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Location
Calvert County, Southern Maryland
Vehicle(s)
1986 Jeep CJ

Mostly stock, 258 c.i.d., T-176 tranny, Dana 30 front, AMC 20 rear, Dana 300 t-case, 31x10.50 tires, 2 inch body lift
I am still trying to understand my project and its lack of power at high speed. So, I did a compression test this morning and got the following:

Cyl 1: 150
Cyl 2: 140
Cyl 3: 142
Cyl 4: 160
Cyl 5: 158
Cyl 6: 162

Frankly, I am not sure what to make of these numbers. By the manual, the acceptable range is between 120-150, with no more than 30 psi max diff between any of the cylinders.

What do you think??
 
Remember, a compression test is a quick test to get you going in a certain direction. As you stated, your cylinders are within spec. What you are looking for is the odd ball number. Re test and see if your results stay close to the same. And again, look for the cylinder that has the greatest deviation from the rest. Secondly, read your spark plugs. You can write a volume of information based on the plugs. Do this, put in a fresh set, drive for a couple of days, pull the plugs and take pictures of them. Keep them in order or we will have to start over. ;)
 
Remember, a compression test is a quick test to get you going in a certain direction. As you stated, your cylinders are within spec. What you are looking for is the odd ball number. Re test and see if your results stay close to the same. And again, look for the cylinder that has the greatest deviation from the rest. Secondly, read your spark plugs. You can write a volume of information based on the plugs. Do this, put in a fresh set, drive for a couple of days, pull the plugs and take pictures of them. Keep them in order or we will have to start over. ;)

Thanks!

What do you make of the fact that three of the cylinders are actually HIGHER than specs? Just a random thing in the test, or maybe a lot of carbon build-up??

Also, would you say that anything is strange with cylinders 2 & 3? They are next to each other and lower than the others, but still within specs.

Oh, the spark plugs are pretty new and I think they look fine but will post a picture of them for a second opinion.
 
Here are a couple pictures of the spark plugs (cyls 1-6, left to right).

These were installed three months ago and have about 900 miles on them.

Now that I look at them, they are somewhat stained, but do not appear to have a ton of deposits.

...I should also add that these were initially installed before I performed the Nutter and reset the timing to 8 degrees.
 
Last edited:
When you get results that exceed the specs, that usually is caused by carbon build up on the piston or oil leaking past the stem seals or rings.
 
When you get results that exceed the specs, that usually is caused by carbon build up on the piston or oil leaking past the stem seals or rings.

Thanks! What do you think about the spark plugs? Look OK?

What's the easiest way to diagnose oil leaking vs. carbon build up? How about the warm, soapy water down the carb trick??
 
Run a little seafoam through the engine/gas and then do a check and you'll see
what you really have.
 
Run a little seafoam through the engine/gas and then do a check and you'll see
what you really have.

Do you use Seafoam exclusively in your gas, or do you use it elsewhere too?

I have tried using the Lucas oil gas treatment product over the course of three or four consecutive fill-ups, and I have been using high test gas. So, I figure if it's carbon build-up then it must be really stuck!
 
They make seafoam for cleaning the oil out also but I don't use that.
The seafoam run through the carb or in the tank will clean out all the :dung: and
it'll smoke like it's on fire while doing it. Follow the instructions on the bottle and
you see what I mean. It's the standard when you want to clean out engine deposits.
 
They make seafoam for cleaning the oil out also but I don't use that.
The seafoam run through the carb or in the tank will clean out all the :dung: and
it'll smoke like it's on fire while doing it. Follow the instructions on the bottle and
you see what I mean. It's the standard when you want to clean out engine deposits.

Thanks! It's worth a shot!
 
Ultimately what I am trying to do is determine whether my lack of power at high speed is caused by: a) an internal engine problem, b) a fuel system problem, c) an exhaust problem, or d) something else I haven't thought of!

Also, there's something else...Is is possible that my rig's failure to hold highway speeds in 4th gear when going uphill could be do to that crappy 2.73 gear ratio set up in the AMC20?

Part of me wants to say that it's NOT an engine problem since it runs so well at lower speeds. But part of me also says there must be an engine problem because it gets poor fuel mileage even on flat ground when it can hold speed...
 
Wonder if the trans is slipping.

Okay, I am really going to show my stupidity here...I didn't think a manual transmission could slip, though a clutch could. What should I be looking for?

To be honest, I know the clutch is not in the best of shape...There is no adjustment left in the clutch and the pedal itself does not return all the way up to where it should be. However, I have not noticed any obviously slippage once the clutch is engaged.
 
sounds to me like a great excuse to upgrade the engine :)

I don't have much input without seeing it but the only things I can think off could be:

poorly tuned carb
a lot of gunk anywhere along the intake track from the filter to the inside of the manifold
Clogged cat? if yours has one


hows does the engine sound? Will it rev out or just fall on its face?
 
sounds to me like a great excuse to upgrade the engine :)

I don't have much input without seeing it but the only things I can think off could be:

poorly tuned carb
a lot of gunk anywhere along the intake track from the filter to the inside of the manifold
Clogged cat? if yours has one


hows does the engine sound? Will it rev out or just fall on its face?

Yes, it does have a cat and a very rusty exhaust system that might even be original.

The engine sounds just fine. Going uphill at highway speeds it continues to hum along even as you push the accelerator to the floor. It's like it just runs out of HP and the speed decreases the further up the hill you go. For example, say you're going 60 mph when you hit the bottom of the hill. By the top, you're going maybe 45-50.
 
Also, there's something else...Is is possible that my rig's failure to hold highway speeds in 4th gear when going uphill could be do to that crappy 2.73 gear ratio set up in the AMC20?

Part of me wants to say that it's NOT an engine problem since it runs so well at lower speeds. But part of me also says there must be an engine problem because it gets poor fuel mileage even on flat ground when it can hold speed...

Even though you only have 31's I think the 2;73's are killing you.
If there not the #1 problem they are way up at the top.
 
The clutch could be slipping, my old cj had the top gear that slipped, if you take off from a dead stop and shift quick like you in a race you'll feel it slip if that's it
 
Even though you only have 31's I think the 2;73's are killing you.
If there not the #1 problem they are way up at the top.

Thanks! I am really wondering just how gutless the 2.73s are, but I have never driven a Jeep with them before. It would be nice to somehow separate them from any other potential engine problems...
 
The clutch could be slipping, my old cj had the top gear that slipped, if you take off from a dead stop and shift quick like you in a race you'll feel it slip if that's it

Roger! I will try to experiment with that today.
 
Thanks! I am really wondering just how gutless the 2.73s are, but I have never driven a Jeep with them before. It would be nice to somehow separate them from any other potential engine problems...

Does anybody have any actual experience with 2.73 gears and larger than stock tires? Should it be able to run up a hill on the highway at 60 mph without losing speed??
 

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