What is the correct way to do an engine compression? I've used two different gauges and apparently: both are defective since their both too high. On a 4.0 L 95 model Jaguar, what would you expect a healthy compression would be on an engine with 98k miles on it? Can you please help me with this? I sent in all the info you required for me for an introduction: if there is something more you need to know, please send ask me.

 

Ratio should be 10.5:1 if I remember right.
To convert the ratio to psi, take the (general) atmospheric pressure of 14.7 and multiply it by the ratio, in this case 10.5 which gives you 154.3 psi.
It should tell you in the handbook, if not download the JTIS from the site.

What readings are you getting?

 

I used two different compression gauges that were zeroed out on each cylinder but i did turn the engine over 6-8 times to get the maximum compression because each stroke brought the compression up and it stopped climbing on the eight stroke. I heard that you should do it 3 to 6 stroke and you should get the maximum readings but anyway: i got a really crazy reading all the way across 257 psi. This engine has 98k miles. The compression seem kinda high for this engine.

I do have a OTP disc that sometimes doesn't want to work and the computer doesn't want to recognize the code on the disc and i reactivated this disc about a dozen times and sometimes it works, sometimes it won't so i decided to order a USB drive that all you have to do is plug and play it. I don't know if it's something i'm doing wrong or my computer, but i don't really know what the computer.

What would cause the compression to be so high? It's been a decade since i used the gauges but i have two and i haven't used them that much. Both have the same readings and i know i zeroed them out between cylinders. I watched the compression go up on each stroke.

Thank you!

 

How many times should you have to turn the engine over before it reaches its peak? On my engine, it was a gradual build-up to 6-8 strokes and i'm not sure if i should've ran that many strokes. I was told it should be 3-6 strokes and if the compression is low, you can put a little oil in the cylinder and if the compression goes up any then you likely have a burned valves.

Is this true or not? What is the proper procedure to give a compression reading? I've been doing this for many years, and i usually go from 3 to 8 strokes and if the compression is over 100 psi or greater, and each cylinder is within 10% then it should run smooth. If it's more than 10% than likely it's gonna run rough and have a misfire. I've been told they have to 100 psi.

Please tell me what is the right way to do this or what you would do?

 

It sounds like you're doing the right thing. I'm not an expert but the number of strokes should not be critical (i.e. I don't think you can do too many) as the compression should be released on each stroke via the valves. The gauge is just giving the maximum attained on any stroke not a cumulative figure.

What I do is
- remove all spark plugs
- disable fueling (remove pump relay)
- insert gauge and turn the engine over


Can you try your gauge on another vehicle to test it?

I've never heard of a petrol engine with that 257 compression. The factory compression for a NA AJ16 engine is 159-169 and with a 10% variation as you say.

The wet test (oil in the cylinder) is for checking piston rings. i.e. if wet test is better than dry suspect worn rings/pistons - if no better suspect valves.

 

257psi is diesel compression territory.
Try and borrow another newer compression tester from a mate if you can, or nip out and buy one from the store, forget using a computer for this task.
They are not expensive, and what might seem like a tester in good condition could be actually faulty.

 

Yeah, the compression gauges are in new-like condition but they are decade old. I'll go buy a Mac compression gauge and retest, but thank you for helping because what i did wrong was i didn't disconnect the fuel and the engine had been setting for a few months and i removed the spark plugs and shot two squirts of oil in each cylinder. Then, i screwed the compression gauge and this probably gave me readings of a wet cylinder reading considering there was fuel going into the cylinder, along with oil.

It's been quite some time since i've done any work due to my health, but i really enjoy working on this jag and i don't trust anyone else to fix it. Not even change the oil in it.

Thank you for your advice!