Compression Test Causes Knock?

[shiro_neko]

I did a compression test on my 89 240sx ka24e. followed these instructions: how-to-do-a-compression-test-on-your-en ... 59838.html

I had a reading of 149-155 on 4 cylinders. after putting things back I started the car. now I can hear noise which increases with engine speed (which wasn't there before when I warmed up the engine). I cant think of something that could fall inside the spark plug holes. So what went wrong? I dont want to do any test to avoid further damage. Im having a mechanic to look at it but as of now Im broke. I just want to know what happened so I wont repeat the same mistakes again. any idea?

[tyrodtom]

I always use a blowgun to clean out around the sparkplugs, before I remove them. I've seen some crap hanging around in those spark plug wells.
Are your plugs tight ? Are your wires on the plugs tight ? Did you drop a plug while it was out ?

It's easy to mishandle a plug and chip the porcelain, which could fall out in the cylinder after you install it.
But there's no way for anyone to know what is wrong, or done wrong until you or someone at least checks it out.

[az240sx]

What is the purpose of step 7. Depress accelerator pedal fully to keep throttle valve wide open? If you don't depress it, does it effect the reading?

[tyrodtom]

If you don't open the throttle, where do you think the air for the engine to compress is going to come from ?

If you don't open the throttle, the readings will be low.

[shiro_neko]

I always use a blowgun to clean out around the sparkplugs, before I remove them. I've seen some crap hanging around in those spark plug wells.
Are your plugs tight ? Are your wires on the plugs tight ? Did you drop a plug while it was out ?

It's easy to mishandle a plug and chip the porcelain, which could fall out in the cylinder after you install it.
But there's no way for anyone to know what is wrong, or done wrong until you or someone at least checks it out.

yup I dropped 2 plugs actually...maybe I should take out the plugs again and check

[csramotorsports]

lol ya might be a good idea

[shiro_neko]

plugs are intact

[Nerfz]

Dumb question I know but I will ask...Are you sure you put the wires back on in the right order? and make sure you did not mess up a wire...

[az240sx]

If you don't open the throttle, where do you think the air for the engine to compress is going to come from ?

If you don't open the throttle, the readings will be low.

Makes sense. Did a compression check and it came out 91, 91, 95, 95, but did not depress accelerator. (have to read FSM more carefully)

Wondered why readings were so low.

[shiro_neko]

Dumb question I know but I will ask...Are you sure you put the wires back on in the right order? and make sure you did not mess up a wire...

yep i did not mess up the wires. engine is running great except for the noise

[Armstec_S13]

have you checked your oil level? did you recently put gas in it and if you did, did you use a cheapo gas station? check your voltage too, these cars are so sensitive when it comes to electrical.

maybe the compression test blew a gasket, check oil for coolant.

[tyrodtom]

Compression test blow a gasket !!!! Combustion chamber pessures are much higher under normal running conditions than a compression test.

[shiro_neko]

oil level is good and no coolant. If the compression test blew something I want to know why. Cause now Im afraid to do a compression test on any engine. Im planning to open the engine soon just doing more research. Cant find my chilton and its only in an overhaul is where that book is really useful

[tyrodtom]

I thought I made it clear a compression test WILL NOT blow a headgasket. The pressures inside the combustion chamber with the engine running ,( you know, spark, fire, explosion) is much much greater than just turning the engine over with the spark plugs out, no spark, no fire, no explosion.

Before you took the plugs out the first time, did you clean out around the spark plugs before you removed them ? When you dropped the 2 plugs, did you check them good enough to see they didn't have something in the electrode area , dirt, small gravel, etc. ? If no on either question, then you might have something loose in the combustion chamber, you put in there.

You might pull the plugs out, turn the engine over, it might find it's way back out the open spark plug hole, or blow in the open spark plug hole with compressed air.

[robt95240]

something else you might check is when dropped the plug that you may have put a hair line crack in one of them that would cause it to arc in the head n not get a good fire through the plug causing it to miss n have a bit of a knock.. id definitely be checking into the plugs or debri in the cylinder anytime i drop a plug i replace it completely cause a tiny crack can cause the weirdest problems

[shiro_neko]

I thought I made it clear a compression test WILL NOT blow a headgasket. The pressures inside the combustion chamber with the engine running ,( you know, spark, fire, explosion) is much much greater than just turning the engine over with the spark plugs out, no spark, no fire, no explosion.

Before you took the plugs out the first time, did you clean out around the spark plugs before you removed them ? When you dropped the 2 plugs, did you check them good enough to see they didn't have something in the electrode area , dirt, small gravel, etc. ? If no on either question, then you might have something loose in the combustion chamber, you put in there.

You might pull the plugs out, turn the engine over, it might find it's way back out the open spark plug hole, or blow in the open spark plug hole with compressed air.

I was just saying if something blew or maybe its just coincidental that it happened during the compression test because beforehand Ive been having overheating problems with the ac on. So I did consider a blown gasket. yes I know that there's no combustion thats why Im thinking of what went wrong. doing the same mistakes can screw my engine up again.

Nope I didnt clean around the plugs when I removed them but I did clean the 2 plugs thoroughly. I dont want to cause any further damage so I wont try turning the engine over. I'll just have to wait until I open the engine. I'll keep you posted.

something else you might check is when dropped the plug that you may have put a hair line crack in one of them that would cause it to arc in the head n not get a good fire through the plug causing it to miss n have a bit of a knock.. id definitely be checking into the plugs or debri in the cylinder anytime i drop a plug i replace it completely cause a tiny crack can cause the weirdest problems

yep next time I'll replace my plugs if dropped