Spark plug removal at operating temperature engine for compression test.

[Velox]

I have a few questions about a compression test.

I have read that you need to do a compression test when the engine is at operating temp.
But isn't it bad to remove sparkplugs on an engine that is hot?

And if looking for a leak/scoring why cant you do a compression test on a cold engine. bacause you are looking at the difference between the cilinders right?

[TarlCabot]

i believe the ASE mechanics like to do them on a hot engine with only 2 or 3 turn overs.... dry first, then wet
i haven't really seen much difference between cold or hot, but definitely don't forget to pull the fuel pump fuse # 17 iirc

[chain_slappins4]

I've seen as much as a 30 point difference between a hot and cold engine

Sent from the S4 with a Tap!!!

[TarlCabot]

I've seen as much as a 30 point difference between a hot and cold engine

Sent from the S4 with a Tap!!!

mine was less than 5 psi, but that was just on my engine... i totally agree it should be hot for best results, but if he is just looking at the spread... then doing it hot and cold would probably give him more information

[b6dolphins4]

I'll do it at operating temp, with a 10 count to get a good reading

[chain_slappins4]

I'll do it at operating temp, with a 10 count to get a good reading

a 10 count isn't necessary, maybe a 4 or 5 count.. a 10 count on all cylinders may kill your battery , but you really want to pay attention to the first stroke and make sure the throttle is open all the way

Sent from the S4 with a Tap!!!

[b6dolphins4]

a 10 count isn't necessary, maybe a 4 or 5 count.. a 10 count on all cylinders may kill your battery , but you really want to pay attention to the first stroke and make sure the throttle is open all the way

Sent from the S4 with a Tap!!!

I know it's just one of those things I do to be sure, havent killed a battery yet. Yeah I prop the throttle open with a wrench when I do it.

[Velox]

but won't you damage the threads removing spark plugs on a hot engine?
and ive read that you can damage the coils if starting when disconected. is there also a fuse for the coils?

[b6dolphins4]

but won't you damage the threads removing spark plugs on a hot engine?
and ive read that you can damage the coils if starting when disconected. is there also a fuse for the coils?

I've done it this way on 2 separate s4's and haven't caused any damage. Not sure about the fuse.

[TarlCabot]

no fuse for coils, that would throw a code if you did anyway... never heard of this damaging a coil, but if you dont pull the fuel pump fuse... you will be washing your cylinder walls down with fuel (not good)

[Velox]

One more question, what thread connector size do I need when doing compression test, is it 14mm?

[924Carrera]

but won't you damage the threads removing spark plugs on a hot engine?
and ive read that you can damage the coils if starting when disconected. is there also a fuse for the coils?

I just removed all my coils when doing compression testing. No reason to leave them connected.

[Velox]

I just removed all my coils when doing compression testing. No reason to leave them connected.

Yes, of course ,was stupid question, misread the article.

Do you know the thread size I need for the compression test, is it 14 mm?

[924Carrera]

Yes, of course ,was stupid question, misread the article.

Do you know the thread size I need for the compression test, is it 14 mm?

Yes, 14mm.

This is a helpful guide regarding the NGK spark plug codes. The correct NGK plug for the S4 is a BKR6 series.

https://www.ngk.de/fileadmin/Dokumen...en_code_en.pdf