Compression Test and Spark Plugs

Question 1

What are three main causes of low compression?

Answer 1

1) A leaking valve
(can be carbon lodged between valve face and valve seat)

2) Worn compression rings or cylinder walls

3) “blown” cylinder head gasket

Question 2

What can cause burning of valves?

Answer 2

Insufficient valve clearance
-valve doesn’t close long enough to transfer to the cylinder head

Question 3

DIAGNOSING LOW COMPRESSION (1)

Engine Spec’s: 150 PSI +/- 10%
Cylinder #1: 150 PSI
Cylinder #2: 160 PSI
*Cylinder #3: 70 PSI
Cylinder #4: 150 PSI

Answer 3

Probable Cause: Leaking valve

Question 4

DIAGNOSING LOW COMPRESSION (2)

Engine Spec’s: 140 PSI +/- 10%
*Cylinder #1: 90 PSI
*Cylinder #2: 105 PSI
*Cylinder #3: 100 PSI
*Cylinder #4: 105 PSI

Answer 4

Probable cause: worn compression rings and cylinder

Question 5

DIAGNOSING LOW COMPRESSION (3)

Engine Spec’s: 160 PSI +/- 10%
*Cylinder #1: 20 PSI
*Cylinder #2: 20 PSI
Cylinder #3: 150 PSI
Cylinder #4: 140 PSI

Answer 5

Probable cause: blown head gasket

Question 6

DIAGNOSING LOW COMPRESSION (4)

Engine Spec’s: 150 PSI +/- 10%
Cylinder #1: 190 PSI
Cylinder #2: 210 PSI
Cylinder #3: 200 PSI
Cylinder #4: 195 PSI

*picture of nasty valves covered in black

Answer 6

Probable cause: excessive carbon build-up

Question 7

What are the 3 basic spark plug material variations?

Answer 7

1) Nickel plated copper - “Traditional”
2) Platinum – 2 versions
* centre electrode only
* or on both electrodes
3) Iridium

Question 8

How can you find the specifications for spark plug gaps?

Answer 8

Under-hood decal or Mitchel

Question 9

Can feeler gauges be used for checking spark plug gaps?

Answer 9

No! Use a gapping tool / gap gauge

(make sure you know what this looks like)

Question 10

Will spark plug gaps be okay to use right out of the box?

Answer 10

No, traditional nickel-plated copper plugs need to be checked. While Platinum and Iridium plugs should not require gapping, check to be sure.