Excessive Idle-Speed Operation and Cylinder Wall Glazing

Excessive Idle-Speed Operation and Cylinder Wall Glazing
You are peering into a combustion cylinder bore—before (left) and after (right) de-glaze honing was performed.

In four-stroke outboard engines, cylinder wall glazing often occurs as the result of prolonged and excessive low-rpm operation. Gum, varnish and other products of (incomplete) combustion adhere or "glaze" to the cylinder walls forming a super-slick bore which in turn prevents proper seating and sealing of the piston rings, robs compression, and generally depletes engine performance.

Quite often the engine history report tells the story when we investigate sluggish engine performance, particularly in four-stroke outboards. Shown below is an actual report of an outboard in service two years, with fewer than 250 hours logged. The unit of measure is minutes of actual operating time.

0 1000 rpms ---------- 6020 minutes
1000 - 2000 rpms ------ 1580 minutes
2000 - 3000 rpms ------ 105 minutes
3000 - 4000 rpms ------ 2045 minutes
4000 - 5000 rpms ------ 3502 minutes
5000 - 6000 rpms ------ 1100 minutes
Above 6000 rpms ------ 25 minutes
Total minutes ----------- 14377 minutes
Total hours --------------- 239.62

Note the time of operation logged at less 2000 rpm equals 126.7 hours–or 53% of total engine running time. In general we consider 2000 rpm and less to be idle speed.

At these low speeds the boat does not rise on plane and the four-stroke engine's oil pan remains mostly submerged and oil remains very cool, never reaching normal operating temperatures. This type of engine profile is considered severe duty and more maintenance is required to keep cylinder glazing from occurring. This profile would require new spark plugs every 100 hours, a full engine de-carbonization, and possibly hotter thermostats.

Prolonged low-speed, low-temperature operation allows un-burned fuel to seep past the piston rings and into the crankcase, thinning and reducing the lubricity of the engine oil, and promoting the possibility of low-hour bearing failure.

Rather than idle around so many hours while fishing, a much better plan would be to purchase an electric trolling motor.

The attached photos depict a heavily glazed cylinder on the left and the same cylinder bore after the glaze was removed by honing. Owners often inaccurately assume a manufacturing defect when an engine with such short service time suffers from lack of compression–but nothing could be further from the truth–the problem lies in how it was operated.

Average price of a cylinder de-glaze job runs about $2,500, parts and labor, and this could easily be prevented by changing the engine's operating profile.

Thanks for your business and Merry Christmas!

Chris Mapp
Coastal Bend Marine
Port O'Connor, TX