Hey Scotty. Hope you doing good. Got a little concern with my Volvo XC90, Diesel. The first startup early in the morning is really hard, It's well below freezing, something like -10 all night long.…

Trip02

Hey Scotty. Hope you doing good. Got a little concern with my Volvo XC90, Diesel. The first startup early in the morning is really hard, It's well below freezing, something like -10 all night long. It cranks slowly for the first few turns and then needs quite a long crank to get started. But when it does, the first few seconds it makes quite a loud knocking sound. Sounds like my BMW M3 15 years ago when the crank bearing went out.

It's only the very first start, for the rest of the day it cranks normally and makes no noise.

I have a theory that 2 or 3 of the 5 Glow plugs are no good, and it's relying on 1 or 2 cylinders to fire it up, then the first few cold turnovers, once it's started, aren't truely creating combustion until the compression raises the surface temp of the piston head and cylinder and that's what creates that knocking sound for the first few seconds after it starts, and I guess the cold air is a lot denser so the first crank cycle is fighting against a much higher compression ratio due to the density of the air. Hey, I could be dreaming.

What do you think?

Comments (2)
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Trip02
Trip02

Sy rob, watched this guys videos. He works on the same volvo we have.

Ricardo OG
Ricardo OG

diesels knock when they are cold, If you suspect that it may have bad glow plugs. There are ways to test them, OHM meter can test if they are good or bad. This video may be helpful. https://youtu.be/4Po_1pNHE1Y