Coolant.

Good point.

 

Today I noticed a whitish smoke on the exhaust pipe when I was on the highway, going up the hill and accelerating. I checked the level of the antifreeze and it went down. When I have short and quiet trips the level does not decrease. After my opinion it is the cylinder head gasket, what do you think?

 

Sorry, that’s not good and you should stop driving the car until it’s diagnosed and corrected. I hate to be the bearer of bad news, but the news is going to get a lot worse if you keep driving the car.

White exhaust smoke usually means coolant is entering the combustion chamber. This explains why there’s no sign of a leak. The gravest danger is this: The liquid is not compressible, so if enough enters the combustion chamber, you’ll bend a connecting rod on a compression stroke, making your repair much more expensive.

Your three year-old engine most likely has a leaking head gasket or (less likely) a cracked block. If you change the engine oil, you’ll probably see that it’s milky, tan colored instead of black.

 

I have checked the stainless steel connecting pipe between the EGR valve and intake manifold today and it's clean, apart from dark colour from the burned gases, I'll try to check other things ...

 

Thanks for your reply, I am thinking if it's like you said shouldn't I see the white smoke all the time ? And my engine oil and coolant are clear , they are not mixing up, I am still looking for a good explanation, maybe it's the EGR , my prius it's running just fine , no knocking sounds, no warning lights...

 

I agree on the previous guess that you've got a head (crack or warp) / head gasket issue that lets coolant leak into the cylinders. Since these cars are notorious for their failing HGs, I'll land on that conclusion.

The cylinders are supposed to be kept isolated from the cooling system, relying on the head gasket to keep its seal. During combustion at full throttle, pressure in the cylinders is > 1000 psi. When cold, the cooling system is at 0 psig (atmospheric pressure). But when the cooling system warms up, it can be pressurized at 15+ psig, depending on what the coolant reservoir cap release pressure is (maybe 1.3 bar, as a guess?). That pressure may slowly force coolant past the HG into the cylinders.

So to explain the sometimes lack of white smoke - when the engine is cold, the cooling system will not be pressurized, and the coolant will be less likely to flow through to the cylinders. Also, when the engine is operating at full throttle, the combustion pressures are higher, the force lifting up on the head in each cylinder will be higher, and any marginal opening in the failed HG may be more likely to let coolant through. So yes, it makes sense, and still points to a HG. But further testing should be done.

When everything is good, pressure in the cooling system will build as the engine comes up to operating temperature. But when there is a HG problem that lets combustion gases from the 1000+ psi cylinder past the HG into the cooling system, pressure can rise much more rapidly and at lower temperatures. That will typically be vented out the coolant reservoir cap, often taking a bunch of coolant with it.

This could all be happening and you might still have no mixing of coolant and oil (i.e., no coolant in the oil, and no oil in the coolant). And your car could be running just fine.

Further tests:
pay very close attention to coolant level
block test for combustion gases in the cooling system
pressurize cooling system and check for coolant entering the cylinders

 

Leak down test.

 

Thanks for that beautiful explanation and for your time!
Ok, I have a new booking on 18 March with Toyota for a full service and I'm gonna ask them for a new pressure test ; in the meantime I'll gonna cover my coolant reservoir cap with a white and clean tissue, looking for any pink spots !

 

Good plan.

One slight update ... I just noticed your profile shows you as having a 2017. So when I said these cars are notorious for HG failures, I was referring to the 2010-2015, which I incorrectly assumed you had. Not sure how much that changes for 2017, but it might be important. With such a new car, are you under warranty?

Also, on Mendel's good suggestion on the leakdown test - yes that's a good one. Most useful positive result would be if a cylinder were pressurized and air was found to be leaking out of one of the adjacent cylinders (cylinder-to-cylinder leak), or bubbles coming through the coolant reservoir (cylinder-to-cooling system leak).

But like many (almost all) of the choices for HG tests, a negative test result on one of the choices does not rule out an HG problem.

 

No traces of fluid sitting on top of the bottom paneling.

 

No "pink crusties"

 

No coolant reservoir leak so I guess is an internal issue, HG or EGR...