Bucking when engine breaking down a long hill

Question that may be nothing.


Car is an 02 mk4 golf, alh engine. Standard transmission, 11mm IP, slightly bigger than manual injectors. Car has only minimal smoke when applying WOT under normal circumstances.



Today I was going down a fairly long hill following a semi, engine breaking the whole way down in second gear with the tach holding at around 2400 rpm. After a bit I noticed that the car was shaking a bit from what I thought at first was uneven road surface and I was noticing it because I was going so slow, but now I think it may have been the engine bucking (one or more cylinders missing).


When I got near the bottom of the hill I began to apply throttle to keep speed up, and noticed a significant amount of blue/blackish smoke coming out the back with very minimal throttle. It cleared up fairly quickly when more throttle was applied and then car was back to normal as I accelerated at the bottom of the hill.



What may have caused this? Is it an issue at all, or just because I was engine breaking for a few kilometers straight and wasn't burning off the fuel or even oil?

The engine cooled down, the exhaust cooled down, and the charge air tract got more oil vapors and such passed into it that normal. That would be my guess.

Could also be that the decel fuel cut was happening for so long the pump got an air pocket and had to push it out when it started back up again.

One of my ALHs did something similar when passing through some mountains with a car in tow behind me, although it wasn't all that noticeable but it did happen. And that was perhaps a similar situation although I was in a higher gear going faster but still probably in the 2500 RPM range.

Ya I was wondering if it wasn't something to do with excess oil getting passed into the intake track and everything cooling off.

Could that be an indication of anything wrong with the turbo? It consumes a little oil over 10,000 km oil change cycle but I had chalked that up to valve stem seals, and leaking valve cover/vacuum seal.

What could the bucking have been?

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The bucking was likely the pump pushing the air back out of the head chamber (my best guess), or perhaps an exaggerated slow-down shudder that many 11mm + manual pump cars can have (both mine do, but one is MUCH more pronounced than the other).