Great balls of fire!

I was driving down a hill at about 10mph in 4-lo 2nd gear, realized I was going too fast, so I hit the brakes and pushed it into first at about 3mph and when I let out the clutch the truck disagreed with me. It sort of slammed (like what usually happens when one downshifts like that!) but then started belching blue smoke from the exhaust pipe! I got onto the highway, drove a couple miles back to school, and the exhaust was clear again. Everything seemed to run fine, oil was good, coolant was good. Any idea what happened? It *sounded* like there could have been a little more latent engine noise (various clicking and clanking) than usual, but I was probably imagining it.

 

When you downshift at a high (vehicle) speed, you make the engine suck air into the intake HARD. If you have the throttle closed or nearly closed, you create a huge vacuum inside the engine. The engine will pull air from any place it can get it. In an older engine with wear, that means it pulls air down past the valve guides and up past the piston rings ( among other places ). When air is flowing backward , so to speak, oil comes with the air. You burned that oil off after the vacuum equalized more. When you pulled the oil past, it probably starved a few parts on the top of the engine for a little bit. That might have led to the louder ticking that you heard.

What you should know is that it's not really good to cause the engine to rev that high on a downshift. If you do it too much, you can stretch rods and put undue stress on bearings, rod and crank journals, wrist pins,, etc. Read that: Rebuild.

Downshifting as an engine braking maneuver is fine, as long as the engine is in a reasonable rev range. That's why vehicles with automatic trannys have built in safety features that won't LET the tranny downshift into a certain gear unless it's below a pre-set groundspeed, even if you pulled the shift lever all the way down into Low at 60 mph.

Mike in AR

 

Good example of why autos are fast replacing manuals...
(and I don't mean it's a good thing..)

 

*phew* I thought that might have been what it was.

Don't worry, I totally understand that when there's a rev difference between the drivetrain and the engine is causes wear on the engine and clutch. When I downshift regularly, I push in the clutch, tap the gas to rev it up, put it in gear and let out the clutch while the engine is still revved some. Usually I don't feel the truck buck at all because I got the revs to meet.

Thanks for the explanation of the vaccuum, though, I learn new things every day

 

Follow up question -

Does this mean when I am going down an incline in 3rd gear using my engine as a brake that the vacuum it's creating is hurting the engine?

 

It's not necessarily the incline , itself, it's your groundspeed. In your first post, you mentioned you were in 4Low range. If you do the math on gear reductions (axel ratio x 2nd gear tranny ratio x 2 -- or more - ) you can see how many times your engine is turning vs. how many times your wheels turn. So if you would normally downshift to 1st in 2WD or 4High at 3 mph , shifting into 1st at 3mph in 4Low will make your engine turn twice as fast.

In fact, the transfer case thats attached to my A340H auto tranny has a 2.66 :1 reduction. First gear in my 4Runner in 4Low is approximately 34:1 (4.56 diff gear x 2.80 1st tranny gear x 2.66 Low range reduction in transfer case). I've never kept the tranny in 1st , 4Low for long, but I'm almost wound out at about 5 mph! It's probably less than taht,, the speedo is not accurate down that slow.


However, the incline probably makes oil and other fluids slosh around in places they normally don't go and might make it more likely to be sucked into the combustion chamber under high vacuum conditions.
Mike in AR