[V6-12v] Motor won't start - G28 - engine speed sensor

[Andrew Muff]

Thanks Tony. I guess I wasn't completely clear when I wondered about valves
hitting pistons. What I meant was when the motor was timed 180 degrees off
as in the case below. When we fixed our mistake, we pulled the cams
completely out of the head, then turned them 180 degrees and reinstalled. If
we had just removed the T-belt and rotated the cams on the motor, they would
have definitely hit the pistons.

 

But your response about the ECU created two questions in my mind.

 

1)      The first question I think I solved after thinking about it while in
the shower (all my best thinking happens there ;-)). It goes like this:
Since the crank revolves 2 times for every turn of the cams, why would it
even matter if the cams were 180 degrees rotated. Since 1 turn of the crank
is the compression stroke and the 2nd turn of the crank is the exhaust
stroke. It's like a drummer playing a 4/4 beat and hitting on the 1 and 3 vs
the 2 and 4. It sounds the same to any listener. Without a melody no one can
tell the difference. By rotating the cams 180 degrees, we effectively
changed the compression stroke into an exhaust stroke and vice versa.

What I realized after a while is that the melody is the spark, if you will.
Basically by having the cams 180 degrees off, instead of the spark firing
when the piston gets to the top of its compression stroke, it's now firing
at the top of the exhaust stroke. Which obviously is not super useful.



2)      So here's where your comment about the ECU comes in. And now I'll
use quantum physics as the analogy, because it's like the ECU is in two
different states simultaneously. You argue below that the ECU doesn't know
we got it wrong, which makes sense when I thought through the whole scenario
in #1. However, the ECU did know we got it wrong because it refused to fire
the spark plugs and refused to send a signal to the injectors to spray fuel
so the engine wouldn't start. So the ECU both knew we got it wrong, but
didn't know enough to flag a code. Moreover, if the ECU knew this it should
have stopped the starter from turning over since it's presumably trying to
prevent engine damage by refusing to start. If we hadn't timed the car
exactly 180 degrees off, it would have been likely that valve and piston
damage would have occurred just from engaging the starter.

 

So there you have it. The only reason I went into this much detail is my
frustration with the ECU not throwing a code. A simple "hey you idiots -
you've got the timing wrong so I won't start" would have saved me several
days of frustration.

 

Cheers,

Andrew

 

PS - and yes, I meant 30v not 32v. Was typing too fast.

 

From: Tony Hoffman [mailto:auditony at gmail.com] 
Sent: Thursday, January 10, 2013 5:27 AM
To: Andrew Muff
Cc: v6-12v at audifans.com; AudiFans (Qlist) S4 Qlist
Subject: Re: Motor won't start - G28 - engine speed sensor

 

The pistons will hit valves in the 12V engine. I just tore one down here in
the shop. I'ts a matter of timing. You are nearly guaranteed if the cams
stop and the crank continues to rotate (as in broken belt) the valves will
bend. However, with them both turning together, you have to time them just
right to bend valves.

 

The ECU deosn't know you got it wrong because the cam position snesor sees
TDC the same time the crank position sensor does.

 

BTW, the multivalve V6 is a 30V, 5v per cylinder. It too, will bend valves.
seen it, fixed it.

 

Glad to hear you got it fixed!

 

Tony

On Wed, Jan 9, 2013 at 10:10 PM, Andrew Muff <andrew at muffster.com> wrote:

OK, first - thanks for all of the suggestions. My buddy and I finally
figured it out. After a suggestion to recheck the timing, we realized that
we managed to have the timing off by exactly 180 degrees. After timing the 4
Audis we own between us about 15 times with no issues, I'm at a loss how we
managed to get this time so wrong. And the only reason I'm admitting to this
is so someone might learn from our mistake.

So here is what not to do when timing your Audi. Do not put on the diamond
shaped washers onto the cams backwards. If you do get one backwards, it's
likely you'll figure it out when you put the locking bar across and only one
side fits on. But if you are really not paying attention you'll manage to
get both of them on backwards. That way with the crank locking pin in place,
and the cam locking bar in place, you'll managed to set up the timing
exactly 180 degrees off.

The good news is that I'm told that on the 12v engine, all of the valves and
pistons will still clear. I'm also told that you won't be so lucky if you
have a 32v engine as the valves will strike the pistons ever so slightly.

Now with a cam position sensor and a crank position sensor, why the ECU
doesn't know that the timing is off and throw a code or a CEL is beyond me.
But I'm happy to report that the car is doing laps around the block as I
type. ;-)