[T3] On a 73 T-3 SB fuel injected auto trans pinging

Jim Adney jadney at vwtype3.org
Sun Dec 10 13:29:10 PST 2017

I was concerned that I was mixing up different IAD front covers when I said 
there was a tiny hole at the PCV outlet. I just went down and looked at a 
'72-3 IAD and, you're right, it does not have a tiny outlet hole. The tiny outlet 
hole is only on the '68-9 IAD with the optional cold start valve. As a double 
check, I also blew thru the '72-3 pipe to make sure there wasn't some 
internal restriction in there and there isn't.

Nevertheless, I don't see that system causing a problem. It's possible, 
however, that the PCV valve in the breather box has dirt in it, which makes it 
stick open or closed at times. That might explain your idle variation. Try 
removing and cleaning it.

I understand that you've already got the valves and timing done correctly.

Here are some other things to try/check/consider:

'72 & '73 MAP sensors and brains have the same VW part #, but they have 
different Bosch numbers. I've always suspected this was because VW/Bosch 
felt that the later part numbers would be backwards compatable, but this 
does not insure forward compatability. You need to make sure you have the 
'73 version of each

The '73 brain has a Bosch part # ending in 048. (The '72 brain is 024.)

The '73 MAP sensor has a Bosch part # ending in 116. (The '72 MAP sensor 
is 106.)

I believe you're already using the correct AH distributor.

These parts were all developed to make the vacuum advance/retard 
distributor used in '72 unnecessary. They work together, as a team.  

Regarding your ping. My first recommendation would be to take the car for a 
nice long drive. If that fixes it, great, if it does not, stop halfway thru your 
drive and top up your tank with the highest octane gas you can find. If this 
fixes or severly reduces the ping, you know that what you have is actual 
ping, caused by compression pre-ignition.

If high octane gas does not change the ping, this is not real ping, but 
something else.

(If you're already using premium gas, try regular. If going to regular doesn't 
make the ping get a lot worse, then this isn't ping.)

Regarding the cyl head temp sensor, just leave it alone. There's more 
resistance in series with that sensor inside the brain. (In the one schematic 
that I've been able to find on the web, there's a 300 Ohm resistor that gets 
connected in series, only at idle.) It's always possible that something has 
gone wrong with the circuit inside the brain, but the brains have been 
extremely reliable over the years.

BTW, I believe all the VW/Bosch cyl head temp senors are the same, except 
for details like the length of the wire, the insulation, and, possibly, the style 
of connector.  

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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