[T3] Question about D jet idle issues 73 T3

[Jim Adney]

On 26 Jan 2016 at 16:05, William J wrote:

>  I used a 72 case and heads I had here and it was the same exact auto trans 
> 1600 as the 73 the case and heads down to the PCV breather were the same . 
> However the stock pistons on both had the dished drop and the new set which 
> were Mahle were flat top yet at that time it ran perfect. I did used the 
> dist from the 72 and the vacuum advance from the 73 because the 72 auto dist 
> looked in better shape yet even timed at 5 BTDC I didn't have these issues. 
> I checked the timing and it does not jump around and is steady .

I don't think you've mentioned how the car runs on the road. That's 
probably more important than how it idles. Cold idle is especially 
unimportant, although being in SoCal, cold really isn't very cold. 
Let us know how the car drives.

Here are a few things you should check:

1) Make sure the mech advance is working. You can do this with a 
timing light while the engine is running; watch the timing marks move 
to the left as the rpms go up, and fall back when the rpms drop. Or, 
grab the rotor and twist it CW. It should move ~15 deg against spring 
pressure and then drop back smartly when you let it go.

You're setting the timing to 5 BTDC, right? On a '72-3, that would be 
the 3rd mark from the left. 

Looks like this:   |    |    |  |

Not like this:  |      | | |

Right?

2) Check the fuel pressure to make SURE it is staying constant while 
driving. Get a friend to watch your gauge while you're driving, or 
use a long hose so you can watch it yourself. If there's dirt in the 
tank, it can clog up the filter, allowing only enough fuel thru to 
maintain pressure at idle, but the pressure can drop as the engine 
demands more gas. I make kits to repair the overflow hose which is 
always broken by now. Driving with that hose broken allows water and 
dirt into the gas tank, which will cause all kinds of problems. The 
kits for a '73 are $33.

3) Consider letting me rebuild your '73 distributor. The FI trigger 
points can wear to the point that they trigger erratically. I will 
check and adjust these in the course of a rebuild. Then you can 
return the '73 (505) vacuum can to that distributor. This costs $50 
plus the price of any major parts, if needed, but major parts are 
seldom needed.

4) Check the resistance of each spark plug wire from dist to end. The 
resistive SP connectors occasionally open up and give poor running. 
Each should measure ~1000 Ohms. Replace any SP connector that 
measures infinity.

5) Pull off one SP wire from the dist at a time. Does the engine slow 
down the same amount from each one? If you find one that doesn't slow 
the engine down, there's a problem there.

6) Check the coil nose, the inside of the dist cap, the rotor, and 
the SP connectors for spark tracks. Replace anything that's tracked.

7) Check the condition of the SP wires. If it is hard and brittle, 
replace it. If you still have your OE wireset, don't throw it away: 
Almost everything there is reusable and better than anything you can 
buy new. I have good quality bulk wire which you can use to replace 
old hard wire.  

8) If you get really determined, open up the brain and check 
resistances of all the components via the FI wiring harness. This is 
a lot of work unless you have one of the Bosch D-jet testers. One 
thing to look for is the fact that Bosch reduced the thickness of the 
pins on injectors at some point. It's only a couple of thousandths, 
but it makes for bad connections at the injectors. The solution is to 
CAREFULLY remove the female pins from the injector connectors and 
squeeze them down slightly so they grip the injector pins soundly 
again. This is difficult, frustrating, and time consuming, so you 
don't want to do it unless you actually have this problem. You should 
not have this problem if you still have the original injectors.

Keith is a smart guy, but he's gone OCD on gas for the past couple of 
years. He may be right, but he's the only one in our group who seems 
to have any problems with modern gas. It's possible that there's 
something especially bad about the gas in upstate NY, but I doubt if  
that's the cause of your current problem.  

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