[T3] 73 type 3 FI ignition condenser ?

[William Jahn]

Sent from Mail for Windows 10

From: Jim Adney
Sent: Wednesday, March 20, 2019 9:02 AM
To: type3 at vwtype3.org
Subject: Re: [T3] 73 type 3 FI ignition condenser ?

On 19 Mar 2019 at 18:15, William Jahn wrote:

> I do have an issue with an erratic idle 

You've mentioned this before, and I've struggled with trying to understand 
what you were talking about, until I realized that my '73 had some sort of 
problem at idle when cold. It's not really a consistent miss, but, instead, it 
seems to sort of stumble on idle when cold. As you have mentioned, it drives 
just fine, it's just the cold idle that's annoying.

 Jim it can be described as a stumble . 

 I never noticed this until one day I had the engine running after a short 1 mile round trip and went to open the tailgate and what I heard was not a steady miss yet one that seemed to affect different cylinders at random , I called it an erratic miss. I not only heard it I also felt it if I placed my hand near the tail pipe. We know a steady miss is one you can almost count for lack of a better term . one can also track it down. Pulling wires or injector plugs depending. 

 Once I discovered this I decided to find out what it was . AS far as I recall it started in 2009 which was right after I put the heat exchangers back on in place of the J pipes . Changed the plugs , all the fuel lines , injector seals , fuel filter and a few vacuum lines. Adjusted the valves , changed the oil . I also had an different pressure sensor than it was supposed to have which had been on this car since 96 when I was trying to pass the Calif smog test. I didn’t know they had different part numbers all I knew was they looked identical all had the black tamper cap and all had the ground tab and this one came off the 72 and was riveted and looked new yet back then it ran great even on the X engine that came in this car and it had the proper ECU. 

 I went through everything trying to locate or isolate this issue down to checking every single lead from the ECU to each FI component and each ground. . At one point I added resistance to the HTS and if I added enough think it was 350 ohm what I found was I could now plug in the IAD temp sensor this miss was gone . Then I added resistance to the IAD sensor circuit ( a resistor across the IAD plug ) with the HTS resistance removed . Basically trying to fool the ECU.  Yet neither action was a fix as the temps went up the miss was back. 

 Then I was given the correct Pressure sensor and this seemed to help quite a bit yet the erratic miss was still present as long as the IAD sensor was connected. Then I checked every single ground from each injector and all the others that connect to the trees on the case upper . I cleaned all of them and tightened every single connector puled out of their plastic plug . All I found was one of those plastic insulators where 2 grounds became one with one lead with only 2 strands of wire intact , I cut it off and spiced and soldered and heat shrink and the miss was gone only to return later. 

 You referred to a cold engine, is this when first stared up? I asked because  I had the engine running for quite a while and the weather at that time was hot . 

 I also recall back in the mid 90’s when I would adjust the TPS to see what affect it would offer and going by the Bentley method which has been decide is incorrected, If I didn’t have the TVS dead on and got in to go home from work it would not idle well at all , had to get out pull the oil bath and get it back to the way Bentley stated and it idled fine again. Also when trying to pass smog if anything in the point gap or timing as well as me adding an adjustable resister to the HTC , if anything was off one bit the emissions  were higher. At that time is was the idle and 2500 RPM with a tail pipe sniffer and it failed at idled by a bit. 

 I just want to add a few things here just to try to describe what happens with the IAD sensor plugged in which is to say quite difficult to put in words. I start the car and this was when it would start without priming the system which is what I do now . It idled at 1300 RPM then after 3 minutes it would drop to 950 . Seems the electric AAR binds yet did close, I’m not really sure why the 1300 RPM . 

 Now it’s out of the initial cold start phase. With the IAD sensor plugged in here’s what happens . I get in the auto trans is in park no load, I push down on the gas peddle and as soon as the RPM is at 1300 RPM the engine begins to falter in RPM , so from 1300 until it reaches  2300 RPM the RPM will begin to fluctuate as I keep a steady movement to increase the RPM , even if I just gun it in that RPM range it’s not smooth . Once I go back and unplug that IAD sensor suddenly I can press on the gas and get a smooth steady and responsive engine RPM rise , I can get this affect at the pedal as well as at the throttle lever. 

 As I said I never felt this driving yet if I let off the gas I would hear a popping , I fixed that as it was an exhaust leak . I still have the IAD sensor unplugged , if I don’t that what I call erratic miss returns at idle and that RPM falter returns. Now the electric AAR closes when it decides to. Now when I start it the RPM is low then rises and depending on how hot the engine is in can be 950 to 1,100 .

 Jim I will try the test you suggest and let you know what I find. 

William

I've developed a theory about this. It started when I noticed a note on one of 
the schematic drawings for one of the later 914 d-jet brains. To follow this, 
go here:  

https://www.sw-em.com/bosch_d-jetronic_injection.htm

Scroll down to the 3rd schematic and look at the note near the lower left 
hand corner (index M,11.) This refers to shorting out R471 except at idle. 
You'll find R471 near the lower left hand corner (index G,28) of the 2nd 
schematic on that web page. R471 is a 300 Ohm resistor (in this 914 version 
of the brain) in series with the cyl head temp sensor, thus it adds 300 Ohms 
to the cyl head temp sensor resistance when the engine is idling. This 
richens the idle mixture.

My theory is that this is TOO rich when the engine is cold, which leads to 
poor idling when cold, like what you've described and I've also experienced. 
However, it remains to be seen whether our '73 d-jet brains have the same 
circuitry.

This is all consistent with my observations that the stumbling idle goes away 
as soon as the throttle is opened a tiny bit, and that the engine runs much 
better, and runs much longer with the pump shut off, with the throttle slightly 
open.

Fortunately, there's a simple test for this. Open the engine cover so you're 
all ready to go, make sure your IAD temp sensor is plugged in, then start the 
engine and note the poor idle. Then go back to the engine and unplug the 
throttle valve switch. Does the idle clear up? Does the idle go bad again 
when you plug the TVS connector back on?

If so, I think we've found the problem.

There are several possible solutions.

1) You could leave the TVS disconnected. That would cure this problem, but 
also disable the FI shutoff during overrun, and also disable the "throttle 
pump" and full throttle functions of the TVS.

2) You could disconnect the idle connection wire from the connector. That 
would fix the problem and also disable the FI shutoff during overrun but 
leave the "throttle pump" and full throttle functions working. (Late FI Type 3s 
were delivered in some markets in this condition. The wire was there, but 
just folded back under the boot in the wiring harness.)

3) You could open the brain and reduce the resistance of R471 to the point 
where it no longer caused a problem. That's the approach I would take, but it 
would take some experimentation to determine a good resistance value.

It's still too cold here for me to want to do much outside testing, but if you 
can do this test and let me know the result, that would be a big help. I could 
go ahead and open up a late 311 E brain and try to find that resistor. Once I 
find that resistor I can check to see if it still has its correct resistance or has 
it drifted up (unlikely.) Then I can get ready to try different resistors in place 
of that one.

It will be interesting to find out if this "feature" is part of our d-jet systems and 
if it is even in brains earlier than the '73, late 311 E brain.

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

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