[T3] Temp sensor and MPS problems on new motor

Jim Adney jadney at VWType3.org
Sun Feb 21 15:46:32 PST 2016


Jessica and Daniel,

Let me start by trying to explain why I don't think you should adjust 
the pressure sensor.

Bosch designed the D-jet system to work and meet certain emission and 
mileage standards. For almost 50 years, those of us with these cars 
still on the road have found that they continue to work perfectly 
well without "tweaking" as long as normal maintenance is done and the 
car is carefully maintained. Unfortunately, by this time most of 
these cars have seen quite a few years of neglect and/or abuse. 

In this case, it appears that this system is tending to run lean at 
times. A simple thing to adjust would have been the fuel pressure. 
The pressure sensor is a particularly bad choice to tweak, because 
it's something that most people can't put back to its starting point 
correctly. As long as the engine is assembled correctly, with the 
same spec components, there's no reason to expect a given amout of 
manifold pressure to lead to a different amount of air getting into a 
cylinder. The most common "reasons" for adjusting the pressure sensor 
are to make up for separate flaws that should be corrected 
themselves. The only reason this isn't done, is that we're unable to 
identify those flaws.

There are certainly cases where adjustment would be necessary: 
different displacement, different cam, larger intake valves, etc, but 
those are all changes that one might make intensionally and expect to 
need adjustments to accommodate the changes. I'm assuming your parts 
are all stock.

You chose to adjust the pressure sensor to try to richen the mixture. 
You could also have increased the fuel pressure. I'd be much happier 
with that approach since it's something you can do with a couple of 
wrenches and you can restore it easily. Both adjustments will make 
your mixture richer across all operating conditions, but I suspect 
the fuel pressure makes a more linear change, while the pressure 
sensor is likely to be quite non-linear.

When you adjusted your pressure sensor, what direction did you turn 
the screw and how far did you turn it. (BTW, the fact that all 3 
screws on the sensors I recalibrated ended up pointing in the same 
direction is a coincidence. I never noticed that.)

In your case, you have data that supports the idea that the engine 
starts to run lean at some point. My impression is that this doesn't 
always happen and it's not always possible to predict when it will 
happen. That makes it, in my mind, an intermittent fault. It makes no 
sense to make a permanent adjustment in a somewhat compensating 
parameter to correct for an intermittent fault. You need to find the 
part that's intermittent and fix it. While intermittent faults are 
always hard to pin down, clearly, this has been really, really hard 
in your case.  

At times I wish you were here, but at times I realize that this would 
really put me on the spot. What if I couldn't fix it even then? Doh!

This is going to be a long post, so bear with me.

The first thing I would try to figure out is if it's really running 
lean. In other words, is an excessively lean mixture causing the 
bucking and missing, or is something else causing missing, which puts 
unburned fuel into the exhaust and confusing the A/F sensor? The 
chart of A/Fs that you posted was interesting, but we have to keep in 
mind that it ASSUMES the given A/F and tells you the outcome. You can 
be fooled if you try to follow the logic backwards. [In other words, 
if A implies B, it does not follow that B implies A. And it's also 
possible that C implies B and that B can happen with neither C nor 
A.]

I don't know anything about A/F sensors, but I do know that they all 
depend on various secondary conditions to infer the A/F. So, let's 
assume that your mix is getting overly lean, but try to keep in mind 
that this may be wrong.

What could cause a lean D-jet mixture? I'll try to make a list. It 
will be as complete as I can make it, but there are bound to be 
things I don't think of or remember.  

1) Air leaks past the IAD. Do you have the correct D-jet intake 
manifold gaskets? The ones that are a paper/plastic/paper sandwich 
about 3/16" thick. The metal Beetle ones will definitely leak.

Did you mount the IAD and the intake runners carefully, so the air 
runners aligned nicely BEFORE the hoses were slid into place, and the 
hoses aren't distorted so they don't leak. If the hoses were 
distorted from a previous careless installation, use nylon cable ties 
on them for a year or so, until they reform to straight.

Leaks directly into the IAD don't matter, because the pressure sensor 
sees those and accounts for them, so the AAR, idle adjustment, and 
throttle position only effect the idle speed, never the mixture. The 
hose from the IAD to the pressure sensor needs to be in good 
condition with no leaks. Hose clamps are unnecessary, but hose in 
good condition is important.

2) Power to the injectors. You have to have good connections to the 
injectors, and that includes the full chain from the battery + all 
the way thru the system and back to the battery -. You've checked the 
connections at the injectors and at the center of the case. How about 
pins 3, 4, 5, 6, & 24 at the brain? And check all the rest of those 
brain connector pins while you're at it. Check each female pin in the 
brain connector to make sure they grip tight and make good contact to 
the PC board in the brain. Use a combination of feeler gauges that 
are the same thickness as the PC board to check each pin.

Make sure the ground strap at the front of the tranny is well 
attached. Make sure the wire from the battery + to the main power 
relay is in good condition. I think you've already checked the relay 
itself, but check both ends of the 2 wires (4 ends total) that carry 
power from that relay to the FI wiring harness.

I know you've been thru your charging system and it appears to be in 
good shape.

3) In any electronic device, the most troublesome parts are the 
connections. That includes obvious ones as well as not-so-obvious 
ones, like soldered connections within the brain. I'd like to get you 
a different brain to try. I have a pile of them, but few of them have 
been thru as much testing as yours have been. They ALL seemed to work 
long enough to start and run a car, and I have no reason to expect 
them to fail later on, as the brains have been extremely reliable. Is 
it possible that both of your brains that won't run the fuel pump 
came from the same place? Is it possible that they were both abused 
in the same way?

Do you still have the Bosch D-jet tester? Did you use it to check ALL 
the FI harness connections? You need to gently wiggle each wire while 
you watch the tester. Any meter indication that correlates with wire 
movement is a sign of a bad connection. Did all your connections test 
okay?

4) Check your cyl head temp sensor. Where the wire comes out of the 
body, there is a steel sleeve crimped over the wire. If the wire has 
been bent there, it can cut thru the insulation and ground out that 
sensor. If it does, that will lean out and kill the engine. That part 
of the sensor is sometimes covered by a teflon sleeve which you will 
have to slide off to inspect that area.  

5) Just to simplify things, I suggest that you unplug both the TVS 
and the pressure SWITCH. I'm not suggesting this as anything 
permanent, but just as a way to be SURE that those circuits are in 
the correct state for normal running. There is also wire 18 that 
connects to the starter solenoid. If your car will start okay with 
that wire disconnected, you could try removing that, too, since 
there's a chance that it's connected to a terminal that stays hot, 
instead of one that's only hot while starting. This is something you 
could have checked with the Bosch tester; did you? If you disconnect 
it, it's probably best to leave it grounded somehow.  

6) You have a constant 28 psi reading on your fuel pressure. That 
tells us that your fuel pump and fuel pump relay are good, but how 
sure are you about the calibration of your gauge? Russ checked the 
calibration of my gauge a few years ago, so I could check yours if 
you cared to send it to me with your brains.

7) It's possible for injectors to get clogged. Have you looked at the 
spray patterns from yours? I don't tend to have problems with ones 
that see regular use as long as they haven't seen a lot of water from 
a broken overflow hose. If the system has seen a lot of water, the 
rust that comes with that can certainly clog injectors. Each injector 
has a little conical brass screen filter in its inlet. That will stop 
the big stuff, but some will still get thru. The good news is that 
the clearances in the spray nozzle aren't particularly fine, so that 
part doesn't tend to clog, but it can still rust. If the inlet filter 
fills up, that will limit the amount of gas that can get thru.  

I have access to an injector cleaner and tester. I can replace inlet 
filters and check injectors to make sure they have good spray 
patterns and give out the same amount of fuel. This is seldom needed 
with our injectors, unless there has been a lot of water in the gas.

8) When I recalibrate a pressure sensor, I set it up with a square 
wave generator and dual trace oscilloscope. I drive the primarys of 
the recalibration sensor and a "known good" sensor of the same part # 
from the same square wave and connect a Y hose to each of them while 
I look at the output from each secondary as I vary the vacuums 
together. This runs each one in the same way it is used in the D-jet 
system and lets me watch the outputs over a range of vacuums in real 
time. With this method, it is easy for me to match the 2 sensors to 
better than 1/16 of a turn of the adjusting screw.  

The weakness of this method is in coming up with a "known good" 
pressure sensor. When I do this, I go thru my pile of sensors and 
compare several untampered sensors of the same part # to make sure 
that the one I pick as "known good" isn't distinctly different from 
average. In your case, I ended up using a NOS sensor. I had 3 on the 
table, and they all fell within less than 1/8 turn of each other.  

One thing that's possible is that the NOS one, being later 
production, was made to slightly different standards, possibly more 
lean.

Of the 3 you sent me, I believe 1 had not been tampered with and I 
didn't find 
any need to readjust it. The fact that it agreed with my "known good" 
unit is 
reassuring. I sealed the 2nd one with grey epoxy and the 3rd one with 
rubber 
cement so you could play with it, as you requested. I hope that's the 
one you 
adjusted.

While I've tried to cover all the bases, I'm concerned that there's 
nothing 
here that I haven't mentioned before. There are bound to be 
possibilities that 
I haven't thought of, and we have to keep in mind that there could be 
a problem 
with your new FI wiring harness as well as problems with the brains 
and the A/F sensor readings. Frankly, I love a challenge and I wish 
the weather was nicer and you were here. It's just so hard working 
thru someone else's eyes.

I hope something here helps. Please continue to post your findings 
and ask questions.

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



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