1971 411

[Guam411]

1971 411, in Guam....I need an ECU......any advice?

Car seems to bee running fine, drove it around transmission smooth, decent HP, but then all of a sudden it died when close to idle.

A few times 0 turn over, let it sit turns over......runs good then again towards idle dies out.

if I turn over don't drive at idle stays on.......


VW mech says ECU, he's pretty reliable........what do you all think?


Ray thanks for your response, I was thinking along those lines.


The correlation is with braking at idle, however I got to a point where it would not turn over. I would love to hear more about the Jetronic trouble shooting.

[raygreenwood]

Hello Guam411!

There is a lot to cover so we will get started.

In general....the ECU on the D-jet system is so tough that having a failed ECU is incredibly rare. With age...yes...you can get a cold solder joint crack here or there on the circuit board...or a burned out component....but in all of my working on what has to be a thousand systems by now...I have never seen that happen. I just know it can and a very few other people have had this problem...over 40 years of working on them.

The only other common causes of an ECU dying are getting wet or a huge voltage spike. Also very uncommon.

In general if it starts and runs at all....its not the ECU. However...its very common that the wires between the ECU and components can be bad or making poor connections.

Here are some basics:

1. There is not one single wire in the system that can be disconnected, broken or poorly connected and run normally. There is no "junk" in this system.

2. There is not one single vacuum line or gasket or boot that is allowed to be leaking even in a minor way. This system uses vacuum to set fuel mixture.

This includes:
The seal between throttle body and intake manifold
The boots on the intake runners
The gaskets between runners and heads
The injectors in the intakes
Every single vacuum line and hose must be vacuum tight.
The vacuum hose to the transmission if you have an automatic
The oil cap on the sedans and the dipstick tube and boot on the wagons
The square cork gasket on the oil chimney
The vacuum liens to the heads.
The 12mm vacuum line between the intake manifold and the oil chimney
The vacuum advance can and any vacuum retard and advance lines

MOST especially....the vacuum line to the Manifold pressure sensor n the left side of the engine bay...and the sensor itself must hold vacuum.

There are more but this will get you started.

Also...this sounds like a common issue with fuel supply. There is a sock filter in the gas tank that usually ends up with a pile of silt around it and it clogs intermittently. It will idle fine...but if you rev it it stalls.

In the trunk...on the ledge between the struts is a round circle of carpet in the center. Remove that and under it is the fuel gauge sender. Remove the five screws, the wire and pull the sender unit out.... and with a flashlight...look down in the tank and you can see the filter sock and if its clogged.

The fuel pressure MUST be 28 psi....maximum 30 psi...and it must be stable. The fuel pressure on the system must be exact.

Also..right off the bat...there is a cluster of three double wires right under the center intake manifold. Those are the central grounds for the injectors. Those can cause this symptom. Their connections must be tight and perfect.

Start with this...there is a lot more. Ray

[Guam411]

Wow great information. I'll start with this and let you know how it's going.

R/Brian

[Guam411]

O.K I thought I was making progress. Spent some money and time ensuring vacuum is tight. Your right it isn't the ECU. I drove it about 30 miles, the first 15 were fine, but then slowly the idle at stops began to fade. I put in in neutral while at stop to keep it from dying out. I made it another 15 miles, even up a steep long hill. Transmission on last gear doesn't transfer smoothly (separate issue maybe). Ended up making it all the way home and just as I got in the driveway it stalled at idle. Tried turning over several times but it would not turn over.
I called the mech, he thinks the valves are leaking possibly. There is some grey smoke, not much from exhaust.

I know a lot of issues but this was the longest it has been driven since I've owned it.

Any suggestions? Continue down the vacuum path? or look elsewhere?

[Guam411]

Sorry....I forgot......I let it get cold and it started up first try no problems. So warm engine idle fail and does not restart when warm.

[Lars S]

[raygreenwood]

[Guam411]

Thanks gents, I will go through the verifications and see how it responds.

[KTPhil]

I also suspect the head temp sensor. Check resistance hot especially (unplugged). I also wonder if it is failing at high temps due to an internal failure that opens up, or even a wire that is doing so. A little time with a VOM and wiggling wires may help check for these faults.

I also have been planning for a long while to obtain a fixed resistor of the correct value for "moderately hot" temps, as an on-the-road substitute should I get a failure on a trip, or to diagnose other mixture issues. With the demise of the local Radio Shack, where else is a good source for this resistor?

[Lars S]

[raygreenwood]

Years ago I posted about the variable resister ballast I made for my 412. I finally found the part number for that resistor.

https://www.radioshack.com/products/1k-ohm-15-turn-pc-mount-cermet-potentiometer-trimmer

This is a 1000 ohm variable resistor with 15 turns/increments (about 60-70 ohms per turn). They are very small..maybe 3/4" long, about 1/$" high and about 1/4" wide. Its rated for low wattage but seemed to have no problem.

There is a 10,000 ohm model with 15 turns as well....600-700 ohms per turn.

https://www.radioshack.com/products/radioshack-10k-ohm-15-turn-cermet-potentiometer-trimmer

I mounted it on a small square circuit board that I put inside of a 411/412 headlight relay box. I also made one that could be plugged into the TS-1 inlet air temp sensor box for diagnostics. I will see if I can post a picture later. Ray

[raygreenwood]

Here are the pictures of that variable resistor mounted on a circuit board for tuning the TS-1. I have one similar on a skinny board for the TS-2

:

Ray