'73 Type 3 Air/Fuel Mixture D-Jet

[fancy pants]

So, I finally got me a squareback (1973, stock FI) and am in the process of easing her back into being a daily driver after being in storage for 2 + years. I have done a complete tune-up, which cured most of the bucking and rough idling. Compression is 135-140 all the way around. Obvious vacuum leaks repaired. Bucking/rough idle had still occurred after the tune-up until I disconnected Temp Sensor 1, which improved (but did not totally eliminate) both symptoms.

Now, it has a rough idle for 2-3 minutes, which then smoothes out, and bucking which occurs rarely - usually at highway speeds. There is also a very fast deceleration (jerking) as soon as I back off even the slightest bit on the throttle. Finally, it seems that power at the upper end of revs is lacking. Starts off strong, but not much there at the higher end of the gears. I can get up to highway speed, but fall back on hills. My speedometer does not work, but I don't think I have been able to get above 65mph. Seems strange.

With these symptoms in mind, and timing, dwell, valves spot-on, I decided to take her in today to be checked on a gas analyzer. HC's were at 900+ and CO 4%, up to 7% with revs.

Now what???

[fancy pants]

Update:

I found that the throttle spring (remember how goofy that was, Gypsie?), was actually 1/2 as long as it should have been. I put in a new spring and the jarring deceleration is cured. I also found out the the vacuum advance unit on the distributor was incorrect. It looks like it may have come from a bus (nipple on left side). I put the correct one on (nipple on right side), which now does not rub on the generator (remember that arcing, Gypsie?). It seems that there is a bit more high end power now.

Occasional bucking continues, and it still seems like there should be just a bit more power at higher revs. Additionally, with the vacuum advance changed, I now get an idle that dives when I let off the throttle before it returns to baseline.

Thoughts anyone?

I found someone in town (DDB) who has a genuine VW D-Jet fuel injection tester. It plugs right into the ECU and you dial in the year of the vehicle and it analyzes your system. I am having this done at 10am Monday. Can't wait!

[bretski]

Just a couple of thoughts...

When you did your tune-up, did you do a points/condenser replacement? How old is the coil? If it's original, it's probably getting tired.

[fancy pants]

Yep - new points and condenser. Dwell is dead on.

The coil, on the other hand - hmmmm.....

[bretski]

Another thought: check and clean all of your ground connections (the root of all FI evil )

[Amskeptic]

Yep - new points and condenser. Dwell is dead on.

The coil, on the other hand - hmmmm.....

The fuel pressure is adjustable. The regulator is located on the firewall. A 13mm locknut and a 10mm adjustment bolt. Clockwise tightens the spring and increases the pressure. For fun, count a half turn in and test drive. This will tell us if the condition relates to a lean mixture (or if the symptoms worsen, a rich mixture).
Colin

[fancy pants]

The fuel pressure is adjustable. The regulator is located on the firewall. A 13mm locknut and a 10mm adjustment bolt. Clockwise tightens the spring and increases the pressure. For fun, count a half turn in and test drive. This will tell us if the condition relates to a lean mixture (or if the symptoms worsen, a rich mixture).
Colin

Update:

Got the FI system tested today, using the VW 1218 tester (not the Bosch tester described in Bentley) and found failure at tests 9 (start-up and warming mixture) and 12 (pressure sensor malfunction). It looked like someone had tried to adjust the pressure sensor, so I replaced it with an unmolested one and the difference was night and day. It now runs smoother, starts up easier, has more power and the bucking is nearly (but still not quite) eliminated. After replacement, I went through the FI test again and still had failure with "start-up and warming mixture".

I am still experiencing a dive in RPM when lifting off the throttle. It dives down to about 300 RPM (maybe less) for a second and then returns to normal idle. One thing that I did notice is that I still may not have the correct vacuum unit on the distributer. Should I have a dual-advance unit with a 73? Would that explain my symptoms?If so, I can't find anywhere on the plenum where retard would go. Hmmmm...

Will adjust fuel pressure per Colin's advice next.

[Maz804]

Hi John!
It was nice meeting you and your family last night at the lab.
As per your request: my dizzy has only 1 vac port and everything seems to be in good working order. I have yet to take her on the highway to see how she performs though... Needs to get the brakes functioning first and foremost.

[Manfred]

Update:

After replacement, I went through the FI test again and still had failure with "start-up and warming mixture".

How are your vacuum hoses? I have found that a good idle has a lot to do air mixture. If you are running lean, it will be hard for you to keep a decent idle until your engine warms up.

I've had trouble getting my 78 bus type IV FI motor to idle and hunting down vacuum leaks has cured my problem.

[Amskeptic]

I am still experiencing a dive in RPM when lifting off the throttle. It dives down to about 300 RPM (maybe less) for a second and then returns to normal idle. Will adjust fuel pressure per Colin's advice next.

Warm or cold or all the time?

Idle dive is often insufficient air flow or too lean. Get idle speed (air bypass screw on the plenum) to 900-950 rpm minimum, make sure timing is correct for your engine! Then let's see if a little extra fuel pressure helps. . . or hinders.
Colin

[fancy pants]

Warm or cold or all the time?

Idle dive is often insufficient air flow or too lean. Get idle speed (air bypass screw on the plenum) to 900-950 rpm minimum, make sure timing is correct for your engine! Then let's see if a little extra fuel pressure helps. . . or hinders.
Colin

The idle dive happens consistently - hot or cold. RPM s 1000 and timing is on (5 deg ATDC). Haven't tried the fuel pressure adjustment yet, but will do so next time I open the engine lid.

I can't verify that ALL vacuum hoses are leak-free, but I have replaced all of those with apparent problems. Will do a more detailed vacuum leak hunt when time allows.

[Manfred]

I also found out there is a very thin gasket at the throttle body.

"Throttle Valve gasket fits between the throttle valve housing (aka throttle body) and the intake air plenum. Genuine. Also interchanges with VW part number 025 133 073."

Seems like this gasket is so thin it could easily need to be replaced every 5 years or so.

FYI this is for a bus. But I'm assuming your FI works the same way as mine.


http://www.germansupply.com/home/custom ... 388&page=1

[Amskeptic]

FYI this is for a bus. But I'm assuming your FI works the same way as mine.

They are distinctly different animals when it comes to vacuum leaks. The D-Jet meters fuel based on manifold vacuum, and the bus L-Jet meters fuel based on airflow above the throttle plate. If a D-Jet has a vacuum leak, the mixture richens up. If a L-Jet has a vacuum leak, the mixture leans out.

The D-Jet plenum has a stamped steel end plate gasketed to a cast aluminum plenum that sometimes develops a leak, the L-Jet plenum itself is stamped steel welded at a center seam. While the throttle body-to-plenum cone shaped rubber gasket clamped by two screws does leak on a bus, the Squareback gasket rarely does as it is a flat gasket nicely clamped by four nuts.

I respect both systems enormously. By the way, apply vacuum to the manifold sensor via its hose from the plenum and make sure that the sensor holds vacuum. The pleats in the bellows can split and leak, giving you a rich lopey idle.
Colin

[fancy pants]

SOLVED!!!

After 3 weeks or so of chasing this thing - tune-ups, pressure regulator replacement, replacing bellows, checking vacuum in manifold sensor, adjusting idle speed, adjusting mixture - I finally figured it out. In frustration I pulled the ECU and found that it had been replaced with a D series, which I believe are for 71-72's. The 73 models used an E or an F series. Found a good E series ecu in town, swapped it, and voila!

No more bucking, no more idle diving, runs smoother (quieter even!), more power... So this is what a Type III is supposed to feel like! I was beginning to wonder what all of the fuss was about...

[Amskeptic]

SOLVED!!!

After 3 weeks or so of chasing this thing - tune-ups, pressure regulator replacement, replacing bellows, checking vacuum in manifold sensor, adjusting idle speed, adjusting mixture - I finally figured it out. In frustration I pulled the ECU and found that it had been replaced with a D series, which I believe are for 71-72's. The 73 models used an E or an F series. Found a good E series ecu in town, swapped it, and voila!

No more bucking, no more idle diving, runs smoother (quieter even!), more power... So this is what a Type III is supposed to feel like! I was beginning to wonder what all of the fuss was about...

Congratulations on your perseverence and diligence and keen appreciation of the alphabet. They are lovely drivers yain't they?
Colin