Volkswagen: fuel injection, 78 bus, air fuel ratio, proper combustion

Question
QUESTION: Mr. Wolff,

I have read through your past answers and am thankful you donate your time.  Though I came close a few times, I have not been able to search past strings on this site, or type2.com, for an answer to my problem.  I have a '78 bus, all original, except for the electronic ignition which is a recent addition (which has not had an effect on the problem).  By necessity I have become fairly proficient at diagnosing the troubles with my van, but am currently stumped.  For the most part, I use the Bently manual.  Some time ago the gas mileage started to decrease.  I had built the motor only 30,000 miles prior.  I cc'd the heads to achieve the proper combustion ratio, and drove it very "cool" with the help of my head temp gauge.  It ran great!  I also use a CB performance air/fuel ratio gauge that assures me, my current problem is in the mixture.  Where I used to get 15mpg city, it's now 10.  highway was 20, but now closer to 14.  The ground connectors on the fuel injection are on tight to the best of my knowledge (I checked them), and the AFM checks out to the book's specs (I even tried a rebuilt spare I've been carrying around--no change).  The head temp sensor is new.  I am going mad.  I pulled the motor suspecting bad heads/rings, but there were no problems: they were tight as new.  And clean, too.  The man who honed the cylinders said that it looked like the rings were getting washed.  Indeed, I think so.  But wherein lies my problem?  Do you have any thoughts?  I don't have a spare ECU, but that'd be my next guess (though I've never actually heard of one of them failing this way).  Sorry to be so winded.  Holler if you have an idea for me.  Thanks!  -- Geoffrey
ANSWER: Hi Geoffrey,
Did you do a test on the injector output?  If so, here are some additional checks:

1) Run engine until oil temp is above 176 degrees
pull the wire off the temp sensor
connect ohmmeter, specified resistance: less than 300 OHM

2) Cold Start Valve Leaking

3) Intake air preheating control bad:   check that with the engine warm or ambient air temperature about 68 degrees warm air supply is closed.  

4) An injector may be open continuously:
Remove injectors but leave them attached to ring main
pull wire off ignition coil terminal 1
operate starter briefly and check if more than two drops leak from each injector per minute.

5) Full throttle contact on throttle valve sticking.
Pull one wire off full throttle contact - see if speed drops.

Hopefully some of these tips will help you out.

Diagnose these before even considering the ECU.

Let me know how you make out.

Ron

---------- FOLLOW-UP ----------

QUESTION: Thanks Ron!

I'll double check the head sensor, but i just replaced it.  I know the aux air regulator is good, and vacuum seems to be tight all around (I've sprayed carb cleaner around the best i could).  And I checked the cold-start and injectors somewhat recently, but could stand to do it again as a double-safety check.  I don't have a full-throttle switch.

Do you think it could be a weak coil?  It sparks, but is at least 10 years old.

Thanks,

-- Geoffrey

Answer
Geoffrey,

As a matter of maintenance, you should replace the coil, however I would not suspect that to be the problem.   I will presume you have the correct Bosch plugs set at the correct gap.

My main concern is the electronic ignition.  These are notorious for 'confusing' the fuel injection. (even if it worked well for a short time)!

The 'impulse wire' (the white wire connected to the negative lead of the coil) control the rate and duration which the injectors open and close.  The impulse is based on the opening and closing of the points.

However, with the addition of the electronic ignition, you have no points.   My guess in light of this information, is that your problem lies in the electronic ignition.

Note:  The stock points / condenser / system on the VW T2 is WAY over designed.   The original set-up in your bus can propel a Porsche at over 120 MPH without even breaking a sweat!

I would check the Wolfsburg West site and get the exact points that match your distributor number and revert back to the original system.

This should solve your problem once and for all.

Again, it is highly, highly not advisable to put an electronic ignition system on these VW's, they sincerely offer no additional performance with a stock engine and fuel injection system.  (except perhaps burning the plugs quicker).

Let me know how you make out!

Ron