Vanagon mis-fire

Our 1981 air-cooled 1.9L fuel injected Vanagon had been running fine for the past two years since the engine was rebuilt. We have probably
put 2000 miles on it since then. Just recently it has started to misfire.
Traced the misfire to cylinder number 2.when I remove the wire to that spark plug she actually idled farily smoothly on the three remaining cylinders. First I tried the obvious by swapping spark plug wires, distributor cap, and coil (we have extras) with no change in the symptoms. Next pulled out the plugs and saw that they were all very sootywith the one from #2 a little more so than the others. Not at all oily, just black carbony soot. While I had the plugs out I checked the compression in cylinder 2 and it was at around 125 psi and holding strong. Whew, looks like valves and rings should be okay.
Put new plugs in and she ran smoothly again.let it run for about ten minutes with no change. Turned it off, went inside to get cleaned up to take her out for a test spin. Started it back up..misfire again. Pulled the plugs out and they are all sooty again.
She's obviously running rich? What would the next logicial course of action be? Check timing? Make sure all resistance measurements from ECU are within spec? Grounds are clean.
Thanks!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Bumbletown wrote:

Measure fuel pressure (28-35 PSI), high end when vacuum off. Measure #3 head temp sensor (2500 Ohms cold, ~100 Ohms hot).
Possible injector leaking?
Speedy Jim http://www.nls.net/mp/volks /
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Check all your vacuum hoses. Check the cold start injector - sometimes it stays on. You can unplug it at the injector no problem - just pull hard. If the injectors did not get replaced when the engine was overhauled, that is a likely suspect. As Jim said, the CHT sensor should have 2500 ohms resistance at room temp. Usually when they go it gets really rich as in black smoke and no go at all. After re-reading your symptoms I think I'd disable the coldstart injector first off and see if it solves the problem. Since all the plugs are dark, it could be something adding more resistance to the CHT sensor circuit like an almost broken off connector on the wire or even some corrosion. I don't remember if the '81's used the oxygen sensor for low speed fuel mix adjustment but if you have one and it's been over fifty thou you may want to change it. Check also for resistance in the exhaust, even a cat clogging up. I'm not sure where they put the coldstart injectors on the vanagons - on my '79 it is just ahead of the distributor closest to #4. In my case if it were either on or leaking, all would be dark but #4 would logically be the worst. If they moved it over to the other side on the early vanagons, that would make sense. Let us know what you find please. - BaH

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I have experienced this sort of thing with a misfire on a cylinder and it seemed that the problem was the wires were not snug in the injector plug.
As to the sudden running rich.I would bet money on the CHT. It is a cheap part...but when you install a new one, if you do, do not be tempted to put anything on it such as anti sieze. Instead, chase the threads with a tape in the head first. It is a weird size, but someone on the list knows what it is for sure, for they told me and i got one from my local tool retailer for about $5. Be patient, for it is tough to get to and thread in straight..........
If it was running fine and this suddenly started happenning. I put money on that cure.
But here is another for you..........have you swapped the fuel filter lately? If it is jammed up, the system has to supply more voltage to the pump, and could be causing this. I experienced this yesterday in my 76, and now i have a leaky injector, too...............
--
Adam P
Charleston,SC Coburn, WV
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Vanagon Man wrote: <SNIP>

<SNIP>
I'm not being picky and this doesn't detract from any of the good advice you gave. But just to clarify a bit: The electric fuel pump *always* gets 12V and essentially delivers a constant volume of fuel to the fuel ring.
The **pressure** is determined by how much fuel is shunted back to the tank. This is controlled by the fuel pressure regulator. This is a mechanical valve which opens and closes slightly in response to pressure. Normal pressure is about 35 PSI. When there is manifold vacuum, the setpoint is altered slightly by a vacuum control in the valve and pressure drops to as low as 28 PSI. The valve modulates this pressure (and thus mixture) depending on engine load (vacuum signal).
Speedy Jim http://www.nls.net/mp/volks /
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Thanks Speedy Jim and BaH for your thoughts.
Didn't have a lot of time to play with the old girl yesterday but did manage to check a few things. I had cleaned off the spark plugs again the night before so I started her up and she started and ran really well for about ten minutes. Then she started sputtering and mis-firing again. Pulled the plugs and they were all sooty again with #2 being the worst.same as before.
Let the engine cool and checked the resistance on the cylinger hear temp sensor. Started off at 1000 Ohms and increased throughout the evening (as the engine cooled I suppose) to 1850 and then I checked again this morning after the engine had a chance to cool over night and the reading was at 2550. Pretty close to your value and the one I got from the Bentley guide.
While I was at it I checked resistances of the rotor, all of the Hall effect gadgets, and injectors and they all fell within spec.
This weekend I'll pull the injectors out to see what kind of spray pattern I'm getting and also the verify that I don't have a leak. I will also check the cold start injector as you had suggested.
Just to verify, if I unplug the cold start injector that will cause it to stay in a closed condition not injecting additional gas assuming that it is not leaking of course.
Thanks again for the advice.
marc
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Bumbletown wrote:

Usually the sensor is either good or bad, but when Hot it should go lower in resistance- down to about 100 Ohms. I don't know if the engine was at full running temp when measured, but it's worth looking at again.
One trick is to substitue a variable resistor (volume control) for the sensor. Get a 2.5K or 5K or even a 10K pot. Wire a ~100 Ohm resistor in series and connect from ground to the sensor wire.
Start at 2500 Ohms when cold, then turn the resistance down as the engine warms up. Take it for a ride and turn all the way down. See if that made any difference.
Speedy Jim http://www.nls.net/mp/volks /
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Gee Jim, now you're giving away "speed secrets!" (How to get an otherwise stock bus to go over 100mph under its own power!) Okay, seriously now... What I wanted you to unplug the cold start or "5th" injector for is to confirm that it is not being held open by the circuit that activates it, the "thermo-time switch" which is notorious for failure. As to leaking injectors, I know it happens but have not experienced it myself.
Along the same line of thought as the fuel filter and whether the injectors were replaced when engine rebuilt, were new hoses installed throughout the fuel rail (the path from gas tank to injectors and back to gas tank?) If a hose on the return side is partially kinked or crushed or even breaking down inside creating a blockage, it will increase fuel pressure. I have had readings of over 100 psi before realizing I had forgotten to remove my fuel line clamp on the return line after servicing. If the vacuum hose for the fuel pressure regulator has detached or is kinked, fuel pressure will be about 30% higher than it should be in non-load conditions, but the idle speed should be higher also if this is the case.
I just checked my Bosch book and saw that the '81 Vanagon does indeed have a Lambda loop in the fuel injection system. If it is faulty, the engine may be running rich due to receiving an improper signal voltage. They are expensive, so check everything else... If you have a wrecking yard nearby, they probably have a bunch of them reasonably cheap - just to verify that it does or does not run the same with a different oxygen sensor - but then again, they don't last forever and the wrecking-yard one could be as shot as yours (if it is.)
If you've never read my vacuum hose posts before - Take physical hold of each and every hose in the engine compartment at each end and confirm that it is firmly attached and that it is not decayed so badly that it has started breaking apart. This is very important as the pre-formed hoses appear to be properly attached even when they have slipped off of their attachment nipple. On a FI engine, vacuum leaks behave differently than on carbed engines and it is not always totally predictable as a combination of incidents may all be contributing to the symptoms. Typical of the detached vacuum hose is any of the large hoses attached to the big "S" hose between the airflow meter / air filter and the throttle body which may come loose while crossing railroad tracks. Usually the engine will die and not restart. Usually it will cause a lean condition. Usually but not always, depending on which hose and where and what else!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.