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
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 sooty…with 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.
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
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
If it was running fine and this suddenly started happenning. I put money on
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...............
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
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).
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.
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.
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!
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