VW Diesel No Start and Cuts Out at High RPM
Posted to Technical Discussion Forum on 11/22/2008
14 Replies
Do you guys remember this one?
http://members.iatn.net/forums/read/msg.aspx?f=forum2&m=197656&fv=3&page=1
Well I've got an update, and fix for it. I'll warn you ahead
of time. This is a long one. Well here goes:
Subject vehicle: 2004 VW New Beetle 1.9 "C" BEW diesel
engine
We do some diagnostic work for a used car lot in town. They
asked me to look at this New Beetle with a CHECK ENGINE
light on, and we agreed with stipulations. The reason for
the stipulations were that we don't normally work on too
many foreign cars, and even fewer VW's. So the agreement was
to do a baseline check and get back to him. His complaint
was an intermittent "cranks but won't start". He stated that
if he turned the key on, and the "glow plug light" was
flashing, then the car would not start. But if the light
came on for a little while then went off, it would start and
run perfectly. Initial testing revealed trouble codes P0727
(engine speed input circuit no signal), P0674 (cylinder #4
glow plug circuit open), P0673 (cylinder #3 glow plug
circuit open), and P0672 (cylinder #2 glow plug circuit
open). We proceeded to test the glow plugs, and the car had
only one good plug. The other 3 had various amount of heater
element missing. The customer was informed of the
possibilities of additional problems and possible repeat
overheating and we replaced the glow plugs. We disregarded
the CKP code due to a comment on iATN that any time the
engine cranks excessively, that code will set. After
replacing the glow plugs, the car did not start right away,
but before we could check anything else, it started and ran
and continued to work normally for a couple of days, so he
picked up the car only to return a couple of days later with
the same complaint. This time the only code stored was the
P0727 (engine speed input circuit no signal). We found the
glow plugs to be working normally, using and amp probe, so
we began to look elsewhere. A simple resistance test of the
CKP sensor revealed it to be partially shorted according to
manufacturer's specifications, so we waited several days for
a new OE sensor. The OE spec is 1k to 1.5k ohms. Our old
sensor read about 650 ohms. We installed the sensor, and the
car did not start for several minutes. We spent some time
trying to decide what to test next, and a few minutes later
the car started normally. It would not act up again for
several days, so the customer drove it again, only to return
with the same complaint, as well as a new one. Now it also
cuts out intermittently above about 3000 rpm. Since it
didn't do that before the sensor, and we didn't notice it
after the sensor, we thought that maybe we were going to get
a break and be able to find something more consistent. At
this time, I tried to get my customer to go to the dealer,
but he would have none of it and wanted me to try to fix his
car. I enlisted the help of the guys on iATN, but apparently
this is not a common engine and I could get no concrete
answers to my questions. We did get a tip about the electric
lift pump located in the fuel tank causing an intermittent
no start whenever the pump did not run, so we tested ours
only to find that it was dead. We talked to our customer and
he said he would only buy it if that would fix his problem.
I told him that there was no way I could confirm that it
would be all of his problem, but that it was indeed dead. He
was afraid of getting in much deeper, since he was way over
his head already. Since I could get no definitive answers
from my friend on iATN, I had a heart to heart with the
customer again. He agreed to allow me some testing time to
try and confirm the lift pump issue. By now, the car was
dead and would not start for about a week while we were
waiting on him to make up his mind about that $500 pump.
I began my testing on a car that I knew little to nothing
about by starting with the basics. What we wanted to know
was whether replacing the pump in the tank was a necessity.
Doing a little research I discovered that this engine has an
electric lift pump in the tank that creates pressure to flow
through the fuel filter and into another pump, called a
tandem pump mounted on the end of the cylinder head, driven
by the camshaft. The tandem pump is basically a high
pressure pump mounted to a vacuum pump. Since a diesel
doesn't create any usable vacuum, it is common for them to
have an engine driven vacuum pump. Since I have seen other
diesels run and start just fine with dead lift pumps, I
decided to test this one to see if it would run with just
the tandem pump (high pressure pump) sucking fuel from the
tank instead of being fed pressure from the lift pump in the
tank. My first test was to apply a vacuum to the line going
into the tank. I could not pull a stream of fuel, but a
whole lot of air. Tracing it back, the problem was internal
to the lift pump in the tank. At this time, thinking I might
have a bad line in the modular assembly, we pulled the fuel
lines and discovered the fuel seemed to be a little thick,
similar to vegetable oil instead of diesel fuel. I removed
the lift pump and took a sample of the black liquid that
should have been clear fuel. Because we still didn't know if
this thing would run with a solid column of fuel, I put a
piece of pipe in the fuel line to give me a solid column and
proceeded to vacuum bleed the line to the engine. After a
little cranking, the engine still did not start. I was
beginning to think that the pressure from the lift pump
might be required, to get this thing to start. Also, I
wasn't sure how much air had been ingested into the
injectors and how long it would take to get it out.
At this time, I installed a pressurized container of diesel
fuel to the tandem pump inlet to simulate the lift pump. I
didn't know how much pressure was needed, but since my
interpretation of the manufacturer's test was that the lift
pump should produce .5 bar (about 7 psi) I started there. I
decided to let the fuel run for a while in case there was a
bunch of air in the injectors since I had already verified
that the inlet was not full of fuel. The car still did not
start. At this time, I decided to tap into the high pressure
port with my fuel supply. This factory port measures rail
pressure, and the only spec we had was with the engine
running. Since we already had a gauge in there previously
when the engine was running, we knew that it would idle
somewhere around 60 psi and raise with rpm up to about 100
psi. I put about 60 psi on it, and the car started
immediately. I tried revving it up at this time to see if
the cutting out at 3k was still there, and it was. So, when
it ran a little, I shut it down, and hooked the feed line
back up to the tank which still had the solid tube in lieu
of the lift pump. Since the engine was now running, and it
was late, I cleared all the codes and let the car run for a
while. Remember, what I was trying to determine was, whether
or not this thing would run without pressure from the dead
lift pump. I went home for the evening with happy thoughts,
and the next morning, the engine would not start. It was at
this time, that I called my customer and told him that
whether or not this pump fixed this car, there was no way
that it would run with this one, since it was obviously
sucking air. At that point he found a cheap pump from the
local Napa store, and when it showed up, we discovered that
it weighed half what the factory one did, because there was
no pump in it. It was just a sender, so he elected to wait a
week to get the OE pump.
When I got the OE pump, I proceeded to test it with the same
vacuum test that I used on the old one. The new OE pump
would not allow a solid column of fuel to be drawn to the
engine either. After the new pump was installed, the fuel
tank was drained of all the black liquid, and the system was
bled out and running, we still had to deal with the cutting
out problem at higher rpm.
We knew that it didn't do that before. We also knew that the
customer said it didn't do that before the new CKP sensor.
We also knew that the new OE CKP had the correct resistance.
After test driving with a VAG COM scan tool loaned to me
from a friend on iATN. Thanks Tony. I verified that when the
engine was cutting out, the rpm on the scan data was
faltering. Rather than just reinstall the old CKP I decided
to do some testing. I remembered previously when the engine
was not starting, we had the scope leads installed in the
CKP connector, and I tripped over the leads, jerking them
loose. Almost immediately afterwards, the car started and
ran. I also had discovered that each wire on the sensor
circuit had about a 2.5v dc bias. So I used this to my
advantage to do a wiggle test of the harness, but everything
was perfect. So I grabbed some patterns and removed the new
OE CKP and reinstalled the original (low resistance) one. A
comparison of the two sensors revealed a little different
configuration and one other thing interesting. The sensors
stuck together magnetically. I thought that it was possible
the reverse polarity of the magnet might be the reason for
the driveability concern. Is it possible the signal is
backwards? So I compared the two only to discover that they
were identical. The only difference was that now the car ran
great. I had installed the original "bad" sensor.
Trying the car over the next several days revealed several
things that gave me answers to some of my questions. When
the car was running, and the lift pump was unplugged
electrically, the car would die after it ran for several
minutes, presumably when the solid column of fuel from the
tandem pump to the tank turned into a mixture of air and
fuel. If the lift pump was disabled and the engine shut off
before the fuel column was used up, the car would restart,
at least within a few minutes. When a solid line to the tank
was installed, the car would not restart after setting over
night. When the lift pump was dead, and the old CKP was
installed (the one that made it run good) no amount of
cranking would set the P0727 code, or any other one.
Granted, I didn't crank it enough to burn up the starter,
but I did try it numerous times and even some very long
crank times. When the car was not starting with the old
sensor, the code would set easily. But here is the kicker,
even though it was setting the code, it was still toggling
the injector control circuits. The CKP low amplitude was
setting the code cranking, but was not preventing it from
starting.
I believe that we started out with an intermittent dead lift
pump, and bad glow plugs. Then installed a defective new OE
CKP sensor, and were then fighting air in the fuel system.
It's been about a month now, and the car runs great. Hope
this helps someone in the future. I'll follow up with a
couple of quick tests that I discovered in my venture.
If you get one of these with a no start, tap into the rubber
lines at the top front of the engine going to the tandem
pump. You should have fuel pressure of about 7-10 psi with a
2 second run KOEO, and anytime you are cranking or running.
If the lift pump does not run, fix that first. It is
essential. Then, on the LH end of the head there is a
connector that plugs into the head the feeds the fuel
injectors. The injectors are pumped up by the cam and
controlled by the PCM. Use an amp probe on the brown wire to
see if the injectors are being toggled. Ours were being
toggled even though we had a no start. You do not need the
CMP for this thing to run. Disconnecting it did not turn on
our CEL, but the engine did die if we accelerated to about
3000 rpm. Our tandem pump would put out about 50-60 psi
cranking, with a no start, and with the engine running. It
would ramp up to a little over 100 psi as rpm is raised.
Just because the resistance is wrong or right does not mean
the CKP sensor is good. It's all about signal amplitude.
Ours ran great at low rpm and started good once we got the
fuel system fixed, but it would cut out and or miss at
higher rpm consistently. It got a little better when warm,
and almost ran good at normal temp. The CKP has three wires,
but is a two wire variable reluctance sensor that has 2.5v
dc bias on each signal circuit and one shield wire. When
tapping into the CKP signal, the sync portion of the signal
from the brown wire goes up first, and the white wire goes
down first.
Here are some shots that I grabbed for your entertainment:
New OE sensor (BAD) at idle
New OE Sensor (BAD) around 3400 rpm
Good CKP (too low resistance) at idle
Good CKP (too low resistance) around 3400 rpm
CKP white wire and injector amps cranking
CKP white wire and injector amps cranking #2
Good CKP brown wire at idle
New OE sensor white wire (BAD) at idle with cmp
Fuel pump on bench
Message to everyone
Engine orientation
Tandem fuel pump
Fuel Sample
Fuel Sample in Glass container
Good and bad new CKP sensors
I'll have to get back to you on which sensor is which. It's
Saturday and my tech is gone.
Enjoy.
Scott from Missouri