Now that I've done some research here on ow the VDO cylinder head temp
gauge works I wonder if I did a bad thing.
I have a 1971 Bay window bus with a sunroof and that means it has a
belly pan which makes it almost impossible to run wires along the
frame, so I have run all of them inside the interior down the left side
and of course that left me about three feet short on the supplied lead
length for the gauge.
Thinking I was doing an ok thing I just spliced the needed length and
checked polarity and hooked it up to the gauge.
Then I found it too difficult to reach into the #3 plug hole so I
attatced the sensor to the intake bolt, well that spot is of little use
I now know but the gauge needle never moved at all.
Thanks for your input.
.......The wire from the gauge to the connection with the sensor lead can be
lengthened I would think. The voltage is low enough so that it shouldn't
matter. The intake bolt location for the sensor is no good though. Under the
#3 spark plug is where you need to put it. Just bend it so the ring shaped
part is at an angle with the shank portion so that it'll fit down under the
......I don't have much faith in mine. For a long time, I thought that my
head temps were way too high until I checked with an IR temp gauge and
discovered that the VDO was reading between 75 and 100 degrees F. too high
in the 350+ range. Oddly enough, the IR gauge and the VDO agreed at 250 deg.
You'd think that VDO would have calibrated them at about 400 instead of 250.
Yeah...they're an odd device to say the least...Actually because it's a
'thermal coupling' device..the temps vary with the ambient
temp..opposite of what you'd think...most are 'zeroed' for proper temp
at 68F....anything lower..and it appears to be running hotter...higher
ambient temp makes it appear cooler!!..Very strange...wish they could
develop a more precise, economical gauge. Pat
Tim Rogers wrote:
They're just a novelty item unless you run either a dually on a centrally
located head bolt on each head, or a quad, preferably using the sparkplug
sensors. When I bought my quad, it was about the same price as two VDO
singles, but came with 48" stainless steel wrapped leads. With the quad
gauge, you can, at a glance, see if any cylinder is out of range of the
others. Ambient temp doesn't matter because it affects all 4 equally and
the actual numbers mean little to nothing. Typical of my '79 bus, #1
would start out higher and the temp rise would be quicker than #2.
Once up to operating range and pulling hard, they would all fall exactly
in line with each other. Cooling down went the same way. I actually honed
#3 cylinder an extra thousandth and it ran very close to #1. Prior to
that, it ran 50 degrees hotter all the time.
Before the quad gauge, I ran two VDO senders: one on #1 & one on #3. I
had a single gauge and used a toggle switch. I usually monitored #3. With
a brand new motor, it was #2 that the injector backed out from and seized
up. If I had been running a dual gauge I might have seen the right side
running hotter. With the quad gauge I definitely would have seen it.
I highly recommend the quad CHT gauges. For FI engines, a CB mixture
gauge with the oxy sensor in the #3 header runner (between the exhaust
port and the heat exchanger) along with a quad CHT gauge will tell you
everything you nedd to know to actually save your engine as they will
indicate what is going wrong in time to avoid calamity, if you're paying
any kind of attention. I was always anti- oil temp gauge, but have since
figured out that the actual failure of my last T4 motor would have been
indicated by an oil temp gauge but was not caught by the CHT / Mix combo.
The failure was in the bottom end (where T4's are never supposed to
fail!) When the oil temp started to go out of normal range, I would have
consulted the CHT and Mix gauges and determined that I had either a main
bearing or rod bearing heating up. I probably could not have saved it at
that time, but would have at least had something rebuildable!
Put your single VDO gauge in a pan of water on the stove. Watch the
needle on the gauge rise as the water reaches boiling. Now point a hair
dryer at the connector end of the sensor wire (not the sensor end which
is in the water!) As you heat the connector end, the reading on the gauge
drops. When in actual use, if your running on hot blacktop and your
engine is running hotter than it should, it is very likely that your
single VDO CHT gauge will have a reading lower than normal.
With the 48" leads on the Westach Quad gauge (from Aircraft Spruce) I was
able to put the connector ends on top of my tranny. I also placed a
sensor for a thermometer there. The Westach gauge is calibrated for a 75
degree cabin temperature. That means that when the connectors were at 75
degrees, the gauge was calibrated to read correctly. If the temp on top
of the tranny was 100 degrees, the gauge would read 25 degrees lower than
actual temp. If the temp on top of the tranny was 50 degrees, the reading
would be 25 degrees higher than actual. The only times it was drastically
beyond the range of +/- 25 was when parked. In summer, the tranny case
absorbed heat from the engine and at times the top of the tranny was over
200 degrees when not moving. Likewise, in winter, I saw the thermometer
in the teens more than once, but once underway, it warmed up to the 50/60
range and likewise in summer, cooled to the 70/80 range.
The important thing to remember is that all four readings are affected
equally. You still look for one running out-of-range. Besides that, where
I drive, the ambient temp is almost never above 80 or below 50 for very
long at a time. It was interesting to see the actual temp trends, but of
absolutely no value in monitoring safe engine operation. Freeway
operation saw temps rise and fall more than 200 degrees in half a mile as
hills were climbed and crested.
As important as the proper gauges is the ability to make on-the-fly
adjustments. Adding a slide-type adjustable resistor to the FI CHT sensor
enabled me to enrichen the mixture as temps increased and when hard pulls
were encountered. It also enabled speeds of over 100mph - coupled with a
dashboard mounted momentary switch wired into the cold start injector, it
enabled reaching that speed rather quickly!
(Not as quickly or smoothly or effortlessly as the Subey though!)
..........I just placed my order today. They have a special sensor lead for
acvws that is 18 feet long and has the correct 14mm ring for sitting under
the spark plug. Its number is SEN-11-8 but doesn't come up on their website
so I had to place my order over the telephone. My total expense is about
$160 which includes about $25 for 2nd day UPS from South Dakota to NY.
.......It's going to be great to have an accurate cylinder head temp at last
to look at while driving.
Huh! I guess the simple basic lack of understanding runs much deeper than
I possibly could have imagined. I wish you the luck that I didn't have:
The luck that the cylinder you place your monitor on is the one out of 4
that fails! For that much $$ you could have had a system that actually
works. Ah, well. I guess it's time to go back into hibernation.
.......Sorry Dave, but my brain could never process four Westach CHT
readings while talking on the cellphone and eating a Big Mac and playing
with the adjustment on that dash mounted potentiometer and lastly trying not
to wander across the centerline into oncoming traffic..<g>...... My Berg
1679 has about 20K miles on it now and still has no changes in exhaust valve
length during adjustment/checking so I'm not too worried about whether 1, 2
& 4 are acting up for now. This Dakota Digital CHT gauge really does look
like a good upgrade over the VDO unit and I'll report back on how it's
working maybe next week after I get it installed this weekend.
That's the entire point! 4 needles all pointing the same direction.
Nothing to process, nothing to analyze! Three needles pointing the same
way, one pointing a different direction, INSTANT recognition and a need
for further analysis. A number on a digital readout requires analysis
every time it is referred to. That analysis means little-to-nothing
without further data to compare with for a benchmark. Simple.
I am not picking on you Tim, really! Just try to understand that you have
more like 4 engines back there on a common crankshaft - because of the
design of the engine. I had a wonderful engine working perfectly and the
two cylinders I had the VDO CHT gauge on were working exactly as expected
when the injector seal on #4 slipped, the injector worked loose, and
brand-new #4 piston welded itself to brand-new #4 cylinder!
The needle for #4 would have gradually gone out of alignment with the
other three needles on the gauge. As it got a significant distance out of
range, I would have shut down and worked at it until I found the problem.
When the rag I use to wipe the oil dipstick falls down into the
crankshaft pulley, first I usually notice the vibration, but sometimes it
wraps itself quite uniformily and doesn't shake that much. All 4 needle
pegged or quickly moving toward pegged is an indication of all cooling
air failing! (Rag in crank pulley / T4 fan!)
Okay, guys. I guess I must be the one who doesn't understand! I'm going to
start using your CHT theory for other checking / monitoring on my bus: From
now on, I will ONLY check the air pressure in the left front tire. That
will obviously give me all the information I need, as all four tires are
attached to the same bus, so I only need to check one.
I think it will work for maintenance also. From now on, I'm going to buy a
single spark plug when I change plugs. As long as I change the one in the
left front of the engine, all the others will be okay.
Maybe it will work for an oil change, too: If I put in one of the five
quarts that will be the same as putting in all 5 quarts, right? Should I
put in the first one or the last one?
I think I'm going to start selling CHT gauges. I'm going to use a voltmeter
and just change the face on them. I'll adjust it so that it always reads
around 350 degres F but add circuitry so it changes around +/- 50 degrees.
I'll put a blue LED in for backlighting. It'll be much simpler to install
than the other CHT gauges and will be a nice thing to look at as you drive
along! It will also be 100% a effective as what people are installing.
I've been taking a totally wrong approach. I've been trying to help educate
people on the matter of CHT gauges when I should have been marketing to the
ignorance. What I want to know is, WHAT is the proper CHT for your engine?
I want to know what magic number you need to see on that gauge. Okay. Now
you have a "magic number" that is the temp you expect your cylinder head to
be. When is there a problem? When the magic number rises by 30? By 100? By
300? How do you know?
Okay, you have a microprocessor calibrated and corrected "magic number" -
so what does it mean without any other reference? It means nothing more
than a pretty gauge face to look at while driving!
If you are so "lucky" to have placed your sensor at the cylinder that
develops a problem, good for you! If you have your sensor on #3 and you
have anything that causes a lean condition on #4, it will NOT show on your
#3 gauge. Even worse, if the failure is on #1 or #2...
Now an exhaust temp gauge will tell you if any of the four has a problem
and it reacts quickly like a CHT gauge (with sparkplug sensor). I really
must recommend that if you are only going to have one gauge, make it one
that will actually tell you something that will be meaningful and perhaps
save an engine. Install an exhaust temp gauge. This way, if ANY or all
cylinders have a problem, it will show on the gauge. You will then shutdown
the engine and diagnose the problem. If you had a 4 cylinder CHT gauge, you
would already know which cylinder actually has the problem, but you chose
not to do that.
If you use a two cylinder CHT at least you can use a head bolt on each bank
and although the reaction time is much slower, you will be able to monitor
the entire engine. The under-plug sensors have the quickest reaction time.
They will be affected by different heat-range plugs, by whether you use
anti-seize on your sparkplugs (a cotroversial subject in itself), by how
tight the plugs are, and the old problem of how much the sensor wire is
grounding on the tin after the first time you change sparkplugs or from
These are all problems that I have encountered and solved and shared my
findings with RAMVA over the years. It cost me close to $6,000 over time,
until I finally understood what a CHT gauge can do for me and what it
can't. Scott Fraser started me off in the right direction many years back,
here in RAMVA. I couldn't comprehend what he was trying to explain to me
then and he warned me that people just don't understand!
Bottom line: 4 cylinder CHT gauge with 48" sensors to put connector ends in
side cabin area. Ignore the numbers on the gauge faces. All that is
important is the #1, #2, #3 & #4 designating which cylinder the particular
gauge face relates to. Now watch the 4 needles rise and fall as you drive
along. What temp is right? IT DOESN'T MATTER! Watch how they relate to
going up hills and down hills. Watch how they relate to EACH OTHER! Watch
for trends. If one stays at the bottom while the others rise, there is a
reason. If one rises beyond the others, shut down and find out why! If
every day they have a similar pattern and one day it changes, shut down and
find out why!
As I said in an earlier post, a 4 cyl CHT, a mixture gauge properly
installed, and an oil temp gauge, properly installed will tell you
everything you need to know to save your engine. Here's an option: install
an exhaust temp gauge and put CHT sensors under all 4 sparkplugs, hooking
them up to a rotary switch so you can select each one individually. You
don't need a CHT gauge, just a voltmeter with a low enough range to read
the output from the thermocouples. If you see an abnormal trend on your oil
temp gauge or your exhaust temp gauge, click through the 4 CHT sensors and
see which one is significantly different from the others. (BTW - expect to
always get about 50 degrees higher reading on #3 than #1, and about another
50 degrees between front and back - that is, 350 on #3 will be 300 on #1
and 250-275 on #2&4 - if everything is working normally!) ((At peak
operating efficiency, they will all come together - as in on a long hard
pull, if everything is working properly, your highest reading will be equal
on all 4. As you start down the other side of the hill, they will cool to
the approximate differences I just noted unless there has been a
modification done to alter that.))
C'mon now - if you have money to throw away, just send it to me - at least
I'm trying to help you save some bucks from my own costly errors and pre-
There are, of course, other issues such as why was I driving a bus over
100mph anyway, did I take all the steps in setting up my engine that I
should have - ie: if I'd done a proper job building the engine, would it
have lasted and would I have needed the CHT gauge in the first place? Were
any of my failures directly related to what a quad CHT gauge would have
told me about? Definitely YES to that one. The ultimate failure that
converted me to the Subey was related to just plain making the engine do
what it was not designed to do - I over-revved it when it was new, but
still got over 300,000 miles out of that bottom end - a tribute to the T4
motor. An oil temp gauge would have told me of the main bearing failure,
but when it happened I was in a state of mind that I would not have heeded
Hope this all helps YOU in some way.
............Nothing magical about an accurate number. The VDO gauge, as
everyone knows, is way off in that regard. This other gauge from Dakota
Digital may prove to be reliable and accurate. I have a Fluke IR temp gauge
to use as a check on its accuracy. I'll come back with that information
later....maybe next week. The actual temp of an aluminum head is useful
because if you go over lets say 450(?) deg F., the head is probably ruined
because of the metallurgical changes that can result in cracking or a
dropped valve seat. The aluminum gets soft and brittle after overheating.
Bob Hoover has written some useful information in regard to that.
...........I run an A/F ratio gauge that would also tell me if one cylinder
was running excessively lean or rich for that matter which is also what
happens when an EGT gauge's reading goes up or down. The problem with that
is that those readings are constantly changing (at least with the L-Jetronic
EFI on my bug) when load and rpms change. If I'm out on the open road, the
readings are more constant. But, most of my driving is around town and in
traffic with a lot of acceleration/decelleration along with uphill/downhill
stretches of road. I'm not too sure that I'd spot a real change in the A/F
ratio in time to prevent a problem like you encountered with the injector
seal. Maybe I'd see it and do the right thing in time.....probably not
though. If it's any comfort to you, new injector seals are on my list of
things to install pretty soon. I did use that A/F ratio gauge to figure out
that the L-Jetronic's CHT sensor might be bad last fall. My mixture was
going off the scale lean as the engine was cutting off during accelleration
in a sporadic manner. It turned out that the sensor was grounding out some
of the time and working OK other times. A new sensor (Bosch #0-280-130-003)
has eliminated the problem and I'm running trouble free so far this spring.
If you use a two cylinder CHT at least you can use a head bolt on each bank
What you said about the mixture gauge constantly changing - pretty much
the nature of the L-Jetronic system - HERE is the REST of the story:
So I've got my CHT gauge on #1 & #3 and figure I'm really smart. I watch
the mixture gauge, and like Tim said, it goes rich / lean / rich /lean
and I got into the habit of adjusting the mixture as I'd drive up and
down hills on the freeway. If the mixture gauge started to stay more
toward lean, I'd richen it. After all, it's a new engine, just breaking
it in, and things are gonna change - right?
What was actually happening was a trend that should have told me to shut
down and check things over. I did flip the CHT over to #1 and back to #3
and there was no significant difference, so I just kept enrichening the
mixture until the gauge reading was right. As the air was gushing in to #
2 cylinder and it was getting leaner and leaner, I was adjusting the
mixture on the other 3 cylinders to make the reading on the gauge
I can only assume Tim is smarter than I and won't start treating the
symptoms if it ever occurs. It's easy (at least for me) to do a stupid
thing when you start getting tunnel vision and don't make an effort to
understand the bigger picture.
Tim, I know you understand all that I have said. As for what the magic
number is, there really isn't one. I ran with temps between 500 and 600
as indicated by the VDO gauge when I first started playing with them
years ago. I ran for two summers of heavy driving with those kinds of
temps and was always fearful of the impending meltdown. It never came,
but a good friend checked it out for me and informed me that my
compression ratio was not quite what I had been told by the other
machinist. I was told it was 8:1 when in reality it was 10.9:1 and that
was why I had the high temp issues. 70 to 90 on the freeway was a blast
though! Don't know how fast I really got, but on the bus speedometer the
needle was passing the "M" in MPH which is beyond center at the bottom of
the gauge and about an inch-and-a-half past the highest number which is
90, about half-an-inch from the zero pin!
Now I have an engine that has peak HP output at 5300rpm, is balanced, has
5 real main bearings and will rev effortlessly to 7300rpm. It isn't a
challenge to go fast anymore, so I have found that my driving has really
mellowed out. I still accellerate up every hill just because it's a
Thinking about putting dual EGT gauge on the Subey just for the helluvit!
You're right about the "neat-o factor" & having something to look at!
Interesting that the L-Jetronic CHT sensor was causing a lean condition.
I have had several go bad and all went rich - very, very rich! Also had
the thermo-time switch activate the coldstart injector while driving in
traffic. Diagnosis was purely luck on that one. About thirty seconds and
back on the road again.
..........I was able to see the A/F gauge go lean when the engine was
cutting off. I run a fuel press gauge so I knew that it wasn't that. I say
grounding out but maybe I've got it backwards (opening up?) Whatever it was,
a new sensor eliminated the problem.
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