VDO head temp in a bus

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.
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.......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 plug.
......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.
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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:

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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&#3 would start out higher and the temp rise would be quicker than #2&#4. 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!)
--
-BaH
Dave Pearson
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Here is an interesting write-up on VDO gauges. Scroll down to the cyl head temp section: http://www.ratwell.com/technical/VDOGauges.html

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temp section:

......Thanks for that link Karl. I'm going to buy one of those Dakota Digital head temp gauges.
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head
..........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.
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Thanks to all for your input. I will try hooking up the CHT gauge again today. As advised I will use them for trend rather than actual. Plus they do fill up an unsightly hole the PO left in the dash.
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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.
--
-BaH
Dave Pearson
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.......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.
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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!)
--
-BaH
Dave Pearson
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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 vibration.
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- enlightenment stupidity!
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 it anyway.
Hope this all helps YOU in some way.
--
-BaH
Dave Pearson
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............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.

shutdown
you
...........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

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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 correct!
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 bus... (!)
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.
--
-BaH
Dave Pearson
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..........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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