TheSamba.com :: View topic

Sun Oct 17, 2010 8:58 pm

I have installed a VDO head temp gauge on my 2 liter dual carb type 4 engine. When I climb a hill that is a steady climb for about 3 miles it will read up to 500 deg F. When I am on on flat ground it reads about 350F. All other indications are that the engine is not overheating. I have and oil temp gauge that reads in celsius and the oil temp runs about 85 deg celsius. I can pull the dip stick and can put the oil on my hand without discomfort. My oil pressure is at 28psi at Idle and holds 45psi at 55mph. Could the head temp gauge be that far off. Oh and yes the sensor is mounted under #3 spark plug.

Sun Oct 17, 2010 9:04 pm

did you cut the ring off the spark plug? We had problems with the plug sealing up correctly with the ring cut off and that caused temps to increase. You could have exhaust gasses seeping out at the plug seat under high loads. Also look at the sender, if it gets ovaled it won't seal properly.

Sun Oct 17, 2010 9:47 pm

Yes I cut the ring off the spark plug as the instructions said to do that came with the gauge. Should I put the ring back on the spark plug along with the sensor ring?

Mon Oct 18, 2010 6:10 pm

Roger Larsen wrote: Yes I cut the ring off the spark plug as the instructions said to do that came with the gauge. Should I put the ring back on the spark plug along with the sensor ring?

Even with the ring cut off the spark plug I experienced similar symptoms to you, but not up to 500F! I'm sure it was leaking exhaust gases. I used a new sensor ring as mine had become distorted and coated the spark plug threads with anti-seize (I know, I should have already been doing that!! :wink: )

Mon Oct 18, 2010 8:13 pm

I have read a number of posts on this forum indicating that VDO cylinder head gauges are know for not being very accurate. Ratwell had a link that he did tests that showed them to read as much as 60 deg F higher than the actual temperature. I also read on the forum of doing the math to calculate the actual temperature. Is there math equation I need to know about?

Tue Oct 19, 2010 3:55 am

test the VDO CHT gauge in boiling water before installing it.

Tue Oct 19, 2010 1:09 pm

Bleyseng wrote: test the VDO CHT gauge in boiling water before installing it.
x2 :wink:

Tue Oct 19, 2010 1:59 pm

from experience, they can be wildly innaccurate. i had one in my bus for a while, read normally 350-400f and i accidentally fried it when i was working behind the dash and hooked it up wrong. i got a replacement and it would typically read 450-550deg, scared the hell out of me to drive at all. i put a DD in and im lucky to see 350 max.

Tue Oct 19, 2010 2:42 pm

VDO CHT gauges are not temperature-compensated & not very accurate... best used for trending and not for absolute head temps.

DD gauge is temp-compensated, as are several aviation CHTs.

Micro 1000 makes a nice gauge. Then there is the do-it-all MGL Stratomaster 8)

http://www.aircraftspruce.com/menus/in/chtgauges.html

Tue Oct 19, 2010 3:12 pm

I am feeling a lot better. My van runs so well and all the other signs look so good. I think I will look into another brand CHT temp gauge. Thanks for the responses.

Tue Oct 19, 2010 5:26 pm

I wrestled with this issue a few weeks ago. We purchased a 71 camper and wanted to take it out camping. During our test runs the VDO CHT (mounted on #3 with spark plug ring installed) hit 425F on flat ground. Yet no other indications were present of engine overheating; same as you I could hold the dipstick, oil P and T were great etc... So I went digging here, type2.com and Gene Berg's site and found the reliability of these gauges can be 200 deg F off! To test I got a candy thermometer and compared with a regular cooking thermometer in boiling water to check accuracy. I then heated up canola oil (I wanted a range of temps higher than just boiling water) in a coffe can to 400 deg, checking the temp with the candy thermometer. Then I placed the coffee can (1/4 full of oil - just enought to dangle the sensor in) under the bus and compared temp reading between candy thermomemter and VDO gauge at various times as the oil cooled to 200 deg F. VDO gauge was 75-100 deg off!! This is the chart we drew up for future reference.

VDO
Gauge \ True Temp
100 \ 54
150 \ 98
200 \ 141
250 \ 185
300 \ 228
350 \ 272
400 \ 315
450 \ 359
500 \ 402
550 \ 446
600 \ 489

With this information we took in camping in the mountains and she ran perfectly. Up hill I'd hitjust under 500 F on the VDO, and on the downhill she'd cool off immediately to 425F, which according to our chart was just fine. The oil (which I changed before leaving) had hardly gotten dark in over 200 mi of driving and I burned none. VDO gauge is really nice to have for trending, and certainly if it spikes to 550 or higher now I'll be suspicious!

Tue Oct 19, 2010 9:15 pm

Thanks Erik for the chart--Looks like a good reference

Tue Oct 19, 2010 11:14 pm

Huge. 500 even for a VDO guage is scary!

Tue Oct 19, 2010 11:35 pm

use a spark plug with a sealing ring on it and some anti-seize. Be sure the sender is not ovaled. I used mine without the ring and the got nervous like you and the temps went way down with a ring, although mine was never that bad. I thought that was strange BUT when I looked at the spark plug threads, I could see carbon all the way up to the body so that means it was leaking. It also can lean the mixture on that cylinder a little which is bad. One last thing - this discussion has come up before here and in other forums - usually one to four aircraft mechanics will chime in and either they mount the senders to the head OR under the plug WITH the ring ON.

Wed Oct 20, 2010 7:28 am

Quote: Huge. 500 even for a VDO guage is scary!

Tell me about it!! I've seen other people's tests with similar results tho. I think I will upgrade the gauge in the near future.

Quote: Thanks Erik for the chart--Looks like a good reference

Not a problem. I've seen other charts out there and they all are different, so don't assume your gauge has the same error.

Wed Oct 20, 2010 7:54 am

All this talk about how inaccurate the VDO gauges are is really a lot of people parroting what they heard. In every case where the VDO gauge was not reading the correct temp, there was a leak between the sensor and the head which leaked exhaust gas and gave an increased reading or the sensor wire was not good. Fix the leak and you get a good reading. Yes, the gauge is not temp compensated. This compensation is measured where the thermal couple wire ends and where the electrical wire starts. If this junction is in your engine compartment, then temp in the engine compartment is the temp that needs to be compensated for to correct the reading. Typically this can be the outside temp. I have checked at least 6 different VDO gauges using boiling water and none of them were off. If the sensor is not good, it will make a bad reading. A bad sensor wire is frayed or losing its insulation or is coming out of the ring. Make sure these things are good and the gauge should read reasonably accurate. I have never had a problem with a VDO gauge that had a good sensor wire and a good ring seal.

Wed Oct 20, 2010 8:51 am

My next plans are to make sure that my spark plug is not leaking. Maybe try it with the ring along with sensor. When I have the plug out I plan on testing the gauge in a similar way that Eric did. He said he tested his in oil which will let you get the temperature to the place you would need to be to test it. Testing in water will only let you go to a maximum of 212 deg F. It seems that the inaccuracy really starts to multiply as the temperatures go up higher than the boiling point.

Wed Oct 20, 2010 9:07 am

Quote: In every case where the VDO gauge was not reading the correct temp, there was a leak between the sensor and the head which leaked exhaust gas and gave an increased reading or the sensor wire was not good.

Bahh... no way can you make such a sweeping assumption. Just look at Erik's post above yours. And read the dozens of accounts of people that were running high CHTs with VDO gauges & how those temps dropped with a better gauge (often times using the same sensor mounting method and wiring).

As for temp compensation, IIRC the VDO is calibrated for 70F ambient temp... if the ambient temp is above 70F, the gauge will read falsely LOW and the difference b/w true ambient temp and 70F added to the gauge reading; conversely, if the ambient temp is below 70F the gauge will read falsely HIGH and the difference subtracted from the reading.

Wed Oct 20, 2010 11:49 am

And just as many accounts where other temp gauges also read incorrectly. Most people do not understand how the gauges work and how they get their signal. Therefore, if it doesn't read correctly, just chock it up to the gauge being no good rather than there is another problem. I am sure that there are some VDO gauges that don't read correctly but there are some DD gauges (and others) that don't read correctly also. But with those, there must be something else wrong. :roll:

Wed Oct 20, 2010 1:25 pm

volksterii has good points! Certainly VDO could make a bad gauge here and there, but why does the bad always seem to be the CHT? Most likely because the setup is the most difficult and it is easier to double check OT. As volksterii stated, the correct setup is imperitive. I made sure the the termo couple connection was out of the engine compartment and out of the way, checked for leakage around the plug etc... Best I can figure, all is good, but still want to check it once in awhile. If I could find another gauge (i.e. not buy), I'd try it with the same sensor wire setup for comparison.

I did want to mention that I chose oil as a test because many of the tests I came across that others ran with water did not show a huge difference between gauge readings and actual temp. Most of these errors become more obvious at higher temps. Oddly enough i did not see anyone testing a gauge other than VDO.