I installed a VDO CHT gauge this winter. My understanding is that it uses a thermocouple so that the gauge reads the temperature difference between the ring that is under the spark plug and the outside temperature, and that they are calibrated for 76 degree outside temp.

Is the outside temperature the other end of the plug (which is in the engine bay at 120-130 degrees) or is it at the gauge itself (which is closer to the outside air temp)? I’ve seen both posted here. If it’s the engine bay, then I would need to add roughly 50 degrees to the gauge all the time.

I think my car is running hot and need help on figuring out why. It definitely runs hot (according to the gauge) in 5th gear with revs between 3000-3500. I know Jake preaches keeping the revs over 3500 in 5th. With my stock 1.8, how high an rpm can I run in 4th gear for a sustained period, say 70-75 mph?

I think it runs in the 350-375 range a lot which according to Jake, “350-375 is warm for flat running and may indicate some tuning issues, bad engine configuration or a cooling system thats missing pieces, etc.” I also think it’s gone over 400 degrees many times. Last year I drove it alot in 5th gear at 3000.

I’m not sure where to begin. I adjusted my valves 500 miles ago, all the engine tin is in place, the cylinder fins are clean, the timing is good, the thermostat functions correctly.

From what I've read on this board in response to the exact same question...that gauge is a piece of shit. it's not compensated for outside air temps, so it's never truly accurate. I have read the best gauge for this is by aircraft spruce and it's a DUAL gauge so you can monitor both left AND right side cylinders. It is also cheaper.

I have an engine that likes to sit at 350 and then glide slowly up to 400 on flat running going around 85 on the highway. I think it's my timing and carb tuning personally.

For your question: every hash mark is 50 degrees. That's all I can tell you.

Hope someone comes along and guides you better. Methinks your timing needs adjusting, just like mine does!

I am purchasing an oil dipstick with the temp gauge on top. I know, it won't let me see the temp while I am driving, but if my VDO gauge starts to peak near the red zone on my VDO gauge, at least I can poull over and see what the real temp is from a more accurate gauge.

I would advise everyone to NOT depend strictly on the stock VDO gauge. It is good for indicating changes from historical temperature ranges, but that's where it's value stops.

pardon my attitude, but how does oil temps tell you anything about head temps?

^^^

It doesn't.

Zach

But I'll know when my oil is toasty warm...

i also have a VDO CHT gauge in a euro d-jet 2.0L. the readings i get are in the 375-400 degree range which raby says is way way too high. the engine has recently been "restored" and has, maybe, 5-600 miles on it so i know the cooling stuff (engine sheet metal, cylinders) is clean (no rats nests) so, so far, i'm calling it "normal" (and hoping the VDO gauge just reads a little high) flame on.......

according to jake, your oil can be normal and your heads can be burning up and dropping valves.

CHT for the win

I recently installed a CHT. I have a 2.0L w/ dual Webers, and the car runs a constant 340 degrees. My oil temp guage takes 10 minutes + to come up off of the peg, and runs just between the T and E in Temp.

Yeah, but I'll know when my oil is..... Oh never mind. I'll look to going the CHT direction

I read somewhere to use VDO CHT gauges switch the plus and minus leads with the CHT wires. Basically put the plus on the negative terminal and the minus on the positive.

When I had the plus on plus and the neg on neg I run in the 290 -350 range

So, just to see which one was right, I pulled out my laser thermometer and the true readings were coming from the switched out ( backwards ) terminals I mentioned first.
I set around 190 -198 at 85 mph and around 210 -275 on a high load up a big hill in fifth. If I put it in fourth and keep the revs up it is about 200.

I find those CHTs hard to believe. Under 200 degrees at 85 mph?? Those sound like oil temps.

My gauge was wired backwards. I got [/u]no[u] reading when it was connected per the instructions, and when I reversed the wires, it worked.

you didnt discover anything, this is how they are supposed to be hooked up. says on the manual that comes with the gauge.

the laser temp thingy doesnt give you a good reading since your CHT loop is measuring UNDER the spark plug, good luck getting a reading in there.

Pull the loop dunk it in boiling water....read the gauge.....100C or 212F.

I don't think the original question was answered with regards to the VDO temperature measurement.

The displayed temp on the gauge reads accurate when the gauge itself is sitting in 76 degree ambient air. A thermocouple can only read differential temperature. So if the thermocouple is indicating a 200 deg differential to the gauge, and the gauge is assuming it is in a 76 deg environment, then it will display 276 degrees on the gauge, and that would be an accurate measurement.

But if the gauge is actually sitting in a 96 deg environment and is measuring a 200 deg differential, then the actual temperature at the thermocouple is 296, but the gauge will still display 276. This is because the gauge is non-compensated. Non-compensated means that the gauge has no idea what the ambient temperature is. In the case of VDO, it simply assumes that the ambient is 76.

In other words, when it is hotter than 76 in the car (where the gauge is located), the indicated temperature will be lower than reality. If it is cooler than 76 in the car, then the gauge will read high.

But if you think about it, you can really only read the gauge to the nearest 25 degrees or so. So as long as it's comfortable in the car (60-85 degrees), the gauge will be accurate enough.

So back to my original question, is my car running hotter than it was built to run, and if so, what should I check? I can get the gauge over 400 easily in 5th gear, and in 4th gear going up a hill. It seems that any highway driving will bring it over 375.

And back to MY original suggestion. Verify the reading on the gauge.

How? If I removed the ring end of the harness and removed the gauge from the car, I suppose I could make up a wiring harness between the ring and gauge and put it in boiling water and record the ambient temp. But that wouldn't necessarily tell me if it was accurate at 350-400 degrees.

Am I way off?

If I checked my valves and they didn't need adjusting, would that be an indication that the motor wasn't running too hot?

Can I suggest to get a dakota digital CHT gauge and stop all this nonsense? all this VDO thing does is create controversy and work like ass...

Which model numbers (for the sender and the gauge) have you used at dakotadigital.com?

http://www.dakotadigital.com/index.cfm/pag...prod/prd347.htm

installed in a 914

http://www.914world.com/bbs2/index.php?sho...c=71741&hl=

Thanks!

Yes that is the temp. I didn't mention that I run a mocal and a external oil cooler with a thermoswitch and a fan.'

My car runs cool even hammering on it without the extra cooler and fan.

Maybe I didn't discover anything, but I'm doing my best to relay what I done and try to help this guy out in some small way.