TheSamba.com :: View topic - How hot is too hot? (Cylinder Head Temps)

Tue Jul 03, 2012 5:09 pm

Ok, i know this has been gone over a million times. But I JUST installed a Dakota Digital Cylinder Head temp gauge this afternoon. Right now it is 91 degrees in Milwaukee, it's hot (as it is in much of the country this week.) After the install I took my bus on a few mile drive, through some city streets, on the highway for a a few miles, then back home on city streets. Here's what I found...

Idle 350 degrees
City driving 360-375 degrees
Highway 395 degrees (less than 2 miles on the highway at 70 mph)

Now i realize it's hot, but those numbers seem awfully high. Have i been cooking my engine the entire time i've been driving it? One issue I can see is that my heater controls no longer work so my only option is to have the Heat always ON, which makes the cab REALLY HOT, but I assume it also really hurts the engines ability to cool by restricting flow (is that a fair assumption.) I'm waiting for the engine to cool down a bit and I'm going to disconnect the heater tubes at the heater boxes and see if that helps cool the engine down (I know it will cool me down...!)

My questions are... are those numbers dangerously high?
Have I been damaging my engine all summer?
How big of an impact does ambient temps have on engine temps?

Thanks all!!

Tue Jul 03, 2012 5:25 pm

You may want to do a comparison with an infrared thermometer. Get the engine hot, read your DD, then pull the plug wire, and use your handy IR temp pistol to read the temp down around the head. Compare this to the DD reading. I'd suspect the DD is reading high, but I've been accused, and convicted of optimism in the first degree.

Tue Jul 03, 2012 5:37 pm

Thanks for the reply. I don't have an IR, though I suppose I could pick one up. I purchased the Dakota Digital CHT gauge because it was supposed to be the most accurate. When I initially hooked it up it read 94 degrees (which was the actual ambient temp outside.)

If the DD gauge is correct, how bad is this?

Tue Jul 03, 2012 5:43 pm

There's a possibility that combustion gases can seep out around the sensor ring, and make the readings abnormal. IR thermometers are 30 bones on amazon. Cheap insurance, and confirmation, plus cats/dogs love the laser pointer.

Tue Jul 03, 2012 5:47 pm

Not a fan of gauges because they will turn your hair gray over nothing. 400°F is pretty typical for highway driving, but if it goes much higher then be concerned. Of course it doesn't sound like you actually drove very far.

Tue Jul 03, 2012 6:06 pm

It has been said that 400 degrees for ten seconds will turn the tempered aluminum head into dead soft aluminum. but many things are said by people more concerned with appearing to know it all than with being right

Tue Jul 03, 2012 6:37 pm

It's hard to say for sure but if you only drove 2 minutes and hit 400F I hope that was uphill with you foot in it. Please give us a detailed description of how you installed the sensor on the motor. Like others have said (and I've personally experienced) if you install it the way DD says (under the #3 plug) it is very hard to get a good seal where the combustion chamber heat won't skew the hell out of your readings because the ring get's damaged as you torque the plug.

Tue Jul 03, 2012 6:51 pm

While we all struggle with head temperatures we can note one thing as far as a type 1 bus engine is concerned, without the cooling vanes spot welded on the inside of the cylinder tins, you will always have that doubt about temperature on the cylinder head. The type 4 I guess has different issues concerning head temperature but that is what I know about the type 1 motor.

Tue Jul 03, 2012 7:32 pm

Well here's a bit of an update.

I checked the sensor. It really is a PITA to get in there without mangling it. I removed it, straightened it as best I could and did my best to get it in and get the plug good and tight. I then went for a reasonably lengthy drive.
This is what i found:

For the first 6 or 7 minutes I thought I had solved it by reinstalling the sensor and plug, as the temps stayed below 290. However, once I got out on the highway the temps shot up quickly. I gave it some pretty decent throttle to get up to highway speed. Temps shot up to about 360 very rapidly. I then put my foot in it and ran her up to over 70 for about a mile (I very often cruise at 70 on the highway.) Temps rose to 404. I backed off once I saw they were over 400 and ran at 60. After a few miles I took an off ramp, turned around and headed home. This time I kept the speed between 60 and 65. Temps hovered around 380-385. Back on city streets to my home temps were 350-360, cooling down to below 340 at stoplights.

It should be noted that Ambient temp during all of this were 90 degrees (according to weather.com) and I removed the heater ducting at the heater boxes.

So, what does everyone think? I think i'll pick up an IR thermometer tomorrow to do some double checking, but I do feel like i got the plug sealed and the sensor good and tight. This makes me nervous. I'm planning an 8000 mile 8 week trip in September/October, sure don't need to melt anything!

**EDIT** I've been doing some hunting through the forums. It appears 400 degrees at freeway speeds (certainly with 90 degree ambient temp) doesn't seem all that uncommon... Is this true? I'm still going to get an IR thermometer tomorrow to check the accuracy of my gauge though...

Wed Jul 04, 2012 2:57 am

So how do the plugs look? I'd say you are about normal for a stock bay but you could richen the mix a hair. Using a LM-1 would be the ticket to set it.

Wed Jul 04, 2012 3:21 am

How's your timing? May want to double check that.
Assuming it's FI, hows your mixture?

Wed Jul 04, 2012 3:57 am

Given it's a 79 (fuel injection) plus added smog items I'd say those temps are pretty much on par to what they should be. As Bley said, richen the mixture a tad and check your timing. Timing should be at 28* fully advanced with vac hose disconnected & plugged.

Those later style heads and exhaust set up aren't doing you any favors...

Wed Jul 04, 2012 6:29 am

By the way...using a laser IR thermometer for the purpose of verifying whether the DD gauge is accurate or not .....will almost always show that the gauge is rading too high because...... the laser IR gauge is totally innaccurate for this purpose.

We use these gauges, pyrometers and thermocouples every single day in our labs at work. The Laser or IR gauge is a very cursory tool with no NIST calibration available for even the very best quality ones because there is no way to calibrate usage technique or conditions.

As the beam travels through very hot or cold atmosphere or space on the way to whatever it is that you want to test the temperature....it gains or loses and distorts. No two of these gauges will calibrate together or read the same even when reading the same spot.
They are good to verify within about +/- 25F in hot conditions like those surounding a cylinder head. I agree...better than nothing for trending....but worthless for verifying a DD gauge. Ray

Wed Jul 04, 2012 1:15 pm

Well, here's another update.

I checked the Timing. It was way off. So I set it to 7.5 with the advance attached per the bentley. I had to drive about 60 miles so this was a chance to see if the timing made a difference.

It didn't.

It's 100 degrees outside today. On the highway I observed temps ranging from 380-410 depending on hills, speed, wind, etc. Whenever I'd notice it over 400 I'd back off. When I got off the highway the engine was popping at the stoplight (sounded like fuel igniting in the exhaust.)

I checked my engine tins, but I'm not 100% familiar with what I should have so I suppose bits could be missing without my knowledge. It does appear that there should be tin underneath the engine, that's not there (underneath the exhaust manifolds/heads. I'll try to grab some pics later...

Wed Jul 04, 2012 1:22 pm

Assuming you are a 1979 Federal model with the correct distributor, a 205P or a 205S and that the point dwell angle is pretty close to 47º +/-3º.....

pull the vacuum hose off the distributor. Hook up the timing light as close to the #1 spark plug as possible and start up the engine. Rev it up until there is no more centrifical advance to be had (ie the timing light no longer will advance no matter how fast the engine runs) and make that point to be 28º BTDC on the timing scale. You will be up around 3500 RPMs when that happens.

Hook up the hose and go for a ride. See if that helps.

Wed Jul 04, 2012 1:26 pm

Thanks for the timing tip. I will definitely give that a go and see if it helps. Here are some photos of my current tin underneath. I'm sure everything on top is present...

2012-07-04 15.17.32 (Large) by riponfrosh, on Flickr

2012-07-04 15.17.59 (Large) by riponfrosh, on Flickr

Wed Jul 04, 2012 2:03 pm

You're missing the sled tins on both sides.

Wed Jul 04, 2012 5:09 pm

VDubTech wrote: You're missing the sled tins on both sides.

I will definitely get on replacing those. Thanks! Would that cause overheating issues? I thought I read somewhere that some pieces of the tin are more for preheating air on cold days, not sure if what role these pieces play.

Thanks again!

Wed Jul 04, 2012 5:25 pm

Without the right bottom tin, cold air will blow across the thermostat and may cause it to close the cooling flaps. If you don't have the right bottom tin, disconnect the thermostat for now.

Wed Jul 04, 2012 9:14 pm

Wildthings wrote: Without the right bottom tin, cold air will blow across the thermostat and may cause it to close the cooling flaps. If you don't have the right bottom tin, disconnect the thermostat for now.

Will do. Would the absence of these cause overheating in any way? The uppper tin doesn't seem to be missing anything, and the engine to body foam seal is fully intact.