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Wed Aug 20, 2008 12:20 pm

So, could someone explain what symptoms a bus might show if it's #3 heat temperature sensor was unhooked?

Wed Aug 20, 2008 12:45 pm

I believe it would show signs of running on cold start all the time as the ecu has no way to determine it has reached operating temperature.

Wed Aug 20, 2008 12:52 pm

If you are talking about the Temperature Sensor II on the #3 cylinder, if it fails or is disconnected your CPU will think that the engine is stone cold all the time and will dump in lots and lots of extra fuel. If it runs at all once it warms up, it will probably blow black smoke and you will get fuel delusion of your oil.

Wed Aug 20, 2008 1:42 pm

in my experience the engine will actually not run at all with the temp sensor disconnected. if it is grounded the engine will run super rich, if disconnected it will not start or run at all.

Wed Aug 20, 2008 1:49 pm

Hmm...well perhaps this question wasn't that hypothetical. Anyway, I just installed new heads on my bus, and it ran great for a little while. It seems like I lost compression on a couple of cylinders, and it sounds like head gaskets. While I was disassembling the bus, I noticed that the temp sensor wasn't plugged in at all. This might make sense for why it wasn't running well.

What it was doing was it wouldn't let me get up above 35 mph, and the exhaust smelled super rich. It wasn't really blowing smoke, but it was hardly running. Also, after running a bit, the brakes would lose their boost. It wouldn't stall, but under load it seemed super bogged down. Where does the sensor plug in? I'll finish changing the gaskets just in case. Oh, and does anyone know if the manifold (where the pipe comes out of the head) shoulder is made of plain steel, cast iron, or stainless. The welds came off, so I'll need to get that fixed soon too.

Wed Aug 20, 2008 2:00 pm

wheelhorse wrote: Oh, and does anyone know if the manifold (where the pipe comes out of the head) shoulder is made of plain steel, cast iron, or stainless. The welds came off, so I'll need to get that fixed soon too.

the manifolds are stainless. the flanges are cast stainless, the pipes are stainless too.

Wed Aug 20, 2008 2:11 pm

So, that means that if I need them welded, I would have to find someone who welds stainless specifically?

Wed Aug 20, 2008 3:42 pm

Welding stainless isn't all that special, anyone can use some Super Missile Weld type rod and do a good job. Would think that having them heli-arced might be the best way though.

Wed Aug 20, 2008 3:48 pm

head gaskets?

Mon Sep 01, 2008 11:01 am

It would make sense that if the CHT sensor is unplugged, it would send the wrong value to the computer, and it wont start?

Mon Sep 01, 2008 1:08 pm

wheelhorse wrote: It would make sense that if the CHT sensor is unplugged, it would send the wrong value to the computer, and it wont start?

yes. sometimes they will start but like with symptoms you state.

Mon Sep 01, 2008 4:07 pm

So, the way a CHT Sensor works is like a switch right. Cold = 0, hot = 1 (or vice versa)? So, if it's not connected, it could be telling the computer that it's always cold? Then could I tell the computer that it's warm by making a circuit? I'm just curious how it works. I probably won't actually do it.

Mon Sep 01, 2008 4:10 pm

I think if you ground it to make the circut you will lose all cold starting functions.

Mon Sep 01, 2008 4:14 pm

hmm
but if it's disconnected at cold start, and then made into a circuit when it's warm?...

Mon Sep 01, 2008 4:19 pm

wheelhorse wrote: hmm
but if it's disconnected at cold start, and then made into a circuit when it's warm?...

I don't know if it is strictly on/off or a linear progression as temps rise. Someone with knowledge which surpasses mine can answer that question, like Robert Bentely who is not with me right now. 8) Why would you want to mess with making a switch for it anyway? The part is only $15 bucks from bus depot, I bought 2 to have one as a spare when I had my '78...

EDIT: I see you don't have a place to screw it in, why not just get a bit, make a hole and thread it so you have the correct placement and setup?

Mon Sep 01, 2008 6:17 pm

The CHT is resistance (ohms) so that cold its roughly 2500 ohms and hot (325F) its 50 ohms.

lower ohms makes the ECU think the engine is hot or warm....so if it grounds out (lower ohms) it runs rich.

914 guys have used a pot to have a variable resistor to set the cold idle better esp. if you have modified the engine.

Mon Sep 01, 2008 6:40 pm

Right now I'm just trying to figure out a way to get the bus back home. I've been stuck in Ann Arbor for about a week with no cash, and I have a 2 hour drive to get home, where I happen to have a new sensor. At the same time, I don't want to tear apart my engine, seeing as I just replaced the heads.