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Wed Feb 03, 2010 2:59 pm

Can anyone help a newbie out?

I have a 77 bay window bus with fuel inj, and the last owner replaced the temp sensor with a manual air/fuel ratio adjustment knob on the dash, so you can make it run leaner or richer. My question is, does this have any negative effects on the engine and should I put the temp sensor back in, and why does the temperature have anything to do with the fuel mix?

Un-relatedly, the engine tin looks to be all in place from the top, but I can see the fins of the cylinder heads from underneath. (on one side it's just in the front of the oil filter) I can't find a good under engine picture, but it seems like I'm missing a couple of pieces.

Thanks

Wed Feb 03, 2010 3:02 pm

Can you take some pictures of your engine? It would help us figure out if you are missing tin.

I've never heard of somebody completely removing the temp sensor and putting a manual adjust in the dash. I would think that the wiring would be screwed up. Are we talking about the Temperature Sensor 2 on the drivers side head?

Wed Feb 03, 2010 3:03 pm

The FI system needs to know what temperature the engine is at to give it the correct air:fuel mixutre. Your PO was trying to fool it with that reostat. it will sort of work, but you need to know what you are doing.

Been here?

http://www.ratwell.com/technical/TempSensorII.html

Your tinwork is OK. It is supposed to look like that over the drivers side rear.

Wed Feb 03, 2010 4:03 pm

504 wrote: Can anyone help a newbie out?

I have a 77 bay window bus with fuel inj, and the last owner replaced the temp sensor with a manual air/fuel ratio adjustment knob on the dash, so you can make it run leaner or richer. My question is, does this have any negative effects on the engine and should I put the temp sensor back in, and why does the temperature have anything to do with the fuel mix?

Un-relatedly, the engine tin looks to be all in place from the top, but I can see the fins of the cylinder heads from underneath. (on one side it's just in the front of the oil filter) I can't find a good under engine picture, but it seems like I'm missing a couple of pieces.

Thanks

As randy noted....the temperature sensor in the head helps to set the baseline fuel mixture in your system...richer for warm-up..leaner as it warms up. The metering flap in teh air cleaner housing sets dynamic fuel mixture load....and the fuel pressure regualator varies the mixture according to need based on vacuum of the manifold and movement of the throttle and rpm.

What the previous owner did is actually excellent and very advanced tuning in several ways....but as Randy noted...you had better know what you are doing...and you better have tested extensively and made notes before doing this.

The problem with heh stock temperature sensor in the head...and also somewhat the temp. sensor in the intake.....is that the size and massof these sensors is very small.
They can have a habit in various climates (like very hot or very cold)...of either gathering and banking heat and not releasing it...which can cause hot starting issues when running errands and stopping and starting frequently....or in the winter in very cold areas....they can cause excessively rich running when they never seem to get warmed up enough.

Sometimes this method of adding a rheostat...or even a fixed resistor package is a very good method of tuning the existing sensor to work well with a stock fuel injection system that is installed on a non-stock engine.

But...it can also sometimes be used by an unknowing owner to correct for problems that should have been addressed other ways:
For example:
In cold weather, if the previous owner has taken the cooling flap and thermostat system off the engine, it may neverseem to warm up right without highway driving. It loses power and eats fuel. A rheostat can artificially push the baseline fuel mixture leaner. Not exactly unsafe...but not proffesional either.

or....if the sensor is out of spec due to age (common)...the PO might be cheaping out on replacing it.....or......there may be wiring issues that cause it to read oddly.....or...ther may be vacuum leaks that the PO was too lazy to search out, causing lean running...and this is a way to richen the baseline mixture.

To start out...leave the rheostat where it is, put a standard stock temp sensor into the head...and go through everything on this engine and set it back to normal. Ray

Wed Feb 03, 2010 6:02 pm

Thanks guys.

I believe the PO put in the manual adjustment to increase fuel economy on the highway. I have the temp sensor still. Are you saying I should re-install it and see how the engine acts?

Ray, what exactly do you mean by put the temp sensor back and set everything back to normal? Should i remove the rheostat, because it screws into the same hole as the temp sensor ( in the drivers side head in the front of the engine)? I haven't played with the adjustment much, but the bus starts every time and seems to drive ok (as far as i know). I also haven't driven it all that much as I've been tinkering with the shifting and many other neglected areas.

Also, i am going to take some pictures of the places I think the tin may be missing. The bus spent its days in Maryland, and now its in new orleans where its 90+ degrees, and I don't want to take any chances.

Thanks again,
john

Wed Feb 03, 2010 8:29 pm

504 wrote: Should i remove the rheostat, because it screws into the same hole as the temp sensor ( in the drivers side head in the front of the engine)?

The rheostat is the thing in the dash with a knob on it. Whatever is in the temp sensor spot is something else. Maybe it's the temp sensor. Pics would help ID it.

Wed Feb 03, 2010 8:50 pm

My guess would be that the stock sensor is indeed in the head and the reostat allows the output of the sensor to be manipulated to fool the FI computer.

While it maybe of interest to tweak it for varying performance reasons, your would also need to know just what the air:fuel ratio is and just what the cylinder head temperatures are running. Too lean is also too hot.

I would go back to the stock set up (remove the reostat after ensuring that the AFM has not been dinked with) if it were me.

But that is just me also.

Wed Feb 03, 2010 8:52 pm

i believe pictures would help in this instance. pics of the "rheostat" and of where the TS2 should be in the engine.

Thu Feb 04, 2010 2:32 am

They got rid of manual chokes on cars because people couldn't figure them out, plus people were forever leaving their chokes partially on. This is the most likely outcome of running a dash mounted adjustment for the mixture as well. You will forget to keep cranking it back and end up running too rich a lot of times, and wonder why your gas gauge drops like a rock. I would think for 99 out of 100 people having a dash adjustment would be more hassle than it is worth.

Thu Feb 04, 2010 6:20 am

There are two possible ways that the rheostat is wired:
(1) In a loop with the original factory CHT sensor still in the head...meaning the rheostat adds resistance to the sensor. This is called "ballasting".
(2) The sensor is removed and the rheostat takes its place.

The CHT is a resistance to ground system. This is why is has only one wire. The electronic control unit is trying to actually connect to ground at that single wire point. The resistance to its reaching ground is the sensor.
Connecting the wire straight to the block would effectively be "0" resistance.

Leave the rheostat in place for the moment in the dash...leave its wires alone...and find out what is screwed into the left side cylinder head. If there is a sensor there ....and it has nothing connected to it...or maybe no sensor at all...you will know that Scenario #2 is being employed.

If the wire from the sensor in the head....is connecting to another wire that leads forward and the fuel injection wiring harness wire that shoul be connected to the sensor is hooked to another wire leading forward....you will know that scenario #1 is being employed.

There is a third scenario....but lets not talk about that right now. Its possible some crafty individual has wired the rheostat directly into the proper area on the circuit board in the ECU....kind of like late type 4 D-jet had for adjusting idle mixture. Ray

Thu Feb 04, 2010 8:32 am

[quote
There is a third scenario....but lets not talk about that right now. Its possible some crafty individual has wired the rheostat directly into the proper area on the circuit board in the ECU....kind of like late type 4 D-jet had for adjusting idle mixture. Ray[/quote]

Now that would be trick and a real pain as you would be forever chasing the perfect idle!

Thu Feb 04, 2010 9:49 am

Bleyseng wrote: [quote
There is a third scenario....but lets not talk about that right now. Its possible some crafty individual has wired the rheostat directly into the proper area on the circuit board in the ECU....kind of like late type 4 D-jet had for adjusting idle mixture. Ray

Now that would be trick and a real pain as you would be forever chasing the perfect idle![/quote]

Yes...that would be the ugliness of option 3.

The important thing to understand about the CHT in D-jet is that it is not...and does not need to be.... a minute-byminute/second-by-second correction device. Thats not what its for.
It sets the basic engine temperature "trend" so basic fuel mixture can be set.... (aside from dynamic load and emergency enrichment for passing and acceleration).

The computer needs to know when the engine is cold for start up....and not just basic cold. For instance the CHT signals cold start up in summer weather and enriches your mixture for starting. But that is still far different from cold start up in freezing weather. It can do both because its a "thermistor".

But....about maybe 5-8 minutes into the start up cycle whether it is summer or winter....the CHT ouput resistance will be roughly the same.....and the resolution of that resistance reading and what the computer does with it....is rather negligable.

OK....let me see if I can make it even simpler:

At say 35F...the CHT may read 2700 ohms.
At say 80F...the CHT may read 2200 ohms
This is summer versus winter at start up.

In 5-8 minutes of running...both could easily be at 1300 ohms. That was teh uniformity I was getting at above.
Along the way...each 100-200 ohm step might have caused a noticable change to baseline fuel mixture. But...encrements of less that about 50 ohms don't really seem to show in engine running. A CHT reading of 1215 ohms compared to one of 1238 ohms would be a bassically worthless difference in the big picture. This all varies a bit according to the sensitivity of each engine, your baseline fuel pressure which is a multipler and a few other details.

What I am getting at is that I could simply use a board with fixed resistors or a rheostat with fixed resistance points of say.....7-10 zones....and it would be as seamless of a transition as the stock CHT without the issue of it banking heat or dropping too low in resistance...like they do..
In reality...you would I think..really rather oil temp...or a interpretation between oil and head temp to trend the overall temp range.

I could have the zones be say....2800, 2500, 2200, 1900,1600,1300,800 500, 200.

I have a 7 position resistor plate right now that works off of a bi-metallic spring to move to each fixed resistor as teh head heats up. I'm not through testing it, but it should work much better than stock CHT. Ray

Mon Feb 08, 2010 3:54 pm

So I got in touch with the PO and took a look at the temp sensor. The temp sensor is there but not hooked up. The rheostat is wired directly into the wiring harness where the temp sensor should be hooked up. There's another temp sensor screwed into the fan housing (not hooked up) that he said was put there as a testing point when he was installing the rheostat. He said I can hook up the temp sensor but i will get 2x better gas mileage if i use the rheostat correctly and lean out the engine. Does this sound like a good idea? Should I get something to monitor the temp of the cylinder heads to make sure i'm not overheating? I read the article on ratwell about cht sensors and his opinion was that the only way to do it is with a multi-meter.

Let me know what you think.

Thanks, John

Mon Feb 08, 2010 9:01 pm

I would hook the CHT back up as stock after testing it. The po setup can really lean it out but too lean mean lots of HEAT. Getting it running correctly, new fuel lines and vac hoses and see how it should be stock, 17 to 22 MPG.