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Thermostat talk: Fooling the PT CPU into thinking cool....

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Old 10 Aug 2009, 02:03 pm
THS_Cruiser03's Avatar
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Join Date: May 2008
Location: North Central IL, USA
Posts: 119
Question Thermostat talk: Fooling the PT CPU into thinking cool....

Hi Mopar Brethren!
(...and yes I tried the search function before posting this)

So one thing I have noticed about my PT is that I do have stronger acceleration when the temp gauge is running about halfway between normal (middle mark) and "C" - I have done a number of acceleration runs in this range and there is definitley more pull in at this temp. range.

I've also noticed in my research over the years that the Whipple SC kit for the NA PT motors uses a 160 thermostat. I also noticed that the Kenne Bell Optimizer computer mod (which is no longer supported or available from K.B.) calls out the use of the 160 thermostat for best results too.

So this got me to thinking I need to do this mod! But do I need to run a 160 stat or just fool the car into thinking it's running hotter than it actually is?

That being said I also know the MPG suffers when you run 160 instead of 195. Since my car is my daily driver I need to have the best of both worlds. My current mods haven't hurt my milagea much so I'd like to keep that trend going with fuel at $2.70 around here. Some would suggest (on this forum even) that running the 180 is the way to go, but I think I have a better idea.

So I decided with my EE friend that I would like to run the 160 stat, but fool the computer into thinking the temp is running 195-200. My plan is to put a resistor pack in line of with the temp sensor (mounted to the water neck). The switch will have bypass and resistance settings, so when switched to the resistance setting it will trim the output voltage of the sensor to mimic the temp. sensor output when running at 195-200. When on bypass it will just be the actual voltage coming off the sensor.

So next I ran a test to capture the sensors output at various engine temps during warm-up, and let the engine fans cycle on/off while capturing the volts at every 5 of change. My PT's stat is stock '03 and has never been changed. Below is a list of the data I collected using a voltmeter and thermocouple taped to the metal water neck:

Temp sensor volts/actual F - Key on, engine off: 4.99v/98.3 F


Temp sensor V --- Actual temp. F at water neck

2.99 95.9
2.98 96.3
2.72 100.0
2.53 105
2.32 110
2.16 115
2.0 120
1.87 125
1.77 130
1.64 135
1.51 140
1.40 145
1.28 149 - Fans turn on for first time... (parked) - Not sure why....
1.18 150 - (fans on continuously temp keeps rising)
1.08 155 - (fans on continuously)
0.99 160 - (fans on continuously)
1.01 164.5 - Fans turn off now....
0.95 170
0.97 175
0.98 170
0.92 175
0.87 180
0.81 184 - Fans come on again
0.85 183 - Fans turn off now.....

0.91 180
0.81 184 - Fans come on again
0.79 190 - (fans on continuously)
0.93 185- Fans turn off now.....
0.81 183

- PARKED CAR WILL MAINTAIN 180-185 F while parked

Unfortunately my wires weren't long enough on the thermocouple to take a drive and see what happens, but I got my data. So I'm thinking my resistor pack has to tell the car to turn on the fans when the engine is running say 170 or the voltage output must be equal to 0.95 volts, then turn off again when the engine temp goes down to about 167.

Here is my problems though -

1. If I do this my car will be applying fuel as if the motor is running 190 plus, but it will really be running about 160-170. This could possibly throw off the accleration improvement I see today when she is in the middle of the warm up cycle. (i.e. little to no hp/torque gain)

2. I thought the stock PT stats were 195, not 200 as I've seen listed in these and other forums. But my car never got to 195 or 200 during my test. However the temps were taken from the surface of the metal water neck (aluminum I think) which may have thrown off the numbers a bit. *Shrug*

3. # 2 had me wondering if the car is taking additional temp. data from some other sensor....

- Such as in the radiator
- The fans themselves
- Exhaust gas temp

Does anyone know for sure where the ECU gets all of it's temp data from on PT's?

A little help on this project would be appreciated from the guys who know!



Base '03, Light Almond Pearl Metallic - Modified 2.4 na w/ported head, 10.4:1 compression ratio & Crane #10 camshafts - 4 speed - Energy Poly bushings - Napa ceramic pads & Ultra premium rotors - Rotora Stainless brake lines - Extreme Acc. Xenon HID 8k low & 6k high

Dark taupe & Ivory Interior, Factory CD w/Sirius Stratus 4, Power Acoustik 840w 4ch, Boss 200w 2ch w 10" bandpass sub, 2 sets Phoniex Gold Matched comp. speakers

Rolling stock: 16" SE A7w 225/50/16 Kuhmo Ecsta AST - One of the nicest Light Almond PT's you'll find
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Old 10 Aug 2009, 08:01 pm
quicksilverdon's Avatar
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Join Date: Nov 2003
Location: Felton, Delaware
Posts: 20,501
Default Re: Thermostat talk: Fooling the PT CPU into thinking cool....

The PT computer sends more gas when temp is low. I don't know what results will be when computer thinks engine is warmer, but isn't.
At least your mod is reversible.
"Anti-intellectualism has been a constant thread winding its way through our political and cultural life,
nurtured by the false notion that democracy means that 'my ignorance is just as good as your knowledge."
Isaac Asimov"

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Old 10 Aug 2009, 08:15 pm
Fanatic Cruiser
Join Date: Dec 2003
Location: San Antonio, Texas
Posts: 535
Default Re: Thermostat talk: Fooling the PT CPU into thinking cool....

I understand what your trying to do but it's not going to work. If you fool the computer into seeing the higher temp then the computer is still going to run the engine at that temperature. The power is because the engine is cooler the computer adds a little more gas.
What you would have to do is actually tell the computer that it is running colder than it is. Then you get into emissions problems and the potential for overheating. If your telling the computer that it is running 20 or 30 degrees cooler than it is then the engine can overheat by that much. Fan comes on at about 210 or so. Once you get to about 225 or so then your pretty boned. Aluminum heads with iron blocks don't like that!
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Old 11 Aug 2009, 08:38 pm
03hothemi's Avatar
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Join Date: Jul 2009
Posts: 30
Default Re: Thermostat talk: Fooling the PT CPU into thinking cool....

These cars have an intake air temp sensor right? IF they do(I really have no honest to god idea if they do or not) try raising the resistance in the signal wire of the IAT back to the PCM. This also works the same if the car has no IAT, but has a battery temp sensor. From what I understand a battery temp sensor functions the same as the IAT if one does not exist. It helps a little with the acceleration on my Ram without throwing codes, or risking overheating. On my Ram, a 1/4 w 4.7k ohm resistor fools the computer into thinking the actual air charge is 15 - 20 degrees cooler than actual temp, fooling the PCM into throwing a little more fuel in.
2005 PTC Limited N/A- All Stock...for now

2003 Hemi Ram - TMR, Flows, Killed the Cat with mechanical O2 sim, K & N, no hat, Taylor Shorty Wires, Tow Pkg

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