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Old 03-26-2011, 02:21 PM   #1
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Bad throttle position sensor

Wanted to add this little bit of knowledge to the board because I couldnt find it with search.
Lt1 with automatic transmission
Symptoms:
Sometimes the transmission won't shift until over 5800 rpms.
Sometimes the car idles higher than normal or the engines rpm are higher than normal while driving.
Car kicks hard when shifting from park to reverse or drive.
Fans do not come on at times so car overheats.

No SES no trouble codes.

The way I figured it out was I was watching free scan on my laptop and noticed the throttle position reading was at 77 while the car was just sitting in my driveway. Replaced my tps and so far all of my symptoms have gone away.

Without freescan you could just unplug the tps while the car is idling high and the idle should drop. This will likely throw 2 codes but if your fans kick on and your idle drops then your tps is likely an issue. That is my experience anyway.
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Old 03-26-2011, 08:37 PM   #2
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Re: Bad throttle position sensor

The reading for your TPS or TPP was "77" whats? The TPS is measured in 0-5 volts. There is also throttle position percent (TPP) which is measured in 0-100 percent. Then there's the idle air control (IAC) which is measured in "counts".
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Old 03-27-2011, 11:49 AM   #3
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Re: Bad throttle position sensor

TPP was 77%, that is the 77 I was refering too. My IAC was 130 at the same time. When I replaced my TPS the IAC counts were lower, around 80.

I did not put a volt meter on my bad TPS.
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Old 03-27-2011, 12:00 PM   #4
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Re: Bad throttle position sensor

The first thing to do when you suspect the throttle position sensor is to check the voltage.
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94 Formula A3+1: 381ci all-forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams custom solid roller / TH400+GearVendors OD/4.11's / Strange 12-bolt / 300-shot N2O / Spohn Suspension / 6-pt roll bar with integrated SFC's / full AutoMeter instrumentation / MoTeC M48 Pro engine management system / a few other odds 'n ends. 800HP/800LB-FT at the flywheel.
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Old 03-27-2011, 12:14 PM   #5
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Re: Bad throttle position sensor

I believe you. But I didn't suspect it until I saw the engine data in freescan showing the throttle position at 77%.

Then I just felt lucky and lazy and decided to replace the sensor and it worked
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Old 03-27-2011, 03:52 PM   #6
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Re: Bad throttle position sensor

Freescan also shows you the TPS voltage..... didn't you bother to look at it? That would have explained why the TPP was reading 77% with the throttle closed.
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Old 03-27-2011, 06:16 PM   #7
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Re: Bad throttle position sensor

Does it show it in the engine data section or in the CSV file? I wasn't aware so I didn't look.
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Old 03-27-2011, 06:27 PM   #8
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Re: Bad throttle position sensor

People have sent me HUNDREDS of FreeScan logs for review. Its in there.
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Old 03-30-2011, 08:08 PM   #9
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Re: Bad throttle position sensor

found tps voltage in column C of the csv output file.
TPS voltage was 3.08 at 67% 3.43 at 78%
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Old 03-30-2011, 08:08 PM
 
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