Should I just replace the TPS on a 89 Mustang GT 5 Speed ? ( UPDATE )

I posted a few months ago about my stang burping / bucking when slowing down ( foot off the throttle ) and sometimes when accelerating.
Last time it did this, it did not throw a CEL and no codes stored with KOEO. Well it happened today right after start up and finally threw a CEL.
for 10 minutes then went off.This has been going on for some time now but only during the first few minutes of start up only when in open loop.
Once in closed loop all is fine and does not seem to happen… I pulled only one code …Code 63 - Throttle Position Sensor (TPS) signal too low, and nothing else stored. Base idle has been set up with the IAC disconnected per the shop manual.

I know how to check the voltage on the TPS from the shop manual from idle to WOT which I have not done yet due to being in hot Florida and we have a mosquito epidemic here with all the rain ( car is under the carport so no garage ) and too late to do it right now unless I want to get eaten alive.This is the original TPS and only thing I changed on his car is a water pump, alternator, fuel pump, plugs and wires a and clutch after 85k to a bad throw out bearing. Car now has 112k and owned since new and not driven too much. Car has never been abused and all stock except for the mufflers to give it a little better sound from stock.

TB, IAC and MAF have been cleaned a few months ago… Should I just replace it or could there be another problem causing code 63 ? Car runs fine when not acting up and plenty of HP to spare. Can have it in 2nd at 25 mph and floor it to about 70 (to redline ) and no hiccups…It has a mind of its own.

I can get one for around 35.00 locally and easy to change. it does not act up on every start up from cold…Just it finally threw a code…As I stated seems to do it only during open loop which is whats bothering me.

Thanks Guys.

I say it’s a cheap enough, relatively educated ( givien the code ) guess, so why not ?
Go for it.

You can check the TPS by measuring the votage out of the TPS.

With the connector plugged into the TPS, locate the signal out wire from the TPS. Take a stick pin/needle and insert into the back of the connector where the signal out wire comes out. This is called back-probing. Now connect a volt/ohm meter, positive to the pin and negative to an engine ground. Set the meter to read 2 V DC. Turn the ignition switch to the run position. Now slowly open the throttle from under the hood and watch the voltage on the meter. The voltage should steadily and smoothly increase. Now slowly close the throttle, The voltage should steadily and smoothly decrease. If the voltage reading acts eratic while opening or closing the throttle, replace the TPS.


Thanks Tester, that was my next step, was going to do that today but it started getting dark and the bugs came out ( mosquitos). For the 35 or 40 bucks, it would not kill me to get a new one as its the original TPS on the engine and just works as a volume control like on a radio or TV telling the ECU what fuel to send to the injectors and timing advance etc within the resistance reading in the TPS which can get dead or dirty spots …as they do go bad…Will keep you informed as I troubleshoot this .Due to time etc can’t do this until Saturday when my buddy comes over as the stang is a 2nd car.What gets me is only acts up when the engine is cold and in open loop. When it does act up, once engine reaches about 190 degrees it stops acting up going into closed loop. and can drive for an hour without bucking etc. I will answer this after we do some checking and come back with a reply…too bad others that ask questions here never respond to a fix here as there are a lot of good mechanics that respond here…Been on this board for a long time sine 2000…Thanks…other suggestions are always welcome !

Can’t wait until the winter months when we top at 70- 75 degrees and no bugs…LOL Love older cars…easier to work on…

Howie32703 in Hot and Muggy FL… Thanks All

I realize this car’s older, but I thought a TPS uses a 5V reference voltage

Ok here is an update…checked tps voltage on the orange ( 5v from ECU ) and black wire ( ground ) and read exactly 5.00 volts. Checked it from the black ( ground ) to green ( return ) and read .924 volts ( out of specs ).

Went through the throttle range from idle to WOT very slowly and back, and the DVM had a smooth reading up to about 4.75 volts with no jumping. Did it several times. I was able to adjust the TPS by loosening the 2 screws and then set it to about .985 volts and reset the ECU by disconnecting the battery for 15 minutes.
( The max I could adjust it to was 1.10 volts which is out of spec. )

It started right up but my idle was around 1100 instead of 750 and took it for a 20 minute ride,
driving normally and doing some WOT’s in 2nd gear. Smooth as silk and no bucking. When I got home idle was back to normal…Checked again for any codes and came back clean.

Now my question is, why did the TPS voltage drop low on its own ? I do not like shot gunning or replacing parts if they check out good now. Is this TPS on its way out or should I just replace it ? I heard from other people that the heat from the engine over time can change the resistance of the TPS. I can see this if the TPS was constructed of carbon like a radio volume control but not if it made out of resistance wire, which I do not know what its made of. Anyone know…Just curious.

The 2 screws holding the TPS to the TB were very tight so it did not move on its own when this problem first started.