Contour Enthusiasts Group Archives Forums General Troubleshooting Funky OBD-II readings...bad TPS?

This is a spin-off of my orignal thread.

I hooked up my laptop today and got some weird readings. With the key in the "on" position, and not running, the TPS is at 16.5% and the MAF was reading 0.117#/min. Shouldn't both of these values be ZERO if i'm not touching the gas pedal AND the engine is off?!?

This would explain why my car won't idle without dying. The computer thinks that the throttle is open so it is dumping gas into the engine and closing off the IAC. This is drowning my Zetec in fuel and not giving it any air. I've gotta swap out that TPS!!!

BTW, what kind of readings should I have for my MAF and TPS at a given RPM?

Here are some of my other readings...

Engine off:
MAF=0.177 #/min
TPS=16.5%
ECT=73 degrees F
IAT=45 degrees F (It was about 40 outside)

Started and holding gas to maintain 2k RPM:
MAF=1.113 #/min
TPS=20.8% (only 4% change?)
ECT=180
IAT=54
LOAD=25.9%
Short Term Fuel Trim=varied from -1 to 0
Long Term Fuel Trim=3.9
Spark Advance = 35.5 degrees
#1 O2 Sensor = varied from 0 to 0.745
#2 O2 Sensor = 0.725

Revving to 3500RPM:
MAF=1.702 #/min
TP=21.6% (less than 1% !?)
Spark Adv = 36.5 degrees
LOAD=29.4%

Idling badly:
MAF=0.366 #/min
TPS= 16.5%
ECT= 185
IAT= 66
LOAD= 41.2% (MUCH higher then at 3500RPM)
Short Term Fuel Trim= varied from 0 to 2.3
RPM = 800
Spark Advance = 28 degrees
#1 O2 Sensor = varied from 0 to 7.35
#2 O2 Sensor = 0.818

FYI, all readings were done with my car in park.

BP

Not sure about minimum MAF % but don't think it can reach zero.

16% TPS is bogus for sure.

You have been resetting the PCM, right?

Never heard how it runs other than idle. Timing belt OK? Checked the vacuum?

Yep, reset the PCM everytime. Vacuum is good, and the timing belt only has 17k on it.

I'm gonna yank the TPS tomorrow and substitute a 5k potentiometer and see what happens.

Waddya think?

BP

Sounds like a plan - since you put everything but the kitchen sink in there already!

The following is from the Ford TSP (Technical Service Publication):

MAF voltage @ KOEO (Key On Engine Off) should read 0
MAF voltage @ hot idle should read .6-.9
MAF voltage @ 30 MPH should read .8-1.5
MAF voltage @ 55 MPH should read 1.2-2.5

TPS voltage @ KOEO (Key On Engine Off) should read .53-1.27
TPS voltage @ hot idle should read .53-1.27
TPS voltage @ 30 MPH should read 1-1.3
TPS voltage @ 55 MPH should read 1.1-1.4

Screw it, I'm done.

I just checked the TPS and MAF voltages, both are within spec.

I'm gonna suck it up and take it in tomorrow.

BP

the tps isn't designed to go to zero, but the MAF will be close if the engine isn't running.. If you have an idle proble, it is most likely caused by the IAC. Also, the IAC opens in proportion to the throttle opening to act as a dashpot (like on older mechanical carburetor engines) to prevent gaskets from being sucked in and failing in the event that you snap the throttle closed at high RPM.

My readings are on my laptop at work, but i know that the TPS was around 0.62. The MAF was also in range.

Besides putting the old TPS back on, or buying a new one, is there anyway to test it?

I'm guessing that I would need and oscilloscope in order to determine if it is running at the right voltage and frequency. Ugh...I am sooo sick of working on my car...

...maybe tomorrow...

It's satruday night. Time to go drinkin'.

BP

We have a 99 Zetec ATX. The min throttle position setting seems to always read 17%. There are no idle problems (yet!)on our car. I don't think you have a bad TPS.

Yeah, with the TPS at 16% it read somewhere around a volt (well within limits).

I'd like to try swapping out the IAC but I really doubt that it is bad. It worked flawlessly when i had it out.

(I hope this made sense..Jeagermeister(sp?) is screwing with me a little.)

BP