QuestionQUESTION: Hello and thank you John please help me, my 1997 LR Disco 1 when you first turn it on it might idle way to high (anywere between 1500rpm and highest Ive seen it go is 3700rpm in park or on the other hand sometimes I start it and it will rev so low that is will stall or almost stall unless i give it a bit of gas. If I put the truck in drive i will go about 50-60 without touching the gas pedal. PLease help thank you for your time. I hope it will not be too expensive to fix because I have fel in love with this truck!!!!!
ANSWER: Hi James,
this sounds like the Throttle Position Sensor (TPS) and you could probably do replace this yourself. There are other possible causes but the symptoms you describe sound like those my '95 Disco experienced many years ago.
At that time, the dealer swapped out the MAF, the crankshaft position sensor, tried to con me into buying a new ECU (which I refused - $3000? NO WAY!!!) Long story - short: turned out to be the TPS and dealer charged me $585. In retrospect, I should have done it myself and saved $300+
Best of luck,
JohnMc
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QUESTION: Thank you so much John for your reply would this also explain it being stuck in high or low revs? If I am driving slow and the revs are at 1000rpm and i quickly goose it and the revs get stuck high as soon as it gets up to like 1500rps it does not want to come back down. Sometimes if I stop quickly as I am stopped completley i will still be reving at about 1500rpms and then it will abrubtly drop down below 1000rpm. When it is stuck in high revs the truck will do 60 without me touching the gas pedal
AnswerHi James,
no problem...here's LR's explanation and MY COMMENTS FOLLOW:
The throttle position sensor is mounted on the side of the plenum chamber inlet neck and is directly coupled to the throttle butterfly shaft. The throttle position sensor is a resistive device supplied with a voltage from the ECM. Movement of the accelerator pedal causes the throttle valve to open, thus rotating the wiper arm within the throttle position sensor which in turn varies the resistance in proportion to the valve position. The ECM lengthens the injector open time when it detects a change in output voltage (rising) from the throttle position sensor.
THE TPS IS A POTENTIOMETER/RHEOSTAT DEVICE THAT IN ESSENCE TRANSLATES PEDAL POSITION INTO DATA THAT THE ECM USES TO MEASURE OUT THE FUEL. THE ECM IS THE BRAINS BEHIND THE ENGINE MANAGEMENT SYSTEM.
In addition the ECM will weaken the mixture when it detects the throttle position sensor output voltage is decreasing under deceleration and will shorten the length of time the injectors are open.
THE ECM CONTROLS THE AIR/FUEL MIX USED IN YOUR COMBUSTION ENGINE.
When the throttle is fully open, the ECM will detect the corresponding throttle position sensor voltage and will apply full load enrichment. This is a fixed percentage and is independent of temperature. Full load enrichment is also achieved by adjusting the length of the injector open time.
WHEN YOUR DISCO IS REVVING LIKE CRAZY, IT COULD BE THAT THE TPS IS SENDING CORRUPT DATA TO THE ECM WHICH IN TURN HAS A LIMITED INTERPRETIVE RANGE. AIR AND FUEL MIXTURES MAY BE RANDOM AND CHANGING TOO FREQUENTLY.
When the throttle is closed, overrun fuel cut off or idle speed control may be facilitated dependant on other inputs to the ECM.
The throttle position sensor is ’self adaptive’, which means that adjustment is not possible. It also means the throttle position sensor setting is not lost, for example, when throttle stop wear occurs.
WHEN YOUR PEDAL IS NOT BEING USED, THE ECM RELIES ON OTHER SENSORS AND ITS OWN SETTINGS TO ESTABLISH AN IDLE SPEED. IF THE TPS IS FAULTY (SCREWY) IT COULD BE SENDING ALL SORTS OF CORRUPT (WEIRD) DATA AND THE ECM IS TRYING TO FIND A MATCHING SETTING.
HOPE THIS IS HELPFUL.
JOHNMC
