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The mkiv Supra Owners Club

Throttle Position Sensor (TPS) intermittent harsh gear changes and idle problems


TubbyTwo

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Sadly not all good news. whilst the gearbox is now fine there is an intermittent problem with the idle when in gear or park when there isn't any load on the auto box. The idle rises and falls almost as if your blipping the throttle, and drops so low at times the car almost cuts out. When this happens the auto box also slams into gear with a harsh jolt.

 

Its a new box from Keron, with fresh fluid at the correct level.

 

I have a spare ECU for the autobox so will swap for that and test, just completely disheartened now, over £700 down in the last few months and still got problems. :(

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Sadly not all good news. whilst the gearbox is now fine there is an intermittent problem with the idle when in gear or park when there isn't any load on the auto box. The idle rises and falls almost as if your blipping the throttle, and drops so low at times the car almost cuts out. When this happens the auto box also slams into gear with a harsh jolt.

 

Its a new box from Keron, with fresh fluid at the correct level.

 

I have a spare ECU for the autobox so will swap for that and test, just completely disheartened now, over £700 down in the last few months and still got problems. :(

 

sorry to hear this mate :( hope you get it sorted.

 

sounds like it might not even be the autobox? if its not the autobox ecu then maybe its the engine ecu? or idle control valve? hate problems like this, the list could build up :(

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I was thinking perhaps ICV, its weird.

 

Another issue is when doing any sort of speed and lifting off the throttle you can at times feel the box almost holding back as if your gently tapping the brakes. You can see the rev counter drop, hold drop hold etc.

 

Its doing my head in now, and I'm getting to the point of throwing the towel in tbh, I just don't have the time cash or interest to start replacing half the car.

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Just been round the block again, car starts up fine, idles fine goes into gear without a massive thud, gearbox is doing almost seamless changes, its silky smooth.

 

Pull up to junction, foot on brake in "D". Idle starts to bugger about, drops to the point where it almost cuts out then shoots up again to 1000/1100rpm then back to 900rpm for a few secs then repeats. On EBC screen vac reading goes from -80 to -57/-65 when this happens. then returns to its usual range of -78 to -80.

 

Press throttle to pull away revs plummet then car takes off now jerking into every single gear quite harsh at times and revs hesitant and seem to hang when off the throttle.

 

Also getting a smell of fuel when then happens.

 

On boost and when driving (aside from harsh gear changes) the engine is smooth and so is the boost through all gears.

 

Going to read error codes at lunch and check engine oil level.

 

Then guess the starting points will be:

 

- Clean ICV

-O2 sensor test/ swap

- ?

- ?

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Sounds to me like it's overfuelling, check your oxygen sensor output with a multimeter. Cold start will be fine as it's open loop, but as soon as it goes closed loop it overfuels and tries to stall. Lifting off the throttle will produce more engine braking than normal and pulling away in an auto will lurch as you can't rev it up before slipping the clutch.

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I can't find the guide to checking the o2 sensor, fail :( Never can find that blighter.

 

Anyway, check pin Ox1 in the diagnostics port. You should see, at warm idle, a hunt between 0.5v and 0.7v.

Any solid figure between 0.8 and 1.0v is running too rich and the ECU can't compensate.

Any solid figure under 0.4v means it's running too lean and the ECU can't compensate.

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just tried the car again and while its cold it doesn't seem to do it, after a couple of miles it starts d**king around again. Will multimeter the O2 sensor tonight and report back.

 

autobox swap went ok no problems, I know the garage well who did the work so no problems there, they also flushed the oil cooler for the box as well. It started this a week before the box was replaced so dont think its swap related.

 

Will swap the trans ecu as well and test.

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So, the car has gone back to its old behaviour from before the new autobox was fitted. plant the throttle in 3rd and it sticks at 3k rpm while the speed needle rises, makes boost but isnt transmitted to the car very very laggy.

 

Just read the error codes and its showing code 41 - Thottle position sensor:

attachment.php?attachmentid=13923&d=1112558346

 

ffs.

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Buggery :( Did they swap the torque convertor as well as the autobox itself?

 

The stalling problem still could be overfuelling, but if the TPS isn't working correctly then it might the the IDL switch line, the signal that tells the ECU that the throttle is shut and it should control the idle using the ICV. If that doesn't work, when the throttle is shut the engine struggles as the idle control valve is also shut and barely any air can get in. It's something you can check with a multimeter at the ECU, using a safety pin as a probe to go through the wiring insulation. You can also check the throttle signal like that, see what the ECU is actually getting.

 

I don't know what a dodgy TPS would do to the autobox control, but it probably won't help it.

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nope old box is gone, it was buggered as it was burning fluid in about a week. new one has made a massive difference until today, guess its picked up on the code. Strangely enough both myself and Ian C error checked the car, and I have been almost every week while fault finding and it hasnt logged anything until today. Perhaps the TPS was on its way out but not gone enough to log a code?

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Well if this started before the work was done it probably wasn't the swap :) Pulling the TPS sensor and testing it with a multimeter is the way to go, as well as the test at the ECU. Reason being, with all your strange behaviour, it might be that the sensor works but the connections at the ECU are loose. When I went through 3 months of trying to diagnose a bad overfuelling condition once, my TPS signal read either 0 or 100% because of this very problem. I had a bunch of other fruity behaviour as well, in the end I just refurbed every connector on the ECU plug and all these ancilliary problems that were confusing the issue went away.

 

I put a guide on here how to check and refurb the pins, it's not too difficult, just fiddly and time consuming. An hour or so in the footwell, yay :)

 

Edit to say - at least we did in fact correctly diagnose a shagged autobox ;) Hopefully the light at the end of the tunnel is no longer your autobox glowing red.

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