First inch of throttle doesn't do anything, then kicks in
 

As thread title really. Basically the first cm or two of throttle travel has no effect but then the power kicks in. The throttle is essentially either on or off, which results in large jerks when going on or off the throttle, and makes maintaining a constant speed impossible. My Subaru dealer had no idea, so I took it to the tuning company that installed the mods on my car (TDi in Thurrock). Within 5 minutes of looking at the car they diagnosed the problem as a faulty throttle position sensor and recommended I take it back to the Subaru dealer for replacement which I did. The dealer was a bit reluctant to do the work, but did so in the end.

The problem persisted for about 20 miles, until i got stuck in stop start traffic, when the engine misfired and the engine light came on. After this the problem appeared to have disappeared. TDi said that this was the ecu reboot and relearning the throttle position.

However, within a couple of days, the problem has returned again. TDi suggested trying to realign the sensor using an OBDII reader, which I'll attempt to do (any suggestions on a good one?), but I have to say, I'm not holding out much hope.

Any ideas?

Errrm, on what year car would be helpful. ;)

02 WRX Wagon. HKS bits; induction, manifold, exhaust, fcon, and boost controller.

Tried leaving the battery disconnected overn9ight to clear the ECU, but hasn't changed anything.

There were no codes thrown when the issue originally happened and this is why the dealer seems nonplussed. If there isn't a code, they don't seem to know what to do. :(

id thought torque developments would have been a bit more helpful

i remember asking tdi to do a cam belt change and they wanted 80 pound an hour labour and reckon it took 5 hours to do as had to remove radiator lol,any way sorry to hear of your problem and hope u get it sorted.

id thought tdi would have been able to align the sensor for u even thou u didnt buy it from them.....

Main dealers techs tend not to be very clever - and tend to follow the service book "script" with the same religious fervour as an indian callcentre employee. Although in this case it sounds like the "specialist" you have visted (and, by inference, spent money with in the past) is no better. I'm surprised the dealer was reluctant to take your money though - charging you full whack for a sensor plus half an hour (or an hour's) labour for a five minute job is easy money, even if they might be doubtful that the job actually needs doing. Suppose they deserve some credit for trying to caution you.

If the ECU hasn't actually thrown the TPS error, I would be interested to know how TDI diagnosed it as faulty.

The throttle position sensor error is usually set if the TPS voltage recorded by the ECU drops below 0.15 or goes above 4.9 volts for more than 0.2 of a second. Given that the working voltage of the sensor is between 0.5 and 4.3 volts (+/- a bit), these criteria are usually sufficient to pick up the two most common failure modes - either short circuit, or an open circuit caused either by the sensor wearing, or a wiring break.

Have you checked, incidentally, that the throttle cable is correctly adjusted - there's no noticeable slack that needs to be taken up?

It's difficult to work out what's going on here. On the face of it it sounds as though the original diagnosis of a faulty throttle position sensor is incorrect, firstly for the reasons above (i.e. lack of CEL) and secondly because you say the problem has now recurred, even with the new pot fitted.

Possibly related, possibly not, is the presence of the Fcon piggyback controller, and the "HKS induction". Can you confirm whether this "induction" is a cone filter or an aftermarket panel filter, and also can you confirm whether TDI have fitted and ensured all this stuff was running safely (i.e. AFRs, combustion validated) before it left them?

In addition, what's the timescale between the car being modified and this problem first presenting? Were the bits put on months ago (with the car running fine since) or was it all done this week?

As a correction to earlier posts, newage ECUs do not lose their trouble code memory if the battery is disconnected. However, TDIs comment about the ECU "rebooting" while you were sitting in traffic is a point of some concern. The only way the ECU can reboot/restart is if it is told to do so via a diagnostic tool. One would expect an experienced tuner to know this. In addition, if the ECU needed to recalibrate its throttle angle data, it would have done so immediately the new sensor was fitted, not 20 miles down the road.

As such, the fact that you've received this explanation both asks a question about their competence, and, in the context where the explanation you've been given cannot be true, asks what is really going on.

TDI themselves will, if they are equipped to fit these modifications properly, surely be able to check whether the throttle position sensor is correctly calibrated. If they can't, how did they diagnose your old one as faulty?

Too many things here that make too little sense to make sense of, if you see what I mean. :confused:

Duly noted, thankyou.;)

Thanks for the detailed response Splitpin. To address your points in order:

1. How did TDi diagnose as faulty? They said they were reading the voltage and they could see that there it was jumping between 0% and 4.73%.

2. I had noticed that the throttle cable was a bit slack when I was reconnecting the battery, so I tightened it up this weekend. This has not made any noticable difference to the symptom.

3. The induction is a dome shaped foam filter. The modifications were all done years ago (when it was only a few months old), before I bought the car.

Has the tps been recalibrated..

Why fit an fcon when ecu is mappable on the car already?

What mods do you have?

The TPS has not been recalibrated since it was fitted last week.

As for the mods, they are detailed above. Pasted here: HKS bits; induction, manifold, exhaust, fcon, and boost controller.

ecu would be throwing error code for tps if it was a calibration issue.

standard ecu doesn't reboot as mentioned, it constantly learns and adjusts.

Simon

I've just ordered an OBD2 reader so will attempt calibrating the TPS myself but I have to say I'm not holding out much hope that this is the issue.

I'm wondering whether the butterfly, or spindle, is simply sticking and then springing open once there's sufficient tension in the cable? Maybe it's damaged as a result of bits of HKS foam mushroom getting stuck on the butterfly...

So did all this start after they had fitted your mods??

I have a saying, "if it worked before you took it into the garage and you had mods done and its not right after, remove the mods and get the garage to fix the problem they probably caused in the first place!"

Tony:D

As I said above, the mods were done years ago when the car was new. The only change I can think of was a new clutch a few hundred miles before the issue occured.

Tim, when you say butterfly or spindle might be damaged, is this something I could check myself?

Perhaps as simple as rotating the throttle spindle by hand and seeing if it moves freely? Maybe remove the throttle cable from the spindle so you can feel the spindle movement more easily?

Got it, thanks.

To update everyone, my ODB2 reader has arrived. THis morning the engine light came on again, and the code was P1507. Seems all I need to do to fix this is clean a valve.

Well, cleaning it doesn't seem to have helped. Does anyone know the part number for this on an 02 WRX? I assume the best next step would be to replace it.

Did you clean the entire assembly, and dismantle it? Might be worth removing the entire throttle body and cleaning it all - they tend to get gunked up inside.

I was unable to remove the motor or actuator from the end of the valve as it used some bizarre pentagonal screws I was unable to find a driver for (and Subaru dealer had no idea what I was talking about!). As such I have no confidence that the flap is moving inside the valve. Both times I have removed it, it has been in an identical position.

I'm not that technical, so if there is a guide anywhere for how to remove the throttle body then I might attempt it.

Thanks.

If you're not that technical, I wonder if it might be better to find a local independant subaru specialist, and have them remove it for cleaning and inspection?

Just spoken to the dealer again, and they have suggested the problem may lie with the neutral position switch. As the clutch has been recently changed, what is the likelyhood that the switch may have been damaged?

Anyway, that looks like something I can visually check myself so I'll have a quick gander.

Neutral position switch "looks" ok.

However, tried some other things as well.

1. The IAVC seems to be working. The valve spins when turning the ignition on, and if you disconnect the power when the engine is running, the idle starts to vary considerably.
2. I disconnected the the power to the throttle position sensor and the issue seems to have disappeared. The revs now take longer to fall (as I assume the ECU doens't realise the throttle position has changed), but there is no longer the jerky on/off application of power.

Does this further information suggest any further ideas?

Thanks all.

Maybe a knackered TPS?