Speedo not working - running rough

[mcelliott]

The speedo on my '01 bug eye wagon has been a little temporamental recently and as of today has totally stopped working. The management light then came on and the car starting to run really rough. I dropped the car into the garage and they ran a diagnostic check which came out with a speed sensor and idle control system malfunction. Are the two linked? Hopefully it is just a new speed sensor that is required but would appreciate any comments.

[ScoobieWRX]

The speed sensor tells the ECU how fast the car is going. The ECU needs this information so it knows when to signal the IACV valve to settle the idle. Idle is slightly higher when you're coasting than when you are stationary. You have two levels of stationary idle as well, one for when there is no load on the alternator (lowest idle) and the other (higher idle) for when you load the alternator with stuff like air conditioning, headlights, etc...

Generally with Subaru it only needs one genuine issue to potentially set off a whole bunch of DTC's (error codes) because so much stuff is linked together. When one bit goes pete tong the rest shows an error in sympathy.

This may be a reason why your car is running rough but i'd have said it's got more mechanically to do with your IACV than the speedo sensor itself. If your IACV valve starts playing up it can cause some rough running low down in the rpm scale on a light throttle but that's not to say some of that could also be attributed to the speed sensor issue.

ETA: You can take the cover off your IACV valve to give it a squirt inside with carb cleeaner. It may well be fuill of crap. You will have to buy the tiny gasket that seals the cover to the IACV housing, its made of thin rubber and they tend to become enlarged over time and stop sealing. If you get a leak from a dodgy IACV gasket you may experience as vacuum leak and that in itself causes running issues.

Alternatively your car may have gone into limp mode due to the speed sensor malfunction thereby restricting it's performance.

Edited by ScoobieWRX on Thursday 20th December 20:39

[markCSC]

My speed sensor went up the creak and I experienced the same problems and error codes. Bought and installed a new sensor (well a second hand one) and all was fine again.

Not to hard to install yourself. If you take it to a dealer they will remove the downpipe to get the old one out and charge you for the privilege (there is no room to undo the sensor when the downpipe is in place). All you need do is use a pair of mole grips. It will bugger the sensor but it doesn't matter if you are installing a new one.
Installation is much easier as you won't be whacking the downpipe went tightening it.

[mcelliott]

Thanks for the replies, part duly ordered (speed sensor) so should be running as good as new before long.

[gm38]

Was going to start a new thread but spotted this one - I have a similar problem on my '97 wagon STi - speedo stopped working, engine light on.

Except after a good look under the car, I spotted a cable hanging down, with a metal 'rod' type connector on the end. I'm thinking this can't be a coincidence! It was hanging down along the exhaust manifold. Is this in fact the speed sensor and it's somehow disconnected itself? It seems feasible as a couple of weeks ago it sounded like something fell off from under the bonnet. No idea what it was and everything was fine until the other day when the speedo stopped working. I reckon maybe I lost some kind of housing and the lead has eventually worked loose.

Question is - where exactly should this thing be connected? I couldn't see anywhere obvious in there.

Any help much appreciated - I'm getting paranoid trying to guess what speed I'm doing!

[ScoobieWRX]

May well be the speed sensor that fell out, rattled its way loose with engine/suspension vibrations. You need to ge under the car and look at the back half of the gearbox to see if there are any disconnected or missing sensors.

[markCSC]

If it has fallen out you should see a hole in the gearbox near the dip stick on the drivers side.

[gm38]

Thanks for the pointers guys - I'll have another look around the gearbox as soon as I can get back under there (might need to get it onto a ramp somewhere to get a good look...)

[markCSC]

sensor should look like this



Unless you have a v1 or v2 which use a cable

[markCSC]

If it doesn't look like that it could be the "cat on fire" sensor (exhaust gas sensor). I had one on my original downpipe but the new one doesn't have a hole for it. Therefore it is unplugged and tied out of the way. This might be what you found

Looks like this

[gm38]

Thanks MarkCSC! Both very useful clues (I like pictures). It is an old model but I think version 3 sti from 1997 (import). The lead doesn't look like the speed sensor pictured - but could be more like your second picture from what I can tell (in which case, this may have nothing to do with my speedo?!) - I'll check it out but may have to be the weekend...