Throttle position sensor?

[mhibbins]

My 91 S3 started behaving very strangely indeed on saturday. Within the space of a mile it went from running perfectly to a real mess. It wouldn't hold a steady throttle (it would kangaroo along) but was smooth on no throttle (steady deceleration) and happy on full throttle (full smooth power all the way through the rev range) but on anything inbetween it would jump all over the place. I took the throttle position sensor off and measured the resitance and it smoothly went from about 500ohms to 5000ohms as I turned the pot so it seems ok. Is there anything else that would cause such symptoms and show deterioration over such a short time? Thanks, -- Mark

[LeeBee]

Sounds like a typical throttle switch fault to me.I think they are best checked with an oscilloscope as these show up any noise on the track.Mine went a while ago and it seemed fine with a multimeter?

[Paul V]

I just had exactly the same problem, the car would be running fine, then cut completely then come back in with a burst. I put it into Dartford Tuning who changed the Fuel Potentiometer, the car is now back to normal.

[TVR Mark]

It does sound like the throttle Potentiometer I've had 3 go in 5 years. The other possiblity is the bypass valve located on the front of the plannum chamber

[Roy C]

Make sure the air meters are clean too. The last time I thought the Throttle Pot was on the way out, it turned out to be dirt in the air meters causing similar symptoms.

[mhibbins]

I dumped it at the local garage (didn't have time to sort it out for myself) and they have fixed it - sort of. They found it was running on at best 2 cylinders and more often on one (eek). The throttle pot should really affect all cylinders equally (I imagine) so that shouldn't be at fault. They swapped the airmeters over but the same cylinders were running. They pulled the plugs and found the rest fouled and soaked. They cleaned it all up and now it's running fine. The problem I have is that I don't know how this can happen while simply driving along. I did cut a chunk off the end off the crank case breather hose as it was kinked and just before the car started to misbehave the remaining hose got hot and collapsed <cries of "ah, the truth is coming out now">. Could this have initiated the fouling of the plugs? I've now replaced the hose with a bit of coolant hose until my proper breather hose arrives. Any comments? -- Mark

[TVR Mark]

Which breather? Have you got 2, 1 on each bank? If you are talking about the breather for the right bank (when sitting in the car) and if it is anythink like mine in goes into the plannum chamber and is under vacuum. Coolent hose will colapse and under extreme driving eg track days will blow oil out of the left breather into the air filter. It can't do the engine much good in normal driving. Edited by TVR Mark on Thursday 8th March 18:57

[mhibbins]

quote:Which breather? Have you got 2, 1 on each bank? If you are talking about the breather for the right bank (when sitting in the car) and if it is anythink like mine in goes into the plannum chamber and is under vacuum. Coolent hose will colapse and under extreme driving eg track days will blow oil out of the left breather into the air filter. It can't do the engine much good in normal driving. Edited by TVR Mark on Thursday 8th March 18:57

That's the one, right bank to plennum. Like I say the coolant hose is only a temporary measure until I get the correct hose tomorrow. -- Mark

[chin up]

Umm, mind if I chip in chaps? My S3C (no air flow mters) runs in a similar manner to that described. I have been planning to replace the pot to cure the problem, but upon reading this I checked my vacuum hoses. I found one which exits the underside of the plenum chamber on the left (or near) side, about one third of the way into the chamber from the back. With me so far? It then runs around the rear of the engine to the right hand side, and joins onto a T-piece. This tube has been completely flattened along its entire length by the suction. It's lazy of me really, but could I ask you guys if you know what this pipe does? It goes without saying that I'll replace it, I'm just busting to know if it will cheer up my rather 'grumpy at idle' S3C. Thanks in advance fellas... chin up

[mhibbins]

No idea what it does but I took the old hose to my local ford dealers and they found it using the code marked on the pipe. Since replacing it I have absolutely no problems with the idling or any of the other symptoms I described below. Given that the hose was about 3 quid and the pot was about 65 quid, I'd say it's worth a go. When you take out the old one, remember what route it takes and put the new hose in the same way making sure you don't ask it to make any sharp turns. It may not look kinked but it will kink and block under vacuum. Good luck, -- Mark

[chin up]

Many thanks for the tip Mark. As you say, 3 beats 65 every day of the week. Ta again, chin up

[johno]

There are effectively two hoses running around the engine bay concerned with oil breathing. The hose from the right hand bank is connected to the rocker cover through the Crankcase emission valve and provides a link into the plenum under the throttle cable attachment. There is also a 'T' piece before that joining another hose which then feeds into the side of the air filter. This is a TVR fix, and replacement air filters do not come with an attachment for this pipe. If the pipe connection to the plenum is lose or the blocked in any way it will upset the mixture in the plenum and therefor up set your set up :-) If the pipe becomes disconnected the plenum will suck in tonnes of air etc etc The connection to the air filter is to provide assistance in meeting emissions. I've re-routed mine out of the air filter as I'm fed up with all the cr^* it throw's onto the flow meters. If your totally unlucky like myself you can have the inlet manifold out twice to try and nail an air leak which two dealerships couldn't find to then work out it's a leaky brake servo instead !!!!!! Chuffed, oh yeh.... Cheers Mark

[johno]

Forgot to mention that I have gone through several throttle pots and now use the Pot for the Cerberra S6 instead of the Ford one as these are only £18 rather than £50 odd. Cheers Mark

[johno]

If the engine is idling upto 1000 rpm ONLY when you are stationary when you put your foot on the brakes but otherwise idles fine then it may be that your servo has a leak. The best way to check is to start with the one way valve that is fitted into the servo. Disconnect the pipe from the plenum to servo and try to blow down the tube. If you can blow down the tube then the valve is knackered. The way the servo works is that there is a vacumm created by the feed to the plenum and this vacuum assists your foot power. The one way valve prevents that vacuum being lost if the engine is stopped for any reason and you need to keep braking. If you switch the engine off you should still have at least 2 to 3 pushes of the brake pedal before you loose the vacuum assistance. If the valve is gone then you will not get this. After you've checked the one way valve is operating correctly then you need to check the servo housing itself. In my case both the valve was knackered and there was a hole in the body. The hole itself was not visible until I really poked at it. Favourite area would be behind the wheel arch where water/moisture gets collected at the bottom of the body of the servo unit. Mine was flaky/corroded and although not visible after I had poked at it I opened a hole that was 2 inches long ! Remember the engine can exert a lot more suction than you can and it will suck air in from anywhere it can get it. If the servo is leaking, it won't need to be a large hole to upset the engine idle and the ECU will continually try to correct the problem. I didn't get a problem as you desrcribe, rather I had a latching idle and no adjustability of the mixture setting by the flow meters. Yours though may be the early stages that mine jumped ! Hope this all helps.... Cheers Mark PS - Let us know as I'm intrigued over that one

[johno]

is the idle holding at 1000rpm or just going up for a few seconds and then coming back down ?

[johno]

This is normal behaviour. As long as the revs do not go up to silly heights then it is fairly standard. For instance if you rev the engine then it won't settle straight down to idle immediately, taking a little while to come down to static idle. It just seems to be the behaviour of the engine.