2001 Fiat Punto Won't Hold Revs for NCT

choons

Hi, need some advise if anyone has experienced this before.

Had an NCT last week and failed due to the tester not being able to hold revs within a limit for the emissions test. Passed everything else and car drives fine.

Car is a 2001 Fiat Punto 1.2 8v. The test sheet reads "Emission test not done - throttle control faulty"

I've tested this myself and can hold high revs steadily but there is no rev counter on the dash. How do the NCT know what revs the car is at?

Any advice? I'll be trying to fix myself. Thanks

dgt

Throttle needs a clean by the sounds of it. Easy job to do

valerossi

Yeah try clean the throttle body with carb spray and tooth brush but don't take it off because you might cause a vacuum leak if you fit it back incorrectly oh don't use to much spray don't ya flood her. how does it idle when warmed up.

choons

This morning I used a toothbrush and carb cleaner but problem is still there. I looked up youtube vids on how to do this.

Normally car idles fine when hot or cold.

After I had attempted the fix today and was sitting in traffic, it revved itself in idle. First time it has ever done this

What's my next move?! Thanks for replies.

shamwari

OP, I'm assuming this car has a throttle cable and isn't a fly by wire jobby.

Can you explain exactly what happens when you hold the car at say 2000rpm? Do the revs fluctuate up and down like a sine wave even though you are holding the pedal position constant?

And specifically: have you adjusted anything on the throttle body, like the throttle plate screw?

Did you spray any carb cleaner down through the idle speed control valve on the front of the TB?

nct tester

throttle body replacement id say. but its a common problem on puntos so might be hard to get a second hand one that isn't affected by this problem. we get the revs in 4 ways, rotophon, battery clamps, obd or magnet.

shamwari

Spraying cleaner down through the ISCV is a no-no on these. What it does is dislodges any crud and flushes it down into the idle valve itself and causes it to jam. This in turn can cause any one of a number of faults. It isn't the end of the world though! By the way, the TB with the accelerator cable is far more reliable than the fly-by-wire one.

choons

shamwari wrote: »

OP, I'm assuming this car has a throttle cable and isn't a fly by wire jobby.

Can you explain exactly what happens when you hold the car at say 2000rpm? Do the revs fluctuate up and down like a sine wave even though you are holding the pedal position constant?

And specifically: have you adjusted anything on the throttle body, like the throttle plate screw?

Did you spray any carb cleaner down through the idle speed control valve on the front of the TB?

Yep, throttle cable.

Since there is no rev counter in the car I can only attempt to hold the revs at a speed that sounds faster than idle. When I try this, it holds steady for 3 or 4 seconds but then starts to increase RPM until I take my foot off the pedal. I have held the pedal position constant while doing this. I haven't held the revs long enough to see if they will come back down/follow a sine wave because I didn't want to damage anything. I'll try holding for a good few seconds today and report back.

Nope, have not adjusted anything!

Is the idle speed control valve the semi circle hole in front of the throttle butterfly in this photo?! I had to google this so I'm not sure. If that is the ISCV then yes, I sprayed a small bit of carb cleaner down!

choons

nct tester wrote: »

throttle body replacement id say. but its a common problem on puntos so might be hard to get a second hand one that isn't affected by this problem. we get the revs in 4 ways, rotophon, battery clamps, obd or magnet.

shamwari wrote: »

Spraying cleaner down through the ISCV is a no-no on these. What it does is dislodges any crud and flushes it down into the idle valve itself and causes it to jam. This in turn can cause any one of a number of faults. It isn't the end of the world though! By the way, the TB with the accelerator cable is far more reliable than the fly-by-wire one.

Interesting, thanks.

shamwari

Yes the semi circle hole is the ISCV. At idle, this valve is open whilst the butterfly in the main throttle behind remains closed. If this thing sticks then you can get funny idling or running faults. At idle, cover the ISCV portion with your hand and the car should cut out. If it stays running then there's and airleak somewhere around the TB base or the manifold. The manifolds are favourite if the car has overheated as they are made of plastic and can warp.

If someone had ajusted the throttle plate screw, the ECU will see a mismatch between throttle position and the expected idle speed and will therefore cut the injection pulses. I've seen this on more than one occasion.

choons

Okay, I put my hand over the valve and the engine cut out so no leaks there.

Today I replaced the whole throttle body assembly with a second hand one and still the same problem. I've tried resetting the ECU as well.

shamwari

choons wrote: »

Okay, I put my hand over the valve and the engine cut out so no leaks there.

Today I replaced the whole throttle body assembly with a second hand one and still the same problem. I've tried resetting the ECU as well.

oh dear. The exhaust isn't blocked by any chance?

Failing that, it'll need the live data looked at to see what's awry.

dgt

Disconnect the lambda sensor (s) and see if you can hold revs then. Don't worry about messing the ecu it'll just run with default values.

If this works, budget for new sensors

shamwari

dgt wrote: »

Disconnect the lambda sensor (s) and see if you can hold revs then. Don't worry about messing the ecu it'll just run with default values.

If this works, budget for new sensors

That's worth a punt(0) but if the exhaust is partially blocked then disconnecting the 02 sensor may actually clear the symptom but not the underlying cause.

A look on live data will see the 02 sensor going low (lean) when the revs cycle downwards during the faults occurence.

After swapping the TB, the op won't be happy if replacing the sensor makes no difference.

Mar4ix

if lambda (before cat) is same as on mk1 engine, i may lend it to check, i have one new and one used. But lambda will take effect on car engine control only when engine is properly warm.

choons

Okay so I tried to tackle the lambda sensor next. It was being covered by a heat shield which had to come off first. Unfortunately the bolts holding the shield on were rusted in place so I sprayed them and the lambda with some WD40. I disconnected the battery and left the car overnight for the WD40 to make its way in. When I reconnected the battery, I said I'd give the car a try to see if anything had changed.

Punto seemed to be holding revs for a few seconds but it still wasn't perfect. I don't know if it was the new throttle body or if removing the battery properly reset the ECU but it was better than the day before. I never got as far as taking out the sensor.

My NCT retest was booked for this morning so I said I'd chance it. Success! Scraped a pass and tester said there is still a problem holding the revs but it stayed steady for just enough time to get an emissions reading. Lambda result was 1.03 which is right on the limit so that needs to be replaced.

I had read online that sometimes on Puntos, you can steady the revs by turning on all electronics like full beams, fan and radio. The NCT lad did this during the test which may have helped.

I've college exams now but over the summer I'll change the sensor, do a full service and new break pads.

Thanks for all the suggestions and replies

tl;dr - New throttle body and reset ECU. Passed NCT but still need new lambda sensor. Thanks.

Mar4ix

thats good new, that you passed nct, but if lambda doesnt work right, and you keep waiting till better times (i mean financially), it may damage other parts .... like catalytic converter or other lambda, which is behind that. also fuel consumption goin to be higher.

when you changed tb , did you changed with all sensors ? or you put your own sensors in it ? sticking revs could be from very simple .. like damaged accelerator cable, or could be TPS (throttle position sensor) .

choons

Mar4ix wrote: »

thats good new, that you passed nct, but if lambda doesnt work right, and you keep waiting till better times (i mean financially), it may damage other parts .... like catalytic converter or other lambda, which is behind that. also fuel consumption goin to be higher.

when you changed tb , did you changed with all sensors ? or you put your own sensors in it ? sticking revs could be from very simple .. like damaged accelerator cable, or could be TPS (throttle position sensor) .

Yeah it's not ideal but will have to wait until I have the time and money!

I changed the whole TB including the two sensors that were built in (TPS and another one). The revs are not sticking, they are increasing themselves without movement from the pedal after the pedal has been slightly pressed.

I got the new throttle body from an ad on donedeal for €20 so there is a chance it is also broken. A fiat dealership quoted me €500 for a new one (:eek:) so I'm happy enough for now.