Accelerator Pedal Potentiometer.

[LB123]

Daughters 1.4 BBZ Fab has an uneven seeming missfire on very light throttle. Only at about 1500 to 2000 rpm steady drive at about 1-5% throttle. i.e. when almost coasting. Idle is steady and acceleration on any more than 10% throttle is smooth.

VCDS shows no misfire code. It has just had new coils plugs and EGR after failed No 3 coil and EGR problems. For the first time since I have been maintaining it is registering no fault codes in any module. I'm puzzled.

 

Is there any tendency for the accel potentiometers to wear the resistance track at the 0 - 5% range to the point of giving an uneven/jumping signal which might account for the jumpy ride when almost coasting? I can't think of any other explanation.

[CWARD]

They can fail as I've just found out with the wife's car. With her car, not a Skoda, it was sometimes unresponsive to the pedal input  but reading up on it can include hunting on a steady input and poor fuel economy. 

 

I replaced hers with one from a breakers but if one of your cars has the same pedal design you could try swapping them over to test before you go spending any money.

[KenONeill]

@LB123 - Mine did fail, which showed up as an initial lack of response rolling back on in low speed corners, and eventually a complete fail which threw a CEL and put the car in limp mode (fast idle at 1_200rpm which was enough to give me about 35mph to get to a garage).

[Breezy_Pete]

Watch the resistance values of the two pedal potentiometers in VCDS measuring blocks while moving the pedal in that range?

[LB123]

Had a long look in VCDS and can't detect a failure. Sampling rate of ECU is obviously very rapid. VCDS much more leisurely. Might not be up to detecting tiny glitches.

[Breezy_Pete]

Maybe.

Worth trying another pedal if you can get one for not much money, I guess.

[sepulchrave]

More likely a vacuum leak causing the issue, therefore bypassing the throttle body, incredibly common on this engine at this age, check ALL the vacuum pipes for splits wherever they fit onto barbs.

[LB123]

3 hours ago, sepulchrave said:

vacuum leak

Will do. Only pipes going into the manifold downstream of the throttle body that I can find are EGR pipe and servo line. Doesn't seem to have an activated carbon filter canister.

Am I right in thinking there is no MAF, only MAP sensor?

[sepulchrave]

2 minutes ago, LB123 said:

Will do. Only pipes going into the manifold downstream of the throttle body that I can find are EGR pipe and servo line. Doesn't seem to have an activated carbon filter canister.

Am I right in thinking there is no MAF, only MAP sensor?

 

Dead right, that servo line ALWAYS leaks somewhere, usually at every joint, cut the split bits off, heat the pipe and push it back on while still hot, it shrinks to fit tightly.

[Breezy_Pete]

Carbon canister is behind rear wheel arch trim, purge valve near expansion tank.

EGR, PCV, purge valve and servo line all feed in below throttle on my BBY, I'd imagine BBZ would be same?

 

 

Hehe, just noticed my servo line has split, just servo side of the non-return valve, top edge of the picture. Thanks!

Edited 12 October, 2017 by Wino

[LB123]

Mine was split in exactly the same position but has been repaired. See previous pic in my EGR thread.

[LB123]

My search for an air leak has failed. No change with the replacement throttle pedal. Still almost like a miss on almost trailing throttle. Still no codes in VCDS.

 

HVAC needs a further look. I'm sure one of the family fabias used to report its HVAC in VCDS. This one doesn't. Enquiry to group 8 (HVAC) reports 'No response from controller'. So it doesn't seem to have a dedicated one. (Checking previous logs from 3 years ago it didn't have one then either. Must have been the 1.4 MPI that had one.) I can't be sure there is not a leak in the hidden pipework or the canister itself.

 

So question - Can anyone tell me when should the valve on the HVAC be open and when closed? 

[sepulchrave]

Do you mean the fuel tank purge valve? HVAC is air conditioning.

[Breezy_Pete]

Yeah, EVAP, I reckon. Not sure about when the purge valve is busy, but I think it's proportional/ PWMed rather than fully open/shut. I'll see if I can dig up any relevant info tomorrow, if I get a minute.

This seems relevant/useful: http://www.samarins.com/glossary/purge-valve.html

Edited 18 October, 2017 by Wino

[sepulchrave]

I think the EVAP is just an open or shut solenoid, it's open when the car is started and closes once you drive away. Check it's not stuck open.

[Breezy_Pete]

I wonder if your PCV diaphragm is bust. Awkward place to check on these though. Try sticking your finger over the little rectangular port where the arrow is pointing, if you can reach it, while engine is idling, minding aux belt!

It shouldn't be sucking.

 

 

Edited 19 October, 2017 by Wino

[LB123]

Ah. HVAC is aircon. Right. That would explain as the old MPI had aircon and the 16v doesn't. Another mystery cleared up.

 

Yes, I do mean EVAP. If I have an air leak into the manifold bypassing the TB but no apparent leaks in the pipes I can reach then EVAP and crankcase ventilation must be candidates. Thanks for the pointers. I will get on it.

[LB123]

Right,Wino. The hole in the PCV doesn't seem to suck. My finger couldn't detect any definite vacuum.

 

EVAP valve is closed with ignition off, and ign on but engine not running and at idle. It stutters (?slow PWM effect) open on about 3% throttle but seems to be closed above about 10% throttle (but its difficult testing and reving the engine outside my house.) Quick dirty test was to block off the EVAP pipe to the TB and the fault seems to have gone. Certainly much better. And no fault light complaining about the blocked off pipe! Yet.

 

I suspect the system is doing what it should. Stuttering open when it should and closed when it should. This also clearly is associated with the same conditions which show the 'almost trailing' throttle judder. Why this should upset the smooth running I don't know for sure but it might be theres a leak under the car or the cannister itself letting in excess air on almost trailing throttle.

I will drive it for a bit and report back but feel optimistic. Many thanks.

Edited 19 October, 2017 by LB123

[Breezy_Pete]

Good work.

You could eliminate the purge valve itself possibly by unplugging the electrical connector and unblocking the hose you've blocked, and see if the fault comes back.  With it un-energised, it should behave exactly the same as if it is blocked off, (unless there's a leak in the hose between it and the manifold). You will get a fault light I should think, but it should clear once you put everything back to normal.

[LB123]

Good and bad news. Fault light came on as the day wore on. Cancelled and returned. But it continued to perform better. The reasoning I think is that I do have one or more air leaks causing the problem. Blocking the EVAP pipe reduced the air entry that is bypassing the TB by enough to improve running. My conclusion is the EVAP is probably OK and the air leak(s) are probably elsewhere and I have failed to identify it/them.

Next step is strip inlet manifold, PCV, TB and the associated pipery within the engine bay and clean and check the lot on the bench. All new gaskets/O-rings when assembling. Will try to test those sensors and valves that are testable and replace if needed.

[sepulchrave]

34 minutes ago, LB123 said:

Good and bad news. Fault light came on as the day wore on. Cancelled and returned. But it continued to perform better. The reasoning I think is that I do have one or more air leaks causing the problem. Blocking the EVAP pipe reduced the air entry that is bypassing the TB by enough to improve running. My conclusion is the EVAP is probably OK and the air leak(s) are probably elsewhere and I have failed to identify it/them.

Next step is strip inlet manifold, PCV, TB and the associated pipery within the engine bay and clean and check the lot on the bench. All new gaskets/O-rings when assembling. Will try to test those sensors and valves that are testable and replace if needed.

 

I agree, but you don't really need new gaskets and o rings, just put it back together with some RTV, this will also give you a chance to get all the oily gunge out of the manifold as well, there was a shocking amount on my 1.4 when I did this and it ran much better afterwards.

[LB123]

Thanks. Good to know. Manifold is filthy inside.

Finally got tired of nibbling round the edges of the problem. There's a leak somewhere and I don't think it's in an accessible place where I can find it.

Will report findings.

[LB123]

Manifold, TB, EGR and all the inlet gubbins stripped and cleaned. I didn't identify a specific leak although some of the 'rubber' seals were a bit crinkled. All reassembled with RTV sealant on every joint including pipework joints.

Problem fixed. Idle and light throttle smooth as silk. Sorted.

 

EGR and TB adapt both ran and OK'd. No fault codes. Every one who helped, particularly with the diagnosis of 'air leak' many thanks. I didn't find it but I fixed it!

 

Time to give it back to daughter No 3.