Polo 1.4 16V Auto Running Lean

[SilverBKY]

I need some help. I know its not a Skoda but its the same family and this Forum seems helpful.

 

I have a Polo 1.4 16V Auto with a BKY engine and 90K on the clock which was running fine until it started losing power and revving alot before changing gear (it has a old style torque converter gearbox). Now it will not rev above 3500 in or out of gear and on the last drive would not manage over 50mpg on the flat. It is running lean as the plugs were white and the pre cat lambda sensor was also white. The symptoms as now very consistent although its degrading to this state was gradual

 

So far I have changed

Auto Gearbox oil as I though this may be the reason for over revving. No change.

Fuel Filter. No change.

Throttle Body. No change.

MAP sensor. No change.

In tank fuel pump. No change.

Spark Plugs. Observed to be white. No change.

Pre Cat Lambda sensor. No change.

 

I have a VCDS and it had codes (which I have left in)

16804 - Catalyst System: Bank 1

   P0420 - 35-00 _ Efficiency Below Threshold

17550 - Load Calculation Cross Check

   P1142 -35-10 Lower Limit Exceeded - Intermittent

16517 - Oxygen (Lambda) Sensor B1 S1

   P0133 - 35_00 Response too slow

18049 - Please check DTC Memory of AC controler

  P1641 - 35-00 -

 

Any ideas ? I am really struggling with this , the clue HAS to be the white plugs which is lean fuelling but why ????

 

John

[Breezy_Pete]

Have you checked the brake servo vacuum hose and all its connections for cracks and leaks?

[SilverBKY]

Thank you Pete.

 

I have just been out and looked at that pipe, it has a junction 1/2 way along which goes off to the air filter. It looks good and I wriggled it with the engine running which caused not even a fluctuation in the revs.

 

The connection of that pipe to the air filter was not super secure but I tried to improve that which I think I have but sadly with no affect.

 

Any other ideas, I'll try anything. (within reason).

[SilverBKY]

It has to be something like that doesn't it. If I pull the servo pipe off under the throttle body and block it up then that whole system will be out of the equation.

[Breezy_Pete]

You know that will make the brake pedal require a hell of a lot more pushing to get any braking effect, I presume.

[Breezy_Pete]

EVAP purge valve hose to manifold and PCV valve ouput hose are other possible sources of airleaks.

[SilverBKY]

But wouldn't they affect the low speed running rather than the full throttle ? I've had a good look at those pipes and they look OK. Pulling that servo pipe off looks potentially terminal with snapping of bits of plastic a danger.

 

There is a crankcase breather pipe around the back there.

 

Is the EVAP valve the little one with a wire going to it on the drivers side strut top ? the PCV the crank breather ?

 

I realise that i have omitted the key info that it starts and idles perfectly.

[Breezy_Pete]

4 hours ago, SilverBKY said:

Is the EVAP valve the little one with a wire going to it on the drivers side strut top ? the PCV the crank breather ?

Yes, well near the expansion tank black lump with in and out hoses, with two wires; and yes there's a hose from the crank breather module on the back of the block that comes up to the inlet mani just under TB.

Edited 18 December, 2022 by Breezy_Pete

[SilverBKY]

Hello Pete and others. I have had the VCDS plugged in and run the engine after warming up and a quick blip on the throttle it settled into this state.

 

My impression is

-the Lambda is about right at 0.44V, I understand the desired Stoichiometric is 0.45V.

-idle speed is good.

-the inlet manifold pressure is 433mBar and is rather higher than the 300mBar I see on a different thread I caught on this forum. I swapped the MAP sensor for another and it is exactly the same and with the engine running reads high 900's which is atmospheric. So all sounds good with the sensor if not the result.

-the Bank 1 Lambda Control of -10% means that the ECU is trimming the injection low to keep the lambda at close to 0.44V

-the throttle drive at 13.7% no idea.

 

With the engine running I tried wafting an unlit blow torch around to see it it would pick up any air leaks apart from finding that lambda control value drops to about -40% when playing on the throttle body it showed nothing.

 

I tried pulling the brake servo pipe off the moulding below the throttle body and blocking the hole, no effect.

 

Like the earlier message above from Pete it appears to be an air leak in the inlet tract but can I find it...... Any ideas ?   Thanks    John

 

[Breezy_Pete]

Does this BKY have an EGR valve? I seem to remember some do, some don't. 

[SilverBKY]

No EGR on this BKY, it appears that some do. I had a look having read some of this Forum.

[J.R.]

9 minutes ago, SilverBKY said:

I tried wafting an unlit blow torch around to see it it would pick up any air leaks apart from finding that lambda control value drops to about -40% when playing on the throttle body it showed nothing.

I like your logic!

 

The obvious suggestion would be the throttle spindle bearings and seals.

[J.R.]

My not so reliable memory tells me that when I used to play around with Lambda sensors the reference voltage was 0.9 or 1 volt for Lambda 1.0, maybe it is lower on the later generation.

 

I keep a moving coil analogue multimeter simply for testing Lambda sensor outputs, whether it be 0.9v, 1.0v or 0.45v what I look for is the voltage rapidly switching high and low around that median voltage.

[varooom]

10 minutes ago, Breezy_Pete said:

Does this BKY have an EGR valve? I seem to remember some do, some don't. 

 

9 minutes ago, SilverBKY said:

No EGR on this BKY, it appears that some do. I had a look having read some of this Forum.

 

Your memory is good, here is where the change occurs

EGR = Yes (▸model year 2006)

[Breezy_Pete]

Inlet manifold to head o-ring seals? Did you try your butane near that junction?

 

[SilverBKY]

Good evening. I have taken the inlet manifold off, which was amazingly easy.

 

The head o-rings seals were hard and old but I am desperate so I using Loctite gasket paste both in the groove in the plastic, with the old seal in there and on the surface against the head and carefully put it back on. I arranged blocks for all the pipes from the EVAP valve, the PCV and the servo, i.e. blocked the holes in the inlet manifold and started it up. after a process of elimination it appears that the fat pipe from the PCV valve (low on back of crankcase) sends the trim over 10%. having blocked both pipe coming from the PCV and the hole in the inlet manifold and warming up it gave.

 

697 rpm, 375mBar. with -2.0% trim and 0.44V on the lambda. So quite an improvement on yesterday.

 

So my hopes were up and a test drive..... which was disappointing, when you are trying to accelerate the trimming leans the mixture giving

 

2268 rpm, 975 mBar with -17% trim and 0.44V on the lambda (the lambda does move about abit and during warm up gives a result of more than a volt which I presume is the heating phase).

 

I am convinced that this leaning off is causing the performance issue.  Any ideas of the next step ?   Please.   John

 

[SuperbTWM]

Do these have a vacuum operated FPR that could be faulty?

 

[Breezy_Pete]

No. Fuel pressure is fixed at 3 bar by combined filter/pressure regulator near tank.

 

 

[Breezy_Pete]

https://www.clubpolo.co.uk/topic/342411-dtc-17550-17558/

 

May be worth a look.

[SilverBKY]

Thanks for the link, I swapped the throttle body early on and it made no difference so I think cleaning the throttle body as that person seems to have done to fix their car I do not think will fix mine, but I can give it spray anyway, I have nothing to lose.

As you may imagine I have been thinking about this, does anyone have the throttle body % opening figures for when the engine is idling and warm. My results last night are as below and the pressure is less vacuum than previous forum discussion at this address. www.briskoda.net/forums/topic/456368-intake-air-leak

VCDS Block 003  680rpm, 373Mbar, 13.7% Throttle Drive, 3deg Ign

VCDS Block 033  3% Lambda Control (Fuel Trim), 0.444V Oxygen Sensor (Lambda)

 

I have suspected the exhaust catalyst is blocked hence drastically reducing the power and unable to rev it due to exhaust flow rate. This would explain 2 things.

1. why the fuel trim is all wrong at higher revs as the ECU is encountering an unmapped exhaust restriction so to keep the lambda at 0.45V has to back off the fuelling.

2. the pressure is high when idling as the throttle body has to be more open than it ideally should be to overcome the exhaust back pressure.

 

With 90K and 17 years on what looks like the original exhaust system it may be on borrowed time.

 

How is my logic or am I getting excited for a solution again ??????    John

[Breezy_Pete]

Sounds like a logical idea, but fairly pricey to replace. Finding an OE type twin cat exhaust section may be a big challenge, most aftermarket replacements omit the precat, I think.

[SilverBKY]

Interesting, you are right I have had a look and I have not found a twin cat pipe. Can I fit a single cat pipe to the car?

 

If I am off line for several days Christmas has got in the way.

[Breezy_Pete]

Yes, the single cat ones must be adequate to pass an MOT; they will have worse emissions early in the warm-up phase of a drive cycle, but MOT tests up-to-temp tailpipe emissions only.

[SilverBKY]

I thought that I would update you on this as I have fixed the car.

 

As discussed above I bought an exhaust pipe front to back so 3 bits, the single catalyst front section and the 2 silencer sections for the rear end. All purchased and delivered for a remarkable £182.

 

They were fitted and the car drives nicely now.

 

The forensic examination of the exhaust showed that the centre catalyst was clean and solid whereas the front catalyst was partly blocked as can be seen on the photo. It is clean on the downstream side. The new exhaust pipe information sheet indicated that this is possibly due to incorrect fuel or poor running. Well it does burn a little oil, but only a little. Attached is a photo of the top of the front cat through the hole it bolts onto the manifold.

 

I am pretty sure that this part blocked catalyst prevented full revs and confused the engine management unit as the inlet manifold pressure sensor and throttle body positions were way different to the map in the ECU.

 

Thank you for your help and advice and if anyone else has such an issue they can shortcut to the solution.

[Breezy_Pete]

Great result and thanks for letting us know what fixed it. 🙂