Intermittent misfire, leak and implausible MAP correlation

BearMcPear

BearMcPear

Member
Hello everyone. After cleaning up the throttle body and the flange its connected to as well as recalibrated the throttle body, my A2 has been behaving quite weirdly. Sometimes it runs perfectly fine until it misfires when pulling away or at idle and then continues to run poorly. Sometimes it misfires terribly when I start it, but runs perfectly fine after a restart. Another symptom is the engine shuddering every ~5 seconds when the oil cap is opened.
A scan showed the following errors:

16685 - Cylinder 1
P0301 - 35-10 - Misfire Detected - Intermittent
16685 - Cylinder 1
P0301 - 35-10 - Misfire Detected - Intermittent
17961 - Barometric / Manifold Pressure Signals
P1553 - 35-10 - Implausible Correlation - Intermittent
17912 - Intake Air System
P1504 - 35-10 - Leak Detected - Intermittent

I thought that perhaps I had made a mistake with putting things back together, so went through it again yesterday. The car feels the same, but after a scan only the cylinder 1 misfire error remains. Could be that it simply hasn't diagnosed the other errors yet.
The symptoms are akin to this thread, however no solution was found there sadly. Does anyone know what the issue could be or how I could diagnose it?
 
Correct throttle body recalibration requires a sound and properly charged battery, otherwise you end up worse off than when you started.

I'm presuming that you used a registered copy of VCDS to do the recalibration?
 
This symptoms sound like the petrol engines' standard response to a misfire issue. The ECU counts misfires on each cylinder and after a certain number on any particular one, it assumes there's an issue with that cylinder and stops injecting fuel into it, to save the catalytic convertor. The count is reset with each restart. Sadly that doesn't help you get any closer to the cause though :oops: .
 
steve_c said:
Correct throttle body recalibration requires a sound and properly charged battery, otherwise you end up worse off than when you started.

I'm presuming that you used a registered copy of VCDS to do the recalibration?
Click to expand...

Yes. The battery might have been low voltage the first time, but I did it again when the battery was normal. Worth hooking it up to a battery tender and trying again?
 
Try a compression test. There seems to be a trend with the 1.4 petrol engine, for one cylinder (usually number 2), to loose compression, likely due to a valve not quite closing. Hydraulic valve lifters seem to be the prime suspect.
Mac.
 
If the symptom is loss of compression on one cylinder, the only way to diagnose whether it's lifter/cam related, or valve seat related, is to remove the cam cover (actually the cam box, as cams and lifters are integral within it), and repeat the compression test.
If compression is OK, it's a cam/lifter problem. If compression is not OK, it's probably time to remove the cylinder head.
I realise this is much easier said than done.
Mac.
 
I had a somewhat similar issue, and a lot of the problem seemed to be down to air leaks. The throttle body on my 1.4 petrol was effectively in two parts, with an aluminium gasket in between. Air leaks between the two parts caused many of the issues you are experiencing.

The other thing that may have been an issue (though I repaired it before I found the leaks in the throttle body) was where a crank-case breather hose (running from the throttle body to the crank case breather at the back of the engine) had rubbed through on a brake line, making a small hole.

I suggest you start by making sure there are no air leaks on the wrong side of the throttle butterfly before you panic about more serious potential causes.
 
mowog said:
I had a somewhat similar issue, and a lot of the problem seemed to be down to air leaks. The throttle body on my 1.4 petrol was effectively in two parts, with an aluminium gasket in between. Air leaks between the two parts caused many of the issues you are experiencing.

The other thing that may have been an issue (though I repaired it before I found the leaks in the throttle body) was where a crank-case breather hose (running from the throttle body to the crank case breather at the back of the engine) had rubbed through on a brake line, making a small hole.

I suggest you start by making sure there are no air leaks on the wrong side of the throttle butterfly before you panic about more serious potential causes.
Click to expand...
Do you know of a good way to check whether it leaks there? I think it could also be possible that it leaks below the bottom flange, as the gasket ring is quite high and doesn't enter its indentation very easily.
 
BearMcPear said:
Do you know of a good way to check whether it leaks there? I think it could also be possible that it leaks below the bottom flange, as the gasket ring is quite high and doesn't enter its indentation very easily.
Click to expand...

Well, I guess the traditional way to check for that kind of leak is to spray something non-damaging around the potential leak while the engine is running. Would have though WD40 would be a safe bet.

Personally I 'tested' it by taking both halves off the car, cleaning them and putting them back together with a little silicon instant gasket on either side of the aluminium gasket. Let it go off for a couple of hours before trying a restart, and after that it was much improved.

Do check that hose as well, as I gather rubbing through on the brake line is really quite common.
 
After disassembling the intake again such that I can try some silicon gasket, I found that the gasket I received instead seems to be three pressed onto each other. 🤔
 
First time after the whole silicon gasket thing, it ran quite poorly again. Went to get my laptop for VCDS and it ran fine of course. 🤦‍♂️
If it stays like this I will take it to a mechanic.
Another thing I noticed is a very slight rubbing sound, as if something is very slightly rubbing against the timing belt. Has anyone else heard this?
 
mowog said:
Ah, gutted. Sorry to hear that - it really sounds like an air leak, but if you can't find one then I suppose it must be something more complex... :(
Click to expand...
Yeah it probably is. Maybe too well hidden for me. Maybe I will take it apart again over the weekend and go for the lots of silicone gasket approach. ;)
 
BearMcPear said:
Yeah it probably is. Maybe too well hidden for me. Maybe I will take it apart again over the weekend and go for the lots of silicone gasket approach. ;)
Click to expand...
Can I recommend a really methodical approach? Strip off the throttle body, both bits, until you just have the mouth of the intake manifold staring back at you. Check that for any possible route for air leaks first. You could perhaps even try blocking it with a cloth and using a compressor to blow air in, in the hopes of hearing if it's escaping from around the manifold.

Feel along the soft vacuum pipe (or is it pipes - can't remember if there's one or two!) all the way down to the back of the engine block, looking for any signs of damage. Again, maybe try blowing into them to show any leaks.

And as you assemble the throttle body, check that it's sealing correctly at each stage. Remember that even a small leak on the wrong side of the butterfly could cause some weirdness if the engine thinks it's getting more air than it should.

IIRC you shouldn't need to add the airbox in order to run the car - having the throttle body open to the air shouldn't make a difference, and it gives you better sight over and around the engine.

I do recall that I also replaced my MAP sensor too, though can't for the life of me remember if it helped. Was cheap though, so didn't mind
 
BearMcPear said:
After disassembling the intake again such that I can try some silicon gasket, I found that the gasket I received instead seems to be three pressed onto each other. 🤔
Click to expand...
This happened with mine - I ordered the metal gasket through TPS and it was brought to the pickup centre a couple of days later. I found that I had 4 or 5 of them - it looks like they are pressed out and stacked for storage, and when they are picked for a delivery it is clear that often more than one is stuck together as you need to really have good fingernails and eyesight to get them apart and I guess the warehouse people are working on a very tight timeline so probably don't waste time ensuring that only one is dispensed each time. If the mechanic fitting them is also under similar time pressure it probably results in more than one getting inserted which presumably doesn't guarantee a perfect seal.
 
mowog said:
Well, I guess the traditional way to check for that kind of leak is to spray something non-damaging around the potential leak while the engine is running. Would have though WD40 would be a safe bet.

Personally I 'tested' it by taking both halves off the car, cleaning them and putting them back together with a little silicon instant gasket on either side of the aluminium gasket. Let it go off for a couple of hours before trying a restart, and after that it was much improved.

Do check that hose as well, as I gather rubbing through on the brake line is really quite common.
Click to expand...
Does the oil filler lid on top of the engine have any residue around the edges? I managed to reduce the frequency of the air leak (implausible value) errors by replacing the oil filler lid with a new one with more springy seal underneath as well as the fuel cap.
 
mowog said:
Can I recommend a really methodical approach? Strip off the throttle body, both bits, until you just have the mouth of the intake manifold staring back at you. Check that for any possible route for air leaks first. You could perhaps even try blocking it with a cloth and using a compressor to blow air in, in the hopes of hearing if it's escaping from around the manifold.

Feel along the soft vacuum pipe (or is it pipes - can't remember if there's one or two!) all the way down to the back of the engine block, looking for any signs of damage. Again, maybe try blowing into them to show any leaks.

And as you assemble the throttle body, check that it's sealing correctly at each stage. Remember that even a small leak on the wrong side of the butterfly could cause some weirdness if the engine thinks it's getting more air than it should.

IIRC you shouldn't need to add the airbox in order to run the car - having the throttle body open to the air shouldn't make a difference, and it gives you better sight over and around the engine.

I do recall that I also replaced my MAP sensor too, though can't for the life of me remember if it helped. Was cheap though, so didn't mind
Click to expand...

Heh yes it is indeed a good idea to wait until I have some more energy and do it properly.
Will also order a MAP sensor, that is a good idea. Even if nothing is wrong with it, it's only 11 euros so may as wel. :)
 
Robin_Cox said:
Does the oil filler lid on top of the engine have any residue around the edges? I managed to reduce the frequency of the air leak (implausible value) errors by replacing the oil filler lid with a new one with more springy seal underneath as well as the fuel cap.
Click to expand...

That is actually a good point. There is some old oil residue around the filler cap. Previously I assumed this was from some clumsy fill-ups, but worth changing that out as well.
 
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