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Discussion Starter · #1 ·
Basically, whenever my engine is cold or hasn't been driven yet, it idles smoothly, the revs are constant and there's no vibration.

However, when the engine has warmed up, the revs will alternate by
50RPM at idle, the car shudders, and you can hear the lumpy idle from
the inside and outside. Giving it a little gas will erradicate the lumpy idle, but
it will return once you off the gas. A normal idling engine should have a constant 'vrooooom' sound, but mine sounds very lumpy when warm. As I say again, this ONLY
happens when the engine has warmed up, never when it's cold, so does
anyone have any ideas?

So I was wondering, whether anyone has any ideas what this is? I've done some research on this site, and the throttle body appears a couple of times, is it likely to be this thats causing the shudder and the lumpy idle? Or are there other components that can cause this?

The engine is all stock by the way, no mods or anything.
 

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Sounds like the throttle body could need a quick cleaning.

Pop off the plastic intake pipe after the airbox and maf. Spray carb cleaner in there, and wipe with a clean cotton cloth. Push the throttle valve open with ur fingers and spray/clean inside and around the throttle. It should be well black everywhere, and shiney metal once u've finished.

I think that should cure the problems, it did on my friends 1.6.

And if it doesn't, theres no harm in trying, as its probably really dirty, and its cheap to buy carb cleaner
 

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Discussion Starter · #3 ·
Alrite, I'll have a look into cleaning the throttle body.

Although, is there any reason why the idle is only rough when the engine is warm or has just been driven?
 

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If there is gunk that's preventing the throttle plate seating properly, it will mess up the air / fuel ratio and the ecu will always be trying to compensate hence the rough idle. When on warm up, the ecu will hold the throttle plate slightly open to cope with the richer mixture so it will not manifest itself.
 

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i am having the same prob think the carb cleaner out and when you clean the tb do you have to disconnect the battery and if so do you have to do a throttle body alignment i have read posts on here that state you need vag com to do the tba but i have also read you can do the thing with the bonnet up and let the car idle for 10 mins does this actually work or do you need vag com
 

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Discussion Starter · #7 ·
Whilst still on this issue, whenever I'm experiencing this problem and I switch the aircondition on, all symptoms seem to disappear.

The revs seen to remain stable, theres no vibration in the car, and there's no lumpy engine noise whatsoever - this happens when I simply turn the aircondition ON. But if I switch the aircondition off again, the fluctuating revs, engine vibration and lumpy engine noise all return.

Is this still likely a throttle body issue? I've not cleaned it out yet by the way.
 

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i may be wrong but when the aicon is switched on it puts more strain onto the engine and i would imagine the throttle will open slightly to compensate this may be why it is running smoother
 

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Discussion Starter · #9 ·
Yeah, makes sense to be honest, looks like it all points to the throttle body.

Just out of interest, my car is still under warrenty (not manufacturer), and it covers my engine. So do you reckon a throttle body clean plus vagcom update would be under warrenty?

I would attempt it myself you see, but this isn't the sort of thing I would do, as I've never touched an engine before in my life.
 

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Discussion Starter · #10 ·
I took the car to a VW Specialist to get some work done, and I told them to clean out & realign the throttle body whilst they were at it.

Now, I'm still having these lumpy idling issues when the engine is warm, and as I've already had the throttle body cleaned and aligned, I was wondering whether any other faulty part can cause lumpy idles when warm. No codes were found when it was on the VAG-COM, so any ideas now guys?
 

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It could also be the air temperature activated intake valve that sits before the air filter box, or as in my case, inside the box. If the valve does not flip over when the engine's warm, it will always supply the intake with heated air from the exhaust heat shielding.

Have a look at my post from a few days ago. I had exactly your problem and it got so bad the engine went in to limp mode.
 

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Discussion Starter · #13 ·
I think I may aswell try the Temp Sender.

Although am I right in saying this is also called the Coolant Temperature Sensor, and should take no longer than five minutes to install?
 

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Discussion Starter · #14 ·
Can anyone else help me diagnose why the car only idles lumpy when the engine is warm?

As I said, have already tried the throttle body, am going to try the temp sensor and check the air intake valve, but anything else I should logically check or replace?
 

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Discussion Starter · #15 ·
Would a faulty lambda sensor cause a lumpy idle when warm?

Reason being, I recently had a faulty crankshaft sensor replaced, and the mechanic said two codes came up on the VAG-COM. A crankshaft sensor, and a lambda sensor. He said the lambda sensor fault was probably bought up because of the faulty crankshaft sensor, and erased the codes. Now, was he right, or could the lambda fault code be a fault itself, and would this contribute to a lumpy warm idle?

I would appreciate any help guys, as this problem has been bugging me for a long time.
 

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Check your vacum hose that goes from the vacum servo hose - assuming your 1.6 has one, these were known to crack slightly and cause leakage of air (used for braking) but is connected to the throttle housing (well in the case of a 1.8T) this caused erratic idle on my 1.8, I changed the pipe and all was ok.

Im not too sure on 1.6 though so anyone else got a take on this?
 

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Same issues on an Audi A4, TDI. Dont know the engine code i'm afraid. All i know is it's R reg (1997 year)

Idles fine when cold, but when it gets to a certain temperature, operating temp i assume, it then idles roughly....throttle body being the usual culprit i presume?
 

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Same issues on an Audi A4, TDI. Dont know the engine code i'm afraid. All i know is it's R reg (1997 year)

Idles fine when cold, but when it gets to a certain temperature, operating temp i assume, it then idles roughly....throttle body being the usual culprit i presume?
No, the TDI does not have a throttle body.

More likely on the TDI is the coolant temperature sensor or even the needle lift sensor on Nr3 injector.

You need to get it read for fault codes and / or have a scan of the ECU measured values i.e. coolant temp. otherwise you could end up spending money uneccessarily.
 
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