Lumpy Idle When Warm?

Mackass

Mackass

Mackass

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22 Jan 2005
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West London
Basically, whenever my engine is cold or hasn't been driven yet, it idles smoothly, the revs are constant and there's no noticeable problems.

However, when the engine has warmed up (usually to 90C), it idles lumpily, the revs will alternate by around 50RPM, and you can hear the lumpy idle from the outside of the car. 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 quite 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?

I've already had the throttle body cleaned out and re-adapted, so this isn't the problem. Back when I had a faulty crank sensor replaced, the mechanic told me two faults came up on the computer prior; a crank sensor (which I've already replaced) and a lambda sensor. The mechanic said the lambda sensor fault may have been trigured by the faulty crank sensor, and told me to leave it for the moment.

So I was wondering, whether a faulty lambda sensor may be the cause for my lumpy idle when the engine is warm? And if so, howcomes the engine only has lumpy running symptoms when it is warmed up, and not cold?
 
my guess would be the idle control valve. Your not experiencing the idling problem when the engine is cold because the engine will idle at a slightly higher speed until the engine temperature is warm enough.

thedazman
 
What about a faulty coolant temperature sensor. If the ECU thinks the car is cold all the time then it would still continue to run richer even when the engine has warmed up, which might account for your symptoms. This would be accompanied by poor fuel economy.
 
I had this problem, or a similar one. It was nothing mechanical at all, just the computer software needed upgrading.
 
My idle on my escort has been bad since i've owned it. Originally it was ok when cold but terrible when warm, so i had the lambda sensor and the maf changed. This helped but its now the other way around, i've got a new idle control valve to put on and this should fix it.
 
dirtydog said:
What about a faulty coolant temperature sensor. If the ECU thinks the car is cold all the time then it would still continue to run richer even when the engine has warmed up, which might account for your symptoms. This would be accompanied by poor fuel economy.
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Exactly what I was thinking. A friend of mine had a faulty CTS on a vts, it produced an extremely erratic idle
 
Cleaning the idle control valve is free to do (or a few quid for some carb cleaner), so its a good place to start. :)

Got to do mine too, I have the same problem!
 
D4VE said:
Cleaning the idle control valve is free to do (or a few quid for some carb cleaner), so its a good place to start. :)

Got to do mine too, I have the same problem!
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*nods* made a huge difference to my engine when i cleaned my ICV out.. crankshaft position sensor also if your car has one.. they tend to get all crudded up

:)
 
The car's a VW Golf 1.6 8V Mk4.

I was thinking the coolant temperature sensor initially, however the water/coolant temperature reading on my display doesn't seem wrong. Because when the engine is cold - the needle will point to the left, and when warm - the needle will point to the right towards 90C area, however I am looking to change this, as it's only £25.

Regarding the idle control valve, I've already had the throttle body cleaned out and re-adapted, is the idle control valve not the butterfly valve in the throttle body, or is it a different part of the throttle body. If so, are these a lot to replace?

Going back to the lambda sensor, read somewhere that it doesn't kick in until the engine is hot enough, is this true or not? If so, could it be this that's causing the lumpy idle when the engine is warm, as that's when the sensor is in operation.
 
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Mackass said:
The car's a VW Golf 1.6 8V Mk4.

I was thinking the coolant temperature sensor initially, however the water/coolant temperature reading on my display doesn't seem wrong. Because when the engine is cold - the needle will point to the left, and when warm - the needle will point to the right towards 90C area, however I am looking to change this, as it's only £25.
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The sensor can be faulty, yet the dash temperature gauge can still read correctly - at least on some cars, I don't know about the Golf specifically :) I'm not saying it is the problem - your symptoms could be caused by lots of things, you just have to eliminate them until you find the cause.
 
dirtydog said:
The sensor can be faulty, yet the dash temperature gauge can still read correctly - at least on some cars, I don't know about the Golf specifically :) I'm not saying it is the problem - your symptoms could be caused by lots of things, you just have to eliminate them until you find the cause.
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Well, the faulty temperature sensor is a common fault with Mk4 Golfs. It's actually the sensor itself which is the problem, and there is a newer version of the sensor which VW now use, I've just got to find out whether I've still got the old sensor plugged in. Probably going to get a new one from the dealers later, they're only £25, so I can rule out a dodgy temperature sensor if the problem still exists.

I'll update you guys on my progress later this evening.
 
dirtydog said:
The sensor can be faulty, yet the dash temperature gauge can still read correctly - at least on some cars, I don't know about the Golf specifically :) I'm not saying it is the problem - your symptoms could be caused by lots of things, you just have to eliminate them until you find the cause.
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My friends VTS temp read fine on the dash, seperate sensor for the dashboard and the ecu it seems.
 
The VW parts department are closed until wednesday as they're taking stock. :mad:

If the new coolant temp sensor doesn't solve the problem, could the lambda sensor (which came up on the ECU scan initially) be another possibility for the cause of the lumpy idle when warm?
 
I was under the impression that a faulty lambda sensor can cause exactly the problems you describe, lumpy at warm idle rather than cold idle. I'll try and find a source to back this up.
 
faulty lambda sensor would (or at least should) affect the engine on anything but wide-open-throttle (at w.o.t the ECU ignores the lambda anyways)

what happens if you hold the revs at a steady 2000rpm with the car parked up in neutral?

on my car when the lambda sensor was wonked, the engine wouldn't hold a steady 2000rpm with no load on the engine, the revs kept rising and falling all the time..

a new lambda sensor will likely be from £30 - £75 depending on the car, i think i paid about £56 for my one last year..
 
I think it would be cheaper to pay to get any fault codes read and test the range of sensors than change them by process of illimination.
 
When trying to hold the revs at 2000, it will reach 2000 revs no problem, but it cannot hold it steadily i.e. the revs will start to fall slowly, even though my right foot is making no movement, is this a sign of a faulty lambda sensor?

Tesla, had the cars ECU read last week, and the lambda sensor fault did come up, but the mechanic said it was probably caused by my faulty crank sensor which I've replaced - he erased the codes and said see how it goes. Now, as I'm still having these warm idle issues, I promptly remembered the faulty lambda sensor coming up, and am trying to see whether this has any link with my warm idle issues.

DiscoKandy, if you're right, then it sounds more and more likely like the lambda, however I'm trying to rule out other components before replacing it - checked and prices are quite dare.
 
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