High idle and poor fuel economy

reverend-frog · 21-02-17, 11:48 AM

My 2.0 16v Coupe (B4, 89k miles) gets about 270-280 miles with regular driving to a 70-litre tank of fuel.

I have noticed that the car idles erratically (around 1300-1500 rpm even when warm) and produces a burning smell when revved hard.

Any ideas as to what might be causing all this and are these issues related?

missfire · 21-02-17, 02:47 PM

Hi, not much to go on but high idle is more often than not an air leak on the induction side of the engine or the idle stabilisation valve playing up/jammed fully open.

Burning smell could be anything from oil to half a cat jammed in your subframe, but 18mpg sounds a bit rough. Has the car had a full service recently of all fluids, filters, spark plugs etc.? Brakes serviced/free? Is the plastic bit on top of the dipstick fully intact? (don't ask, but have seen that one recently, car being run with no oil in it)

reverend-frog · 21-02-17, 03:15 PM

Quote:

Originally Posted by missfire

Hi, not much to go on but high idle is more often than not an air leak on the induction side of the engine or the idle stabilisation valve playing up/jammed fully open.

Burning smell could be anything from oil to half a cat jammed in your subframe, but 18mpg sounds a bit rough. Has the car had a full service recently of all fluids, filters, spark plugs etc.? Braked serviced/free? Is the plastic bit on top of the dipstick fully intact? (don't ask, but have seen that one recently, car being run with no oil in it)

The car is very loud on start up from the engine bay, almost like there's a blow at the manifold. Might be the leak you mention...

And the dipstick collar is loose, but I've never noticed any leaks.

Car is also probably due a service too - but I suspect the issue is something less straightforward than that. Yes 18mpg is appalling!

My Corrado VR6 was a good 7mpg better than that

mikes2 · 21-02-17, 06:24 PM

Could be a cracked manifold or just the gasket. bit of an undertaking to get the manifold off

missfire · 21-02-17, 06:30 PM

I mean high idle can be the engine sucking extra air via a leak into the engine, on the induction/air filter side of the engine, causing a lean mixture, that could create a burning smell, as the engine will run hot. On an engine like that I'd be doing a very thorough service, new lambda probe and possibly a new CAT also, unless there is obvious signs of oil being burned. If the dipstick plastic bit is the correct length, you'll get the correct oil level is what I mean, I have one here that it was broken and the previous owner/garage was reading a low oil level and thinking it was full. If it's intact at least the level will be correct. Loud engine bay could be an exhaust manifold leak, or exhaust downpipe/flexi join leak, but if the engine is serviceable, has been looked after and otherwise working fine the main things that will effect the MPG are the service items, lambda, potentially blocked CAT, binding brakes, temperature sensors, etc. but it's almost impossible to diagnose online with no other information. How many miles has it done?

missfire · 21-02-17, 06:38 PM

Quote:

Originally Posted by mikes2

Could be a cracked manifold or just the gasket. bit of an undertaking to get the manifold off

oh, sorry, writing stuff while you are too, again!

reverend-frog · 22-02-17, 02:08 PM

Thanks for all your replies so far

Quote:

Originally Posted by missfire

I mean high idle can be the engine sucking extra air via a leak into the engine, on the induction/air filter side of the engine, causing a lean mixture.

I changed down from fifth to third with the windows open the other day and heard a distinct popping from the exhaust, like unburned fuel. Would this tally with a lean mixture?

Quote:

Originally Posted by missfire

If the dipstick plastic bit is the correct length, you'll get the correct oil level is what I mean, I have one here that it was broken and the previous owner/garage was reading a low oil level and thinking it was full.

I had the oil level warning come on recently actually - obviously I topped it up.

Quote:

Originally Posted by missfire

How many miles has it done?

89,000

missfire · 22-02-17, 02:23 PM

Popping is unburnt fuel which is commonly linked to a rich mixture, but can be because of poor combustion also, due to poor condition service items, like spark plugs, also leads etc. So unburnt fuel could technically get through even if the mixture is correct, but that popping over time could also 'pop' your exhaust gaskets and joins, which might be the case?
A few quid in service items could narrow this down a whole lot, to for example, a bad lambda or temp sensor.

reverend-frog · 22-02-17, 02:26 PM

Quote:

Originally Posted by missfire

Popping is unburnt fuel which is commonly linked to a rich mixture, but can be because of poor combustion also, due to poor condition service items, like spark plugs, also leads etc. So unburnt fuel could technically get through even if the mixture is correct, but that popping over time could also 'pop' your exhaust gaskets and joins, which might be the case?
A few quid in service items could narrow this down a whole lot, to for example, a bad lambda or temp sensor.

Yeah, I wasn't going to spend any money on this car but it's costing me about thirty quid in unnecessary fuel a month, so false economy not to get it serviced by the sounds of things

missfire · 22-02-17, 02:31 PM

Definitely!

21-02-17, 02:47 PM	#2
missfire making wooden things Classic Audi Club Member Join Date: Dec 2009 Location: cow land Posts: 8,154	Hi, not much to go on but high idle is more often than not an air leak on the induction side of the engine or the idle stabilisation valve playing up/jammed fully open. Burning smell could be anything from oil to half a cat jammed in your subframe, but 18mpg sounds a bit rough. Has the car had a full service recently of all fluids, filters, spark plugs etc.? Brakes serviced/free? Is the plastic bit on top of the dipstick fully intact? (don't ask, but have seen that one recently, car being run with no oil in it) Last edited by missfire; 21-02-17 at 06:17 PM.

21-02-17, 06:24 PM	#4
mikes2 4 ring whore! Classic Audi Club Member Join Date: Nov 2007 Location: Polmont Posts: 2,617	Could be a cracked manifold or just the gasket. bit of an undertaking to get the manifold off __________________ 1991 90 quattro 20v sport 1991 90 quattro

21-02-17, 06:30 PM	#5
missfire making wooden things Classic Audi Club Member Join Date: Dec 2009 Location: cow land Posts: 8,154	I mean high idle can be the engine sucking extra air via a leak into the engine, on the induction/air filter side of the engine, causing a lean mixture, that could create a burning smell, as the engine will run hot. On an engine like that I'd be doing a very thorough service, new lambda probe and possibly a new CAT also, unless there is obvious signs of oil being burned. If the dipstick plastic bit is the correct length, you'll get the correct oil level is what I mean, I have one here that it was broken and the previous owner/garage was reading a low oil level and thinking it was full. If it's intact at least the level will be correct. Loud engine bay could be an exhaust manifold leak, or exhaust downpipe/flexi join leak, but if the engine is serviceable, has been looked after and otherwise working fine the main things that will effect the MPG are the service items, lambda, potentially blocked CAT, binding brakes, temperature sensors, etc. but it's almost impossible to diagnose online with no other information. How many miles has it done?

22-02-17, 02:23 PM	#8
missfire making wooden things Classic Audi Club Member Join Date: Dec 2009 Location: cow land Posts: 8,154	Popping is unburnt fuel which is commonly linked to a rich mixture, but can be because of poor combustion also, due to poor condition service items, like spark plugs, also leads etc. So unburnt fuel could technically get through even if the mixture is correct, but that popping over time could also 'pop' your exhaust gaskets and joins, which might be the case? A few quid in service items could narrow this down a whole lot, to for example, a bad lambda or temp sensor.

22-02-17, 02:31 PM	#10
missfire making wooden things Classic Audi Club Member Join Date: Dec 2009 Location: cow land Posts: 8,154	Definitely!