LCR rough idle potential causes/fixes

Onni92

Active Member
Feb 19, 2014
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The next thing on what seems like an ever expanding list of things that need fixing on my LCR is the slightly rough idle. Basically the revs stay constant apart from every now and then (around every 30 seconds) the revs will fall for a split second and the engine seems to struggle for half a second before returning to where they should until it happens again.

I have done a bit of searching around at the usual causes and so far i have had it in the garage to check for vacuum leaks which they fitted a blanking plate for the secondary air filter that had been removed and called it fixed (not sure if they checked all the other hoses etc)

i also sprayed some electrical contact cleaner into the MAF sensor through the mesh (i didnt have the right tool too take the actual sensor out of the housing) As well as cleaning the throttle body by spraying a load of egr/air intake cleaner and wiping with a micro fibre cloth which someone pressed the accelerator so it was open.

Since doing all of this i'm sure sometimes when i first start my car and drive off in the morning it hesitates slightly, but only right at the start, its fine after a minute or so.

Does anyone have any suggestions as to what could be causing this? if its required i can try and capture this on video but i hope i have explained it well enough!

Im thinking something along the lines of getting it booked into a garage for a proper smoke test to see if there are any vacuum leaks or possibly buying a new MAF sensor?
 

Onni92

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Feb 19, 2014
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Ohhh and i dont know if it is relevant but i have also had a code thrown up saying that it is running lean, this is what lead me to believe it was a vacuum leak however the code has only ever thrown up twice and it just so happened to be when i filled up using premium unleaded, whereas i usually just use regular. at first i thought this was a coincidence but its happened twice now within a few miles of filling up with premium where i can drive hundreds of miles on regular unleaded without a code being thrown up for running lean.
 

andycupra

status subject to change
I'd unplug the maf and see if this helps.
Poorly mafs can cause issues like this. But then so could a number of other things.
When you unplug the maf you will get the esp light on and will not have traction control. Don't worry this will all be Ok once you plug back in.
 

Onni92

Active Member
Feb 19, 2014
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I've just unplugged the maf sensor and it definitely got worse whilst it was unplugged! The revs were dropping pretty much continuously rather than every 30 seconds or so!
 

mty12345

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Jun 17, 2011
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More than likely a split pcv hose under the inlet manifold. That's where I'd start. Also the car should always be run on 98 octane super unleaded, preferably shell v power as it's 99 octane compared to the 97 octane of other supers. It will run on standard unleaded but only at reduced performance. When swapping between super and regular the car will take a while to adjust to the proper petrol. It will be trying to produce full power with super unleaded in there, that's probably why you only get a lean code then.
 
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pedro1989

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May 2, 2014
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Bridgend, South Wales
More than likely a split pcv hose under the inlet manifold. That's where I'd start. Also the car should always be run on 98 octane super unleaded, preferably shell v power as it's 99 octane compared to the 97 octane of other supers. It will run on standard unleaded but only at reduced performance. When swapping between super and regular the car will take a while to adjust to the proper petrol. It will be trying to produce full power with super unleaded in there, that's probably why you only get a lean code then.

You must have that on permanent copy/paste by now Mty :)

I'm in for a split hose/air leak being the problem too.
 

Onni92

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Feb 19, 2014
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A couple of the hoses under there did look a bit grubby, how do I go about checking it? Can I undo the Allen bolts that hold on the metal bracket thing for easier access or is that not an option?
 

mty12345

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Jun 17, 2011
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Yeah i should have it ready for copy/paste hahaha :) Would make life easier :) but yeah thats the best thing to do mate, it's still a bit fiddly but you can move the bracket enough to make it easier. The dirty hoses will likely be where the leak is. They will be the cause of your lean code as well as the rough running. My split pcv hose caused both those things, not had either for months now i've replaced it.
 

briscoe

Active Member
Apr 16, 2013
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Mine did the same sorted it with silicone hoses though :) Common problem on these and I'd start with as already discussed the pcv pipe work under the inlet manifold. Three hoses under there prone to split (mine all 3 were split). Can take the pipe work out without taking the manifold off as well ;)

Also check the top breather hose on the right hand side of the cam cover another common split pipe usually in the join where you can't see it.
 
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Onni92

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Feb 19, 2014
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I will be sure to investigate this at the weekend! Would it be any trouble to ask for a photo with the hoses that are prone to split circled or something? Would be really useful!

Also if I do happen to uncover some split hoses how do I go about replacing them! Are the part numbers on them so it's just a case of a trip down to euro car parts and changing them out?
 

mty12345

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Jun 17, 2011
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Yeah mate I'll sort out a photo for you later on. Most of the hoses have got a part number stamped on them,the ones without a number are usually just standard vacuum hoses.
 
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Feb 1, 2007
1,602
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Nottingham
More than likely a split pcv hose under the inlet manifold. That's where I'd start. Also the car should always be run on 98 octane super unleaded, preferably shell v power as it's 99 octane compared to the 97 octane of other supers. It will run on standard unleaded but only at reduced performance. When swapping between super and regular the car will take a while to adjust to the proper petrol. It will be trying to produce full power with super unleaded in there, that's probably why you only get a lean code then.

You wouldn't normally get a lean code from swapping fuels.
Ive done it a few times and never got a code.
 

Onni92

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Feb 19, 2014
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Thank a lot everyone! Really helping me out! Once I have the picture, depending on how much the hoses are I might just swap out all of the ones that are prone to leak!

Once this is sorted I will only have a couple more issue that need fixing which I'm sure I will post their own thread when it's time (coolant is leaking somewhere, air con is leaking so it couldn't be regassed, and I damaged the wiring replacing an abs sensor which will need to be looked at by an auto electrician)
 

mty12345

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Jun 17, 2011
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You wouldn't normally get a lean code from swapping fuels.
Ive done it a few times and never got a code.

Yeah mate that's what i was getting at but obviously didn't do a very good job of making clear what i was trying to say,lol.

I meant that with the regular unleaded it's probably pulling the timing back and adjusting the boost/fuelling to run on it. But once the ecu realises there is a tank of 98octane in there, then it will aim for full performance and maybe thats why the lean code only showed up when he put 98 octane in.... not because of the fuel but jut that that was when his split pipes would be letting in the most un-metered air etc. That or maybe it was just coincidence.
 
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mty12345

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Jun 17, 2011
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Couldn't find a photo of the exact hoses mate, once you undo the bracket with the arrow and move it out the way as far as you can, then using a torch you'll see some fairly chunky hoses behind where the bracket was, directly under the manifold itself. They are the most likely ones, especially the 2 that have a kind of elastic strap around them. Also check the smaller vacuum hoses that are bolted to the underside of the bracket while you've got it off.

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What the hoses look like (the 3 on the right)

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Hope thats some help
 
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Onni92

Active Member
Feb 19, 2014
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So I've had a look and on of the pipes in the area circled was covered in oil however when I sprayed some egr/air intake cleaner (I'm guessing it would be the same as spraying carb cleaner) the revs didn't change at all! So I'm a bit stumped as to my next move! I have pretty much every hose I could see a blast and nothing changed the revs.

One thing that I did notice was the fact that one of the bolts on my manifold was missing which could obviously cause a leak however spraying that did nothing to the revs either! There is definitely a hissing sound coming from that general area so something has to be leaking right?!
 

mty12345

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Jun 17, 2011
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as traumapat says the only way to be certain is a smoke test. I'm not sure intake cleaner will show anything as it might only be leaking under boost or overrun conditions etc etc. Either that or it could be a fairly small leak, so that the amount of cleaner drawn in at one time won't affect the revs anyway.

If there is a lot of oil on those pipes that is usually a good indication of a leak either in that pipe or the one next to it. Have a feel along the pipes where the oil is and give them a gentle squeeze to check for splits. The one that goes most often is the "S" shaped one in the middle of the pic above. They are all PCV pipes and carry hot oil vapour and the heat and il itself attacks the rubber over the years and it perishes
 
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Carlw1994

Active Member
Jan 25, 2016
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Just got a seat Leon cupra 2003 1.8t 20v 268 bhp with full custom map and dyno seem to have a vacuum leak as it reads -.8 on gauge . Where are the typical points for cracks or splits ? Any help would be highly appreciated , thanks in advance
 

mty12345

Active Member
Jun 17, 2011
3,727
451
bristol
Just got a seat Leon cupra 2003 1.8t 20v 268 bhp with full custom map and dyno seem to have a vacuum leak as it reads -.8 on gauge . Where are the typical points for cracks or splits ? Any help would be highly appreciated , thanks in advance

All the hoses mentioned above can split. PCV pipes under the inlet maniold are usually the main culprits.
 
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