Hunting idle and low mpg

[coziam]

Hello, so I am new to forums and I have looked to see if this has been discussed before, but I can find anything.

So i have a hunting idle only on cold starts, and lately I have noticed my mpg has dropped from around 23-25mpg around town to 10-14mpg.

Since owning the car I have had r tech stage 1 with injector clean and intake valves walnut blasted plus new pcv and dv. Timing belt and cam chain replacement. New coolant temp sensor. New pre cat o2 sensor. New maf sensor.

I am wondering if I might have a small vacuum leak that may have gotten worse over time. What would be the most common vacuum leak? Also I have a obd eleven scan tool that is showing no codes.

Thanks for your help in advance.

 

[Frostensen]

Same problem with 2008 1.8 tsi... around 14mpg in city traffic (no highway). Can't figure out the problem. Everything seems to be running fine but I can't get my head around that gas mileage.

 

[coziam]

Same problem with 2008 1.8 tsi... around 14mpg in city traffic (no highway). Can't figure out the problem. Everything seems to be running fine but I can't get my head around that gas mileage.

I used to get 22-24mpg city 32-34mpg motorway. Now I'm getting 16-19mpg city on a long run and 27-29mpg motorway. I have checked fuel trim and ltft is 9% and I'm getting evap incorrect flow error code, but I changed the n80 purge valve last year. https://youtube.com/shorts/bQPox-ZxOcI?feature=share you can kinda hear the hunting at the end of this video I did.

 

[Frostensen]

This is probably a dumb question, but is that gas or diesel?

 

[coziam]

It's petrol

 

[Frostensen]

Have you tested injectors for possible faliure and evap hoses for cracks (there are 3 I think)?

 

[coziam]

This was doing this last year on the cold start, before it went to rtech performance to get the injectors cleaned with new filter baskets and have the intake valves walnut blasted. My main concern is the dramatic drop in mpg. I have purchased a new charcoal canister, n80 valve and the silicone pipe kit for it. I will keep you updated on if this has any effect on the issue I'm having.

 

[Frostensen]

Thanks for that, I'd appreciate it if you would keep us updated.

 

[coziam]

So I changed the n80 valve, charcoal canister, replaced the hoses with silicone ones and I also bought a new fuel cap 'while I'm there'. This hasn't changed the idle hunting, but I would say it's had a small effect on fuel economy. I have bought an OEM engine cover and I'm going to see if the MAF sensor placement is having any effect. I have none branded intake hoses.

 

[Frostensen]

 

[coziam]

Changed to an original engine cover and still no difference. I'm thinking of getting some new spark plugs next.

 

[Awesam]

Hi I'm new here! I've just bought a 2007 Leon FR which is suffering from the same symptoms (idle hunting and low mpg).
I was considering the R-Tech walnut blasting inlet cleaning service to see if that would make a difference. My car has only 69k on it.

 

[coziam]

Hi I'm new here! I've just bought a 2007 Leon FR which is suffering from the same symptoms (idle hunting and low mpg).
I was considering the R-Tech walnut blasting inlet cleaning service to see if that would make a difference. My car has only 69k on it.

Hello,
Mine only does the hunting at idle on the cold start, is your the same? I did take mine to r tech and had the injectors cleaned and the intake valves walnut blasted and it had no effect on my car. That isn't to say it wouldn't have effect on your car. Have you checked the live data for fuel trim? That can identify a vacuum leak. If there is unmetered air going into the engine, the ecu will try and compensate with more fuel, making the mpg go down. It would be a good idea to check that first. Also check your pcv valve, you can check the one way valve, I'm not sure if you suck or blow on it to check. Let me know how you get on.

 

[Awesam]

Yes it the same from a cold start. As live data lol I'm not that savvy however I do have OBD11 and VCDS. What I have noticed that the seal around my oil dipstick missing so planning to change that to a yellow one. Will check the PCV valve and let you know.

 

[coziam]

Yes it the same from a cold start. As live data lol I'm not that savvy however I do have OBD11 and VCDS. What I have noticed that the seal around my oil dipstick missing so planning to change that to a yellow one. Will check the PCV valve and let you know.

I have a obdeleven and it's under, control units, engine, live data and if I'm right it's lambda control (mixture adaptation). It's just strange to me it only hunts on cold start. https://youtube.com/shorts/bQPox-ZxOcI?feature=share
You can kinda hear it on this video I posted a while ago to see if I could get any help. Does yours sound like this?

 

[Awesam]

I'm usually inside the cabin and i think mines maybe worst. I'll take a vid next I do a cold start.

 

[coziam]

I'm usually inside the cabin and i think mines maybe worst. I'll take a vid next I do a cold start.

It does sound way worse in the cabin but if you could video a short video post on YouTube and send the link I will do one of mine aswel from the cabin. This weekend I am replacing all the o rings on my vacuum pump to see if that has any difference.

 

[Awesam]

So did take a video but the sounds not great so will retake it.
I ran a DTC reader and found the code P2187 bank 2 - running lean on idle so not surprised.
Also whilst getting cam follower changed we spotted a split in one of the small hose neat the HPFP. So will get that change when it goes in for a oil change.

 

[coziam]

Well at least you can be certain it's a vacuum leak of some kind. This could be the dipstick you mentioned previously. Other common vacuum leaks are the diverter valve and the pcv valve. Let me know how that goes.

 

[Awesam]

I've ordered a new dipstick and Garage with get the pipes when its in for an oil service. PCV Valve looks fairly. Never thought it could be the DV before as I have experienced any lag when flooring the accelerator. Car has already a stg1 map but I'm keen on taking it to R-Tech and getting the remap and Walnut blasting package.