# Suspect misfire on idle and light load



## Phil69 (Sep 13, 2006)

Hi all,
Went out for a drive today and after a bit of a play, I came to a junction and noticed the idle had changed. I can only describe it as being similar to a misfire as the idle is lumpy and almost sounds a little like an Imbreza engine going wub-wub-wub-wub-wub as the idle pulses.

It does this either at idle or when on very light load.

What I dont understand is that when accelerating or under harder loads, the engine runs perfectly fine and pulls just as hard as before without problem.

The exhaust system is a HKS twin system so the front 3 cylinders and rear 3 are seperate all the way through yet when feeling at the back, the pulsing seems to be equal on both sides meaning the fault isnt on 1 cylinder only so plugs, coils and coil loom are out of the question - these were my 1st thought.

ECU is a G4 link and the car is a 1995 R33 GTR.

Any thoughts please guys??


----------



## [email protected] M/S (Feb 16, 2002)

running lean? will make the car sound rough but wont cause a full misfire


----------



## Phil69 (Sep 13, 2006)

I dont have any way of checking the map myself so not sure. If it is something to do with fuelling, what could've caused the change considering the rest of the rev range seems to still be ok?


----------



## R32 Combat (Jan 29, 2004)

Are you running O2 feedback? A bad lambda will do that.


----------



## Phil69 (Sep 13, 2006)

When the G4 Link ECU was fitted, they also fitted a 'closed loop system' so I'm assuming this would use a Lambda?


----------



## MIKEGTR (Jul 8, 2005)

Inlet manifold gasket


----------



## [email protected] M/S (Feb 16, 2002)

> Inlet manifold gasket





doesn't make a difference normally to a map based system

running closed loop thou if the sensor gone bad, The link can work out of the Sensor gone bad thou.

When I set the car up with closed loop I tune the car a to run a tad rich and trim to leaner correct AFR.


----------



## ShanGTR3318 (Mar 30, 2012)

Just a suggestion from an experience i had on my previous car. 

Have you tested the coils? I had the same issue on my 350z, I also had a separate exhaust system and i had the exact same symptoms as what you described. One coil was on its way out. After replacing it, everything was normal again.


----------



## Phil69 (Sep 13, 2006)

They havent been tested yet but as explained before, my twin exhaust system is seperated down the whole car with 1 pipe per 3 cylinders yet the pulsing is equal to both pipes. For the fault to be felt both sides, 2 coils would had to of broke down at the exact same time which although isnt impossible, its not very likely.

Its booked in with MGT Wednesday as they fitted the ECU and closed loop. Could look into fitting another set of coils as fault finding while there but will see what they say 1st.


----------



## Phil69 (Sep 13, 2006)

Any other suggestions before I take it in tomorrow??

ECU, map or Lambda are looking possible but what else could cause this?


----------



## [email protected] M/S (Feb 16, 2002)

Just let Garth @ MGT have a look at it, hopefully something simple.


----------



## Phil69 (Sep 13, 2006)

Yeah, thats the plan. I was just hoping to try and make sure I'm not risking any damage by driving it the way it is as I have a 2 and half hour drive to get there.


----------



## ShanGTR3318 (Mar 30, 2012)

Cant think of anything else. I really hope its nothing major, good luck with it. I hope it gets sorted. Let us know how it goes


----------



## Phil69 (Sep 13, 2006)

I WANT TO CRY!!!!!

Got the car to MGT ok yesterday and everything started as a mystery. Fuelling was spot on, all plugs and coils were firing but there was no compression in number 1 despite the fact that under load, it seemed to drive fine!

Took the plugs out and using a fibre-optics mini camera, watched the valves as the engine was hand rotated. We could see that the exhaust valve wasnt seating correctly. Rocker covers and cams came off and it was found that the valve shim had unseated and was holding the valve open. Luckily the valve hadnt been bent so everything got put back together and we took it out for a test drive.

All went well....UNTIL....we did a couple of long pulls then BANG!! - Clouds of thick oil smoke out of the exhaust. It mostly appeared from the pipe running from the rear turbo so the obvious thought to start with was the rear turbo had blown. However, whilst limping it back home, it quickly developed a heavy misfire throughout the rev range and the smoke got worse and worse before I ended up having to pull over and call the AA recovery as the smoke was blocking the entire road and was dangerous for myself and others.

I'll do some more tests today when I get a chance but yesterday, Mark drove to where I had made it to and unplugged injectors in turn to identify where the misfire was. It appears that the misfire is on number 1 again....even though the smoke is coming from the pipe that flows from the rear turbo and cylinders 4, 5 and 6. The only possibilty I am hoping for is that the Iridium plug in number 1 has given up due to the previous fault as it looked coked up causing the misfire and that the oil smoke fault is just the rear turbo. The other option Mark suggested is that I've done a piston ring and that oil getting past it drowning the plug causing the smoke and misfire.

Obviously, a turbo recon is cheaper than a bottom end rebuild as I'm capable of removing/refitting turbos but not ever tried rebuilding an engine.


----------

