Random misfires

[ExiledWeegie]

Hi, I hope someone can help because I'm now stumped on this one. Car has got random misfires across all 4 cylinders, at idle every now and then you can feel and hear it misfire, driving around normally its usually fine, full throttle and you dont feel anything but around half throttle you can feel it misfire like it hesitates then goes again. In VCDS measuring blocks 015 and 016 i can make it shows misfires on the counts by just blipping the pedal, it happens on all 4 cylinders, some more than others. so far heres what I've done...

full new pcv system smoke tested and fine no leaks
smoke tested turbo system and no boost leaks
new Bosch maf sensor
all new Alpha competition coils
new NGK laser platinum plugs gapped to 0.28
cleaned throttle body and adapted through VCDS
cleaned intake temp sensor

I'm just about to put a new coolant temp sensor in today when it arrives, basically because I've read it can cause problems like this, but I have no clue where to go from after this and I cant keep throwing money doing the parts connon at it. any ideas?

one thing as well is the idle speed used to be around 740-760rpm, after cleaning throttle body and intake sensor it sits around 800-840rpm and occasionally drops to 740 then bounces back up.

 

[ExiledWeegie]

just put a new throttle body on it, and a new pcv check valve and it seems to be running fine when on boost and part throttle but the idle speed is still higher than usual and still getting misfires randomly across all cylinders at idle. any ideas? could it be the harness? and if so can i just cut and replace or does it need to be the whole thing?

 

[ExiledWeegie]

just checked the harness that i can see and it all looks in good condition, cleaned up the ground on the rocker cover, taped it all back up and its still the same random misfires at idle.

 

[mty12345]

Difficult one, Have you checked the fuel system? If it's happening on all 4 cylinders it must be something common to all 4, so i think you can rulle out spark/ignition. Check the Charcoal cannister pipes are ok, valves are working and soleniod is ok. What are your LTFT and STFTs in live data.

I know you said you smoke tested the pcv system, but have you physically inspected the pipes for splits. Have seen it before where the splits only open up under vacuum/boost, and the smoke testers don't produce enough pressure to open the splits. There are also some one-way valves in the pcv.

 

[ExiledWeegie]

thanks for the reply, Ive just today put 4 new injectors in the car as my attempt to service the ones in it didnt go to plan, ive also replaced the pcv system with silicone hoses and a aluminium check valve all from creation motorsport, ive also deleted the suction jet pump and its pipework so theres not much of the pcv system left and it shows no leaks at all. the boost hoses are all nearly brand new with mikalor clamps and are all well tightened (FMIC) it also boosts to 20psi peak and holds steady at 15psi and the boost gauge shows no drops that indicate a boost leak. Ive got another set of plugs on the way and Im going to replace the pre and post cat sensors and see if it makes a difference. I havent logged the short and long term trims and ive forgotten what they were so ill try get them logged asap (the coolant divertor has broken at the minute) as i cant start the car.

 

[mty12345]

Ah right, sounds like you've covered most bases. Hopefully fuel trims might give some clue as to what is causing the misfire.

 

[ExiledWeegie]

Ah right, sounds like you've covered most bases. Hopefully fuel trims might give some clue as to what is causing the misfire.

I logged these earlier from vcds, im not upto date on what exactly to do on vcds so hope this can be interpreted. I noticed on measure blocks 111 that lambda was all 0.0% and never changed when the car was idle or driving does this mean anything? and now im getting lots of misfire on cylinder 4 and the odd misfire on other cylinders, im at a total loss with this.

 

[ExiledWeegie]

did a compression test today and was shocked with the number from a 21 year old car

cylinder 1 - 180psi
cylinder 2 - 180psi
cylinder 3 - 175psi
cylinder 4 - 180psi

changed the map sensor and number 4 injector and the misifres have mainly stopped and the car feels so much better, still the odd misfire at idle but never on a single cylinder and it disappears just as quick as its picked up, im resetting the battery now to see what fuel trims are. when i took it for a drive earlier they were..

idle -0.2 (minus)
partial -3.1 (minus)

so came down a bit, hopefully getting somewhere now, thank f***!!!

 

[ExiledWeegie]

took its a quick 5 mile run and the fuel trims are...

idle 0.0
partial -2.3 (minus)

noticing quite a bit of burbling from the exhaust when off throttle, i do have a small leak but its after both sensors, could that make a difference to the burbling and slight rich fuel trim?

 

[mty12345]

Fuel trims are fine, anything within +/-10% is ok. If changing the 4th injector has improved things then would just see how it goes. What fuel are you using?

 

[ExiledWeegie]

Fuel trims are fine, anything within +/-10% is ok. If changing the 4th injector has improved things then would just see how it goes. What fuel are you using?

I use Shell Vpower in it since I got it. changing injector 4 took away all the misfires from cyl 4, it was quite bad, showing 68 misfires within a minute, now i can let it idle for ages and it will pop up with 1 random one on a random cylinder it shows for like a couple seconds then clears again so deffo much better now. I hope im close to having it running the way it should. Ive still to stick a new set of plugs in it so hopefully that will stop the random misfires but if their only happening at idle I can live with that right now.