Misfire/stutter At Idle/low Revs (neo Rb25det).. Have Searched

[revolt]

A few weeks ago my S2 Stag developed a very slight misfire at idle. Previous to this, it has always been super smooth (have to look at tacho to see it's running).

The misfire gradually got worse and is now quite noticeable at idle and when accelerating from idle to about 2800rpm, it is a constant stutter/misfire. 0-100km/h takes about 12-13 seconds. Engine-wise, the car is stock besides the coil packs mentioned below and a drop-in replacement K+N filter.

I have tried all of the following (after researching about 50 similar threads):

- Replaced Coil packs with Super Sparks

- Replaced Spark Plugs with BCP6ES @ 0.8mm and 1.1mm

- Check base ignition timing. 15 deg @ 650rpm, using a pluglead directly into Cyl 1 and also checked with timing loop wire

- Unplugged the coil packs one-by-one while engine is running and all are firing (revs dropped and idled even worse when they were D/C)

- Cleaned AAC Valve and AFM

- Unplugged/wiggled around all connectors

- Checked ECU for codes (No errors)

- Checked voltage drop from battery to all of the earth points on the engine. All are 0.00 or 0.01

- Sprayed Start ya Bastard around every intake and vacuum line. Revs did not change

- Tested TPS. With Ignition ON, but car off, idle is 0.45v, WOT is 3.9v. With car running, idle is 0.41, WOT is 1.4v (Is this a possible cause?)

- Tested AFM. With Ignition ON, but car off, reading is 0.22v. With car running, it is 1.14v

What else could it be? Coolant Temperature Sensor? CAS? Injector problem?

It does not have a boost gauge. If I get one, will I be able to tell if it has vacuum leaks by its reading at idle?

The car uses Castrol Edge 5-30 fully synthetic, changed every 5000kms, the 02 sensor was changed 20,000kms ago. It has 145,000kms on the clock.

Spent most of the past 3 days on it, as well as testing random things since it started showing symptoms.

Thanks for any advice

Edited January 2, 2010 by revolt

[TMW-R34]

A few weeks ago my S2 Stag developed a very slight misfire at idle. Previous to this, it has always been super smooth (have to look at tacho to see it's running).

The misfire gradually got worse and is now quite noticeable at idle and when accelerating from idle to about 2800rpm, it is a constant stutter/misfire. 0-100km/h takes about 12-13 seconds. Engine-wise, the car is stock besides the coil packs mentioned below and a drop-in replacement K+N filter.

I have tried all of the following (after researching about 50 similar threads):

- Replaced Coil packs with Super Sparks

- Replaced Spark Plugs with BCP6ES @ 0.8mm and 1.1mm

- Check base ignition timing. 15 deg @ 650rpm, using a pluglead directly into Cyl 1 and also checked with timing loop wire

- Unplugged the coil packs one-by-one while engine is running and all are firing (revs dropped and idled even worse when they were D/C)

- Cleaned AAC Valve and AFM

- Unplugged/wiggled around all connectors

- Checked ECU for codes (No errors)

- Checked voltage drop from battery to all of the earth points on the engine. All are 0.00 or 0.01

- Sprayed Start ya Bastard around every intake and vacuum line. Revs did not change

- Tested TPS. With Ignition ON, but car off, idle is 0.45v, WOT is 3.9v. With car running, idle is 0.41, WOT is 1.4v (Is this a possible cause?)

- Tested AFM. With Ignition ON, but car off, reading is 0.22v. With car running, it is 1.14v

What else could it be? Coolant Temperature Sensor? CAS? Injector problem?

It does not have a boost gauge. If I get one, will I be able to tell if it has vacuum leaks by its reading at idle?

The car uses Castrol Edge 5-30 fully synthetic, changed every 5000kms, the 02 sensor was changed 20,000kms ago. It has 145,000kms on the clock.

Spent most of the past 3 days on it, as well as testing random things since it started showing symptoms.

Thanks for any advice

Far out, my car (34 GTT) also has an rb25detneo and is beginning to get quite a noticable missfire, i am getting some new coilpacks next week as it still has factory ones in there.

I hope i dont have the same problem, the 1st thing i would have checked would have definately been the coilpacks but you said you have just done that.

Perhaps the fuel pump/filter could do with replacing??

[revolt]

Didn't think of the filter. Will give that a shot. The current one has only done about 15,000km tho.

Far out, my car (34 GTT) also has an rb25detneo and is beginning to get quite a noticable missfire, i am getting some new coilpacks next week as it still has factory ones in there.

I hope i dont have the same problem, the 1st thing i would have checked would have definately been the coilpacks but you said you have just done that.

Perhaps the fuel pump/filter could do with replacing??

[Chris Wilson]

Have you tried removing the air flow meter and washing it through inside with brake cleaner? Sometimes the hot wire gets contaminated ESPECIALLY if you have ever had an oil impregnated "performance" air filter in it. The other easy thing to try is either `scope the output of the O2 sensor to see if it's dead or lazy, or just fit a new one.

[TMW-R34]

Didn't think of the filter. Will give that a shot. The current one has only done about 15,000km tho.

Yeah thats not really a lot, perhaps and hopefully you got a dodgy one and replacing it will fix it.

Best of luck.

[revolt]

Yeah, cleaned the AFM with Brake Cleaner. Will look into the O2 sensor. Not very old either though.

[tridentt150v]

try swapping out the AFM? could be the dreaded dry solder joint.......which I'm informed you can repair

[revolt]

Yeah, that is another possibility, because it reads 0.22v with the ign ON, but not running.. others have said around 0.5v is normal?

try swapping out the AFM? could be the dreaded dry solder joint.......which I'm informed you can repair

[gotRICE?]

Have you tried unplugging the AFM while its idling?

It should run pretty smooth with it unplugged at idle IF it was the problem

[geraus]

Have you checked all your VACUM hoses for no leaks?

[revolt]

Have you checked all your VACUM hoses for no leaks?

Yep, I noticed on the Driver's side in between the engine and stub axle (on one of the 4wd doo-dads) there is what looks like a vacuum hose that comes up to near the oil filter.. the other end is not connected to anything, but I can't see where it should go? Is this just a breather line or something? The other thing is there is a single pin sensor on the oil filter housing that is not connected. My guess is it's for an optional oil pressure sensor..

Unplugged the AFM while running and the car idles better. When I rev it with the AFM off, it revs smoothly up to the limp home 2500rpm limit, where it misfires like crazy. Is that 2500rpm misfire normal with no AFM? If so, my guess is my AFM is rooted since it's a lot smoother with the AFM off.

EDIT: OH! Very important thing I just figured out. It only does it when it's warm. I always drive sedately til it's up to temp, so I didn't notice but today I decided to flog it and see what happens.. it was perfect for about 5-10 mins.

Edited January 3, 2010 by revolt

[paulr33]

are you sure TPS at WOT is 1.4v when running and 3.9v at ON but engine not running?

this would make the car run like ass if it was true

double check this please

[Flargh!]

Yep, I noticed on the Driver's side in between the engine and stub axle (on one of the 4wd doo-dads) there is what looks like a vacuum hose that comes up to near the oil filter.. the other end is not connected to anything, but I can't see where it should go? Is this just a breather line or something? The other thing is there is a single pin sensor on the oil filter housing that is not connected. My guess is it's for an optional oil pressure sensor..

Unplugged the AFM while running and the car idles better. When I rev it with the AFM off, it revs smoothly up to the limp home 2500rpm limit, where it misfires like crazy. Is that 2500rpm misfire normal with no AFM? If so, my guess is my AFM is rooted since it's a lot smoother with the AFM off.

EDIT: OH! Very important thing I just figured out. It only does it when it's warm. I always drive sedately til it's up to temp, so I didn't notice but today I decided to flog it and see what happens.. it was perfect for about 5-10 mins.

ECU adjust fuel according to water temp i think. Perhaps the sensor is faulty and it's still running on cold start settings which would be rich i suspect??? No idea if that would cause your drama though.

[paulr33]

if the water temp sensor is dead the engine check light will be on

if its out of whack and working (ie giving a valid reading) the ecu will assume the water temp is, what the sensor says

so if the sensor is out of whack the ecu wont know any better but if the sensor is dead the ecu will turn on the engine check light

[gotRICE?]

Yep, I noticed on the Driver's side in between the engine and stub axle (on one of the 4wd doo-dads) there is what looks like a vacuum hose that comes up to near the oil filter.. the other end is not connected to anything, but I can't see where it should go? Is this just a breather line or something? The other thing is there is a single pin sensor on the oil filter housing that is not connected. My guess is it's for an optional oil pressure sensor..

Unplugged the AFM while running and the car idles better. When I rev it with the AFM off, it revs smoothly up to the limp home 2500rpm limit, where it misfires like crazy. Is that 2500rpm misfire normal with no AFM? If so, my guess is my AFM is rooted since it's a lot smoother with the AFM off.

EDIT: OH! Very important thing I just figured out. It only does it when it's warm. I always drive sedately til it's up to temp, so I didn't notice but today I decided to flog it and see what happens.. it was perfect for about 5-10 mins.

When i did my FMIC install i forgot to plug the AFM back, and managed to get about 300m down the road before it started spluttering and spitting. So i guess i kept the revs low untill then Sounds like you "may" have a dodgy AFM. See if you can borrow a friends that is known working?

[revolt]

Retested the TPS. 6k RPM was 1.2v..... 3k RPM was about 0.7v. Idle is 0.4v

[revolt]

Bah, double post. But with the engine off, it's 0.45 at idle and 3.99 at WOT

Edited January 3, 2010 by revolt

[Flargh!]

Bah, double post. But with the engine off, it's 0.45 at idle and 3.99 at WOT

Perhaps thats your problem then.

Mine is 0.44 at idle with engine off and 3.90 at WOT

Engine running i have 0.41 at idle and 3.79 at around 6000 rpm.

[revolt]

Perhaps thats your problem then.

Mine is 0.44 at idle with engine off and 3.90 at WOT

Engine running i have 0.41 at idle and 3.79 at around 6000 rpm.

Ah, awesome! (well, sort of.. still have to buy one, but at least it might be solved).

Will pull it off and inspect it. Thanks for your help everyone! I will try to visit the forums a bit more and help others.

[paulr33]

yeah its wierd that when the engine is running it doesnt go anywhere near the normal values

that would make the car run like ass as the ECU has mapping and fuel enrichment etc based on TPS (and AFM, but TPS is also used)

so whilst your floor it, the ECU doesn't detect you are flooring it and thinks the engine is only under some minor levels of load even though AFM is probably showing normal full voltage