rb25 non turbo rough idle

[robnicko]

hi, new poster here seeking some help with a 1999 R34 GT neo 2.5ltr non turbo. Car is completely stock apart from a pod air intake.

Issue is the idle has turned rough and exhaust smells like fuel not being burnt properly, power is down yet still fires up no problem and drives although the roughness is felt throughout the rev range.

2 weeks ago I replaced the spark plugs with new NGK PFR5G-11, head gasket has been ruled out, old plugs looked good - no fouling or damage but replaced anyway.

Upon putting it all back together it fired up fine and idles smooth and drove perfectly for about 15 min and then the rough idle came back. It has on a couple of occassions gone back to normal temporarily but then goes rough again.

There is no check engine light and I have done the pin on the diagnostic port and no errors present. I have checked the IAC which was very clean, water temp sensor, all vacuum lines and cleaned throttle body which was very clean anyway. I have also tried unplugging the O2 sensor and that made no difference. There is no contamination around the spark plugs (oil / coolant etc)

Today I put new coils in it and again upon starting it was like a new car for about 15 min and then it went rough again - no check engine light or error codes

All injectors are firing and sealing properly, ive cleaned and reset all connectors and checked for any wires that may have rubbed through causing a bad earth and also confirmed all fuses and relays are fine. Also, the car has not stalled at all, just runs rough

Im a bit stumped so hoping someone who may have dealt with a similar issue can advise where to look next?

 

thanks

rob

 

   

 

Edited December 19, 2021 by robnicko

[GTSBoy]

So....it's fine when you just put it back together and goes off when it has warmed up for 15 minutes. That would suggest that it is either something that is temperature sensitive (ie, wiring/connectors for injectors or coils) getting the shits up when they heat up, or possibly something that is becoming significant once the engine/ECU reaches a particular temperature. When I say "ECU" in that sentence, I'm actually talking about the engine, but as seen from the point of view of the ECU program. For example, when cold cold, the idle speed is controlled by the cold start fast idle valve. The ECU has no control over idle speed until the fast idle valve closes down, which it does by direct thermal means (ie, it sees the coolant temperature flowing through and physically closes). The ECU is then in control and the IAC can affect idle. Also, when certain threshold coolant temperatures are reached, the ECU changes from one ignition timing strategy to another. That change could switch the engine's operation over from "the problem is there but you can't detect it because the timing makes it run a particular way" to "now the timing is normal and now you can notice the problem".

All of this means that you might need to use some freeze/cooling spray to chill out loom connectors while it is stumbling, to see if it cleans up. Spray it on coil & injector plugs, CAS plug, AFM plug, IACV plugs, etc. You might need to do some more direct diagnostic work, where you deliberately get it hot then quickly change to cold (spare) components, or control how the idle air gets to where it is supposed to go, watch the ignition timing to see if it is becoming horribly retarded when the stumble starts, etc etc.

[robnicko]

Thanks for your reply GTSBoy

Ill try the freezing spray as you mention and see if it helps isolate the issue.
I forgot to mention that i cleaned the MAF sensor with CRC MAF Cleaner as well as its plug and the CAS / IACV plugs - basically every plug on the motor I have unplugged & cleaned with contact cleaner before re seating. Alternator is charging correctly as well

I understand what you mean with the cold enrichment / operating temp different 'tune strategies'

When its running rough the engine is lightly shaking at idle and as rpm increases the shakiness also increases. When its normal you cant even feel it running, extremely smooth. Im starting to think something to do with the CAS possibly however no codes are present and it always fires up no problem and even this morning after a cold start was running rough from the get go. 

To check the timing on this motor is it possible with a standard strobe timing light? To connect to the #1 coil is not possible as the intake needs to be removed to access the coils....

Nissans are new to me so its a quick learning lesson and a little frustrating!

[GTSBoy]

On 12/20/2021 at 2:40 PM, robnicko said:

To connect to the #1 coil is not possible as the intake needs to be removed to access the coils....

This. You need to interpose a spark plug lead between the coil and plug.

[robnicko]

update,  ive done the cold spray on all connectors and no difference.  put the original coils back in and same deal - rough idle, shaking and reduced power

changed fuel filter, confirmed all injectors are working.  car starts and idles no worries - just rough / shaky which increases with rpm and doesnt smooth out

still no error codes coming up when bridging the diag port

took it around the block but had to use the auto like a manual on uphills

no coolant loss or oil and no white residue on the oil filler cap when cold

exhaust at tailpipe is 'fluffy' and stinks.

next will be to unbolt the CAT once it cools down and see if it is blocked however I did this a couple of weeks back and it was clear and no loose bits when shaking it

any ideas beside a match in the tank?

[joshuaho96]

Do you see a check engine light when the rough running begins after 15 minutes? I would check the ECU capacitors. Have you checked mechanical and ignition timing?

[robnicko]

no, there isnt any check engine light appearing & no codes when bridging the consult port plug either.  only code that appears when bridging the consult port is 55 (no malfunction) which is 5 slow & 5 quick flashes

 

 

Edited December 23, 2021 by robnicko

[blind_elk]

Have you determined which cylinder(s) are causing the rough running? Compression test?

[robnicko]

Havent done a comp test however will look at doing one when I get a test tool

I dont beleive theres any 'mechanical' problem with the motor as its not using any oil, water or making unusual sounds - just very shaky and low on power.  when opening up the throttle it sounds like its bogging down

Tomorrow will remove all coil packs & plugs and inspect the plugs again although theyre brand new

tried a new coolant temp sensor and no difference

unplugged O2 sensor & no difference

unplugged MAF and idle drops and had to give throttle to keep engine running  (cleaned with MAF cleaner again)

unplugged TPS & engine stalled

Checked CAT again - no rattles and in good order

overnight i unplugged battery & all electrical connections on motor and re plugged in everything just now & no difference

Crimped return line on fuel rail to increase pressure to injectors - no difference

Also have drained fuel and put 20l of premium in with injector cleaner - no difference

Checked timing again and all good @ 15degrees

 

 

 

[blind_elk]

At the risk of being repetitive, the O2 sensor will NOT be the cause. The ECU only reacts to O2 when up to temp and at cruise conditions. At idle, there is not enough gas flow; at normal driving, there is too much variability of gas flow.

Have you done the standard diagnostic test to determine which cylinder(s) cause the rough running? (disconnect each coil in turn, see if the idle changes)

Sometimes, depending on the timing light, checking timing can give incorrect information - the indicated timing is doubled, so you are actually running at 7.5 BTDC at idle.

[joshuaho96]

FYI, you can do the cylinder balance test through Consult/NDS if you have it instead of letting raw fuel go through the exhaust.

[robnicko]

well, what a way to spend Boxing Day

Ive narrowed it down to the coil pack loom so will get a new one. the connectors are very brittle and a couple crumbled when I took out the loom today.

for now I have used zip ties to secure the plug to each coil pack and the car now goes great (this is with the original coils and spark plugs).

The connector for Cyl 3 (looking from front to rear) crumbled the worst and is still not making the best connection however ive secured it as best as possible until the new loom is purchased so a light rough idle is still present but nothing compared to this morning - id say the car is about 90% good now

also it finally threw up error code 12 and it was the solder connections on the MAF so I i resoldered and thats all ok now

BlindElk - unplugging each coil on this motor is difficult as once the plenum is installed and other things its near impossible to access the coils whilst engine running. That being said I've removed and refit the intake and get to the plugs that many times I can do it in about 15min  :)

Thanks for your input to those that replied

 

 

[joshuaho96]

On 12/26/2021 at 12:48 AM, robnicko said:

well, what a way to spend Boxing Day

Ive narrowed it down to the coil pack loom so will get a new one. the connectors are very brittle and a couple crumbled when I took out the loom today.

for now I have used zip ties to secure the plug to each coil pack and the car now goes great (this is with the original coils and spark plugs).

The connector for Cyl 3 (looking from front to rear) crumbled the worst and is still not making the best connection however ive secured it as best as possible until the new loom is purchased so a light rough idle is still present but nothing compared to this morning - id say the car is about 90% good now

also it finally threw up error code 12 and it was the solder connections on the MAF so I i resoldered and thats all ok now

BlindElk - unplugging each coil on this motor is difficult as once the plenum is installed and other things its near impossible to access the coils whilst engine running. That being said I've removed and refit the intake and get to the plugs that many times I can do it in about 15min  :)

Thanks for your input to those that replied

 

 

At least you figured it out. Thankfully mechanical machines are a lot easier to diagnose and fix than biological ones. I highly recommend getting a Consult cable and Nissan Datascan for diagnosing this kind of weirdness in the future. You can do the "unplug coil" test with a simple click of a button on your laptop instead of dealing with 25+ year old connectors crumbling in your hands.

[robnicko]

the odd thing is the consult port on this car doesnt have the tx/rx wires installed for whatever reason.

I had a friend connect his laptop with Nistune software and consult cable however it wouldnt communicate but anyhow seems ive located the issue so should be good once i get the new loom.

quick learning curve coming from Rotaries & Landcruisers but in the end air / fuel & spark make engines go and spark was the issue here