Rb25 misfire – not the usual suspects

fella

fella

Member
Right then folks, I have been a long time member on here, but as you can tell by my post count, I do a lot more reading and learning than contributing!

However as the title suggests I have a problem with the RB25DET in my C35 laurel and it has got me utterly utterly baffled.

Basically without going on too long I imported the car late 2010 and it had had a standard RB25DET with a manual gearbox popped in, in Japan (not very well tho to be fair).

I changed the headgasket about 6 months ago to a cometic rascal (for the higher boost later on) after shearing exhaust manifold studs and popping the head off to drill em out, other than that the engine is standard internally.

The misfire started late last year, and basically even when cold occurs when the car starts making any kind of positive boost pressure (not that a I drive it on boost when its cold but needed to check).

BUT it also gets worse as the engine warms up to point where it will miss on idle and you can hear it dropping a cylinder now and then,

Once warmed up it gets to the point where it is extremely difficult to drive as it will miss ALL the time and sound like its pinking its tits off on virtually any throttle.

Now the troublesome bit…………….. so far I have checked and changed what I think to be the main possible culprits and am completely baffled.

Basically I have:
Replaced the coil packs with new yellow jacket ones
Replaced the wiring loom to the coil packs and checked the continuity of the wiring loom too
Replaced the spark plugs (twice), now on nice standard NGK BKR6EYA copper jobs gapped to 0.8mm to reduce the resistance as much as I dare.
Checked the Cam Angle Sensor, but have just replaced it anyway (made NO difference)
Checked the throttle position sensor (reads fine on my AVCR fuel controller between 0-100%/0-5V)
Checked and cleaned the (standard RB25) AFM (reads fine on my AVCR fuel controller between 0-100%/0-5V)
Checked for boost leaks – plugged the throttle end and the turbo inlet end and filled the system with air from a compressor and used a soap spray to check/fix ALL joints. But only running stock boost at the minute on the standard tubby
Checked cylinder compression - all 6 are 9.5:1 or slightly above
Added new/additional earth points from/to the head, batt, block, chassis and inlet manifold
Checked the fuel pump voltage (getting 11.8V with ignition on)
Changed the fuel filter (grasping at straws at this point)

Sorry for EPIC length of this post but its really starting to peck my head now ………… any all thoughts/suggestions are very welcome folks.
Thanks for reading.
 
We had a fault on our old r33 gtst it wasn't firing on one cylinder we did the old new plugs new coils etc turned out we tried a ecu from another gtst and it cured it,

Have you got another ecu you can try?
 
combattuk said:
We had a fault on our old r33 gtst it wasn't firing on one cylinder we did the old new plugs new coils etc turned out we tried a ecu from another gtst and it cured it,

Have you got another ecu you can try?
Click to expand...

That's a bit of a long story too, but basically no I don't unfortunately.

But the ECU thats in it (from the part number) is from an R34 gtst auto! So how it runs at all is a mystery!?!? It has been working okay till now ........... not brilliant but not missing.

I know that its not right, but its got funny uncommon pinouts (so no plug and play) and I want to do that last and just get a nistune setup done.

But I was beginning to wonder if it had started to go funny on me (kind of running out of other ideas), cheers.
 
it can all of a sudden go we must of had a short in the wiring somewhere one min it was fine next thing the miss fire, I hate electrical problems trying to find the gremlin's crawling around lol, might be worth a try find somebody with a spare ecu you can quickly bung on
 
Thinkin you might be right tbh, am leaning towards either an ECU fault, some sort of ninja stealthy air leak on a manifold gasket or somet, or at worst a mechanical prob like a valve or that gets worse when hot (hoping not tho)
 
Was out in the laurel yesterday testing if the new (used0 cam angle snor made any difference to the misfire (which it didn't) and the clutch slave cylinder popped, so came home on a recovery truck :cry:

............. but sorted that today and whilst under the car I found 3 wires dangling (like I said the conversion looks to have been done by a muppet in Japan), that all turned out to be ignition live! ( 1x12V and 2x3-4V)

So have been back out again this aft to see if taping off the bare ends of these made any difference ............... NONE WHATSOEVER!

It basically still sounds like an impreza once you try to give it throttle/boost. But if you nurse it and run on vac, you can get it all the way round the revs to the redline without a sniff of a misfire (it takes its time tho!!).

But it really does get noticably worse when the engine is getting up to temp ......... maybe an air leak is getting worse as stuff expands with the heat ........... or maybe I've stop diagnosing and started guessing out of desperation?
 
Compressions checked wet or dry?? (with or without oil dropped into cylinders) I know you checked compressions and I hate to be a pessimist but it Is an RB! Being able to nurse it through revs also suggests the usual rb failure. Hope I'm wrong dude but I'd guess you've lost compression on a cylinder? Good luck!!
 
http://www.driftworks.com/forum/dri...issota-500bhp-1jz-r33-gtst-drift-project.html

Have a quick glance over this guys misfortunes... Sounds similar and I've had a misfire on an rb25 and i could also nurse it, ended up fitting new pistons (number 5 buggered) and did the same again. Its worse when warm prob because everything's expanding when it's warming up and the compression drops further.

But firstly you need to Identify which cylinder is missing then swap a plug, and injector from another cylinder, see if the misfire moves, if so you've found your fault. If not test the loom and spray some carb cleaner or wd40 around all the inlet gaskets and listen for a change in engine note. In which case youve got an air leak. Failing that, piston/rings/valve or head gasket... Prob piston/ring on cylinder 5 or 6:( if in doubt-Fit a 1jz instead!!
 
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Dori-points said:
Compressions checked wet or dry?? (with or without oil dropped into cylinders) I know you checked compressions and I hate to be a pessimist but it Is an RB! Being able to nurse it through revs also suggests the usual rb failure. Hope I'm wrong dude but I'd guess you've lost compression on a cylinder? Good luck!!
Click to expand...

Cheers for your input chap, I must be honest I am starting to think that this might be a bit more serious (read serious as expensive!) than I was hoping, as all of the simple to change/bolt on bits keep passing all the tests and proving to be okay, and I'm running out of external components to test!.

The compression test was done with the engine stone cold and the cylinders dry, so kind of ruled out rings, but now you mention it, I should probably try a comp test with it warm now as that is when the problem is at its worst.

Fingers crossed that this doesn't prove to be the source of the problem!
 
Dori-points said:
http://www.driftworks.com/forum/dri...issota-500bhp-1jz-r33-gtst-drift-project.html

Have a quick glance over this guys misfortunes... Sounds similar and I've had a misfire on an rb25 and i could also nurse it, ended up fitting new pistons (number 5 buggered) and did the same again. Its worse when warm prob because everything's expanding when it's warming up and the compression drops further.

But firstly you need to Identify which cylinder is missing then swap a plug, and injector from another cylinder, see if the misfire moves, if so you've found your fault. If not test the loom and spray some carb cleaner or wd40 around all the inlet gaskets and listen for a change in engine note. In which case youve got an air leak. Failing that, piston/rings/valve or head gasket... Prob piston/ring on cylinder 5 or 6:( if in doubt-Fit a 1jz instead!!
Click to expand...

I tried to do the whole 'swap the plugs swap the packs' thing, but it hasn't really showed any great pattern tbh ........ which again has me baffled?

maybe somebody could see somet obvious I'm missing, or just a fresh set of eyes would be enough, but I swapped the plugs for new ones a few weeks ago, ran it missing for as long as I could/dare, to see if any plugs fouled and took em back out ............... it goes from cam pulley to bulkhead from RIGHT to LEFT...............

IMAG0485.jpg


then cleaned em up and popped em back in, swapping the slightly fouled plugs (4&5) into cylinders 1 & 2 and went again, then whipped em back out for another look .................... (cam pulley to bulkhead from RIGHT to LEFT again)

IMAG0502.jpg


and thought bingo!

So popped some new copper NGK's (BKR6EYA's gapped to 0.8mm) in expecting the prob to have gone, but no!

so marked up and swapped the coil packs around (1&2 with 5&6) to see if even though they were new, if maybe they were duff, but again the cylinders that seem to have darker plugs don't follow anything I've swapped around (cam pulley plug on the left this time, sorry!) ............

IMAG0539.jpg
 
sooooooooooooo ....................

Even tho it is officially BASTARD freezing here in Huddersfield, I've been out on the drive today, and added to the list of things that are NOT wrong with my shitty RB25.

Like I mentioned I have replaced the head gasket recently with a cometic rascal, as the head was off to remove a snapped exhaust manifold stud.

So had one of those moments where I thought (and maybe grasped at a straw or 2!) that perhaps the uprated head bolts I popped back in where stretch bolts and needed re-torqueing .......... nope....... still ALL at 102Nm.

But whilst I had some of the upper plumbing off I thought I may as well check the mechanical timing hadn't changed somehow and was causing all this grief ........ you guessed it, not that either.

IMAG0541.jpg

IMAG0542.jpg

IMAG0540.jpg

You can't really see the crank pulley pic cos my 'smart' phone disabled the flash as it was too cold ....... sad when your phone is clever enough to know its too cold to be outside but your not!!


So last but not least (this is without doubt goin to get abused ..... and deservedly so tbh!) I rigged up a 'smoke tester' style setup, involving a coffee jar, some clear pipe, some ebay super special smoke bombs and a shit load of duct tape!

IMAG0544.jpg


It actually worked quite well, I couldn't find any vacuum leaks, but if I opened the throttle flap I got smoke recircing out through the port I had disconnected to put the smoke through in the first place, so knew it was gettin into the inlet manifold and would have leaked if it could (did replace inlet and exhaust mani gaskets when the head was refitted).

Just for a bit more background (as if this post needs any more writing!!!), when I was doin the head gasket I had a gander at the bores and couldn't see anything wrong with em, I didn't use a run out gauge or any formal measuring device, but there was no lip on the bore that could be seen/felt and all I did was give the pistons a de-coke with carb/injector cleaner and a stiif (not wire!) brush, blow em clean and then flushed the oil as best i could could with new stuff once it was rebuilt.

BEFORE:

IMAG0363-1.jpg




AFTER:

IMAG0391.jpg



Gonna try to hunt down an ECU that has the right pinouts to suit my plug and on payday (woooo!) I'm gonna get a cylinder leak down tester to see if the compression test missed somet ........... other than that I'm stumped ......... again!
 
Dori-points said:
Which cylinder is misfiring buddy??? You need to narrow it down really?
Click to expand...

that's just it ............ I'm not really sure!

Thought the plugs would have told me but it seems to change, and my ECU does't seem to have the LED and screw arrangement for the fault codes I'm used to with other nissans of this vintage ........... just holes were they could be!

IMAG0531.jpg

IMAG0532.jpg
 
On the odb plug thing you can put a paper clip across 2 of the pins which will make the engine management light flash the ecu codes up for you, if you google it it should find a topic that will tell u the location and fault codes etc
 
try your ecu in someone elses car, if it runs like shit you found the prob. if not i suspect a wiring fault somewhere. then its the long arduos task of trying to find it
 
the r34 ecu is a funny fella.. and is very finnicky about things..

have you connected consult to it??? i bet there is a myriad of faults..

nistune is a VERY good idea on this ecu.. it works brilliantly.. i have done many many r34 gtt's on that setup.


fella said:
That's a bit of a long story too, but basically no I don't unfortunately.

But the ECU thats in it (from the part number) is from an R34 gtst auto! So how it runs at all is a mystery!?!? It has been working okay till now ........... not brilliant but not missing.

I know that its not right, but its got funny uncommon pinouts (so no plug and play) and I want to do that last and just get a nistune setup done.

But I was beginning to wonder if it had started to go funny on me (kind of running out of other ideas), cheers.
Click to expand...
 
I had this EXACT problem but on an rb20det and it turned out to be split breather hoses replaced the breather hoses and all my problems went away!
 
You say you checked for boost leaks, ie...

Checked for boost leaks – plugged the throttle end and the turbo inlet end and filled the system with air from a compressor and used a soap spray to check/fix ALL joints.

But that doesnt count post throttle, ie inlet manifold.

I had a problem with my RB26 that sounds very similar, and that turned out to be the inlet manifold gasket, even though no soapy water no nothing could spot it. Even RB26 experts checked it over, changed loads of electrical things as they were also sure there was no leaks.
In the end we changed them anyhow and it was fixed.
 
combattuk said:
On the odb plug thing you can put a paper clip across 2 of the pins which will make the engine management light flash the ecu codes up for you, if you google it it should find a topic that will tell u the location and fault codes etc
Click to expand...

ah ......... my management light doesn't work either! (remember it was converted by a Japanese muppet and bought by a British muppet)

On my list of shit to put right with this frickin car, but before this prob, I didn't bother cos I knew the wrong ECU in the wrong car with the wrong gearbox would just have it on all the time. Do remember seein somet about that being a way to extract the fault codes tho ..... might be a worthwhile day of my life to get it working to see what isn't working elsewhere.

Not even sure where abouts on my cluster it should be tho (never seen a club S laurel dash before)......... will try to get a pick later as I'm off out to see if the consult I got hold of works (not expectin much).
 
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