Engine sporadically running rough

[HXD]

I've recently started experiencing some weird, inconsistent symptoms with my engine. Occasionally, the revs will suddenly dive into the low hundreds and the engine will be spluttery and struggle to make power. This can last for a few seconds or sometimes even a few minutes. Typically, it's gone by the time I next start the car up again.

The car has just had a major service and a completely clean MOT, with no error codes showing via the ODB-II port (print-out from the garage's diagnostic computer), and zero symptoms whilst it was with the garage and the test centre. So I'm a little stumped as to what on Earth might be going on.

One thing to note is that a couple of weeks ago, the petrol gauge failed to show the car was empty – instead stuck around 1/4 showing on the dash – and the engine did suffer some fuel starvation for a very short drive as a result. This spluttery, difficulty-making-power behaviour I've had 3–4 times since (as described above) feels the same as when the car was fuel-starved.

Also this might be purely incidental, but it has had a tendency to happen more when the car is wet, either because of the weather or immediately after it has been washed, but not every time it has been wet (either the body work or the atmospheric conditions). This may just be me being superstitious though; it's so wet so often this time of year this could just be pure coincidence.

So I'm a bit perplexed. I wondered if it might be corroded wiring somewhere, or possibly an intake leak or pressure loss. But because it's not consistent, I'm not sure either of those make 100% sense to me—grateful to be proven wrong though! I wondered if it might be a coil or two going, but again I'm not sure because the car goes back to running buttery smooth afterwards, no matter whether I'm pootling around town or giving it a blast on the motorway. I wondered if it might be an airlock caused by the fuel starvation, but I'd have thought those would've long cleared by now (it's done over 100 miles since).

Could it be as straight-forward as the fuel pump having sucked up some crap from the bottom of the tank when it was fuel-starved, and now the pump is dying? I used a correct dose of Redex immediately after the fuel starvation, just in case any lines might have had crap in them, and it's used almost a full tank of fresh 99RON since then (but still with the sporadic periods of misfires every now and then).

Anyway, your thoughts appreciated! It's starting to make me not want to drive my car…

[Dave G]

Sorry to hear you've had problems.

This could be a number of things...  Spark plugs, coil packs, fuel filter, fuel pump.  However, if it were a coil pack going, then most likely it will be just one cylinder (unless you were really unlucky) so the car would run, just a bit rough.  From what you describe, it sounds like all cylinders are affected so I'd suspect a fuel starvation issue.  As no codes have been logged in the ECU, then it might be off the radar of the ECU.  So maybe check fuel filter or fuel pump pick up (probably not easy to do though) to see if they are blocked.

Other areas could be something like a crank position sensor or coolant temperature sensor going duff, that will make the car run very poorly but should be picked up by an engine code.

Puzzling...

Dave

[HXD]

Thanks for the suggestions, Dave, appreciate it. Great to understand more about what can and can't get logged by the ECU.

Brand new plugs went in as part of the major service, and my mechanic confirmed there was nothing untoward about the old ones when they came out. So I don't think it'll be the spark directly, but something "further back" in the process.

It really is struggling to make power when this happens though, so I'd say it feels much more like perhaps a whole bank or multiple random cylinders affected for a period when it does start spluttering. Given the fuel starvation issue previously, I'm leaning more towards a fuelling issue. I've no idea when the fuel filter was last changed, so I may get that changed when it goes in for its planned gearbox + diff oil changes in March.

Edit: Having just dug out and flipped through my 350Z/G35 Haynes manual, I see what you mean about the fuel filter not being easy. I didn't appreciate it was also in the same housing as the fuel pump and pickup. I'm now wondering if I shouldn't just get the whole assembly replaced, including a 'better' aftermarket fuel pump.

[Dave G]

So shouldn't be spark plugs then if they've just been changed (and assuming that they were installed properly), so agree, it does sound like a fuel starvation issue.  Yeah, the fuel filter is part of the pump/pick up assembly so not as easy as replacing an in-line one!

Could be a good shout to replace the whole fuel pump assembly as if the fuel pump ran dry for a period of time then it might have overheated (I think the pump will use the fuel to cool it) and got damaged.  If your pump isn't making good pressure then that would cause the engine to run rough.  The fuel pressure regulators are mechanical so again, won't be picked up by ECU...  Also, you originally mentioned that the gauge didn't show empty so changing the whole unit will help with that issue reoccurring.  Just an expensive punt though...

[HXD]

Thanks Dave, that's really interesting about the overheating aspect for the pump. I'd never thought of something like that.

I've done some research over the past week or so, and (I may be jinxing myself pre-emptively here) it doesn't look all the complicated to replace the pump yourself. I'm aiming for an OEM+ approach with any replacements I make on my car, so was considering something like a a Walbro 255 aftermarket pump. It looks like you need to remove some plastic 'sighting' teeth around the bottom edge of the Walbro by hand, in order for it to fit flush in the original 350Z fuel assembly, and there's a little bit of wiring/splicing to do for power. But otherwise it looks pretty straightforward to do.

Sod's Law, it's not happened again in the last week since posting. But I'm fairly certain the engine feels down on power now, and doesn't rev up as smoothly or quickly as I remember - which I suppose you could attribute to lowered fuel pressure. If I had a garage, I'd be sorely tempted to Destroy It Myself immediately, but might wait for the milder weather so I can attempt this on the driveway. Either that or bite the bullet and ask my local mechanic to do it, if the spluttering starts again.

[PaulW]

As you mentioned it happens when it's wet and you have aftermarket air filter I'd have a look at the maf sensor. 

[Dave G]

18 hours ago, PaulW said:

As you mentioned it happens when it's wet and you have aftermarket air filter I'd have a look at the maf sensor. 

Good shout...!

[HXD]

23 hours ago, PaulW said:

As you mentioned it happens when it's wet and you have aftermarket air filter I'd have a look at the maf sensor. 

Thanks, that sounds like a cheaper and much less messy place to start at least. 😅

I'm thinking a can of Liqui Moly MAF cleaner plus an air filter cleaning & oiling kit would be worth ordering beforehand? The filters are about 5–6 years old now, no idea when/if they were last oiled—they were already fitted when I bought the car.

[PaulW]

Rather than clean the maf I'd replace it;

https://www.autodoc.co.uk/ridex/8053911?gshp=1&gclid=CjwKCAiAr4GgBhBFEiwAgwORrethAV3wkN6LF-UTNdeidG1d23PI83QsqUDcGeYIKoprLk-qEz4ZshoCa4cQAvD_BwE

Never had a lot of luck cleaning them

[HXD]

So an update on progress with this issue…

Last weekend I thoroughly cleaned the pair of HKS Super Hybrid air filters, which seemed to be somewhat flat and quite grimy in the centre layer. I soaked them in K&N's specialist filter cleaning solution, let them sit for 10mins, then thoroughly rinsed them with cold water through the back of the filters only. I did this twice, then left them to dry on the boiler overnight. I then lightly misted the front of each filter with air filter spray oil the following day before refitting them in the car.

I also thoroughly blasted both MAF sensors with cleaning solvent and allowed them to dry naturally, being careful not to touch anything other than the connector socket with my fingers. I also did this process twice, just to be sure.

After putting everything back together, there were no issues on startup and drive, which was reassuring. However, after driving in drizzly conditions for about 15mins, the car did start to slowly develop very mild spluttering again. Which is definitely an improvement over the more sudden and aggressive dips I experienced before.

So my thinking is that cleaning and oiling the filters has helped, particularly in reducing water induction, but that there is still a fundamental regulation issue somewhere. And for the sake of continuing in the theme of "cheapest fix first" I'm thinking a replacement pair of MAF sensors might be next—thanks for the link, @PaulW.

[Dave G]

The chase continues...

Thanks for the update.  Hope the change of MAF sensors cures it for you.

[HXD]

So still no change in the car's performance after trying one new MAF sensor to begin with. It's still sporadically dipping into Limp Mode once in a while, and also started running a bit richer on the fuel mix of its own accord. It's not showing any signs of overheating, and it cranks and starts easily as it always has. My regular local mechanic has had a good look again at the car today, but he still can't see anything obvious either; there are no codes being logged by the ECU, and therefore very little to go on.

With no real clues left as to what is happening before getting into four-figure engine teardown territory, I spoke to Scott at Abbey Motorsport this afternoon to see if they have any ideas - they remapped and dyno'd the car back in 2018, prior to my ownership.

Long story short, they think it might be early signs of the oil gallery gaskets starting to deteriorate. They've given me some diagnostic tests to do once the car comes back from my local garage, but I've a suspicion this might be where things are headed.

Will continue updating as things progress.

[Jeff-W]

Thanks for the update - sadly, you may be heading into some dark territory here.

[Dave G]

Oh no, that doesn't sound good.  I hope the diagnostic tests will help you find out if that is the problem.  Is the suspicion that oil is getting into a cylinder somewhere?

[HXD]

Not into the cylinders, as there is no sign of smoke or sooting at all. The car had its first major service in a few years in February, and sailed through the MOT at the samne time with no problems at all in either case; the old spark plugs came out so clean, my mechanic wasn't sure we should bother replacing them, but brand new NGKs went in anyway.

I think the concern is that one of the gallery gaskets might be just starting to break down, but certainly not catastrophic as this problem is so sporadic and doesn't last for more than a few moments when the engine does misbehave. My mechanic did a full exterior inspection earlier this week, including underneath, and couldn't find anything obviously wrong with the car anywhere. He did say the CO2 is notably increased over when it passed its MOT, but that's about it, and the car has never misbehaved for him any time he's had it.

I did the tests advised by Scott at Abbey yesterday, and the oil PSI once the car is at temp and been driven for a good 25mins - motorway and A-roads - looks to be around the 15 mark, which I believe isn't unusual or immediately indicative of a gallery gasket problem.

Waiting to hear back from Abbey for possible next steps...

[paul_k]

Water or rust in the fuel?

[HXD]

That's an interesting thought, but it's had at least 2 tanks of fuel since this first started, so I don't think it'll be the fuel itself.

I've booked the car in with Horsham next month - they're half the distance away, and Abbey didn't come back to me - so hopefully I'll know more after they've run some tests.

[HXD]

Right, after what feels like a small eternity, I've finally been able to get my car to and from Horsham Development, and let Jez & co. take a good look over my 350.

Sporadic rough running / random limp mode

They couldn't replicate that very weird, very sporadic rough running issue either - they had a hosepipe in all the nooks and crannies underneath with the car running and being rev'd, specifically paying (spraying?) attention to wherever there could be exposed electrics... and absolute nothing untoward happened (typical).

Incidentally, I've not experienced this odd behaviour again since the beginning of April either. Really strange. I've changed back to using Shell V-Power recently, instead of Tesco's Momentum that I've relied on for most the year prior. So maybe it was a fuel-related problem somehow? No-one really knows still, but hopefully this problem is gone for good.

Over-rich fuel mix

Horsham did find the source of what was making the car run rich, however. There is apparently a known side-effect of UpRev tuning that leads to "drift" in the fuelling calculations over time, leading to a progressively over-rich mixture of fuel being fed into the cylinders. This was easily remedied in the first instance by simply disconnecting the battery for approximately 5 minutes, which I believe causes the logging in the ECU to be reset.

H-Dev also spotted a blown exhaust gasket (£10+labour) which they replaced and then tightened everything up. And what a difference! The car is back to its usual purring, buttery smooth, nicely urgent self once again.

Whilst the UpRev issue might slowly arise again over time, Jez has sent the logs from my car to UpRev in the States for analysis and suggestions. Once that comes back, hopefully UpRev have a more permanent solution than the simple battery-disconnect reset (fingers crossed that isn't a complete reflash and retune though).

Oil gallery gasket

No issue with the oil gallery gasket either! H-Dev did say it's starting to dip ever so slightly below Nissan's recommended minimum PSI of 15 at hot idle, so it does sound like it might be just starting to go a little bit. So the car will get a full OEM metal gasket replacement kit and H-Dev's torque bolt upgrade when it returns in a few weeks.

Very happy - not to mention, very relieved!

[Dave G]

Great that you've got some expert advice and have you car back to its previous best! Just shows the value of supporting the fabulous independent garages that continue to sprinkle their magic dust on our cars! 

Pleased for you too! 👍 

[HXD]

Quote

...the car will get a full OEM metal gasket replacement kit and H-Dev's torque bolt upgrade when it returns in a few weeks.

Which is tomorrow, finally! Looking forward to getting this ticked off my car's To Do's.