Frequent Lean condition RB25DET

Hi as title mentions I have an ongoing problem with my Skyline.

For a long time now my car has sporadically been running lean.

In the beginning it was noticeable at idle but seemed to be better when driving.

When it ran bad my AFR gauge was reading between 16-17 at idle and the car began to run rough but felt better when free-reving.

Sometimes I could drive for an hour or more without any problem and then all of sudden it would start to run lean again.

I have noticed that it seem to get better sometimes when i shut the engine off and restart it after some minutes.

As time have passed this problem has become much more frequent.

Now I can start the car up and it will be fine for a while, but almost as soon as I begin to drive, it will start to progressivly run lean and struggle.

as mentioned above it was just noticeable at idle first but now afr will drop to 16-17 when cruising too.

My gauge has been reading full lean sometimes at idle aswell.

my Afr gauge is not connected to my ECU it's just a standalone gauge.

im running an Apexi power fc. car was mapped 4 years ago and afr has been fine with this map before.

I don't run with closed loop activated.

The engine is mildly modified rb25det.

main things that is upgraded is turbo, injectors, fuelpump and airflowmeter and supporting mods.

recent things I have done and checked are.

* supplied the fuelpump with full battery voltage with a relay kit and fixed proper grounding points for it.

I have visually checked the connectors to the fuel pump assemlby.

* Filled up tank with new gasoline, the car had been standing in storage for 9 months until recently

* checked base fuel preasure. It was reading 38psi without vaccum. I also replaced the fuel preasure regulator soon after with a Nismo adjustable one and set that up to 40psi base preasure. one thing though everytime I have checked the fuel preasure the car has been running fine unfortunately so have not been able to verify the preasure when it runs poor.

* removed old sketchy alarm system that was in the car since I bought it. and mended all bad eletrical connentions there.

* replaced fuel filter with genuine nissan part.

* replaced CAS with new genuine Nissan part

what else.. spark plugs was replaced 2 years ago along with a PRP VR38 coil conversion kit. spark plugs are NGK BCPR7ES gapped at 0.7mm.

Injectors was replaced with Nismo 555cc 5 years ago at the same time the ECU went in.

Fuel pump was replaced 8 years ago with a Tomei 248/lph drop in asembly.

Im running on pump gas 98 octane. here we have 5% ethanol in the petrol. how much 5% would affect injector condition when storing for around 9 months im not sure about.

Any input or tips would be greatly appriciated!

Thanks!

Klas,

Based on all the info you have provided, I'm assuming you are still running on MAF? You have mentioned you have noticed the problem slowly getting worse? Have you cleaned MAF sensor? It is possible that you are getting a scewed reading. As the MAF uses a little heater element and as air passes by depending the intake air temp it determines air flow by how fast the heater cools down, so if you have a dirty MAF it may take longer for the heating element to cool, so the ECU will think that it is not getting as much air flow that you truly are, and it will start to pull fuel back.

You can 100% ignore all of that if you aren't running MAF sensors lol.

-Tobes from New Hampshire USA

Thanks for the tip!

Yes, I do run a Z32 MAF sensor that was replaced at the same time as the injectors and turbo, forgot to mention that.

But no I haven’t cleaned it so I will look in to it.

So I inspected the MAF and looked at the output signal while driving and couldn’t notice anything abnormal like an odd voltage drop or value jumping around etc.

I have noticed though that after replacing the fuel preasure regulator and setting it up at the same fuel preasure as the stock one (40psi) that the car seem to run leaner more often than before, even tough it did run lean with the stock unit as well. I notice it at light acceleration and when transitions occur.

It feels like the afr is jumping around a bit and is hard to dial in. It dosen’t seem to help much when giving it more fuel at those cells.

The car still ran into low 10’s at higher rpm when I did a full pull which i’ve reached previously.

How lean the car runs seem to variy a bit but I haven’t really done a full pull when the car is running at it’s worst so could affect high rpm as well.

Wideband O2 sensor drift perhaps.

Wonder if you have a boost leak to the fuel pressure regulator which is causing your fuel pressure to shift around.

So other than the wideband, is there other signs of it leaning out?

Particularly in transitions the car will feel like its hesitating as you try and accelerate initially, and if its too lean on idle it will likely idle funny, either rough, or more likely the idle rpm's will be lower than normal or both.

As if there are no other signs of it shifting, sounds to be more like a sensor issue. Wideband sensors rely on them being at the right temp for them to read correctly. Its why they have heaters in them to make sure they heat up initially and also never drop below the min operating temp. Which might explain why it reads better at WOT as its getting the sensor hot enough to read the correct value. Its not uncommon to have a wideband sensor fail

Most will have a calibration process, usually removing the sensor, the powering ignition on (engine not running) then they calibrate in free air.

@DynoDom @salco1987

I think I can rule out the Wideband. Because It is very noticeble on the behavoiur of the car. When the afr reaches around 16-18afr when idle and cruising I feel the car loosing power, stuggle to accelerate. I’ts not like crazy misfire more like it is running out of fuel and hesitates. And like I mentioned above my wideband is not connected with my ecu so the car dosen’t take the lean reading into account. Stock 02 sensor is turned off

Rechecked my fuel preasure yesterday at idle and it showed the same preasure that I set the car at (39psi base). Don’t know what the fuel preasure does when driving under boost though. And my Fuel preasure vaccum is not connected to my boost controller I took the source from another intake port

So what sensor is the ECU using?

Under idle and transient and cruise it will need some reference for itself. Even if long term closed loop fuel control is on, it will still use the wideband for short term trimming.

However the AFR being out by that much is suggesting something more mechanical rather than tune or ECU. As its out by around 20% which is massive

Your fuel pressure base of 39PSI is odd too, it's usually 43.5 PSI for most cars.

How did you set your base fuel pressure?

The car should be idling, then remove the boost reference line from the fuel pressure regulator, set the pressure using the screw, usually to 43.5PSI or 3 Bar. Then reconnect the vacuum line. At idle you should see the fuel pressure drop several PSI once you reconnect the line, this is due to the intake manifold being in a slight vacuum at idle and shows the fuel pressure regulator is doing what its supposed to.

@salco1987

I belive the power fc uses the values in the injector map for reference when running in those light load areas when the narrowband 02 feedback or closed loop is turned off.

The car has run with good afr previously in those cells that are now lean.

I measured the fuel pressure like you describe, after the fuel filter, engine on idle with vaccum disconnected from fuel reg and blocked of the other end of the vaccum hose.

I’ve heard between 2.5-3 bar is normal?

I'm not an RB expert so might need to leave that to somebody else. But does appear they expect to see 43PSI with the vacuum line disconnected (3 BAR), and often hover around 32 PSI at idle with it connected.

So if you've checked that, its something else.

The ECU is unlikely to have learned a 20% lean state and stayed there. So my bet would be its not the fuel map or transient maps in the ECU.

I'm still going with its something mechanical.

MAF is a good candidate, i'd suggest cleaning it with some MAF sensor cleaner. If its sending bad data, the ecu will send the wrong amount of fuel. But it could be faulty so might not be a bad thing to try and replace, even with one from a wreckers to rule it out

99.9% confident it's a faulty fuel regulator - I might have missed it, but you didn't say what it is, nor if you've checked it's not a counterfeit item, as there are many fakes being sold on-line*. A fake may have worked quite well when new, but those symptoms are what I'd expect with one that's 'sticking', whether from wear or other factors.

*There are guides from some of the bigger manufacturers on how to tell fakes from genuine regulators.

@salco1987 Yeah it sure does feel like something more than just a tune issue. I will try to test another MAF

@Gord I thought so too that’s why I replaced it. It’s a Nismo adjustable one. Basically a stock unit but adjustable. I Bought it new from Rhdjapan which they in turn get from the manufacturer. Im confident it’s a genuine item.

Maybe the preasure regulator is set a bit lower than it actually should be at 39psi like my oem one was showing when i measured the preasure, instead of 43.5psi.

But it’s weird that the afr varies so much i think.

I will try do a compression test aswell.

I mean you could I'm plug MAF and watch your numbers, throttle response will be terrible but! If you have a mechanical fueling issue or intake leak echnically you would still be able to see weird numbers for AFR... But if the AFR stay steady( even tho it's probably going to show rich).

A lot of late 80s early 90s Japanese port injected engines run closer to 2.5 base or intentionally somewhere between 2.5 but under 3 bar, base pressure is likely fine.

I wasn't of the impression that Apexi used O2 feedback at all but have only really looked in FD3S application in detail, I don't believe my friend's 200sx has closed loop either.

If there was a leak in the FPR reference line it would run rich at idle (higher reference pressure leaking towards atmosphere from vacuum) not lean. Then lean in boost leaking down towards atmosphere from manifold pressure. The magnitude of the AFR shift would respond with how far in vacuum/boost you were.

I'd be replacing with a step colder gapped down iridium plugs (fine wire generates a more aggressive field which jumps the same gap at a lower voltage, many peoplerun 0.5-0.6mm in boosted applications) and seeing if you could get someone with the same coils to help you swap them out for a full drive cycle/heat soak test. Ignition electronics can have intermittent failures with heat cycling. Probably worth checking all coil plug wiring and earthing too.

~17.5:1 AFR reading from stoiciometric (or close) or 16:1 from a slightly rich idle is pretty much a perfect one cylinder misfire (1/6th shift in reading) which points to one coil/plug or less likely injector driver or sticking injector. If you have or can borrow a non-contact IR thermometer ("temp gun") pointing at the exhaust manifold may help you diagnose if it is a single cylinder (if chasing on load on a dyno). You can often buy them even at a chemist/pharmacy these days.

My money is on a fouled plug or non genuine or tired coils from firing larger than required gaps although they should be doing it easy on a mild setup.

Back to basics fault finding is the core of this stuff, we have much better tools available to us now than people did 40 years ago, like wideband sensors and thermocouples or ir temp sensors. Always think about what the numbers are telling you in terms of miss rate or % versus cylinder count and what you can do to check for changes, you will not damage anything at idle, pulling an injector plug one at a time while someone watches the wideband while it's "running" lean will likely find which cylinder your fault is on (no change in reading/engine rpm/sound) with zero additional tools or equipment. You can also hold a screwdriver, other long tool or stethoscope to each injector at idle and hear if it is cycling to pretty well rule out sticking injector/failed IGBT. That simple test up front may have saved you a lot of time and money replacing other components, although it can be easier said than done with intermittent faults.

Just as trivia, even if base fuel pressure was 3 bar (which it almost certainly wasn't unless his aftermarket pump was overpowering the factory reg), using the orifice flow equation for the injectors fuel pressure would have to drop to 30psi from 3 bar for that afr shift, and in reality from a lower base his fuel pressure would have to be in the 20s to see that swing.

A small head gasket failure can give a start/idle cruise miss and clean up on load but mild tuned RBs aren't known for it and your plugs are old.

If I was your neighbour and you described those symptoms:

Rule out/reduce likelihood of more significant mechanical failures such as head gasket/piston/rings/valve/seat/spring/lifter.

Before even starting the car:

1. Look under oil fill cap/on dipstick for evidence of milkshake from oil/coolant mixing (head gasket failure) or extreme fuel contamination from piston/ring pack failure(detonation).

2. Open highest coolant system pressure cap, look for significant gas pocket from combustion gas pushing out coolant, soot particles or significant opacity or discolouration of coolant from combustion gas, also check overflow for evidence of coolant consumption or ejection.

Assuming that all looks OK.

1. I'd be pulling the plugs and inspecting them while cool for evidence of wear, fouling and wetness. This will likely point to one cylinder. (I'd replace them with gapped irridium or platinum if possible), if not just new plugs of appropriate reach/heat range.

2. Probably worth pulling the CAS plug, efi relay or fuse or fuel pump relay or fuse at this point and turning it over to listen for any obvious difference in compression (valve/seat/spring/lifter damage or piston/ringpack) or ejection of material with plugs out. Compression test if at all possible with borrowed or bought tester. (At this point we assume either compression looks ok across the board or tester unavailable)

I would then test each coil/plug. CAS connected, all plugs in, can either just crank or run engine. If cranking only remove fuel pump relay/fuse, otherwise if you want to do it running for stronger spark install and connect all plugs/coil, disconnect the injector plug from the same cylinder as the coil you are testing (engine off). While injector plugs are off (one at a time) engine you can check for system voltage on one terminal and a pulse to earth on the other(this should rule out injector wiring/injectir driver fault). Remove one coil, connect a spare plug in the coil and earth the plug to the head/manifold and get your buddy to crank/start and watch for spark (turn off), repeat on all cylinders. (This will most likely find your fault if it wasn't just a worn/fouled plug). If you are yet to isolate fault all looks OK, go on:

Listen to all injectors via screw driver/stethoscope with engine idling if possible/remove one injector plug at a time to isolate cylinder if all ignition system tested OK and miss/high AFR still exists with new plugs. May identify stuck injector at this point.

Listen to each side of head for any ticking for potential damaged valve/spring/lifter if you were unable to borrow or buy a compression tester.

If it's still missing at this point and it doesn't appear to be ignition or fuel I'd pull cam covers for a look.

If you haven't resolved or identified which cylinder and likely fault after that I'd be very surprised.

My best bet is spark plug or coil on one cylinder followed by small head gasket failure on one cylinder drawing enough coolant to misfire in vacuum but not generally on load.