RB25 believed to be leaning out under boost, opinions on my diagnosis

[KendallH]

Swapped in an S2 RB25DET about a month and a half ago. Have been completely unhappy with the performance.

Mods: ISIS intake w/ Q45 TB & TPS, top mount tubular mani, Master Power 57 Trim, FMIC, Greddy RS, 38 mm external WG running .5 bar spring no boost controller, adjustable fpr set around ~48psi

Symptoms: Slight missfire at idle, strong missfire at 10-15% throttle (jolts me forward in my seat), lack of power, boosting at 5500 (I attribute that to the large turbo and being untuned), spark plugs showing lean (no wideband), slight break up under boost, backfire when decelerating in gear which becomes very strong (think fireballs that light up the entire street) after coming off boost.

My diagnosis: I think that due to high under hood temps the rb generates the maf is getting warm and not reading airflow correctly. My theory being the actual sensor wire is warming due to atmospheric temperature and not getting cold enough to read the airflow coming through it resulting in the lean condition. Unfortunately I do not have a wideband at this time to measure the AFR, but like I stated earlier the old plugs I pulled showed signs of running lean and did not even smell like fuel. I've installed an adjustable FPR to boost fuel pressure to remedy the issue but it had no noticable affect on the condition. I also swapped out the Autolite spark plugs that came with the motor for NGK BCPR6ES-11 regapped to .8mm (.031).

Does my diagnosis have any ground or does anyone have any other suggestions?

[Kingtal0n]

Swapped in an S2 RB25DET about a month and a half ago. Have been completely unhappy with the performance.

Mods: ISIS intake w/ Q45 TB & TPS, top mount tubular mani, Master Power 57 Trim, FMIC, Greddy RS, 38 mm external WG running .5 bar spring no boost controller, adjustable fpr set around ~48psi

Symptoms: Slight missfire at idle, strong missfire at 10-15% throttle (jolts me forward in my seat), lack of power, boosting at 5500 (I attribute that to the large turbo and being untuned), spark plugs showing lean (no wideband), slight break up under boost, backfire when decelerating in gear which becomes very strong (think fireballs that light up the entire street) after coming off boost.

My diagnosis: I think that due to high under hood temps the rb generates the maf is getting warm and not reading airflow correctly. My theory being the actual sensor wire is warming due to atmospheric temperature and not getting cold enough to read the airflow coming through it resulting in the lean condition. Unfortunately I do not have a wideband at this time to measure the AFR, but like I stated earlier the old plugs I pulled showed signs of running lean and did not even smell like fuel. I've installed an adjustable FPR to boost fuel pressure to remedy the issue but it had no noticable affect on the condition. I also swapped out the Autolite spark plugs that came with the motor for NGK BCPR6ES-11 regapped to .8mm (.031).

Does my diagnosis have any ground or does anyone have any other suggestions?

1. You need a wideband, .
2. What have you done for a tune? It needs tuning, that is my diagnosis from over the internet without seeing it.

[KendallH]

Yes, I know I need a wideband and a tune badly. Its currently running the bone stock ecu. I imagined that even with the different turbo setup, running stock boost would be OK, being I've seen people crank RB25s up to 10-12psi on a stock setup.

[Zacho]

Yes, I know I need a wideband and a tune badly. Its currently running the bone stock ecu. I imagined that even with the different turbo setup, running stock boost would be OK, being I've seen people crank RB25s up to 10-12psi on a stock setup.

Stop driving your car. You could have done irreversible damage by now. For the same boost pressure a larger turbo flows more air. I hope you have larger injectors.

[Brok3n_RB25]

A stock boost number (7psi for RB25s) doesn't mean that it's flowing the same amount of air. Think of it like a hose. You put a regular garden hose at 7psi and it has a set flow. You put the same pressure through a fire hose and it's going to flow a hell of a lot more. You NEEEEEEEEED a tune. You might have already done irreversible damage just as the previous post states.

[DJDANGER24]

/\What he said/\

Tune the bitch and quit half-assing it

[cotbu]

My theory being the actual sensor wire is warming due to atmospheric temperature and not getting cold enough to read the airflow coming through it resulting in the lean condition.

Not saying you don't have a mafs problem?!? just your "theory" is wrong! If you have ever taken the temperature of the hot wire element, you'd know what I mean.

[Brok3n_RB25]

Oh, and unless you're overheating... under hood temps won't effect you that much.
Granted heat soak is a bitch, but I daily drove my RB this past summer here in Phoenix.

[r33nota240]

sounds like ur timing is off.

[r33nota240]

or a mix between timing and tps sensor.

[shoguner]

Yes, I know I need a wideband and a tune badly.

Sounds like you know what you need. Now go do it!

[S13-RB25]

Hello all. I'm new to zilvia. I recently swamped and rb25 into a 92 s13 coupe. Right now i have a greddy downpipe, greddy intercooler, greddy bov, greddy air filter, greddy intake manifold, nismo fuel pressure regulator, his wastgate actuator set to 13psi of boost walbro 255 lph fuel pump. After changing plugs i started getting a hesitation its progressively gotten worse. Ive swapped plugs back out, tried another fuel pump, changed fuel filter, mafs, crank angle sensor, wired a separate relay for the fuel pump, different coils, checked and repaired boost leaks, regrounded the pcm and coils, tested the knock sensors checked the injectors to be sure they weren't clogged, tried more and less fuel pressure, quadruple checked the timing, changed the tps, tested the coolant temp sensor, lowered the boost... pretty much everything besides the ecu. Starts hesitating under boost but normal driving is fine. Sounds like a bee r but it keeps revving. Im worried I'm going to destroy the engine with how violent it hesitates and backfires. I'm pretty much out of ideas and am looking for more. Ive already spent over $500 trying to fix it. If anyone can give me some insight as to what's going on I'd be very grateful. Sometimes the problem isn't there but its very rare. I dont have a wideband but o2 sensor voltage at wot is .920mv

[fliprayzin240sx]

does it hesitate all the time (warm or cold engine)? When you said you swapped out the plugs, what plugs did you run and what gap? Also, did you double check the timing on it? I've seen the CAS up front come loose and advance the timing from the baseline and run like shit.

[S13-RB25]

Ngk bkr7eix-11 tried em at 1.1 gap and .8 gap. It had autolites in it. That's what i put back in. Its holding 60psi of fuel pressure. Every now and then it'll run right. But rarely. I have alternator volts at the injectors and ignition coils. I think either sparks breaking down or its loading up with too much fuel but i see no smoke. It seems i see a lot of people with this type of problem but can never find put what they've found wrong. Maybe its the ignition driver in the ecu. Just hate to spend more money on something that might not fix it. At 13 psi and stock Turbo maf is reading 4.80 volts haven't a clue if that's right. I checked the timing several times. Dead on. And it does it no matter the temp hot or cold.

[Sileighty_85]

Ngk bkr7eix-11 tried em at 1.1 gap and .8 gap. It had autolites in it. That's what i put back in. Its holding 60psi of fuel pressure. Every now and then it'll run right. But rarely. I have alternator volts at the injectors and ignition coils. I think either sparks breaking down or its loading up with too much fuel but i see no smoke. It seems i see a lot of people with this type of problem but can never find put what they've found wrong. Maybe its the ignition driver in the ecu. Just hate to spend more money on something that might not fix it. At 13 psi and stock Turbo maf is reading 4.80 volts haven't a clue if that's right. I checked the timing several times. Dead on. And it does it no matter the temp hot or cold.

Autolites are garbage

Get some NGK's and drop the gap down to .028

[S13-RB25]

Yea i think so too that's why i put the ngk plugs in it. That's when all my problems started lol. What ngk plug do you recommended? I tried iridiums.

[Sileighty_85]

iridium's are fine, but having too big of a gap will blow out the spark.

[S13-RB25]

Yea ive gapped them down too. Forgot to meantion that.

[EjMalernee]

Might be a coil going out and acting funny. What are you tuned on or is it stock ecu.

[S13-RB25]

I replaced the coils. Yes with used one but i really think it would have changed something. Knock sensors have 2.5v with the engine running and 560k ohm resistance with the engine off. I'm beginning to think maybe they are showing their age and are over sensitive, picking up knock when its not there and its enriching the f/a ratio and over retarding the timing. I am running the stock ecu that came with the swap but it didnt do this right after i turned the boost up. I was running 13 lbs for 3 months without a single problem. Only thing i did was put iridium plugs in it. But again i think that was a coincidence. I'm thinking of wiring in resistors to eliminate the knock sensors and try it. If it clears up ill replace them. Dont quite have any other ideas.

[epson1]

you know whats cheaper than replacing all that stuff?? a wideband....

[Rb25d]

you know whats cheaper than replacing all that stuff?? a wideband....

60psi is to much. Should run around 38psi of gas

[Croustibat]

60psi is to much. Should run around 38psi of gas

Upping a 3 year old thread with a useless comment. Isn't that punishable by death ?

[Kingtal0n]

Upping a 3 year old thread with a useless comment. Isn't that punishable by death ?

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