Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum - View Single Post - RB25 believed to be leaning out under boost, opinions on my diagnosis

KendallH · 11-25-2011, 09:28 AM

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?

11-25-2011, 09:28 AM	#1
KendallH Zilvia Addict Join Date: Jan 2011 Location: Clearwater, FL Level: Master Troll Age: 33 Posts: 871 Trader Rating: (3)	RB25 believed to be leaning out under boost, opinions on my diagnosis 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?