Help Needed: How Can 3.5 Be Running Both Rich And Lean? - Nissan Forums : Nissan Forum
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post #1 of 4 Old 06-10-2015, 05:45 PM Thread Starter
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Help Needed: How Can 3.5 Be Running Both Rich And Lean?

To 'Bottom-Line" it... I'm running rich on the front bank and lean on the rear bank on a '05 Altima 3.5 and we've seemingly exhausted reasons as to why.

Backstory: Bought cherry '05 Altima SE 3.5 auto about 3 years ago w/22K original miles. Now has 63K miles. It still seems like a brand new car. I've done a lot of 'work' on the car, both cosmetic and mechanical (just about every N/A bolt-on that one can do other than headers) and no 'upgrades' have disappointed. Have never thrown a code.

About 6 weeks ago I had a good Nissan performance shop install JWT S2 cams which, as most of you know, is a relatively mild street grind - also retained the stock valve springs. The only other 'change' I had done at the time of reassembly was to replace my stock UIM and elbow with my reworked SSIM UIM and elbow. I use the NWP phenolic spacers as well and have had the UIM off and on several times before this install with no problems.

From the very beginning of re-starting the engine it was idling rough and it was throwing a P0300 random misfire code - which was all expected. The idle was reset to 900 but the P0300 persisted. Since I had an Uprev license from a previous dyno tune, 245 whp, I took it in to hopefully sort things out. The tuner, to his credit, would not tune the car because he was seeing dramatic differences between the two upstream sensors... the front one was remaining static and shifting between rich and lean while the back one seemed to be operating normally with constant adjustments shown by the differing amplitudes on the graph. He suggested to first check for vacuum leaks.

I had a smoke leak test performed to locate any possible leaks and corrected a cracked breather hose between the two valve covers and a leak between the UIM and elbow (forgot sealer on one side - DOH!) and, after much testing, replaced all upstream and downstream 02 sensors (no leaks found in exhaust system either). Right after changing the two upstream sensors the SES light went out but came back after a few miles of driving. Also checked valve lash (perfect), plugs (20K miles and 'normal'), compression (210/cylinder), and everything else we could think of, including the dealer with his diagnostic equipment, and still having rough idling and P0300 code. What we've been able to narrow this down to is this - the front bank is running lean and the rear bank is running rich and for the life of us we can't figure it out. The rough idle is due in large part to the ECU trying to make adjustments. One person even posited the possibility that one of the cams on the front bank is slightly different than the others but I've ruled that out as a possibility since it is an extreme outlier.

I'm looking to anyone here who can suggest a possible cause to pursue other than reinstalling the stock cams. My next course of action 'we', all the other technicians I've worked with on this, agree to take is to reinstall my stock UIM and elbow(w/VIAS delete), since that was the only other change I made with the cam install, and to switch the front injectors with the rear injectors to see if it could be an injector-related issue. Any other suggestions... please?

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'05 Altima SE 3.5 5sp A/T w/AEM CAI, NWP Phenolic Spacers w/Port Match, Hi-Flow Cats, Racing Y-Pipe, NWP TB, JWT S2 Cams, Custom Bilt (with a difference) SSIM Mod-UIM/Elbow, Catch Can, MagnaFlow Cat Back, Eibach Springs, KYB Shocks/Struts, Stillen Big Brake (PBR) Kit-Front, Powerstop Rotors-Rear, Hawk Ceramic Pads, Goodridge Steel Braided Brake Lines, Falken Azenis, 5mm wheel spacers, Strut Bar, Traction Bars Uprev Tuned.

Last edited by halfscot; 07-22-2015 at 01:50 PM. Reason: Made a correction
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post #2 of 4 Old 07-22-2015, 01:48 PM Thread Starter
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Update: finally discovered cause. After speaking with a JWT technician, the best CS ever, he said he'd seen the exact same thing on a Z that was being built and it was from one of the exhaust cams being installed one tooth off. Well, I hired a 'retired' Nissan Master Mechanic to come to my house and we took off the front engine cover and the cam sprockets in just over two hours. It was quickly determined that the rear bank, the one running lean, exhaust cam was indeed one tooth off. When everything was reassembled it started right up and puuuuurrrrrred like a kitten. It was just like the JWT technician said - There was absolutely no reason that cam, the S2, would not idle smoothly as that was what the grind was designed to do for my Altima configuration (5 sp auto).

No more throwing it in neutral at stop lights. No more SES light throwing a P0300 random misfire code. No more huge flat spot at the top of the power curve. What a relief! Now I can get the Uprev tune that I couldn't do because of the condition it was in.
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post #3 of 4 Old 07-22-2015, 02:11 PM
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Wow, that is one unusual problem!
You must be elated that it's finally resolved.
Congrats!
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post #4 of 4 Old 07-22-2015, 06:05 PM Thread Starter
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Absolutely right on! This Altima of mine is my 'baby'. With only 64K original miles and mostly garaged it's still like a brand new car, only 'better' than the original. I was losing sleep at night over this one. I can't say enough too about the Nissan mechanic I hired. He was/is incredible in allowing me to work with him and how quickly/competently he was with the work only he could do. From now on he's my 'go to' guy for any other major work I will want to do.

'05 Altima SE 3.5 5sp A/T w/AEM CAI, NWP Phenolic Spacers w/Port Match, Hi-Flow Cats, Racing Y-Pipe, NWP TB, JWT S2 Cams, Custom Bilt (with a difference) SSIM Mod-UIM/Elbow, Catch Can, MagnaFlow Cat Back, Eibach Springs, KYB Shocks/Struts, Stillen Big Brake (PBR) Kit-Front, Powerstop Rotors-Rear, Hawk Ceramic Pads, Goodridge Steel Braided Brake Lines, Falken Azenis, 5mm wheel spacers, Strut Bar, Traction Bars Uprev Tuned.
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