Helping my brother diagnose his problem. I'm spent with all my knowledge already. Hence, hoping to get some input from you guys.


It's a stock 95 auto maxima with 5 speed swap and a warpspeed y-pipe and k&n intake. We've tested it on the auto ecu and got 3 auto related codes. Plug in the spare 5 speed ecu, received 0505 code, it's a no code.

Basically, we can drive the car fine till we gas it down to WOT. The car will reach redline in close loop no problems, but when we pedal down to WOT's open loop, it's hesitating, barely able to rev past 4-5k rpms. The problem is worsening everyday.

We've replaced coils, plugs, maf sensor, coolant temp sensor, TPS, check KS, replaced O2's, cleaned up the IACV and manifold. Checked compression: all above spec. Even the injectors are good by sound test. We aren't really familiar with why the difference between close and open loop would be so significantly affecting the performance. So, any input or ideas is appreciated.

 

My first and second guess would have been the coilpacks and MAF sensor but you already tried those...Did you measure coilpack resistance to make sure they are all within spec? What brand plugs are you running? How old is the fuel filter?

 

i would guess it is some problem with the ECU. Maybe you got a bad one or maybe a bad wiring harness

 

We replaced the coilpacks with another set from a perfectly running car. So, it's not those for sure. Running platinum G-power plugs, Oem replacements, and Copper V-power, same results. Fuel filter replaced about 2 years ago, it can be the case, we'll look into that soon.

ECU should be fine, we have four 4th gen maxima's right now, exchanging parts around proved that it's not likely the ecu problem. I'm thinking wiring harness.

I still can't think of what can cause this WOT faultiness...

 

Hook up a wideband and get an air/fuel reading?

 

man, he ain't buying a wb. Mines already bunged up, so it's not coming off again.

 

Well I'm stumped...

What's the vacuum read at idle and WOT?
How about changing the KS out anyway, just to completely eliminate it.

 

try the mass air meter, I cleaned mine with electrical contact cleaner and it worked for me.

 

Did you check the fuel pressure and replace the filter?

 

Yeah, I have it come down to one of these:
1) fuel filter
2) cat converter failure, maybe partial clogging, but not likely, it doesn't feel like it.
3) mass air sensor, but we did change to another one, and there's no effects.
4) knock sensor, but it would normally throw a ghost code.

We did pull a code that says 0505, I've had that code in my own car long before when the rear O2 after cat wasn't hooked up right. But it never hindered my performances at all.

Well, he dropped the car at the shop this morning, we doubt that the mechanics can fix this, though. I'm already pretty notchy at diagnosing due to my project car. So, any addition inputs are still appreciated.

 

LOL ... 0505 means "No codes"

 

If you would read up on my 1st post, I have already defined it that 0505 means "No codes". What's your LOL about?

Though 0505 means no code, it doesn't exactly mean that the engine is perfectly fine. Not long ago, I replaced the rear cat O2 and wrongly wired it, it gave an 0505 code, which means no code. But after I rewired it correctly, the code never came back up.

From what I know, 0505 means no code because the ECU registers a diagnose of undefined failure. The rear O2 sensor failure would trigger it because as long as the front 2 banks' O2's are in working order, rear O2's not used by the ECU to correct its close loop stoich ratios. This gives the ECU no reasons to throw a code, so it's no codes as in 0505. Now, if the rear O2 is bad and then one or both of the front O2's becomes faulty, then the car would register all the O2 sensor failure codes.

 

First off there's nothing "stock" about that car anymore.
Sounds like you have an ignition problem. Coils would be my first guess, as a weak spark will show it's sign under hard accelleration (WOT) conditions.
Did you put 6 new coils in the car? Have you checked the spark plugs? Are they the correct plug and is the gap correct?

 

Replaced igntion coils with perfect running ones.
Got 2 spare sets of NGK spark Plugs, none of which made any differences.