After spending days watching video after video on KA24DE Altima videos on performance issues, I figure I would give some of you an opportunity to share your own experiences and potential solution(s). The Altima has 268K miles - does not burn oil. It purrs like a kitten during idle, completely smooth. Once warm, engine idles at ~725rpm. When revving during idle, engine is completely smooth and responsive. The problem occurs under a load condition (during driving, brake torquing) between 1000 to 2200rpm, only when the engine has warmed up. I mention this because if the engine is cold, it does not bog during that rpm range and accelerates normally. Additionally, fuel mileage is dreadful (~14mpg). After hitting 2200rpm, the engine seems to overcome whatever the issue is and proceeds to accelerate normally.

When examining the data with a graphing OBD2 scanner, I notice the following:

• No permanent or pending codes (including running lean) per OBD2 - no CEL
• Short term fuel trim is pegged at 25% (indicating ECM is compensating for a lean condition)
• Load percentage is reported at 100% during the bogging condition
• TPS is reporting correctly at tip-in and wide-open throttle (WOT) during operation
• O2 is reporting at near 0.1V during the bogging condition (confirming lean condition)

Maintenance performed, miles ago:

• NGK spark plugs, < 1K
• NGK spark plug wires, < 1K
• Fuel filter, < 1K
• Distributor and rotor, < 10K
• New primary catalytic converter, < 10K
• New exhaust manifold gasket, < 10K
• New upstream O2 sensor, < 10K
• New water pump, < 1K
• New thermostat, < 1K
• New radiator, < 1K

Tests performed:

• OBD2 - no codes, pending or otherwise
• Catalytic converter test (inlet temperature is always lower than exhaust temperature), both primary and secondary
• Exhaust pressure seems fine when brake torquing engine (no constriction at the tail-pipe)
• Blow-by (none, checked by removing oil filler cap and subjectively feeling for air)
• Alternator voltage, 13.1V
• Mass airflow (MAF) readings, idle and 2500rpm consistently within FSM specifications
• O2 upstream, voltages consistently within FSM specifications
• O2 downstream, voltages consistently within FSM specifications
• Able to go into closed loop based on O2 data, OBD2 shows switching occurring during operation
• Engine coolant temperature (ECT) sensor, responding normally and consistently during temperature rise and fall
• Throttle position sensor (TPS) is correctly and consistently reporting
• Spark is strong with the new plugs and wires
• Rear wheels rotate freely without dragging

Tests to be performed (need another weekend):

• Fuel injector ohm readings
• Actual compression test for variance between cylinders (don't expect anything seriously wrong due to lack significant blow-by air)
• Fuel pressure test, unfortunately do not have a gauge for this (yet)

I theorize that it could be 1 or more weak injectors (with higher ohm readings) where the engine isn't mis-firing due to lack of fuel delivery but the injectors cannot sufficiently supply fuel based on air readings. The excessive fuel use could be the ECM trying to compensate for an overall lean issue based on the poor performance of 1 or 2 cylinders - the good injectors would be spraying more fuel to trying to zero out the lean condition. That being said, I'm leaning towards a weak fuel pump but am trying to avoid buying yet another piece of diagnostic equipment but I'm running out of other things that it could be. Has any of you had a similar issue with their Altima and found a solution? Most of the issues I've seen have to do with poor idle and bogging, solved with IACV replacement, MAF replacement, or something else that I've checked out to be working. Even with high mileage, it demonstrates what the KA24DE is capable of if it has been taken care of throughout its life.

Your thoughts would be appreciated.

You have indicated running very lean, fix THAT.

That’s my point. I’m trying to figure out why. Vacuum is good. Intake gasket is good. Intake pipe is good.

You can have a vacuum leak to affect idle that does not affect overall vacuum enough to tell it.

Clogged injector(s). Bad pump. Not having a fuel pressure gauge can melt your motor. Unmetered air leak.

.................DP

Well here we go, checking the fuel pressure is a must! When was the last time the fuel filter was changed? Did you add any kind of fuel additives to gas tank? (the reason for this question is water forms in the tank when winter sets in the form of water droplets in the tank and
new gas when filling on cold day. Sometimes the water will be carried in the gas and burned by the engine, but not all of it does. Some
of the water or matter cant get through the filter and starts to clog limiting the pressure, this shows up under load of the engine and act like it bogging down (fast loading will show more of this problem). If you slowy depress the gas pedal will the engine keep accelerating? If it does chances are its clogging when the higher load is a applied. Another possible problem is the iac valve not opening or staying open, it located on ka24 on rear side of the intake manifold rear back , a two wire connector attached. I had one of the connector have a wire break off and cause what your seeing, take a look . The test set from a frieght store run is 19.99 and get 20 percent discount coupon from the web site. hope this info helps. sky5031