It might be the AMM, although it doesn't seem to make sense if you only replaced the WP.
Check this out:

Air Mass Meter Diagnosis.

Setting Base Idle and Mixture on Bosch LH2.2 Cars. See the FAQ section above for tips on base specifications.

Simple Functional Diagnosis of AMM. [Editor] The classic test of AMM failure is to disconnect it; if the car runs better, then the AMM is at fault. But here are the OEM tests per the OEM manual for Bosch 2.4 (the-016 AMM):

Check the ground point of the AMM:

ignition off; disconnect AMM
connect ohmmeter between ground and terminal 1
should be 0 ohms
if not 0 ohms, check the ground point on the intake manifold
Check signal from AMM:

clean off sensor connector, removing any corrosion
-start engine
connect voltmeter between ground and back of terminal 3 on back of connector
should read approx 2.3 volts.
if not approx. 2.3 volts, substitute another AMM
Testing AMMs and Calibration. This is a response from Python Injection (rebuilders of AMMs) to a question about testing AMMs.
From: Joe Evert, Director of Engineering, Python Injection
Subject: Re: Technical Question about AMM
The reason the OEM doesn't give a test procedure for calibrating the air mass meter is two fold. First of all BOSCH Hot Wire Air Mass sensors are not linear devices like a throttle position sensor. The output does not change the same amount for a given air change. To make it short, most linear sensors will give you say... 1 volt for 100cfm air flow, 2 volts for 200cfm air flow, 3 volts for 300cfm airflow etc. This is a linear device. Bosch hot wire sensors are not like this. They change a great deal at low air flows but once the air flow increases past a certain point, say 50% of what the engine can draw, they change very little. This makes the sensor very accurate at low to moderate air flows and good enough at high air flows. Just a small amount of inaccuracy at low RPM and the vehicle will run terrible. If the voltage for a given air flow is off by 100 milli volts at low RPM the car will barely run. At high RPM A 100 milli volt deviation will not even be noticed. Because of this it makes it next to impossible for the technician to accurately diagnose the air mass in the field. We use a calibrated flow bench that measures the exact CFM air flow to then compare the voltage to. This is not practical in the field because temperature, altitude, humidity and the mechanical condition of the engine will affect how much air the engine is drawing in. So just to say that the Air mass should have xx.xx volts at idle would be completely false since all these parameters must be accounted for. Also even if a range is given just a small amount of deviation in the output causes poor performance.

So what are you to do? For on-vehicle diagnoses the best way is just unplug it at idle; if the vehicle runs better it is most likely bad. This is because if the air flow sensor is off voltage at high RPM it will also be off at idle. Also if you are experiencing repeated failures you probably have a defective air box thermostat. This little thermostatic bulb is located in the air filter box and controls the hot air into the engine. When this fails it fails in hot air mode and routes hot air from around the manifold into the air intake. This will destroy the air mass meter in no time.

Contaminants on Heated Sensor Wire. [Editor] It is rare but possible to see contaminants build up on the tiny sensor wires inside the Bosch air mass sensor due to dirt, spider webs, a disintegrating air filter, etc. In some instances, the hot burn-off cycle will not remove these contaminants. When this happens, it skews the ECU transfer function such that the MAF overestimates airflow at idle (causes the fuel system to go rich) and underestimates airflow at high airflow (causes the fuel system to go lean). This means that Long Term Fuel Trim will learn lean (negative) corrections at idle and rich (positive) corrections at higher airflow and may be the cause of high NOX readings on test. To remedy, spray the sensor wires inside very carefully with carb or brake cleaner to remove them, then reinstall and turn the engine on and off to activate the burnoff cycle.

Most folk start with the disconnect test, " So what are you to do?"

I got it at http://www.brickboard.com/FAQ/700-900/