2002 Mountaineer rough idle

My 2002 Mountaineer V8 has a very rough idle after the engine comes up to temp. When the engine is cold is idles fine. The Service Engine Soon light is on. I tried cleaning the Mass Air Flow Sensor, but it didn't make any difference.
Any ideas?
How do I tell if the MAFS is defective?

Dave

You should start by having the diagnostic codes pulled and posting the code(s) here. Many of the local chain auto parts stores will pull codes at no charge. You need to be a bit careful about taking guidance on those codes from the salesperson though. Rarely does a code mean a specific part is bad, but rather they indicate a problem in a system and may indicate which part has identified the issue.

You should also carefully inspect the vacuum lines under the hood for cracks or for being disconnected. Also, did this just start after some service was performed on the vehicle? Is so, that could give you a general area to start your inspection.

-Rod

Rod,

The codes retrieved are:
P0316, P0300, P1131 and P1151.
I looked for any loose hoses or wires and haven't found anything yet.

Thanks,
Dave

The P0300 is a code for a random misfire. Per the factory service manual, the possible causes for this code are:


Camshaft Position (CMP) sensor
Fuel level below 1/8 tank
Stuck open EGR valve
Blocked EGR passages.

The P0316 identifies that the misfire occurred in the first 1000 engine revolutions following the engine start. Possible causes for this code are:


Damaged Crankshaft Position (CKP) sensor
Ignition system
Fuel injectors
Running out of fuel
Fuel quality issues
Base engine
Damaged Powertrain Control Module (PCM).

P1131 indicates a lack of H02S - bank 1, sensor 1, switching. Sensor indicates Lean.

Similarly P1151 indicates a lack of H02S - bank 2, sensor 1, switching. Sensor indicates Lean.

For each of these codes the possible causes are:
Electrical:


Short to VPWR in the sensor or harness
Water in the harness connector
Open or shorted HO2S circuit
Corrosion or poor mating terminals or wiring
Damaged HO2S
Damaged PCM

Fuel System:


Leaking fuel pressure regulator
Low fuel pressure or running out of fuel
Vapor recovery system

Induction System:


Air leaks after the MAF
Vacuum leaks
PCV system
Improperly seated engine oil dipstick

EGR System:


Leaking gasket
Stuck EGR valve
Leaking diaphragm or EGR Vacuum Regulator (EVR)

Base Engine:


Oil overfill
Cam timing
Cylinder compression
Exhaust leaks before or near the HO2S(s)

As you can see, there are some common denominators here. I'd suggest you start with the free ones - oil level, check for vacuum leaks. If you have a fuel pressure tester and/or vacuum gauge, check them for being within specification. If you're low on fuel, fill up with a good fuel. Also keep in mind that the Differential Pressure Feedback EGR (DPFE)/EVR have been common issues on several late 1980's and 1990's Fords. I haven't seen a lot of issues with them in the 2002 model year, but that may just be due to the relative newness of the vehicles still.

-Rod

When the engine is cold, the fuel pulse is longer!

How's the idle with the Air cond on?
How old is both the Air filter, and fuel filter?
Did you clean the IAC?
How many miles are on the upstream H20 sensor.

The MAF sensor only throws a DTC when it's a hard fault.
Even though you cleaned it, it still could still be bad.

Tests:
Fuel pressure test
A Smoke test is for checking unmetered air leaks (vacuum leaks) on your PCV, EGR, and Air Intake system.
Crank/cam sensor test

The MAF sensor only throws a DTC when it's a hard fault.
Even though you cleaned it, it still could still be bad.

Not completely true, that's the purpose of "Pending" diagnostic codes. A scan tool can pull pending codes for intermittent faults, such as an intermittent MAF sensor, which I've personally experienced.

True that even after cleaning it, it could still be bad. The better test for the MAF at this point would be to use a scan tool in datastream mode to monitor air flow as reported by the MAF, and while doing so, also monitor bank 1 and bank 2 lean/rich status along with the fuel trim values.

-Rod