Surging, mis-fire, rough idle and Lambda light on

[jammer5]

I have a 1999 S80 T6 which has had a replacement XeMoDeX ETM fitted about 12 months ago. This sorted out my previous problem with the ETM failure that most people will be familiar with.

I now have a problem with the vehicle where the engine is misbehaving At start-up, it revs to ~4000rpm, surging as it does so, for maybe 30 seconds before it (usually) settles down to idling/running rough. This is frequently accompanied by the Lambda light on the dashboard. Sometimes it goes into limp-home mode. Other times it continues to run with a mis-fire. Sometimes I can drive it for about 15 minutes without trouble, but then it reverts to either limp-home or mis-firing.

It is sometimes possible to "reset" the car by pulling over to the side of the road and switching everything off for 10 seconds. Restarting usually gets the vehicle drivable again (but not for long).

I have bought a cheap OBDII scanner (ELM327) which reveals only two diagnostic trouble codes: P0121 and P0303
P0121 is a Throttle Position Sensor error code.
P0303 is Misfire on cylinder 3.

I find it hard to believe the XeMoDeX unit is faulty however the diagnostic flow-chart that they sent me seems to point to this being the case.

I have not replaced the MAF sensor yet either although from reading other posts on these forums, I suspect this could explain some of my symptoms. I don't want to go through the hassle and expense of sending my ETM back to Canada (from NZ) and waiting for a replacement, unless this is absolutely necessary so I would like to explore other possibilities first.

Can anyone suggest what else might be throwing up the Lambda warning light, and causing a mis-fire/rough idle ?
Can I try anything to eliminate the MAF from suspicion ? It's not throwing any codes.
Should I be replacing Sparkplugs, HT leads first?

Any advice gratefully appreciated.

[boosted5cyl]

Sounds more like a combination of fouled plugs as a result of a vacuum leak. I'd forget about the MAF for now.
Yes, start with new plugs, basic copper plugs, gapped to .28". Verify the gap is .28" before installing, you will likely have to adjust it. Also, swap the coils on cylinder 3 and another cylinder, say 1. There are no HT leads, its a coil on plug design.
Clear the codes, drive it, observe the behavior, re-read codes (if any) and go from there.

P0121 corresponds to two different Volvo ECM codes: ECM-903C & ECM-959F

ECM-903C Reconfiguration emission system. Information

Diagnostic trouble code (DTC) information ECM-903C

Condition
This diagnostic trouble code is for informational purposes only. The diagnostic trouble code's only purpose is to inform that the reconfiguration, which may affect the emission system, has been active or is active. The diagnostic trouble code is always stored as the result of another trouble code. Troubleshooting is not required.
Also for upgraded software, released during year 2006 or later, this diagnostic trouble code is used as an information diagnostic trouble code. The diagnostic trouble code will only, however, be stored in case the substitute value, throttle shut, is or has been activated. The diagnostic trouble code is always stored as the result of another trouble code. Troubleshooting is not required. This applies to, for example, diagnostic trouble codes ECM-9150, ECM-9160, ECM-9190, ECM-91A7 or ECM-91B7.
Identifying upgraded software ECM/ETM
Note! For upgraded software, released during 2006 or later. There are certain instances when diagnostic trouble code ECM-903C is stored without another diagnostic trouble code that describes why the substitute value, throttle shut, is active or has been activated. If this is the case, try replacing the throttle unit.

Substitute value
None.
Possible source
Faulty signal:
other diagnostic trouble code (DTC) is stored.
This diagnostic trouble code is generated in combination with other trouble codes. Information on the possible root cause is described.
Information about combined diagnostic trouble codes (DTCs)

Fault symptom
Driving/Poor performance/lacks power
Driving/Poor performance/lacks power/At take off
Driving/Hesitates/surges/Unsure when/at all times
Driving/Hesitates/surges/During acceleration
Driving/Hesitates/surges/During deceleration
Driving/Hesitates/surges/At steady speed
Cruise control/Does not engage
Text window and warning symbol/Red symbol and text message
Idling/Uneven idle
Warning lights and chimes/Electronic Throttle System (ETS) indication/no indication
Warning lights and chimes/Malfunction Indicator Light ("Check engine" light) indication/no indication

ECM-959F Accelerator pedal sensor. Faulty signal, B6284T

Condition
The accelerator pedal position sensor has two different internal sensors, where one generates an analogue signal and the other a pulse-width modulated (PWM) signal. The sensor for the pulse-width modulated signal (PWM) is connected to both Engine control module (ECM) and Electronic throttle module (ETM). The sensor for the analogue signal is only connected to Engine control module (ECM). Then both control modules send out the information on the CAN-net, so that other control modules have possibility to use the information.
Electronic throttle module (ETM) compares the analogue signal that comes directly from the accelerator pedal position sensor with the pulse-width modulated signal coming from the Electronic throttle module (ETM) via the CAN-net. If these two signals differ by more than 10 % for longer time than 0.5 seconds when the engine speed is higher than 600 rpm, which then stores diagnostic trouble code ECM-959F, Faulty signal. At times when the analogue signal is in the process of changing, e.g., during acceleration, a greater deviation between both signals is allowed.
For upgraded software, released during year 2006 or later, the above detection time is increased from 0.5 to 0.9 seconds.
Identifying upgraded software ECM/ETM
For more information about the electronic throttle system and related components such as the electronic throttle module, accelerator pedal position sensor, brake light switch, clutch pedal position sensor, and brake pedal position sensor: Design Function

Substitute value
Closing of throttle during braking.
The accelerator pedal position is ignored when the brake pedal is depressed.
Cruise control off.
Cruise control off during entire operating cycle in which fault was detected. If the fault is no longer present, the function is available the next time the car is started.
For upgraded software, released in 2006 or later, adjustments have been made to substitute values. In driving cycles when the fault is active, then slow opening of the throttle will occur and the throttle will close at braking.
Identifying upgraded software ECM/ETM
Possible source
loose connections or contact resistance in the terminals on the digital pulse width modulated signal cable to the electronic throttle module (ETM)
fault in the PWM signal between the engine control module (ECM) and the electronic throttle module (ETM)
interference on the CAN signal wiring
defective electronic throttle module (ETM).


This diagnostic trouble code is generated in combination with other trouble codes. Information on the possible root cause is described.
For turbo engines and 6-cylinder aspirated engines: Information about combined diagnostic trouble codes (DTCs)
For 5-cyl. normally aspirated engines. Information about combined diagnostic trouble codes (DTCs)



Fault symptom
Poor accelerator pedal (AP) response
Cruise control does not function at all.

You may be having issues with the throttle but that doesn't really explain a misfire, rough type running maybe if its hunting, but thats not quite the same thing. For the cost of a few plugs its worth knowing you have fresh ones in there. Check ALL of the vaccum pipes, including the small pipes that go from the metal wastegate pipe to each turbo, these often rupture.

[jammer5]

Thanks for the comprehensive reply. I'll start with the plugs. Symptoms sound more like the first ECM-903C code but I'll check everything. Much appreciated.