Volvo 850 Code 121 - Faulty MAF not faulty

[steve92]

I have a Volvo 850 1996 with an ECU LH-Jetronic 3.2 Engine: B5254S

A week ago when leaving to work the engine stumbled and shut off when leaving the parking lot like I was lifting the clutch too quickly but I didn't. Now it has been a whole week of investigating the issue the engine has.

Fault code: 121 MAF sensor value abcent or invalid.

Tested the MAF and it looked like it really was faulty due to the output signal voltage jumping all over the place. So I replaced the MAF with a new one but still the engine has this same issue and is still giving a code 121 after clearing it a bunch of times and even disconnected the battery for an hour.

So the next thing I did was testing if the MAF gets voltage, key in ON position without ignition, 12V input detected in MAF connector pins 2/3 (the sensor has a 4-pin connector). This seemed to be working as intended so next thing I did was testing the 3 cables going to the ECU that they are not shorted out or disconnected, and they were not. They were not shorted out to eachother either and the ground wire had a contact with the negative terminal on the battery as intended.

After a while of investigation I went to take apart the Main Relay (fuel injection) because according to a wiring diagram of Jetronic 3.2 the oxygen sensor, the MAF and the injectors were all connected to that main relay power source. I thought meaby that device was malfunctioning but it wasn't. The insides were as shiny as brand new device with no cracked soldering joints and even tested it with the engine running to see if it sparkles, and it didn't.

The ECU is in the safe mode but the engine wont stay running after startup. (stalling within 5-10 seconds)
So to keep the engine running I turned the throttle a little every few seconds to keep the engine running while investigating the Main Relay.
While giving that throttle a very delicate 3-5 degree turn I heard a sparkling sound coming somewhere from the middle of the fuel injectors.
I heard this sparkling on every turn of the throttle for 10-15 turns untill suddenly it didn't sparkle on the last turn and shot the RPM up to approximately 1100RPM for 2-3 seconds after which I didn't need to turn the throttle anymore to keep the engine running and it was somewhat normally sailing at 790-840rpm on idle. (tiny bit higher variable on rpm compared to a working system)

Fuel line pressure was tested and it had no problems. The whole fuel delivery chain should be fine.
Except having one noisy injector with a cold engine that Im already planning to check/replace or clean at some point.

However the only code is 121 and I don't know what to test here anymore. Can a failing ignition coil meaby cause a MAF Fault Code 121? As somehow I think it might have something to do with the sparkling I heard.

Any thoughts would be helpful.

[RickHaleParker]

Clean the MAF connector.

23: MFI 1-2-1 MAF sensor signal. Signal faulty.

DTC information MFI 1-2-1

S145881 Condition
Since the computed mass air flow is determined largely by the injection period, the system is dependent on the MAF sensor signal. The DTC is recorded if the control module detects that the signal is lower than 2.7 V or higher than 8 V.

Substitute value
The control module assumes a fixed load signal value (limp home) when the DTC is recorded. The adaptive and idling control functions are inhibited, and the ICM and AW 50-42 TCM load signal is adjusted to the full-load value.

Possible source

• Open-circuit in signal or supply lead.
  Signal lead short-circuited to ground or supply.
  Contact resistance in connections.
  MAF sensor faulty.

Fault symptoms
None.

[misha]

If your replacement MAF is NOT oem Bosch,it will not work.The fact is that the car will run better with an old one.
Disconnect the MAF and start the engine.If it runs better than you still have MAF problem.
Note that Maf sensors for Jetronic & later Motronic system are NOT compatible,neither the inserts.
They look pretty similiar and have the same number of pins but they are NOT interchangeable.
You need exactly the same Bosch MAF insert,with the same part numbers like old one.
Did you checked the codes from ignition ecu also?