P2431 and secondary air pump short run-time....

[Crispy]

New here. '06 Envoy 4.2 w/88k. Getting my new Envoy sorted out; I have bonded with the front suspension and differential (ball joints, tie rods, diff seal).

I'm getting a repeated P2431 CEL and have an air blower that runs for only 2-3 seconds. I have checked the valve and it closes tight when the blower turns off. I removed the lines on the pump and have no moisture.
What would cause a 2-3 second run time of the blower?
Seems like a valve/sensor issue but I don't want to throw parts at it. I was wondering if a weak relay could have a hard time staying pulled in? The run time is very consistent at 2-3 seconds every time.

Thanks in advance, C.

 

[The_Roadie]

The PCM should be commanding the relay for the full time, so I can only see it being the relay, socket, or something in the turbine pump shakes a poor connection open when it winds up.

 

[Crispy]

I guess I could probe the relay coil and if it's hot for longer than the pump runs, there's my culprit. So the PCM doesn't alter run time for different temps? Or put another way, is blower run time a fixed time? Can you tell me the easiest place to probe the relay coil? Wire ID/color? Thanks, C.

 

[AtlWrk]

Welcome!

Fortunately I don't think the relay or pump are the root cause of your problem. GM did a fairly remarkable job with creating very specific DTCs for the electrical subsystems.

P2431 says that the air pressure as indicated by the sensor in the valve is too high--either with the pump off (pressure should be close to ambient) or with the pump on (pressure should be at a well defined level and not unrealisticly high).

It is possible that the PCM is seeing an abnormaly high pressure when it turns on the pump and immediately shuts it down and sets the P2431 so as not to damage the pump. This could be an electrical fault with the sensor or a malfunctioning valve.

Are you comfortable with a voltmeter? If you measure across pins 1 (dark blue wire) and 2 (yellow wire) on the check valve you should see less than 0.5V (ignition ON, engine off).

Another thought:
The SAI valve opens and the pump runs for about 30 seconds starting immediately at startup. At the end of that period the check valve closes and pump continues to run for 2-3 seconds so that the PCM can verify a further increase in air pressure indicating that the valve has in fact closed. During those 2-3 seconds the air pump gets noticeably louder (think clogged shop vac). For the longest time I was mistakingly thinking this was the only time the pump was on because the first phase was drowned out by the engine.

Point being: if the 2-3 seconds of the pump you're hearing starts 20-30 seconds after engine start it is likely just the second "valve check" phase of operation. This is normal.

(Sorry I know this was a lot of information )

 

[The_Roadie]

Ack, your analysis is a lot better than mine. I have an '04 without the pressure sensors on the solenoid valve. The theory of operation in the shop manual for my system is simpler.

 

[Crispy]

Thanks for that, all good info. I do know my pump waits for about 10-15 seconds after startup and then runs for the 2-3 seconds I described. I also know my valve opens and I get exhaust noise, then closes, I think, at the same time as the blower stops but I will double check that.

So is that .5 volt the sensor output to the PCM for ambiant?

I like your theory about the PCM seeing too high a pressure because the pump acts just that way; right when it's getting spooled up, it shuts down.
I'll check the pump-off-time aginst the valve closing and try to add something to this diagnosis. Thanks again, C.

 

[AtlWrk]

Sorry I misread the DTC troubleshooting. The .5V is the maximum that the sensor signal (pin 2, yellow wire) should be with the ignition ON and the connector disconnected. It's just a check to make sure the the signal wire isn't shorted to voltage. I'm not sure what the sensor output voltages should be as the DTC troubleshooting guide assumes you're using a high end scanner that gives you the pressures, not volts. I could try and get these measurements for you but it might be a few days before I have a chance.

 

[Crispy]

Since I just passed inspection, I'll be working on this until I fix it and hope to help. I appreciate your coming up with a ham & egg'er approach to diagnosing these crazy systems.
I just crawled under for another listen and the pump does sound deadheaded from the start with a very high pitch. I know the valve opens and closes but I don't know if there is flow.
The pump starts a full 15 seconds after engine start. Is this normal or too late? Thanks again, C.