Hunting down a P0401 code: MAP & MAF Values, etc.

[Ramcharger310]

My 01 SSEi with 126K miles, threw a DTC P0401 code.

I'm on a diagnostic hunt and was wondering if my MAF & MAP are acting right.

Trying to rule out the MAP and MAF sensors.

Cold engine, At idle



Reving to 40mph



Cruising at 40



Speeding to 50


Cruising at 50 mph


Speeding to 65mph


End of trip, Hot engine, engine off

[95naSTA]

The cold engine idle shot wouldn't be the best data point since the IAC might still be open. Your MAP numbers seem counter intuitive to me when comparing the "Reving to" vs. "Cruise" values. I would expect the vacuum to drop as you accelerate. The progression of the MAF values makes sense but I'm not sure what they should be at those loads/speeds. Also while the MAF does have a definitive lb/min it's been found in the 3800 tuning community that there are different MAF tables for the same sensor depending on year and platform. So they're more of a ballpark than actual.

Ultimately the MAP values seem weird to me but I'm not sure otherwise.

[harofreak00]

I agree your MAP sensor data looks suspect. They are very cheap compared to the MAF sensors, I'd slap a new one in.

[Ramcharger310]

What would the AC Delco or GM part # be for the MAP sensor?

Amazon doesn't let me add the SC info and keeps recommending the little square one when I need the larger rectangle style.

Is this it? https://smile.amazon.com/ACDelco-213-15 ... B000C9JX4I

https://smile.amazon.com/ACDelco-213-32 ... B000IYTB4O

[harofreak00]

The SSEi's come with a 2-bar MAP sensor, the OEM part numbers for the ones I have on the shelf are #16254539.

Check out the reviews and comments on those Amazon links, they are cheap knock offs even though they are "AC Delco". I have good used OEM ones, PM or email me andrew.suderman@gmail.com

[Ramcharger310]

I'm going to hold off on replacing the MAP sensor right now and tomorrow will try this trick to determine if it is a clog or a vacuum leak.

1. With the EGR valve removed from the intake manifold, have a helper start the engine.

2. You'll notice one of two things:

The engine will start and run smoothly (as if the EGR valve where still there in its place).

Or the engine will start and idle extremely high and/or stall immediately.

3. If the engine starts and idles very very high have your helper immediately turn off the engine.

Let's take a look at your test results:

CASE 1: If the engine started and idled smooth. Then this confirms beyond a shadow of a doubt that the EGR passages are clogged inside the intake's plenum.

This also confirms that this is the reason for diagnostic trouble code P0401.

You'll need to remove the intake manifold's plenum to physically remove this carbon blockage. I have used a piece of coat hanger wire as a snake (along with a lot of Carburetor Cleaner Spray) to remove this blockage and you'll need to do the same.

CASE 2: If the engine started and idled very high and/or stalled. Then this confirms that the EGR passages inside the plenum ARE NOT clogged.

https://easyautodiagnostics.com/gm/4.3L ... ve-tests-5

[Ramcharger310]

So the engine stalled.

I am going to look at fuel trims and check out the egr tube for cracks, next.

I did change out my PCV valve and replace a vacuum hose that goes to the vacuum manifold to the boost bypass valve.



I also put in a new AC Delco air filter and found this:

[95naSTA]

So going back to the MAP sensor, assuming you can see the data you posted earlier with the key in run and engine off, why not try seeing what the MAP reading is with the sensor its self unplugged from the engine and open to atmosphere. If its not zero or close to it, you know its bad.

[Ramcharger310]

So going back to the MAP sensor, assuming you can see the data you posted earlier with the key in run and engine off, why not try seeing what the MAP reading is with the sensor its self unplugged from the engine and open to atmosphere. If its not zero or close to it, you know its bad.

I bought a manual hand pump that does bought vacuum and pressure from Amazon.

I plan to do the test of apply vacuum and see what volts it generates.

https://easyautodiagnostics.com/gm/2.8L ... ter-test-1

2
Connect your vacuum pump to the MAP sensor's vacuum nipple. If you had to disconnect the MAP sensor from its electrical connector to remove it, reconnect it to it now.

3
With your multimeter in Volts DC mode probe the wire that connects to the MAP sensor connector terminal labeled with the number 2 (in the illustration above).

NOTE: Remember, the MAP sensor must remain connected to its 3 wire connector.

4
Ground the black multimeter test lead directly on the battery negative (-) terminal.

5
Turn the key ON but don't start the engine. At this point your multimeter should register 4.7 Volts DC.

Now, pump the vacuum pump to apply vacuum to the MAP sensor. The multimeter should register the following voltages at the following vacuum values (they may differ a little on your specific 2.8L S10/S15):

1.) 0 in. Hg ...... 4.7 Volts.

2.) 5 in. Hg ...... 3.9 Volts.

3.) 10 in. Hg .... 3.0 Volts.

4.) 20 in. Hg .... 1.1 Volts.

Repeat this test step several times and each time, you should see the same values on your multimeter.

Also, If I unplug the vacuum hose from the MAP sensor, wouldn't it be reading atmospheric barometeric pressure and not be 0?

[95naSTA]

Those voltages aren't going to match since that's a sensor for an n/a engine. You can see it's maxed out at roughly 5 volts from those steps and it's a 5v sensor.

When the MAP sensor is open to atmosphere, you'll have no vacuum, or zero inches of mercury. I've always understood MAP sensors to read in PSIG where 0 PSIG is 1 ATM.

[Ramcharger310]

Ok, so I unplugged the vacuum harness from the map sensor.

I turned the key on, engine off.

It read zero. I then applied vacuum at 5 in Hg, then 10 in Hg, and still got zero readings on my ObdLink scan app. It was changing my boost levels.

I did break the map sensor connector tab on the pigtail, however the connector seemed pretty snug in there.




Edit:

So I went back out and grabbed a multimeter and put the red probe on the B middle wire of the map connector and black on a ground point on the car. At 0 vacuum, I was getting 0 volts. I then attached the vacuum pump and applied vacuum and still got no electrical VDC response.

3rd edit:

I ordered some wiring piercing test leads off Amazon. When they get here, I will redo the test.

[Ramcharger310]

Ok, so the wire-piercing probe adapters came in.

https://easyautodiagnostics.com/gm/3.8L ... p-sensor-1


New test results.

I put the red probe on C wire, black to ground and turned key on engine off and got 5 volts.

I then moved red probe onto middle B wire. I connect the vacuum pump and at 0 in Hg, got this result




I then applied 5 in Hg of vacuum and got this:




I then applied 10 in Hg:




And at 10 in Hg as I was taking a pic, the readings started to go wonky:

https://youtu.be/Q3Smf_XjS5U

The volts were fluctuating and would not hold steady.

Lastly at 15 in Hg:



Edit: Would these specs apply to the L67?
http://www.darkside.ca/node/116