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MAP sensor testing

1996 Chevy S-10, 2.2L, 84K miles



Check Engine Light came on today, Autozone read the code as P0107 - MAP sensor circuit low input. Using a multimeter (w/ignition ON, engine OFF), I tested reference voltage at 5V and signal voltage at about 4.5V, both totally within spec. Started the engine to test signal voltage at idle, and it read significantly lower than spec at below 1V, when it should be 1.5V to 2V.



Is that enough to say the sensor is bad?

Yup.
Outstanding work.

That would be called, “Being out of spec”.

Tester

Sounds like it’s time for a new MAP sensor. Testing is critical. So many people just throw parts at a problem. You did the right thing by testing the sensor. Now replace it.

Unless, unless, the MAP (Manifold Absolute Pressure) is really low. Check manifold vacuum with a test vacuum gauge. If vacuum is low, the intake manifold is leaking; or, the engine is so worn that it can’t draw that much vacuum.

I’d thought about that, but what I’ve read tells me that the voltages I was getting were completely out of its operating range; it was generally dancing between zero and one volt. I’d have to imagine that if these readings were from a vacuum leak, my idle would be dancing all over the place as well, but it’s rock solid.

I think this is my fault anyways, since while I was replacing the fuel injectors, the MAP sensor was absent-mindedly baking in the sun the entire time. I briefly noted how hot it was when I picked it up to reinstall, not really thinking about it. Better find some shade or a box next time.

If the problem were a leaking intake manifold, leaking vacuum hose, or low manifold vacuum, the MAP reading would be too high at idle. You can always draw a set vacuum on the MAP nib with a manual pump and see what the reading is. Also put a vacuum gauge on the intake manifold to see its condition. In all likelyhood the MAP sensor has a internal circuit fault. When you replace the sensor, open the old unit up and look at the circuitry. It probably is the same Thin Film Integrated circuitry used in the infamous Ford TFI igniters. A crack in the substrate can cause an intermittant open in the circuitry. Leaving the sensor in the sun did not hurt it as it probably gets a lot hotter during a heat soak after shutting down following a hard run.