MAP sensor mechanical testing

Ever since I watched the Hummer H3 Premium series, I've been intrigued with the idea of using the MAP sensor as a sort of built-in FirstLook Sensor. They won't have the same kind of sensitivity, sure, but maybe it'd work well enough?



I've been on the lookout for good test subjects ever since. This '08 Honda Fit had 80K-ish miles on it, with a perceptible misfire cold. I diagnosed a valve clearance issue, and suggested an adjustment. Before doing the work, I got this waveform. Yellow is the MAP, green is a #1 ignition trace.



It's the sawtooth pattern in the positive spikes of the MAP that caught my eye. They have a somewhat consistent low-low-low-high pattern, especially near the end. Is that indicative of the problem? :huh: I was confident of a valve adjustment issue regardless of this waveform, so I did the adjustment and retook the capture.



The MAP pulses aren't as consistent as I was hoping. But I think they are MORE consistent overall. In my head, I was expecting a very uniform pattern from the MAP, but I'm not sure if that has to do with the engine, the sensor or my scope setup.

Does this test stand on it's own? :huh: I'm not sure... I see a difference between them, I'm just not sure I'd be confident in selling work based on this capture alone. Thoughts? Comments? Haterade?

I think it is a viable test. But like anything new it takes time to interpret the data. I have been trying to find a donor GM map sensor, Just so I have one sensor that I don't have to worry about different readings and sensor differences.

Here is a capture of a GM 5.3L with a bad #6 cylinder deactivation lifter. The green trace is the #6 coil trigger. unfortunately I didn't get to repair the vehicle to have a known good.

I really only see the difference in hindsight, I think it would be hard to see the first capture and say that it needs a valve adjustment. Something to play around with for sure.

What pointed you in the direction of the valve train? Noisey? Vacuum?

Ro-longo wrote: Here is a capture of a GM 5.3L with a bad #6 cylinder deactivation lifter. The green trace is the #6 coil trigger. unfortunately I didn't get to repair the vehicle to have a known good.

Interesting capture... I never thought to try this on an AFM engine. :huh: I always just went in-cylinder, or just pulled the valve cover off for a visual. :lol: But, I can think of some Monte Carlo's where pulling the covers isn't as easy as it is on trucks.

I threw the overlay on there, just to see what it looks like. The rise in MAP signal at 250mS correlates with the #6 intake stroke. That would lead me to believe that it's the #6 intake valve stuck closed, instead of the exhaust?

Noah wrote: I really only see the difference in hindsight, I think it would be hard to see the first capture and say that it needs a valve adjustment. Something to play around with for sure.

Agreed, I'll keep playing with this when I can. This Fit wasn't as bad as some I've seen, so I expect the waveform would be more obvious on more serious cases.

What pointed you in the direction of the valve train? Noisey? Vacuum?

A lot of it was the symptoms plus the mileage, to be honest. IMO, any of these 1.5L's with over 75K miles can benefit from valve adjustment.

I also looked at scan data for obvious fuel control issues. I read years ago in the Snappy Troubleshooter that less than 1V from the MAP sensor with the engine hot, idling, with no loads is a good spec, and I've largely found that to be true. With no vacuum leaks, of course. This one was around 1.1V before the adjustment, and .9V after.

Tyler wrote: I threw the overlay on there, just to see what it looks like. The rise in MAP signal at 250mS correlates with the #6 intake stroke. That would lead me to believe that it's the #6 intake valve stuck closed, instead of the exhaust?

Every time I see an overlay used like that I can't help but wish I had access to one and to answer you question, yes, it is the intake valve stuck closed. I have only fixed one AFM lifters, and that one I had to replace the camshaft,.

Ro-longo wrote: Every time I see an overlay used like that I can't help but wish I had access to one and to answer you question, yes, it is the intake valve stuck closed. I have only fixed one AFM lifters, and that one I had to replace the camshaft,.

Nice! Glad to hear I was looking at the waveform correctly. :lol: Maybe this is the way I should be testing for stuck lifters...

Overlays are great, no doubt, but I have a hard time integrating it into my diagnostic process. Having a Modis, I have to get the waveform, go to a PC, kick the service writer off, copy the waveform off the Modis, and MAYBE see something valuable. :dry:

Of course, the solution here is a Pico and a sweet laptop.

Tyler wrote: Of course, the solution here is a Pico and a sweet laptop.

Agreed, All I have is the Vantage Ultra, once its paid off it's time to upgrade to the PICO