MAP Sensor

[Dave]

Hey guys,

Back story ….
My car was running rich but idled, ran and accelerated fine.

I put a new O2 sensor in and now it idles rough.

I tested the MAP sensor and it takes almost 2 seconds for the voltage to drop.

The service manual states that the “change in voltage should be instantaneous”.

Would that be considered “instantaneous”?

Since the car ran fine before the sensor change, I am assuming that I uncovered another sensor problem and not a mechanical one.

-Thank you.

Topic

[Steven Cummings]

The voltage change on your meter should be within less than 1/2 second (i.e. very fast) as you are moving the throttle.

If your voltage change appears to be lagging, you could try cleaning the air intake hoses and sensors with MAP cleaner

For your oxygen sensors you changed, did you save your old ones? Try putting them back in. Also, on oxygen sensors, only ever by OE or equivalent. Universal rarely work well, especially with newer cars.

[Dave]

I reread it again…it just said voltage change – not that it had to go to that voltage instantaneously.

I did clean the MAP sensor with MAP sensor cleaner.

The voltage changes immediately, but it takes almost 2 seconds to get to the proper voltage.
So, It’ll be at 4.7v (idle)…add vacuum, 3.5, then 2.1v in a little over a second.

My car is a ’95 Cav.

Get this, with the O2 sensor plugged in (new one) the car idles rough. Unplug it, and it idles fine.

I checked the voltage of the O2 sensor and it’s giving similar values as OEM only a little lower by about 50mV.

When I rev the engine, sometimes it goes into negative values – the old one (OEM) didn’t do that. The old one only got as low as 300mV.

The OEM O2 sensor is very white and crusty (the outside protective “cage”) and I can only imagine what the actual sensor looks like.

Unfortunately, my car has hit the age where I am pretty much stuck with ‘universal’ stuff. 🙁

The thing is, I am working on the car running rich (failed emissions with high HC).

And if it were a plug and wire and coil problem, wouldn’t the car run rough regardless of fuel mix?

The EGR valve works fine – bench tested – the holes are very small so I’m having issues putting something up there to test the whole system.

I did check for leaking wires (water mist) and I didn’t see anything obvious. They were replaced a couple of years ago with new plugs (best of the best Champion), btw; so they are not OEM.

[Steven Cummings]

Ok. Your MAP sensor sounds fine. It finding the sitting voltage instantly isn’t going to happen, but it should detect a change instantly and it sounds as though it is.

I understand about your O2 needing universal…happens with older vehicles. Have you looked for other sources of vacuum leaks around the PCV valve, intake manifold, fuel ports and valve covers? The white stuff is just some carbon buildup, not a big deal. Can you pull one of your plugs and see how it looks (or all of them to see if one looks worse than the others)?

Edit: I’d go through the performance and rough idle sections of the troubleshooting FAQ section just to come up with ideas to look at beyond the above mentioned.

Good luck finding it though!

[Dave]

I sprayed brake cleaner around the intake manifold and nothing. As soon as the engine cools, I’m gonna look at the plugs.

[Dave]

I pulled the plugs – all were black but dry.

The #1 connector stuck on the plug – even after twisting it! I rammed it back down deep.

The #3 well – not the plug – just the well – was filled with this brown goop. I cleaned it out the best I could.

The valve cover is not leaking nor are there any leaks around that plug.

I drove around the neighborhood, got the car up to op temp (I think. The temp gauge is where it is when at operating temp) and … no miss.

WTF? The brown “goop” only covered the metal part of the plug, but could that have shorted it?

My exhaust doesnt smell like gas anymore, either.

Anyway, I’ll be driving a further distance later and I’ll see how it goes.

[Steven Cummings]

That would be interesting if it were. It could have not been allowing the spark plug to be completely sealed as well. Let’s hope that was all the problem was. But if it were, then you’ll want to double check for an oil leak somewhere around that spark plug (i.e. valve cover)

Keep us updated

[Dave]

Well, it ran fine. I’ll test drive it some more tomorrow.

I can’t find any oil leaks.

The only thing I can think of is that I accidently spilled some in there during an oil change – the distick/fill is right next to it.

[EricTheCarGuy]

Before you go chasing theories that involve computer controlled components, first check the basics. Including engine mechanical as that is so often overlooked. Anytime you have a performance issue you start with the basics NOT computer controlled components, especially if you don’t have a CEL for them. More info on this and diagnostic procedures can be found here.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

[Dave]

Well, I passed emissions with flying colors.

Thinking about it over night, cleaning the oil out of the plug well didn’t really do anything. Oil doesn’t conduct electricity, after all.

I think that when I pulled the wires and plugs, I accidentally fixed the problem.

How?

Well, the #1 connector stayed on the plug and I had to jam it back into the boot – I couldn’t pull the boot back to re-crimp it.

And the car ran fine after that because I did not do anything else after that.

In other words, I think I accidentally fixed a bad wire – because otherwise, everything looked fine. Well, the #1 plug had some white deposits along with the dry black deposits that the other plugs had and I guess that is a sign that something is happening in that cylinder that isn’t happening with the others.

After replacing my white crusty O2 Sensor, the catalytic converter, and “fixed” the wire, my emissions improved greatly.

My HC ppm went down from 147 to 18
and the NO_x went down from 1057 to 392 on the 25/25 test.

Pass. banana:

[Steven Cummings]

Congrats! That’s outstanding and glad your problem is fixed. Too bad about spending money on the catalytic converter. At least you’re not in CA as that would have required a dealer cat ($640 from GMpartsdirect.com)

[Dave]

Lesson learned again – start simple. Especially, since I have had this problem before! :blush:

What suckered me into thinking it was computer/sensor related was the fact that the engine ran fine(although rich) with the O2 sensor unplugged.

As a matter of fact, if it weren’t for failing emissions, I wouldn’t have known there were problems.

In my ignorance, I just assumed it was another sensor signaling that there was more air coming in (MAP) that there was.

And the thing is, if the #1 connector didn’t stay on the plug (twisting didn’t help), I would not have accidentally fixed the problem.

My next step would have been to test the coils and the wires if things didn’t work.

[Author]

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