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Old 02-22-2018, 11:52 PM   #1
raffivegas
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1997 SL1
Wrench MAP sensor testing questions

Preface:

I've had a P0172 since 2013. Every year during the month of emissions, it turns off and I pass emissions.

I've changed a large number of parts on the vehicle, a 1997 Saturn SL1. Bought it with 131k miles, it has 182k today. Here's the P0172 thread with currently no resolution:

http://www.saturnfans.com/forums/sho...d.php?t=198683

The thread covers all the parts I've changed. Yes, I have the brass ECTS sensor. Had a leaky FPR, bought a new one. Checked for clogged cat/backpressure tested, replaced fuel pump, fuel filter, etc etc list goes on.

I've always suspected the MAP. I watched a few vids, including Richpins, on how to test the MAP. I don't have a vacuum pump, but I'll be ordering one soon.

IN THE MEANTIME, I tested in the following way, and I would like to know if my test was a valid test, and if so, I need help analyzing the results of the test.

The test:

-I back-probed the middle (assuming its the carrier) pin on the MAP sensor (with the other probe on my meter to battery ground).

-I checked voltage at idle, around 1 to 1.2 volts. Revving and holding at any RPM, 2k, 3k, 4k, doesn't matter, if revved slowly, the voltage stays put, around 1 to 1.2 volts. If I blip the throttle quickly to a high rpm, like from idle to 3k very quickly, I get voltage up to 3 volts, but then it dips right back down.

I've made the assumption the voltage should hold at the increased voltage rate given the held increased RPM, for example (arbitrary numbers here), 2V if held at 2k RPMS and 3V if held at 3k RPMS, etc.

In my assumption, I've also believed if the voltage is dropping but I'm holding the RPMs, there's a leak somewhere.

I initially believed my MAP to be faulty, so I went to Autozone and bought a Duralast MAP. It didn't come with the rubber bushing which goes around the nipple. On that condition alone, they agreed to let me bring it back if it doesn't hold a vacuum.

The thing is, I feel like my existing bushing is in pretty good shape, and I've transferred it to the new MAP. If the new MAP is dropping in voltage at a higher than idle static RPM, does that indicate a vacuum leak elsewhere in the system?

If it indicates a vacuum leak elsewhere in the system, where should I begin checking for the leak?

I've done the soapy water bubble test on the exhaust system several times. I've also sprayed the entire intake manifold area with carb choke and seen no change in rpms.

I'd also like to note I now also have a P0108 since replacing the MAP just an hour ago. Drove the car around for 30 minutes, parked, let it sit, got on the freeway, etc. Not sure why I have a P0108 with a new MAP, and not with the old one I've had for 4 years.

As always, thanks.

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Old 02-23-2018, 12:27 AM   #2
OldNuc
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1998 SC2
Default Re: MAP sensor testing questions

Sounds like the pins B and C Have become inter connected or that AZ MAP is incorrect for your car. These cars do not run well on anything except a GM MAP with 103 stamped into it at the connector bulge, you find them in the JY and the one you took off does sound bad. RPM with no motion does not markedly change engine steady state vacuum.


**Have you ever actually leak checked your exhaust system? I men using this method.
Exhaust leak check.
--Locate a low pressure high volume air source, reversible vacuum cleaner, large fan, leaf blower or what ever you can find.
--Loosely couple to cold exhaust at the tail pipe, or direct fan at tail pipe.
--Spray every inch of the exhaust between the head and CAT outlet flange with a mixture of 2 or 3 drops of dish soap and water in a spray bottle.
--Pay close attention to the lower flange, lower support clamp, and flex connector.
--The flex connector is under the protective braid so it requires quite a bit of solution to show any leaks.

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Old 02-23-2018, 12:39 AM   #3
fdryer
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Default Re: MAP sensor testing questions

From the service manual.
Attached Images
File Type: jpg DTC P0108-1.jpg (156.3 KB, 6 views)
Attached Files
File Type: pdf DTC P0172.pdf (100.9 KB, 3 views)
File Type: pdf DTC P0108-1.pdf (148.0 KB, 3 views)

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Old 02-23-2018, 01:30 AM   #4
raffivegas
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Default Re: MAP sensor testing questions

Quote:
Originally Posted by OldNuc View Post
**Have you ever actually leak checked your exhaust system? I men using this method.
Exhaust leak check.
--Locate a low pressure high volume air source, reversible vacuum cleaner, large fan, leaf blower or what ever you can find.
--Loosely couple to cold exhaust at the tail pipe, or direct fan at tail pipe.
--Spray every inch of the exhaust between the head and CAT outlet flange with a mixture of 2 or 3 drops of dish soap and water in a spray bottle.
--Pay close attention to the lower flange, lower support clamp, and flex connector.
--The flex connector is under the protective braid so it requires quite a bit of solution to show any leaks.
Yes, several times, per your suggestion in the past. When I say several times, I mean like 4 times during different years, with a ton of soap, using an electric mattress pump blowing air. I still have my lower support clamp intact.

Last edited by raffivegas; 02-23-2018 at 01:37 AM..

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Old 02-23-2018, 01:37 AM   #5
raffivegas
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Default Re: MAP sensor testing questions

Sorry about the bold red, I thought I was making an inline comment. So what do you mean by "RPM with no motion does not markedly change engine steady state vacuum." ? should a higher HELD RPM not have a change in vacuum? Should a higher help RPM read higher voltage than lower RPM? How can I test if pins B and C have become interconnected? I have 3 MAPs, the one from when I bought the car, another one I pulled from the JY 2 years ago, and the AZ one I just bought tonight.

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Old 02-23-2018, 01:39 AM   #6
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Default Re: MAP sensor testing questions

Quote:
Originally Posted by fdryer View Post
From the service manual.
I have an OBD app on my tablet with one of those bluetooth OBD connectors. I'll hook it up tomorrow to see what voltage I'm getting at idle, thanks.

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Old 02-23-2018, 09:24 AM   #7
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Default Re: MAP sensor testing questions

Quote:
Originally Posted by raffivegas View Post
Sorry about the bold red, I thought I was making an inline comment. So what do you mean by "RPM with no motion does not markedly change engine steady state vacuum." ? should a higher HELD RPM not have a change in vacuum? Should a higher help RPM read higher voltage than lower RPM? How can I test if pins B and C have become interconnected? I have 3 MAPs, the one from when I bought the car, another one I pulled from the JY 2 years ago, and the AZ one I just bought tonight.
The MAP responds to the vacuum in the intake and outputs a voltage proportional to that vacuum. Engine manifold vacuum is 18-20 inches Hg at idle and will be in that range for just about any steady RPM as long as the engine LOAD is not changing and when you are sitting still the load is not changing markedly. You will see a vacuum change when the throttle is either snapped open or closed.

Google engine vacuum readings using a vacuum gauge.

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Old 02-23-2018, 11:32 AM   #8
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Default Re: MAP sensor testing questions

Quote:
Originally Posted by OldNuc View Post
will be in that range for just about any steady RPM as long as the engine LOAD is not changing and when you are sitting still the load is not changing markedly. You will see a vacuum change when the throttle is either snapped open or closed.
got it, thanks.

I noticed this morning my scan software doesn't have a way to measure voltage to the MAP. Do I back-probe the hot line and see what its getting there with my multi-meter?

The connector to the map is brittle and the plastic latches have chipped away. I'm wondering if I should cut the connector out and replace it with one in better shape from the JY.

at idle, my short term trim is at -40% and my long term is at -10%.

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Old 02-23-2018, 12:32 PM   #9
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Default Re: MAP sensor testing questions

Trim has no meaning with a screwed up MAP. Go to JY and get a real GM 103 map and connector, solder and insulate splices.

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Old 02-23-2018, 12:42 PM   #10
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Default Re: MAP sensor testing questions

Most readers display values interpreted for friendly info. A map sensor usually displays values in either kpa or inches of hg (mercury), scientific notation. To display 'raw' values such as voltages isn't in line with readers/scantools. Using sea level pressure of 14.7 psi/29.99 in hg/101kpa, readers should display one of these values when ignition is turned on, engine off. Naturally aspirated engines suck air when running so a vacuum is produced. A reader should display lower values at idle where the highest vacuum is produced with closed throttle (29.99 in hg [absolute vacuum] would drop to between 15-20 in hg). Good readers should be able to display values whether live, captured in freeze frame mode or logged when setup for recording.

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Old 02-23-2018, 01:09 PM   #11
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Default Re: MAP sensor testing questions

Quote:
Originally Posted by OldNuc View Post
Trim has no meaning with a screwed up MAP. Go to JY and get a real GM 103 map and connector, solder and insulate splices.
will do this in the AM tomorrow and will report back, thanks.

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