Intermittent MAP stuck at 2.9 inHG ( P0107 ) solved, finally - Chevy Impala SS Forum
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post #1 of 11 (permalink) Old 01-10-2017, 05:37 PM Thread Starter
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Intermittent MAP stuck at 2.9 inHG ( P0107 ) solved, finally

Bought 96 Impala for a good price because it had all the usual issues. You all educated me and got me through it for cheap, so while I hope this doesn't happen to you, I'd like to add this fiasco to the database.

Relatively unmolested 96 DCM, 168K, P0107 showing. I used to make my rent money with electrical diagnostics, especially emissions stuff, and this is one of the strangest one I've ever seen - which makes sense because it's my car.

Summary: Bad MAP connector, possible intermittent bad ground from ECM, MAP sensor intermittently bad. Story is as follows.

Wiring connector to MAP was crunchy, back-probing 5v ECU reference wire found no 5v, $9 to Rock Auto for pigtail. 5v now seen at connector.

0107 returns.

MAP input (inHG) on scan tool , readings bouncing around as usual, good 5v reference and ground:

Engine off: 29.4
Idle: 9.4
WOT: ~0.0

Scratching my head, I took it on a 2 hr drive to see family.

During drive: Engine tone changes, scan tool shows MAP signal stuck at 2.9 inHG.

Pull over, good 5v power, good ground, sending MAP signal.
Blip throttle from engine compartment with DVOM attached, signal wire fluctuating in voltage, approx 1.7v.
Scan tool PID still shows it stuck at 2.9 inHG.

Motor idling a little rough but not bad, exhaust smelled a little rich. Got back on road, 20 mins later MAP starts switching normally again, I felt it when it came back online. Later on the drive it got stuck for rest of the drive.

Over the next few days, it would do it and I'd pull over and check everything: Identical symptoms. However, one time it started working again the second I touched the voltmeter to the ground wire from ECU to MAP. Aha...

Checked and cleaned all grounds. Stripped the factory wires, the very thin ground wire had a hint of greenish corrosion on the bare copper wire. Kinda like green acne. I ground MAP to solid battery ground, leaving ground wire from computer disconnected.

P0107 returns with this exact same, randomly stuck at 2.9 inHG foolishness. Exact same behavior (will go days without getting stuck, then get stuck for random length of time), except now I only see millivolts on the signal wire. I have a good ground, good 5v, but only millivolts on the signal wire. When it is switching normally, I see normal voltage on signal wire.

So, only true failures exhibited was no 5v before pigtail replacement, now no activity on signal wire. I have never seen a bad GM MAP sensor before, but it would be exactly my luck.

$20 to Amazon for the bottom-shelf Standard ( AS5T ) vs the $46 ACDelco because I am that skeptical it's the MAP sensor.

Code has been gone for three weeks and 300 miles, haven't caught it getting stuck. Gas mileage slightly better.

I think it was the pigtail and the sensor. I am still on the fence about the role that the sensor ground from PCM to MAP played. Intermittent ground can do weird stuff.

Hope this helps someone navigate this horseshyte.
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post #2 of 11 (permalink) Old 01-11-2017, 10:12 AM
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all of us living in a winter of corrosives for ice removal will suffer corroded wires if the insulation has been damaged. the wires can be damaged from uncaring techs, people using carb cleaners/brake cleaner on the electricals. these solvents attack the plastic wire coverings then the corrosive chemicals eat at the very fragile copper.

on my truck the ABS is mounted behind the left front wheel. some how a very small section of the insulation got damaged like the head of a pin. the corrosive chemicals got into the wire and it was a 12 ga wire to the ABS pump motor well that small break ate thru that copper I had to cut out and solder in a new wire then used heat shrink and liquid tape to seal it up. then the abs light did not come on again.

I myself have never had to replace a map but it can happen . I do know that if the engine back fires it does stress out the map.. on my 96 impala at idle 500rpm the voltage out put is 1.2 volts.. the map seal can get messed up also esp if its exposed to solvents . a low vacuum at idle will cause a higher output voltage.

vacuum should be 17-20 inches..at 500-600 rpm..
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post #3 of 11 (permalink) Old 01-11-2017, 10:48 AM Thread Starter
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on my 96 impala at idle 500rpm the voltage out put is 1.2 volts.. the map seal can get messed up also esp if its exposed to solvents . a low vacuum at idle will cause a higher output voltage.

vacuum should be 17-20 inches..at 500-600 rpm..
Agreed on all of those things, has anyone found a source for the MAP seal? I had to reuse the OE, which I was not happy about, but isn't leaking... yet

Quick confirmation question if you have a moment:
New MAP is showing 11.5 inHG of pressure at idle, warmed up, 585 rpm.
Barometric is 29.5 today. Barometric pressure - intake pressure = Vacuum, correct?
29.5 - 11.5 = 18, which is in spec, right?

Headers, flowmasters, otherwise stock, no codes, new O2's.

My vac gauge decided to STB today, which is why I'm trying to do it this way.

Thanks
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post #4 of 11 (permalink) Old 01-11-2017, 12:28 PM
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Quote:
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Agreed on all of those things, has anyone found a source for the MAP seal? I had to reuse the OE, which I was not happy about, but isn't leaking... yet
Yes, I believe there is and its a Harley Davidson Part.

Let me see if I can find the Link with the info for you.

EDIT - Yep, here's the link....

Harley davidson map sensnor seal same as our car

Just as an FYI and see you're fairly new to the forum. I just used the "Search" option and entered "MAP Sensor Seal" and it took less than a minute to get the link. It was the 1st thread to pop up.

If you know what you're looking for, try a Search and you may have to Dig through a few threads to find it. However, this can be a good thing as you often see other useful info that could help later.

EDIT2 - Forgot to add, thanks for taking the time to post up your experience troubleshooting this issue. Could be helpful for others.
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Last edited by 4DoorSS; 01-11-2017 at 12:39 PM.
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post #5 of 11 (permalink) Old 01-11-2017, 12:35 PM Thread Starter
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Yes, I believe there is and its a Harley Davidson Part.

Let me see if I can find the Link with the info for you.
You are the man, looks like it's part # 11291 . Looks like average price with shipping is ~$10, I'll try to find it at a local dealer first.

Again, thanks.
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post #6 of 11 (permalink) Old 01-11-2017, 03:21 PM
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You are the man, looks like it's part # 11291 . Looks like average price with shipping is ~$10, I'll try to find it at a local dealer first.

Again, thanks.
my local Harley shop said 2 bucks ..
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post #7 of 11 (permalink) Old 01-11-2017, 06:24 PM
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18in is good .. should also be steady needle .. a solid 18 with no fluctuations means the valves are good also. I had a used vehicle , buick 350 cu .. had a bad valve needle did a lot of dancing. still ran good though ..
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post #8 of 11 (permalink) Old 01-13-2017, 11:30 AM Thread Starter
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Quote:
Originally Posted by 4DoorSS View Post

Just as an FYI and see you're fairly new to the forum. I just used the "Search" option and entered "MAP Sensor Seal" and it took less than a minute to get the link. It was the 1st thread to pop up.

EDIT2 - Forgot to add, thanks for taking the time to post up your experience troubleshooting this issue. Could be helpful for others.

Thanks, I thought I was expert in searching this forum but hadn't found this one yet.

And it's the least I can do considering how much time and money the contributors to this forum have saved me.

Cheers
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post #9 of 11 (permalink) Old 01-13-2017, 11:33 AM Thread Starter
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18in is good .. should also be steady needle .. a solid 18 with no fluctuations means the valves are good also. I had a used vehicle , buick 350 cu .. had a bad valve needle did a lot of dancing. still ran good though ..
Many thanks, I'll check for jittery needle as soon as the new vacuum gauge gets here.

This site has a set of great animations for what the behavior of your vacuum gauge means. Thought I'd share it for the folks who might be wondering what we're talking about

How to Use and Interpret a Vacuum Gauge
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post #10 of 11 (permalink) Old 01-13-2017, 01:10 PM
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Many thanks, I'll check for jittery needle as soon as the new vacuum gauge gets here.

This site has a set of great animations for what the behavior of your vacuum gauge means. Thought I'd share it for the folks who might be wondering what we're talking about

How to Use and Interpret a Vacuum Gauge
I did use this chart when carbs were used many times. now with these new vehicles and not many vacuum lines it gets used maybe once a year.
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