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1994 Seville STS
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Discussion Starter #1 (Edited)
I have been fighting with intermittent MAP codes P032(open MAP Signal) and P071(intermittent MAP signal). Occasionally there is a stall with those codes showing in history with the P095 (Engine Stall Detected).

The stalling led me to change the MAP sensor with a new GM one six months ago. No stalls for six months but persistent continuing MAP codes P032 and P071. When the problem of unannounced stalls re-emerged recently I had my mechanic re-check the connections to the newer MAP sensor and clean the oil that may have infiltrated the MAP connection. He also added some sort of silicone protectant to guard against further oil interruptions. As part of the review of the health of the MAP sensor I checked the Vacuum data PCM readings on the dash board. The PD02 was 99 kpa at key on position and 48/49 kpa at idle. Is the 48/49 not supposed to be closer to 30 at idle (stronger vacuum)? It worries me that this higher number suggests my engine is suffering from overall lower than perfect vacuum from age.

Is that true or is 48-49kpa at idle a reasonable number? Does it signify the MAP needs replacement again? Or am I using a good MAP sensor on a declining vacuum engine. Maybe I need a new timing belt and big time adjustment.

Thanks to all like Ranger and others who understand this stuff so well.
 

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2002.5 F55 CORSA STS, 2014 Explorer XLT FWD
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If the engine is a Northstar it does not have a timing belt - it has 3 chains - crank to idler, idler to right and left cylinder heads. Remove the oil filler cap and look down in there. Chain.

I have no idea of the OBD1 MAP figures, but a vacuum gauge should show above 20" at idle, maybe 14" - 16" at highway cruise speeds/loads.

"Predicting" vacuum is a moving target because it changes instantly as the gas pedal moves. Low idle vacuum is usually an indicator of vacuum leaks or retarded timing (difficult to do in a totally electronically controlled ignition system).

You could remove the ICM and coils, clean everything, check the ICM ground wire for integrity, ??? new plugs and plug wires ???.

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White Diamond '03 DHS (with DTS floor shift)
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If I remember correctly, the readings you got from the PCM data are in kPa (kilopascals), not inches of vacuum.
http://www.aqua-calc.com/what-is/pressure/kilopascal

He also added some sort of silicone protectant to guard against further oil interruptions.
Not sure where he used this, but be careful, silicone should not be used anywhere near any intake components. It is instant death to O2 sensors.
 

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Master of the Dark Art of Diagnostics
2003 DHS - two-2002 DHS, 2003 SLS, 1995 Sedan DeVille, 1989 Coupe DeVille
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19,867 Posts
If I remember correctly, the readings you got from the PCM data are in kPa (kilopascals), not inches of vacuum.
http://www.aqua-calc.com/what-is/pressure/kilopascal


Not sure where he used this, but be careful, silicone should not be used anywhere near any intake components. It is instant death to O2 sensors.
=========================
He also added some sort of silicone protectant
probably just silicone grease -
 

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1994 Seville STS
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113 Posts
Discussion Starter #5 (Edited)
Thanks Submariner. I had my guy do a smoke test and and found and fixed a very minor vacuum leak. However afterwards when he did a mechanical vacuum check through the brake system it showed 14 inches of Mercury at idle. He thought it should never be below 18 or 19 inches mercury at idle. You say it should be above 20" at idle. I definitelly have low vacuum from the sounds of it. Maybe there is a retardation of the timing.

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Hi there Ranger and thanks for responding. My OBD1 data readout is 99-102 Kpa with engine not on and 48-49 Kpa at idle. I wondered if it should be 30 Kpa at idle. I don't know how to analyze the relationship to timing and possibly impact on MAP sensor. With a mechanical vacuum check through the brake booster connex showing low vacuum down at 14 inches of mercury at idle I have a problem. Does that problem of low vauum set off the MAP codes and occasionally lead to a stall?

On the grease applied to protect against oil infiltrating the MAP my mechanic assured me it was the proper stuff to use.Not simple silicone grease but some expensive dielectric stuff used very sparingly.


If I remember correctly, the readings you got from the PCM data are in kPa (kilopascals), not inches of vacuum.
http://www.aqua-calc.com/what-is/pressure/kilopascal


Not sure where he used this, but be careful, silicone should not be used anywhere near any intake components. It is instant death to O2 sensors.
 

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White Diamond '03 DHS (with DTS floor shift)
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The FSM is the only place I know of to get that PCM data info. It will tell you exactly what those numbers are.
 

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1994 Seville STS
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113 Posts
Discussion Starter #7 (Edited)
After digesting all the data and converting from kpa to inches of mercury I definitely have low vacuum in my North*. Question is can a low vacuum set off the MAP codes (P032(open MAP Signal) and P071(intermittent MAP signal)? Can it lead to the intermittent stall every few days? How does the valve timing retardation play into that?
 

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White Diamond '03 DHS (with DTS floor shift)
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Those codes are pointing to a faulty MAP sensor. Start by replacing it.
 

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1996 El Dorado and 1991 Cheyenne Pickup
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140 Posts
May sound strange, but clean your throttlebody, been there... Unrelated to your low vacuum though, but sure related to the codes you're getting and stalling.
 

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1994 Seville STS
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Discussion Starter #12
Thanks for that el_dorado_mex. I will have my TB cleaned with the correct cleaner.

I also will change out the MAP Sensor again per Ranger's instincts.

On a semi- related issue my CAT is rattling madly at idle while in gear with brake on. Replacement CATs here in Canada with labor exceed the value of my 94 Seville. Can I straight pipe it without fear of new codes given it is OBD1 and there is no sensor after the CAT (just before it)? In my area of the world annual smog inspections have been phased out.
 

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White Diamond '03 DHS (with DTS floor shift)
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Check the heat shield over the CAT. That may be what's rattling. It would be far better to have that welded (if that's what is rattling). Straight piping it will set all sorts of codes because it's health is monitored by O2 sensors fore and aft. Since you are subjected to emissions testing, you COULD straight pipe it if you can put up with that damned amber SES light in your face.
 

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1996 El Dorado and 1991 Cheyenne Pickup
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140 Posts
Also don't think obd1 should throw any codes. But you can easily find out, look for a downstream o2 sensor (behind cat), if none there, no codes will show and you just take it out and enjoy the 2 horses gained :D Agreed on checking heat shield first, easy fix. On the other hand, if your cat is blocked you may get stalling issues and low vacuum...
 

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1994 Seville STS
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113 Posts
Discussion Starter #17 (Edited)
Had the CAT removed and am glad the noisy rattle is gone. The CAT was broken in pieces and the heat shield was not the problem. There were no oxygen sensor codes or any other new codes thrown as a result of straight piping.

I had hoped maybe the intermittent P032(Open MAP Sensor) and P071(Intermittent MAP Sensor) and occasional stall with P095(Engine Stall detected) would also disappear with the freer flowing exhaust but no such luck.

I am waiting for a second new MAP sensor to arrive. I am also wondering if there is a way to rule out other causes that could throw a MAP code. I had my mechanic check all the connections when he did a vacuum leak test last week. All was normal in his view although he mentioned that I could try to replace the "harness" if I could find one. Is the harness just the group of wires that the MAP connects into? This stalling only happens a few times each day at low revs but it is a danger when in traffic. If I feel the stall coming I can avoid it by gunning the engine and raising RPM above 2000. I am very frustrated by the MAP problem being so difficult to zero in on. A fix seems impossible. I am really hoping I bought a bum MAP Sensor last go around and the new sensor is all that needs be done.
Any other ideas most welcome.
 

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1996 El Dorado and 1991 Cheyenne Pickup
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Maybe the codes are no ghosts and you really have intermittent contact issues. You could try and measure each cable's (3 of them if I'm right) resistance from plug sensor to plug pcm while wiggling the harness. If you get to see resistance variation I'd change out the individual cables affected, rather than the whole thing, cheaper and easier.
 

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1994 Seville STS
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Discussion Starter #19
Thanks everyone. New MAP Sensor seems to have done the trick. No codes no stalls and much better fuel economy. Props to Ranger for instincts on it being a bad MAP sensor even though I had three MAP Sensors in seven months. My mechanic is going to take the parts dealer to task for the bad MAP as here in Canada it was $197.00 Can Dollars
 

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1996 El Dorado and 1991 Cheyenne Pickup
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Thanks for coming back and sharing what fixed your problem. Wouldn't have thought you really would need to buy the same part 3 times to finally get a working one :eek:
 
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