Wierd MAP reading - The 1947 - Present Chevrolet & GMC Truck Message Board Network

LeesTruk · 12-10-2015, 01:37 PM

In diagnosing a driveability concern with my LS swap, I was looking at the data stream on my scanner. The MAP is showing between 11.2" and 12" at 675 rpm. Thought that was a little low for the cam I'm using, so I put my shop vacuum gauge on the nipple on the plenum. It was registering a steady 19" at the same time. Thinking I had a bad sensor, I replaced it. Guess what! The new sensor reads the same. I know the rear of the plenum is a vacuum source for the power brake booster, so I blocked the vacuum line. No change. Any ideas? As a side note, the truck goes instantly into limp mode when trying to drive. All of my peripherals seem to be okay, diadnostically. Would a bad PCM send a bad ref. to the scan reader, or should I be looking elsewhere?. I've got no vacuum leaks that I can find. Thanks

**Hart_Rod** · 12-10-2015, 02:45 PM

Is the MAP sensor calibrated correctly within the tune?

LeesTruk · 12-10-2015, 05:30 PM

Can't answer that question. Until I can get some help from someone with HPTune that can read my system, I'm shooting in the dark, here. The initial reflash was to get the truck up and running with a base tune, but the truck can't even do that. It will idle all day long but will go into limp mode as soon as I try to drive it. I have switched voltage to the trans. and I can ground each shift solenoid manually (PCM connectors removed) and hear them operate, point to point wiring is good, with no shorts/opens/grounds on all of the operating devices. And, no DTC's...

I may have someone knowledgeable coming over soon to help, that has Tuner knowledge. Keeping my fingers crossed that this actually happens

aknovaman · 12-10-2015, 07:15 PM

I am confident that Mike can fix it using hp tuners, patience

LeesTruk · 12-10-2015, 09:31 PM

AK, you have NO idea how appreciative I am to have Mike come over. I'm running out of hair to pull out....

aknovaman · 12-10-2015, 10:31 PM

It is a very long 5 minute drive for him to travel to see you............

clinebarger · 12-10-2015, 11:26 PM

Your MAP sensor readings are very close to your Gauge readings....

Converting your Gauge reading to Absolute pressure reading that the Data Stream is reporting...

29.92-12=17.92

12-10-2015, 01:37 PM	#1
LeesTruk Registered User Join Date: Mar 2015 Location: Manchester, CT Posts: 433	Wierd MAP reading In diagnosing a driveability concern with my LS swap, I was looking at the data stream on my scanner. The MAP is showing between 11.2" and 12" at 675 rpm. Thought that was a little low for the cam I'm using, so I put my shop vacuum gauge on the nipple on the plenum. It was registering a steady 19" at the same time. Thinking I had a bad sensor, I replaced it. Guess what! The new sensor reads the same. I know the rear of the plenum is a vacuum source for the power brake booster, so I blocked the vacuum line. No change. Any ideas? As a side note, the truck goes instantly into limp mode when trying to drive. All of my peripherals seem to be okay, diadnostically. Would a bad PCM send a bad ref. to the scan reader, or should I be looking elsewhere?. I've got no vacuum leaks that I can find. Thanks

12-10-2015, 02:45 PM	#2
Hart_Rod ************* Join Date: Apr 2005 Location: Pensacola, FL Posts: 17,858	Re: Wierd MAP reading Is the MAP sensor calibrated correctly within the tune? __________________ Rob - https://www.instagram.com/hart_rod_c10 As Iron Sharpens Iron, So One Man Sharpens Another. Proverbs 27:17 FOR SALE: DBW pedal bracket - http://67-72chevytrucks.com/vboard/s...d.php?t=651123 FOR SALE: Hood Brackets http://67-72chevytrucks.com/vboard/s...d.php?t=679945 1987 Silverado SWB - 34.5K original miles http://67-72chevytrucks.com/vboard/s...d.php?t=801834 1969 SuperBurb - http://67-72chevytrucks.com/vboard/s...d.php?t=200387 1968 Farm truck - http://67-72chevytrucks.com/vboard/s...d.php?t=358692 1968 SWB - http://67-72chevytrucks.com/vboard/s...d.php?t=551258 1948 Chevy - http://67-72chevytrucks.com/vboard/s...=122164&page=3

12-10-2015, 05:30 PM	#3
LeesTruk Registered User Join Date: Mar 2015 Location: Manchester, CT Posts: 433	Re: Wierd MAP reading Can't answer that question. Until I can get some help from someone with HPTune that can read my system, I'm shooting in the dark, here. The initial reflash was to get the truck up and running with a base tune, but the truck can't even do that. It will idle all day long but will go into limp mode as soon as I try to drive it. I have switched voltage to the trans. and I can ground each shift solenoid manually (PCM connectors removed) and hear them operate, point to point wiring is good, with no shorts/opens/grounds on all of the operating devices. And, no DTC's... I may have someone knowledgeable coming over soon to help, that has Tuner knowledge. Keeping my fingers crossed that this actually happens

12-10-2015, 07:15 PM	#4
aknovaman Registered User Join Date: Aug 2013 Location: Tecumseh Posts: 355	Re: Wierd MAP reading I am confident that Mike can fix it using hp tuners, patience __________________ Current project 1959 Willys = GEZR TRUK Geezer is a Latin compound word made up of two roots. Gee meaning Urine Stain and Zer meaning your pants are open! FMCDH GBU and GBA http://www.youtube.com/user/aknovaman/videos

12-10-2015, 09:31 PM	#5
LeesTruk Registered User Join Date: Mar 2015 Location: Manchester, CT Posts: 433	Re: Wierd MAP reading AK, you have NO idea how appreciative I am to have Mike come over. I'm running out of hair to pull out....

12-10-2015, 10:31 PM	#6
aknovaman Registered User Join Date: Aug 2013 Location: Tecumseh Posts: 355	Re: Wierd MAP reading It is a very long 5 minute drive for him to travel to see you............ __________________ Current project 1959 Willys = GEZR TRUK Geezer is a Latin compound word made up of two roots. Gee meaning Urine Stain and Zer meaning your pants are open! FMCDH GBU and GBA http://www.youtube.com/user/aknovaman/videos

12-10-2015, 11:26 PM	#7
clinebarger Senior Member Join Date: May 2009 Location: Fort Worth, TX Posts: 6,370	Re: Wierd MAP reading Your MAP sensor readings are very close to your Gauge readings.... Converting your Gauge reading to Absolute pressure reading that the Data Stream is reporting... 29.92-12=17.92

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