94 sierra vacuum problem?

Smitty94 · 10-10-2009, 07:54 AM

im new to this forum, but i spent hours searching the archives and couldnt find an answer to my problem. i have a 94 gmc sierra with a fresh 355, the problem i am having is that its throwing a map sensor high voltage code, and i have almost no vacuum pressure. but when i unplug the map sensor all my vacuum returns. i plugged the line running from the tbi to the map sensor and hooked a vacuum pump to the map sensor and set it at 10psi vacuum and it idled fine, but no vacuum still.

Kennyray · 10-10-2009, 02:20 PM

do you have the line hooked up to the right port. another posibblty is that the port is plugged, use a small drill bit or wire to clean it out. let us know how it goes.

Smitty94 · 10-17-2009, 11:45 AM

well all the ports are clean, ive come to the conclusion that the cam is to big, it has 7 pounds of vacuum at an idle, soo here comes the vacuum canister, one off of an s10 gives me 2 pounds of vacuum...but now i need to know how vacuum it needs at an idle...i was thinkin somewhere around 15

gmctrucks · 10-17-2009, 11:58 AM

What kind of cam did you put on it write down the spec's also what else did you do to the engine? Wasn't the cam made just for your truck or are you trying something new? I once placed a cam from Chet Hubbard from Cali on my Firebird it sure came out nice with my other add-ons one could hear the cam just a little bit which I wanted.

Smitty94 · 11-06-2009, 03:18 PM

sorry its been awhile to respond works been wierd lately i tried a new cam its a comp cams .470 int .470 exh. and 224 duration @.05 110 degree lobe seperation, ive got world product sportsmans 2 heads that were originally 64cc combustion chambers, but have been decked down to a 57cc. eagle crank 3.48 stroke with .030 over flat top pistons. i seen an ad for rhodes lifters that gain atleast 5" of vacuum that i might try. i talked to jet performance, but they said that a tune wouldnt help.

Smitty94 · 11-06-2009, 03:26 PM

o i forgot to mention the the vacuum at an idle is erratic, it bounces around between 5 and 10" of vacuum. i am starting to think that the cam is too big, im considering a lt1 stock cam for it.

10-10-2009, 07:54 AM	#1
Smitty94 Junior Member Join Date: Oct 2009 Location: mi Posts: 4	94 sierra vacuum problem? im new to this forum, but i spent hours searching the archives and couldnt find an answer to my problem. i have a 94 gmc sierra with a fresh 355, the problem i am having is that its throwing a map sensor high voltage code, and i have almost no vacuum pressure. but when i unplug the map sensor all my vacuum returns. i plugged the line running from the tbi to the map sensor and hooked a vacuum pump to the map sensor and set it at 10psi vacuum and it idled fine, but no vacuum still.

10-10-2009, 02:20 PM	#2
Kennyray Senior Member Join Date: Oct 2005 Location: Birmingham, AL Posts: 895	do you have the line hooked up to the right port. another posibblty is that the port is plugged, use a small drill bit or wire to clean it out. let us know how it goes. __________________ None of the secrets of success will work unless you do.

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10-17-2009, 11:45 AM	#3
Smitty94 Junior Member Join Date: Oct 2009 Location: mi Posts: 4	well all the ports are clean, ive come to the conclusion that the cam is to big, it has 7 pounds of vacuum at an idle, soo here comes the vacuum canister, one off of an s10 gives me 2 pounds of vacuum...but now i need to know how vacuum it needs at an idle...i was thinkin somewhere around 15

10-17-2009, 11:58 AM	#4
gmctrucks Senior Member Join Date: Nov 2006 Location: Houston,Texas Posts: 4,645	What kind of cam did you put on it write down the spec's also what else did you do to the engine? Wasn't the cam made just for your truck or are you trying something new? I once placed a cam from Chet Hubbard from Cali on my Firebird it sure came out nice with my other add-ons one could hear the cam just a little bit which I wanted.

11-06-2009, 03:18 PM	#5
Smitty94 Junior Member Join Date: Oct 2009 Location: mi Posts: 4	sorry its been awhile to respond works been wierd lately i tried a new cam its a comp cams .470 int .470 exh. and 224 duration @.05 110 degree lobe seperation, ive got world product sportsmans 2 heads that were originally 64cc combustion chambers, but have been decked down to a 57cc. eagle crank 3.48 stroke with .030 over flat top pistons. i seen an ad for rhodes lifters that gain atleast 5" of vacuum that i might try. i talked to jet performance, but they said that a tune wouldnt help.

11-06-2009, 03:26 PM	#6
Smitty94 Junior Member Join Date: Oct 2009 Location: mi Posts: 4	o i forgot to mention the the vacuum at an idle is erratic, it bounces around between 5 and 10" of vacuum. i am starting to think that the cam is too big, im considering a lt1 stock cam for it.