Engine Vacuum ports and how they work

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24 Aug 2013 18:44 #15699 by 928mac
I am still chasing poor idle, running, drivability issues after my conversion to the electromotive Tec GT system that I bought from Carl at 928motorsport. Not trying to blame anyone.
I have had this over fueling condition/poor MAP reading since the conversion so its not bent valves

The symptom is a unconventional Map sensor reading.
I am sure the electrical is correct and I am using a 1 bar Map.

At first I thought it was the flappy acting up which I can set to any rpm with the lap top, so I was checking and testing the solenoid and flappy when I found that I had low vacuum in my line that I feed from under the intake

What I found is at an Idle my vacuum is low at about 11 Hg and stays low as I bring the Rpm up untill about 1500 and then it jumps to around 21 and will fluctuate normally between 16 and 21 depending on the engine load or throttle position.

There are 2 ports on top of the throttle housing under the intake and I was using the forward port to supply manifold vacuum, is this not correct?
I checked the rear port and it is very low or zero until the throttle opens and it slowly climbs. That one I was using on the tank/emissions vacuum pot on the passenger side.

I did not have a large enough Tee or connector to check at the brake booster.

I have no MAF sensor and the system works off of a MAP, a air intake temp sensor and the TPS.
With low Vacuum the map reads high and over fuels.

Thanks in advance for you comments

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24 Aug 2013 23:37 #15702 by the flying scotsman
The port that should be used on top of the tbody is the drivers side.....the other port is for the air pump diverter valve.

The ports are covered or not as the case may be by the position of the throttle plate which is set by a screw and lock nut. If not set correct you'll have the symptoms you describe until the plate uncovers the tbody hole and diverts vacuum to the port.

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25 Aug 2013 00:10 #15703 by 928mac

The port that should be used on top of the tbody is the drivers side.....the other port is for the air pump diverter valve.

The ports are covered or not as the case may be by the position of the throttle plate which is set by a screw and lock nut. If not set correct you'll have the symptoms you describe until the plate uncovers the tbody hole and diverts vacuum to the port.


Thanks Malcolm.
I only see the two on top, is there 1 on the drivers side of the throttle housing
Dam I may have to pull the intake.
This would explain why no matter what I do, it wont run right across the range.
Its either great mid and top end or great up to 1/4 throttle.
1st two months it was the wrong map supplied.

edit
Ok I just reread what you wrote.
I am using the port on top that points back and toward the drivers side.
The other one points forward toward the passenger side.
so what is my next step?

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25 Aug 2013 07:13 #15706 by the flying scotsman
if you know the ports are clean pull the intake and rebuild the tbody with double Oring brgs and reset the throttle plate position.

a search of RL will find a thread where I was involved at length helping a member complete a similar project

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