Hot upper plenum

Steele83

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Jun 24, 2020
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here is the back story I have a 1988 Mustang GT 5speed hatch, I have done a complete EGR delete bypassing the coolant putting in just a delete spacer added a phenolic spacer so that the throttle linkage still worked, Converted to Mass Air calibrated for 24lb injectors, upgraded to 24lb injectors, 70MM throttle body, Cold air intake, Under drive pulleys, all new plugs wires cap rotor and ignition coil. On the test drive from converting to Mass Air I could smell the engine getting hot (temp gauge was not working, currently changed out the sensor) I pulled over shut it off to check things out. as I looked and felt around I noticed that the Upper Plenum was very hot like so hot that couldn't leave your hand on it. at this point I found no coolant flow I removed the thermostat and did another test I let the car idle and again the Upper Plenum gets hot quick nothing else does I can put my hand on the lower intake between injectors and its not that hot neither is the water pump, Valve covers. After shut down I could put my hands on the headers before I could the Upper Plenum. so my question is why would the Upper Plenum get that hot without exhaust running through it? The cold air pipes are not warm it doesn't start getting warm until the delete spacer.
 
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Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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The delete plates don't really block off the EGR gases. They stop the flow into the combustible air, but there's nothing stopping the heat from the spent exhaust gases from transferring up through the lower intake tube and into the plenum. Is it enough to cause the entire plenum to heat up? Hard to say. The EGR port in the lower intake is much smaller than what goes through the upper intake and the spacer. Lack of actual flow will slow down the transfer of heat, but eventually it will travel up.

If you really want to block off the EGR, you need to do it at the heads with intake gaskets that block it, or between the lower and upper intake with a gasket, or razor with the fat edge removed RTV'ed down here.

I had a 93 cobra intake on my car, with the coolant lines removed, and a functional EGR, and a plenum spacer, and my upper intake got pretty toasty. Latest build keeps the EGR intake, but i added the coolant lines back on. When i go megasquirt, i'll delete it for good likely by using the razor blade between the upper/lower method.
 

Steele83

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Jun 24, 2020
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The only part of this answer that stumps me is the fact that the lower intake is considerably cooler if the same spent exhaust gases are going through there shouldn't it be just as hot?
 

Mustang5L5

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Feb 18, 2001
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The only part of this answer that stumps me is the fact that the lower intake is considerably cooler if the same spent exhaust gases are going through there shouldn't it be just as hot?

Not necessarily. The EGR gases only come from one head, in the center, and only travel a short distance mostly vertically though a 3/4” dia port. It might be warm in the immediate area over the port.
 

Steele83

New Member
Jun 24, 2020
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Thank you for your time I will try blocking off this port and see if it lowers my upper intake temps. Have you any experience with Mass conversion? I have completed the install with new sensor and a used A9l computer upon restart that dang thing lopes like it has a cam at idle and is undriveable basically during driving it gets stuck around 2500 RPM and drives itself. if you really try to hog into it she starts spitting and sputtering bad. I check to make sure the mass has power/ground and is sending signal but cannot put my finger on why it runs like crap. I thought some of it may be from the super hot intake gases not atomizing or vapor locking I will know more after I block that port but in mean time any other insight would be great. I have checked all the vacuum lines for leaks part of me feels like the upper plenum may be leaking since it got so hot
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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Yup,

I converted my car to mass air from speed density. I didn't have any issue though as it started right up. I purposely left my EGR intact though.

If you are inadvertently injecting EGR gas into the intake at idle, it will cause the car to run like total crap. Make sure you don't have a vac leak