MP inlet plenum

scskydivin

SCCoA Member
I installed a new MP inlet plenum on my 94 auto sc a couple weeks ago. Aside from those pesky EGR bolts, the installation was fairly simple.

While I had the supercharger off, I went ahead and drained the old sc fluid, cleaned the case up pretty good, and filled it up with almost 2 tubes of new fluid that I got from Supercoupe performance.

I properly hooked up all the vaccum lines, put new gaskets on the MP EGR adapter, the EGR itself, and the throttle body, resealed all the supercharger parts (rear at plenum, top, and the 3 intercooler tubes) with black Fel-Pro gasket maker.

Before I put on the MP plenum, with traction control off, I could spin the tires quite freely at takeoff, with the stock inlet plenum.

Now that I've got my 1st Magnum Powers part on my car, I was really excited to see, and feel, how great my car really was.

I've been driving it to work everyday now for 2 weeks, and it won't spin the tires at all from a dead stop. Not even a chirp! It takes a small moment to get wound up, to about 15 M.P.H. Almost sounds like it's bogging down, and then it LAUNCHES like a bat outta hell!!! Feels like my head is gonna fly back to the rear window deck & spin hops the wheels almost sideways when it shifts into 2nd gear. Baaaarrrrk bark bark bark bark! Very nice. It's just that there's NO performance off the line. It kinda sucks.

Any thoughts?


Otherwise, the car is running fine.
 
Are you seeing the normal amount of boost on the gauge when you hit it? Sounds like the vacuum hose going to the fuel pressure regulator may be unhooked or in the wrong location. It should be coming off the return plenum, not the MP inlet plenum.

David
 
Somehow or other you've generated a tune issue. Time to get a QH and figure out what it is. There is nothing inherent with the MP plenum that would cause a loss of low rpm power.
 
Somehow or other you've generated a tune issue. Time to get a QH and figure out what it is. There is nothing inherent with the MP plenum that would cause a loss of low rpm power.

David,

What about unplugging the battery overnight, and forcing the computer to re-learn everything? Not sure if it'd work, but worth a try. :eek:

-Corey
 
David,

What about unplugging the battery overnight, and forcing the computer to re-learn everything? Not sure if it'd work, but worth a try. :eek:

-Corey

Within a short time he'll be back to where he started if that is the problem which I doubt. The one thing to understand about the computer relearning, is that it is always learning and adapting. Its not just a thing that happens over a couple of startups and then stops.

If you are having to rely on "resetting" the computer to have the car run right, then you have problems with either mechanically or the tune, or both. If all is well, I should be able to go to your car and force open loop 100% of the time with no adaptive, and the car runs near perfect.

Fraser
 
The battery was disconnected for a week while I replaced the timing cover, ECRM, radiator and, the plenum. I checked the vaccum lines and everything seems good & tight. The car is running and idling fine around town and runs right down the freeway. Just don't have the low rpm power anymore. As far as creating a tune issue, How did I do that by installing the plenum? My garage is up to date. And I've never had a tune done. I'd like a tune if I need one, but if I have to do it by myself, I haven't the slightest clue of where to even start.
 
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Are you seeing the normal amount of boost on the gauge when you hit it? Sounds like the vacuum hose going to the fuel pressure regulator may be unhooked or in the wrong location. It should be coming off the return plenum, not the MP inlet plenum.

David

My boost from a dead stop starts at 5-8psi w/o much power. When it gets going (we're talking 1 or 2 seconds), it's 12psi & works it's way up to 15psi quickly and holds it there.
 
My boost from a dead stop starts at 5-8psi w/o much power. When it gets going (we're talking 1 or 2 seconds), it's 12psi & works it's way up to 15psi quickly and holds it there.

How much fuel pressure are you seeing with engine off key on ? How much pressure with engine running at idle ? Is the pressure holding after the fuel pump shuts off ?

David
 
How much fuel pressure are you seeing with engine off key on ? How much pressure with engine running at idle ? Is the pressure holding after the fuel pump shuts off ?

David

Feul pressure:

Before engine start key off: 0psi

engine off key on : 36psi

engine idle : 35psi

engine off after idle : 36psi
 
Assuming that the fuel pressure gauge you are using is accurate I would think there is something is going on with the fuel system. It should be at 32.5 at idle and 39.5 with the engine off.
 
Assuming that the fuel pressure gauge you are using is accurate I would think there is something is going on with the fuel system. It should be at 32.5 at idle and 39.5 with the engine off.

The guage I'm using is mounted on the feul rail. I bought it from supercoupe performance. It is quite small.
 
Fuel pressure should be consistent with your boost gauge, lower than 39 when in vacuum (roughly 1psi for every 2" of vacuum). If you are seeing the same pressure when running as when not running, then you have probably not connected the vacuum reference on the FPR. It is dangerous to your engine to attempt to drive the car without a functioning boost/vacuum reference FPR.

Check that it is connected to the back of the return plenum and that there is no leak there.
 
Drove the car 23 miles to work today. 19 of it on the freeway. My odometer moved zero miles. Jeez, if it's not one thing, it's another. Anyway, my previous gas mileage calculation was wrong. Considering my odometer's not working. The speedometer is working fine, so far.

I read the feul pressure when I got to work, It was 40psi on the nose. When I shut it off, the pressure was 35psi, read to the best of my ability.
 
Drove the car 23 miles to work today. 19 of it on the freeway. My odometer moved zero miles. Jeez, if it's not one thing, it's another. Anyway, my previous gas mileage calculation was wrong. Considering my odometer's not working. The speedometer is working fine, so far.

I read the feul pressure when I got to work, It was 40psi on the nose. When I shut it off, the pressure was 35psi, read to the best of my ability.

Sounds like the vacuum hose between the fuel pressure regulator and the return plenum is leaking or disconnected. 40 psi is what you should be seeing with engine running at idle and vacuum hose disconnected from regulator.

With vac/boost refrence hose connected to regulator and 20 incles of vacuum, you should only be seeing around 32 psi. This means your observed pressure is too high for idle, and too low for boost. The bigger concern, is that fuel pressure is supposed to increase pound for pound with boost. If that isn't happening it will bog when first hitting the throttle, then take off pretty hard when rpms increase, but without the additional fuel pressure the engine will go very lean at WOT.

STAY OUT of BOOST until you verify that fuel pressure is increasing under boost.

David
 
Sounds like the vacuum hose between the fuel pressure regulator and the return plenum is leaking or disconnected. 40 psi is what you should be seeing with engine running at idle and vacuum hose disconnected from regulator.

With vac/boost refrence hose connected to regulator and 20 incles of vacuum, you should only be seeing around 32 psi. This means your observed pressure is too high for idle, and too low for boost. The bigger concern, is that fuel pressure is supposed to increase pound for pound with boost. If that isn't happening it will bog when first hitting the throttle, then take off pretty hard when rpms increase, but without the additional fuel pressure the engine will go very lean at WOT.

STAY OUT of BOOST until you verify that fuel pressure is increasing under boost.

David

Sounds like I get to spend another weekend under the hood :(. I'm wearin out.... Guess I need to pull the sc off & really inspect the vaccum line from the feul pressure regulator. So that I'm clear on this, The return plenum is the one that goes into the rear of the intake after the bottom IC tube, correct? Or is it at the short rubber hose that connects the previous mentioned piece to the MP plenum with a little throttle body like job on it? The more we talk about it, the more confused I'm getting :rolleyes:. I have been running the car with the feul pressure at those readings for 30,000 miles. I had no idea that was an issue....
 
Sounds like I get to spend another weekend under the hood :(. I'm wearin out.... Guess I need to pull the sc off & really inspect the vaccum line from the feul pressure regulator. So that I'm clear on this, The return plenum is the one that goes into the rear of the intake after the bottom IC tube, correct? Or is it at the short rubber hose that connects the previous mentioned piece to the MP plenum with a little throttle body like job on it? The more we talk about it, the more confused I'm getting :rolleyes:. I have been running the car with the feul pressure at those readings for 30,000 miles. I had no idea that was an issue....

You don't need to pull the supercharger. Just locate the fuel pressure regulator, then make sure it has a vacuum hose attached to the top of it. Trace the vacuum hose to make sure the other end is connected to the return plenum.

Yes the return plenum is the casting that bolts to the intake manifold and couples to the intake plenum at the bypass valve with a short hose and a couple clamps. It also connects to the lower IC tube. The back side of the return plenum (near firewall passengerside) has a nipple for attaching the fuel pressure regulator vacuum hose. If the hose is connected on both ends, remove it to inspect for cracks, kinks or a clog in hose.

Plan B, just bring the car to my house and I'll help you check it out.

David
 
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