RSFreak

My local bone yard has several 3.1 Camaros (and a Firebird). I was thinking about getting a 3.1 upper intake plenum for my 2.8 mainly so I could delete that plastic IAC tube. Will my 2.8 IAC work with the 3.1 upper?

RSFreak

20 views with no responces? Someone must know the answer to this.

Drew

iTrader: (58)

If what you were saying made any sense at all, you'd probably have more replies.

The plastic iac tube? Last time I checked the MPFI 2.8 uses the same style IAC and TPS as most TPI cars, while the 3.1L uses the later style compact IAC and TPS. Neither configuration uses a plastic tube for the IAC.

Theoretically, a 3.1L intake plenum would work on a 2.8L, but there would really be no advantage since the plenum really has nothing to do with the IAC. It does however have to do with the EGR system on the 3.1L. If you wanted to change IAC styles from early to late you would want to change the throttle body, not the plenum. Incidently both early and late style TPS/IAC use the same wiring, just different plugs. Clip and splice the late style plugs into your harness and you could plug and play, but honestly I still can't see any reason why you'd want to do that.

Perhaps you should try posting a picture of what you're talking about, or consult a service manual for your car and figure out the proper terminology.

RSFreak

Drew, perhaps you should do a little more research before flaming me. I am very familiar with the LB8 but not so much the slight, but important differences in the LH0.

On the LB8, YES, there is a PLASTIC tube from the throttle body (from the IAC housing to be exact) which runs under the upper intake manifold and into a fitting (the same fitting used by the PCV valve) at the rear top of the lower intake manifold. When the IAC valve opens, engine vacuum pulls air through this tube into the intake manifold. The 3.1s use a slightly different IAC setup which eliminates this tube. Perhaps the only difference is the throttle body. I don't know (which is why I asked). Perhaps I should have asked, "is the 3.1s IAC setup compatable with the 2.8?" or "will the 3.1 throttle body work with the 2.8 intake and retain IAC functionality?" Since I know how the 2.8s system works, but not the 3.1s I thought I might find someone on here who would know the answer.

Instead I got you.

Street Lethal

iTrader: (16)

I'm tryind to draw myself a mental picture of your throttle body and intake manifold. On mine throttle body, the IAC housing has to vacuum lines leading to the fuel pressure regulator, as well as the vapor canister. Ahead of the throttle body (on the rubber intake tract), there is a plastic line that leads to the valve cover (as you already pointed out, the same type of fitting for the PCV)...

In any event, swapping manifolds wouldn't cause a problem at all... so long as your PCV is still functional. From one of the diagrams that I'm looking at it right now ('89 2.8), there is nothing computer related over by the rear upper side of the lower intake half. So long as it gets plugged, you'll be fine, however, idle might be affected slightly, so it'll need to be reset.

RSFreak

Thanks Street Lethal. Attached is a pic of my intake. The large black plastic tube on the pass side of the TB coming off the IAC housing at about a 35* angle and running under the upper intake is what I am referring to (see arrow). The 3.1s don't have this. But can I just swap the TB as the IAC air bleed is obviously routed differently? Or is the 3.1s manifold a part of that system as well?

madathlon

Thats the PVC tube.. And you need it if you are in a smog controlled area..

RSFreak

Interesting, 'cause if you block it off, the engine will not idle.

BTW, as you can probably see from my pic, I have no smog equipment on the car.

madathlon

Ya.. the IAC pulls its air from the Block.. if you follow it back it goes to the driverside valve cover.. My PVC tube got clogged up and it would start.. took me days and a lot of hair pulling to figure it out.. And on the smog stuff.. I didnt know if U had it or not.. but the IAC does need that to run for some stupid reason.. And I dont know ur smog laws up there.. That way I said "if" ur in a smog controlled area..

RSFreak

Actually, the PCV valve (on drivers side valve cover) and IAC are attached to the same vacuum source on the intake manifold. However, that tube from the IAC to the intake is not part of the crankcase ventilation system. The IAC doesn't pull air from the block, rather vacuum from the intake pulls air in through the IAC valve to keep enough air in the intake to keep the engine running when the throttle body is closed. I purposely blocked off that tube once and started the engine just fine but the engine would not idle without the TB being cracked open slightly. Thus, your post confuses me a bit.

madathlon

Diffrent year may have a slighty diffrent set up// mine runs right to the valve cover.. Im also in Ca and have a CA smog set up..

RSFreak

The '88 LB8 should be the same as mine, though California car may be different(?) Its not easy to see, but the tube from your PCV valve connects to a "Y" branch at the rear of the intake manifold. One branch of the "Y" goes to the IAC tube and the other connects to a vacuum port on the intake manifold. Other branches off these tubes go to various emissions controls, but those are long gone on my engine so I have no point of reference.

Street Lethal

iTrader: (16)

My 3.1's manifold has no affiliation with the throttle body in that particular way whatsoever. Other than the two vacuum lines previously mentioned (regulator, and canister), the IAC bases it's incoming reading solely through throttle bore. Bolting the 3.1's throttle body to your 2.8 shouldn't be a problem, as long as the idle is re-tuned afterward.

When you cracked open the throttle blades, was this done by hand, primarily? Rather than immediately swap to a 3.1 throttle body, I would first consider taking apart the stocker to see if the IAC housing (internally), has a passageway leading to the throttle blades as well (which I would imagine that it definitely does). I'm assuming that your engine wouldn't hold it's idle (after you plugged that line) because the idle screw needed to be raised.

If that is the case, I'd remove that plastic line, plug up the vacuum fittings associated with it, raise your idle speed manually by hand, wait for the engine to reach normal operating temperature, then re-set it's idle...

If I'm misunderstanding the query, please elaborate for me once more...

Street Lethal

iTrader: (16)

Although I understand the theory here, I still have to question it's principle though. If the engine stalls with the disconnecting of the plastic line (during idle, with it being plugged), this would indicate that the throttle blades are almost completely shut. Which I find very strange. Mainly because idle shouldn't be affected at all in this instance (remember, all air enters through the throttle blades, regardless if some of it is delivered through the IAC, or all of it directly through to the intake manifold, the same amount of air is entering the combustion chambers either way)...

Ultimately, IAC settings are predetermined from the factory (much like carburetion). Once we start plugging vacuum lines, idle speed immediately needs to be recalibrated. Controlled idle speed is determined by the factory based on total air flow including throttle valve, PCV, IAC passages and calibrated vacuum leaks. If one of these are altered at any given time, the engine gets thrown for a loop (because of the predetermined settings), and will more than likely stall...

However, the engine can simply be retuned to compensate for the changes. I think you stalled, not because you weren't getting enough air into the engine, rather, because it through the ECM for a loop (much like when we raise, or lower a controlled idle screw with a screwdriver, the ECM get's very frustrated). Once that plastic line was removed, you would have needed to reset your idle speed immediately...

RSFreak

I comprehend everything you said except:

This is not true when the engine is idling. Think about it, how does the engine idle when your foot is not depressing the gas pedal (ie, throttle blade closed)? The air that the engine requires to idle enters the IAC through a passage in the TB to the right of the throttle blade (see arrow). Apparently the 3.1 IAC deposits this air on the other side of the throttle blade, it kinda does an end-around and bypasses the blade. The 2.8 IAC deposits this air into the afore mentioned tube and into the intake manifold.

Street Lethal

iTrader: (16)

You are absolutely right in saying that, as I didn't explain that part properly. I was so tuned in to the concept of the IAC only measuring air flow based on vacuum pull from the intake manifold (which is why I had your explanation confused initially, as I was confused by the concept of the engine pulling air through the IAC from a non-prevalent opening), that I didn't realize you were aware that air was still being pulled in through the front of the throttle body. When I stated throttle blades, I honestly meant the front of the throttle body. Thats why I was confused. I was saying to myself, if the IAC only has a passageway leading to the intake manifold, then the engine would immediately stall with the throttle blades being completely closed. I misunderstood your explanation...

RSFreak

Its all good.