3400 TB swap IAC mod for 3500 plenum etc

[IsaacHayes]

When swapping a 3400 TB (56mm) onto a small port 3100 you gotta mod it so the IAC passage gets air to the motor. (You will also need to port the plenum to take advantage of it, but better to upgrade the whole intakes).

Same deal when using a 3400 TB on a 3500 plenum with the TB adapter. (you also need to use RTV or cut your own single layer thin cardboard custom over-sized gasket).

One of the pictures below explains it. The other one shows the size difference of the TB plates. Enjoy.

[Guest]

I didn't have to do that when I put a 3400 TB on my small port top end.

[IsaacHayes]

Hope this clears stuff up for anyone looking to use the 3500 plenum and TB adapter with a 3400 TB. You could use a stock 3100 TB but then you sacrifice area in exchange for no modifying.

Also when I transfered my shaft and linkage to the 3400 one from my old 3100, I re-used the washer that held it in. I used a deep dish 10mm socket and tapped it back down. There is some play side to side but the TB plate doesn't touch the inside of the TB. Winding the spring was easy too, one wind is all it takes.

I don't have the 3500 on yet but wanted to get the TB swap out of the way so I know it works fine and there would be no issues and that's one less item to do on the day I do the swap.

I re-used my old 3100 gasket (56mm opening actually) and it had a high idle. I checked for vacuum leaks with lots of propane and brake cleaner, but there wasn't one. The IAC just needed to re-learn since I guess the position is different. After a few starts now it idles right at ~700rpm like stock.

Notice the 52mm plate has a small hole in the center and the 56mm one doesn't. Odd huh?

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Can anyone tell me if this bracket gets in the way with a 3500 plenum swap? What to do with it? Cut it off? It's the pipe that goes to the water pump housing.

[IsaacHayes]

Raven, did your small port plenum have a notch for the IAC chamber? Or did the 3400 TB have it cut out like my 3100 TB shows? I'm wondering if later year small port plenums have the notch?

Still, you gotta do this for the 3500 TB adapter if your 3400 TB is like this one.

[mfuller]

Yes, you have to modify that bracket on the coolant hard line....or at least I had to.

[IsaacHayes]

Did you cut it off or did you bend it out of the way? Or did you bend it and make it work? Thanks!

[bszopi]

On my 62mm TB hack job from, what... 2002? I just ended up cutting that bracket off, and have run it that way since with no issues.

[IsaacHayes]

Ok great! Thanks. I figured it'd be ok if it survived from the tugging I was doing on it to take the TB off without removing the plenum. Where does that hard line go BTW? I assume it attaches to a rubber hose right?

[whitelightnin04MC]

It should be going to the heater core. Mine reattached to my 62mm tb no problem. I'm guessing that this will be one more thing that "just hangs" after I throw a 3500 upper on there.

[IsaacHayes]

Yep the 3500 neck is in a different spot. The bracket has to go! It makes taking the TB off a pain anyways since the stud that goes through to the TB hangs on it when you try to just take off the TB and not plenum first.

[whitelightnin04MC]

Yeah I know what you mean. Heyya, how much does the tb neck move during the swap. I need to make sure my CAI duct is going to route nicely into my cold air box. I'm really close to ordering a new 3500 upper from GMPD. The j-yard by me had one in michigan. It was close to 100 bucks. I don't mind spending the extra 30 for peace of mind.

[Guest]

Honestly I don't recall what the plenum looked like, but I don't recall there being that notch, I have seen it on other plenums though. I'll have to check the TB, I still have that. It idled fine anyway.

[IsaacHayes]

Whitelightining, Did you check car-part.com? I got the upper with shipping for like 80 or something and it had the EGR, TB, MAP, etc all attached still. Call some places there and see what it would be. I think I emailed one and was quoted then when I called I said I had emailed previously and the guy said oh yeah we decided on xx right? (which was lower than the email) and I was like YES! I think I saved like 10 more bucks or something haha.

Raven, Hmm it may be that your 3400 TB IAC passage is styled like the old 3100. What year TB and plenum is it? It may be possible that the gasket thickness is enough to let air through for idle, but that's severely limiting the passage. I'm curious what year my TB is, maybe the older ones had the passage...

[whitelightnin04MC]

I've checked car-part and the couple of places that had one never emailed me back.

[IsaacHayes]

Then call them. Only one place emailed me back and it was the cheapest place. On the LIM I had to get I just went straight to calling.