Few 360 TBI problems

So over the winter i installed a howell tbi kit on my 360. It runs alright but i have been having a few problems. Cold it runs pretty rough about the same as my carb did. Once it warms up it seems to idle fine. When driving and I come to a light or stop sign the idle seems to surge some for a few minutes. Also if you rev the engine the rpms take a second or two to come back down. This can be annoying when shifting through the gears. I think that i may have a vac leak but i cannot seem to find anything. How likely could it be that my intake is leaking in from inside the valley area into the runners. It is an eldelbrock performer intake. But the surging idle and the revs that dont come right back down seem to be my biggest issues that i cannot figure out and could use some guidance on.

Thanks
Pat

The problem I had with my Howell was that the throttle plates were closed too far from the factory. That caused the IAC to always be open (this creates a whooshing sound, like a vacuum leak). The fix was to pop off the cap over the throttle plate adjustment screw (front driver side of the TB), unplug the IAC, close the IAC with a manual tool (Mine is a Thexton THX 398 ), and adjust the idle to 500 RPM with the throttle plate screw. Then plug in the IAC and let the computer raise the idle to 750.

Worked like a champ for me. The tool is expensive, around $150-250 depending on brand, but I already had it. Maybe find a GM dealer or older shop that has one to help you.

Bob
tufcj

i adjusted it per howells instructions. jump A and B on the diagnostic connector and turn on the key, this closes the IAC and then set the idle. If i did this wrong think that could be causing the surging idle and revs hanging up?

Did you set the idle by adjusting the position of the throttle plates? Mine was slow to return to 750 RPM idle before the adjustment, I don't remember any surging. The only time it still idles up is when I lug it for an extended period while 'wheeling. Once the load is gone, it takes 10-15 seconds for the idle to stabilize. That's because the IAC opens trying to hold 750 while lugging it down.

It could be a bad input from the coil also, where it can't get a good sense of spark, so it doesn't know it's RPM.

Bob
tufcj

ill have to try the adjustment again tomorrow and make sure it closes the IAC all the way. It has that same slow return to idle like you mentioned. Im getting my coil input from the same connector that my tach attaches to on my HEI it seems to work alright.

well i tried adjusting the IAC again today and when the iac is disconnected and closed all the way i dont seem to get the problem with the slow return to idle. I reset my idle and took it for a test drive and the problem seemed to come back again. I keep thinking vac leak, but not really sure anymore, maybe some sort of timing issue with my advance or something?

Even if the TB is brand new, it doesn't mean there isn't a problem with the IAC. If you get a rushing air sucking thru a hose sound, the IAC is open. If it's slow to close, it could have a bad stepper motor. Could be a vacuum leak somewhere else, but if it goes away with the IAC closed/disconnected, my money is on the IAC.

Bob
tufcj

I have a old IAC sitting around ill have to see if that one is any different, may pick up another one tomorrow as well. Im off work so ill have some good time to mess with it.

Even when i was running my carb i have a rough idle when cold. Once it got hot it would smooth out. I always guessed i had something leaking that would seal itself up once it expanded. What are the chances the intake valley gasket is leaking and im sucking air in from the crank case.

Thanks for the help so far.

well i messed with this thing again today, been to busy with work. Anyhow I still cannot get rid of this problem. If i unplug the IAC it revs just fine. just getting frustrated with this thing. The idle does seem to surge some times, which still leads me to a vac leak, which i have not been able to find. Could i be getting a leak in the valley gaskets?