Mastiff

I just installed a new throttle body from RV Morse. This is a mild engine with high vacuum at idle. The symptom I have is that if I slowly open the throttle to something like 1100 RPM, it'll sort of stay there, or inch down very slowly. With the engine off, the throttle control feels very smooth and it closes firmly. When it's running, I can push it closed lightly and the idle will come down as expected.

My theory is that strong vacuum suction is holding the blades open a bit. Is this a known thing, and if so, is there a solution. Any other theories? My crusty old throttle body didn't have this problem.

DynoDave43

iTrader: (11)

I've seen a lot of vehicle with carbs and throttle bodies. But I've never seen 1 with so much vacuum that it sucked the throttle open. I guess if you had no, or virtually no throttle return spring pressure.

Dewey316

Worn bushings can do stuff, the vacuum can't hold it open, remember that half of the vacuum is acting on the side of the butterfly that would try to close it , it balances itself out.

It also may be that your idle setting is way off, and so to adjust the idle the IAC is having to do huge movements or can't keep up. Go through initial IAC setting and make sure it is close.

Mastiff

Good point on the suction acting on both sides. I guess I'll get out the laptop and see what's going on with IAC and TPS on the new throttle body. I don't expect that there's any issues with the bushings, etc. on this fresh rebuild.

Dewey316

The easiest thing to do, is with the car off so the IAC is parked, unplug it. Then start the car up, and see where your idle is at.

Mastiff

With the IAC unplugged, I think it goes wide open. When I tried it, it was sucking air through IAC port like crazy. I've got EBL, so I just hooked up my laptop and set the throttle screw so it was reading a few % on TPS, and IAC count was around 45-50 at idle. I don't know if that's perfect, but seems like it's in the ballpark anyway.

Dewey316

Iac count is a good thing to watch, I always try to adjust my throttle with the IAC parked, so that the engine naturally tries to idle at about 400-500 rpm, just running, but below even the lowest computer controlled RPM, then the iac should take over but won't have to do major swings.

The big question is, did doing that adjustment help the situation you started with?

Mastiff

Yeah, it seems to be better. In hindsight, I may have panicked early. Maybe it was fine, but the vehicle needed an on/off cycle to get the IAC in a known position. It really did seem like a vacuum thing, because the return springs seemed unable to fully close the throttle, even though I could do it by hand. Maybe this is possible if IAC is doing something crazy. In any case, I seem to be good. Thanks for the help.