Bad IAC valve?

[ponkotsu]

This seems simple, but just want to ask about it. I realized that my engine RPM is slow to come down to a normal idle speed. If it's in P or N and I rev it up to 2500, it stays there a couple seconds and comes down slowly, and in steps. Even in normal driving if I come to a stop it will step down, 1000k 750, 600. So I was thinking about the IAC valve. I disconnected the plug and tired it. The engine will idle very slow, barely at all, but with it unplugged if I rev the engine to 2500 it drops back down to 500rpm almost immediately. Does this test pretty well confirm the IAC valve is bad?

This is a brand new Hitachi IAC valve so I never even stopped to think it would be bad but I've heard these have a kind of high failure rate. If you read on my other thread I've been having a heck of a time with a rough idle issue. Starting to think this is it.

 

[ponkotsu]

Gotta be an intake leak even though I used brand new Felpro gaskets.

Watched some great how to videos. Tested the IAC with my scanner software. Seems like the engine goes into an intermittent major vacuum leak.

Sometimes when I unplug the IAC the engine chugs down like it should. Sometimes it doesn't change.

I noticed that the IAC duty cycle was 21%, which is actually ZERO by Ford standards. (This tripped me up on earlier diagnosis). Desired RPM was 624 and actual was 650 or so when it should be 400-500. So I unplugged the IAC, and no change in idle at that time. Pinched off the bypass hose for the IAC valve and very slight change in RPM. IAC is closed and functioning properly. So I went through and systematically pinched off every vacuum hose and am confident there is no air getting pulled in from any of the hoses on the upper manifold. I even went around with a listening tube and can't find anything.

Then I started tweaking lower manifold bolts, and what do you know, the RPM moved! Looks like there is a problem with the manifold gasket after all. Hard to imagine the gasket is leaking because it's such a great gasket. So I'm thinking I have warped sealing surfaces, or a crack somewhere. Not supposed to use RTV with these gaskets, but there is an exception to every rule, and I've actually heard experienced mechanics say it's actually a good idea in some cases.

This engine does have that bizarre lower intake manifold with some shafts running through it for 8 butterflies. Possibly the leak is there...

 

[ponkotsu]

Seems that it was the intake leaking still. I used the good Felpro gaskets and cleaned it properly. Today I reinstalled with some blue RTV and it's running way better. Still just a bit of roughness for a couple seconds when I come to a stop while the engine is finding it's happy place. But the engine is fairly promptly going down to 600rpm and idling very smooth. I'll give it a few days and see if the ECM learns the settings and smooths out completely. Could be the intake is still leaking and I might need to be more agressive with the RTV. I suspect the mounting surfaces aren't flat. Its aluminum block, heads and intake, so there could be some warping after 150k miles. Looking at the Felpro gasket it's kind of like two half o-rings one on each side. I can see that if it's not getting compressed enough the intake vacuum could suck it in and create a leak.

 

[Number4]

You didn't mention the engine size. On the V8, at idle and warmed up, I'm pretty sure the RPM is 500 - 600.

 

[ponkotsu]

I'm pretty sure I've got the intake sealed up tight now. Much improved but still some issues.

IAC count runs anywhere from 22 - 35 at idle, which according to Ford diagnostics is normal. When IAC count is at 22 and I disconnect it, the idle doesn't drop much. It will idle around 550 - 600. I've heard that 400-500 is normal, but that is a grey area for sure.

With the IAC connected if I rev the engine the R's seem to come down too slow and it runs kind of rough at times. Seems like it comes down in steps and gets rough at each step. Monitoring the PID it seems like the computer is commanding it to go down.

Idling in Drive it's usually very smooth so that kind of rules out any remaining vacuum leaks. It's generally only rough when idle speed is changing.

With the IAC DISconnected and I rev the engine, the R's drop back to 550 almost immediately. So I'm starting to think I have a bad IAC after all.