djsquare

Long story short I port and polished my throttle body which included filling the ports and drilling them out. I drilled out the IAC port larger to eliminate any idle issues. I also removed my air box fins and baffles as well as removed my rail and injectors, sensors and EGR and gave them a nice clean.

After putting everything back together, I started her up and it idled perfect with much better throttle response. I shut it off and turned it back on only to get a fluctuating idle +-200RPM.

If I'm coasting the RPM will be high around 2000RPM until I come to a complete stop, then it lowers and starts to fluctuate again.

So it idles and runs fine every first start or if I dont start it for a while, but if I turn it off and start it back up, the idle will fluctuate up and down.

I know it's the IAC, I just can't figure out why it's doing this.
I tested the IAC and it does move when cycling the ignition on and off.
Check for vac leaks with carb cleaner, none to be found.

I did a computer relearn and a TPS and IAC relearn but still the same. It is getting slightly better with driving but wont go away. All the sensor readings look fine on the scan tool.

Any suggestions?

Buzzmanb12

iTrader: (11)

Sounds like a slight air leak after the MAF to me, check all your air intake tubes and hoses.

djsquare

I thought it was a leak but sprayed carb cleaner over everything really good and no changes in RPM. Wouldn't a vacuum leak cause it to idle funny all the time though?
It runs completely fine first start and will run fine until I turn it off and back on.
I'll keep searching for a possible leak, but it doesn't seem like a leak to me.

gMAG

iTrader: (1)

I had the exact same issue (without doing the TB r&r). The cause was the IAC. I reused the old, stock one and it worked like a charm.
Could some material from the IAC port cleaning have gotten stuck there?
Maybe this was just a coincidence and the IAC went bad.

djsquare

So I went back and had a good look at the engine and managed to find a spring. The spring is from my IAC sensor. I was testing it off the TB yesterday and it popped apart while testing it, must have not noticed the spring. Putting the spring back in made no difference buuuttttt......

I went at it again with the carb cleaner and found a small leak at the IAC sensor!!! Removed it and low and behold there is no O-ring. I really hope this fixes my issue, i'll give an update when I get an O-ring after work from Lordco.

djsquare

Turns out that was not the problem. Couldn't locate any more leaks. Did a vacuum test and it was a steady 10.5 in.hg which according to my mityvac vacuum tester it should be around 20in. hg.
Also made the TB port holes a bit bigger but turns out thats not the issue.
Still trying to figure it out..

Darth_V8r

iTrader: (4)

Did you end up creating a gap around the throttle blade with your porting? With the blade fully closed can you see light around it?

djsquare

Nope, I stayed a half inch away from the blade.
I shouldn't have to dig too deep as the car was fine before I did the TB P&P.
I did have mice in my engine over the winter so maybe they chewed through something, I'll check all my vacuum lines and wiring when I get a chance.
The only thing related to the P&P that I can think of is not making the IAC port big enough, but I tested it by unplugging the IAC and pushing it in and idling. The idle was around 2000+rpm so it cant be that.
Man I'm confused!

djsquare

I did investigate the wiring and vacuum lines better just now and found a few things! All the sensors seem to be reading so my guess is my wiring is fine.
My IAC is showing at 0 for some reason on my scanner.... And no reading for my TPS unless its a reading from the intake manifold.

The PCV plumbing at the back is very cracked and soft which is probably creating a vacuum leak. I'll try replacing it soon.

Another leak I found: There's a very thin small line running from the wiring harness to the back of the intake manifold that seems to have holes in it causing another leak. (Probably from the dam mice)

djsquare

Even if there was a bad vacuum leak, the idle should still be fine with these engines though am I right? So that should not be the cause...

Darth_V8r

iTrader: (4)

If you were running a SD tune I would agree with you. But unmetered air is unmetered air if the MAF is connected

djsquare

I replaced my pcv lines and fixed a HVAC line 3 days ago and so far the fluctuating idle has returned once. I'll have to do another vac test to see if I have better numbers now.

Darth_V8r

iTrader: (4)

Sounds like progress anyway. Idle surge is a real pain in the ***

djsquare

It's a weird issue. Only happens at operating temperature and never on the first ignition cycle. Only happens when you've got it warmed up and you shut the car off and turn it back on. Even then it happens rarely, but it still happens. It's much better now as the surge is only about 100 RPM.
Last week I was getting groceries and started it back up only to find the surge again, came to a stop light and shut it down for 30 seconds and started it back up and the problem went away. SO WEIRD! But happy because it rarely happens now.

djsquare

I know this post is ages old, but I still refer back to LS1Tech for help.
The problem was fixed about 2 weeks after my last post.
I think the IAC/CPU had to relearn it's values as the TB is different now.

The problem got better and better til it finally went away on its own.

I did have cracked vacuum lines from my EGR and HVAC, fixing those must have helped a lot as well. Didn't find them at first as they were at the back of the engine.

​​​​​​Still using the P&P TB that I did and still working great however I'm not sure I recommend JB welding your TB as it could potentially come apart one day with all the expanding and contracting of the metals. But going around 4 years strong and can't complain. The difference in throttle response is quite amazing. 👌