High Idle

[Aeroking]

Re: High Idle

the ignition module, cap, rotor, wires have about 1500 miles on them.

the truck holds the 1800rpm idle until i shut it off. After a 20 minute drive, when i put it in park (before shutting it off) it goes right back up to 1800. Foot-on-brake in drive, it sits at 1000rpm.

i'll make a DM file tonight and review it.

 

[Aeroking]

Re: High Idle

When you have DM hooked up, be sure and record engine temp, MAP, AFR, O2 voltage, IAC counts, and spark advance. .

what kinda values am i looking for?

 

[Aeroking]

Re: High Idle

ok, double checked my base timing, it is rock solid on 0 degrees.

Datamaster says:

(at idle 1600rpms)

- Spark Advance 22.2
- MAP inHG 12.27
- Vac. inHG 16.57
- IAC pos starts at 35, and promptly drops to 0.
- TPS 0%
- TPS volts .65
- idle RPM 838 (RPM 1625)
- O2 mv hangs around 830

this is on a semi-warmed engine

 

[sly dvl]

Re: High Idle

Your TPS voltage is higher now then it was before. You need to get that down.
Perhaps your IAC motor is dead. Try this:
Ign. on, unplug your IAC Motor, start the engine and allow it to stabilize at whatever RPM it decides, then with the eng. still running plug your IAC back in and see if your idle changes at all.
Since your IAC Counts are zeroing either the ECM is trying to lower the idle using the IAC and it's not responding (cause it's broke) or it is truly zeroed but there's still too much air getting around the throttle plates.
You said you bent the throttle stop arm (why you didn't just back the screw out I don't know) but did you bend it the right way?
Zero counts means zero air is being allowed through the IAC passage - fully extended. High counts mean higher air - retracting.

 

[Aeroking]

Re: High Idle

spark advance is way to high for this cam. i dropped it to 12 degrees advanced, and the idle dropped 400rpms.

i'm gonna keep dropping it


this is the timing it was running

SBC 350s run idle advance around 6 degrees. i'll drop it to 8 and see where i am

 

[WyoSyclone]

Re: High Idle

x2 on Sly Dvl's recommendations..... your timing map looks fine. I would also suspect your ECM is telling your IAC to move and it's not moving. Your vacuum is low but it's probably because your IAC is stuck open and passing too much air.

Your TPS voltage is higher now then it was before. You need to get that down.
Perhaps your IAC motor is dead. Try this:
Ign. on, unplug your IAC Motor, start the engine and allow it to stabilize at whatever RPM it decides, then with the eng. still running plug your IAC back in and see if your idle changes at all.
Since your IAC Counts are zeroing either the ECM is trying to lower the idle using the IAC and it's not responding (cause it's broke) or it is truly zeroed but there's still too much air getting around the throttle plates.
You said you bent the throttle stop arm (why you didn't just back the screw out I don't know) but did you bend it the right way?
Zero counts means zero air is being allowed through the IAC passage - fully extended. High counts mean higher air - retracting.

 

[Aeroking]

Re: High Idle

x2 on Sly Dvl's recommendations..... your timing map looks fine. I would also suspect your ECM is telling your IAC to move and it's not moving. Your vacuum is low but it's probably because your IAC is stuck open and passing too much air.

i've exhausted the IAC tests, and is fine. my problem lies elsewhere.

retarding the timing 20 degrees dropped me 1000rpms off the high idle.

could the distributor be 1 tooth off, and still read 0 degrees static?

for reference, on every startup, the IAC pos starts at 35, then over a period of a 90 seconds or so, it drops to 0. but it starts at 35 on every startup,

the throttle linkage tab was bent so that the blades would close more. whether they were already closed all the way or not, i'm not sure, i can always bend it back if needbe. The bent tab doesnt even contact the throttle stop. if i bend it back, it will rest on the throttle stop.

 

[sly dvl]

Re: High Idle

0 degrees is 0 degrees. even if you're out by 1 tooth or 10 teeth if you're getting 0 then it's fine.
So when you plugged the IAC back in the idle dropped a bit, or it didn't change?
I don't have a lot of experience with the tuning software, but I still believe your problem is something basic. Your desired idle is correct, your IAC counts are 0 so the ECM is trying to lower the idle. It's gotta be something mechanical like a vac leak or a bad IAC.

Your throttle cable isn't holding the plates open is it? It should be slack at idle.

 

[Aeroking]

Re: High Idle

0 degrees is 0 degrees. even if you're out by 1 tooth or 10 teeth if you're getting 0 then it's fine.
So when you plugged the IAC back in the idle dropped a bit, or it didn't change?
I don't have a lot of experience with the tuning software, but I still believe your problem is something basic. Your desired idle is correct, your IAC counts are 0 so the ECM is trying to lower the idle. It's gotta be something mechanical like a vac leak or a bad IAC.

Your throttle cable isn't holding the plates open is it? It should be slack at idle.

The IAC counts begin at 35, then go down to 0 (which means the IAC starts properly, then closes because it thinks the engine is getting too much air, and it closes to try and pull the idle down, but bottoms out on 0, and cant close any further). ( i say "open" and "close", open meaning counts go up, close meaning counts go down, might be the opposite, but the IAC does start at 35, then drop to 0)

the throttle cable doesnt hold the plates closed, the spring on the linkage does.

 

[WyoSyclone]

Re: High Idle

The IAC counts begin at 35, then go down to 0 (which means the IAC starts properly, then closes because it thinks the engine is getting too much air, and it closes to try and pull the idle down, but bottoms out on 0, and cant close any further).

the throttle cable doesnt hold the plates closed, the spring on the linkage does.

The IAC counts that you are seeing are not what the IAC is doing, it's what the ECM is telling it to do.... since the IAC is not successful, the ECM keeps instructing it to drop (the reason it ends up at zero). If the IAC was moving, the idle would be changing.

Here's what the factory GM manual says on high idle.... notice they have a special tool that cycles the IAC through it's movement.

Hope this helps! Keep with it... but go buy an IAC

 

[Aeroking]

Re: High Idle

ok, i'll break down and buy another IAC, this one is only a year old. but i'll buy another, just to factor that out.


thanks for keeping with me guys, aside from the high idle, the truck runs fantastic, and i've tuned it smoothly to 10psi. It pulls solid through the rpms, and feels strong. :tup:

BRAVO to code$59, i love it.

 

[WyoSyclone]

Re: High Idle

ok, i'll break down and buy another IAC, this one is only a year old. but i'll buy another, just to factor that out.


thanks for keeping with me guys, aside from the high idle, the truck runs fantastic, and i've tuned it smoothly to 10psi. It pulls solid through the rpms, and feels strong. :tup:

BRAVO to code$59, i love it.

Well, before you buy another one Brandon, let's look at some facts that we have... your vacuum is low 16.xx. You have put a new IAC on it already. You can easily test the IAC with a multimeter by checking the resistance across the coils (40 to 80 ohm across coils).

So, if the IAC tests OK... it could only be a few things (read those bullet points on the first page).....
1) vacuum leak (including PCV system).
2) bad IAC - easy to test
3) I think a vacuum leak could also lower fuel pressure, causing it to go lean, idle faster.
4) Not sure if a bad TPS driver circuit in the ECM could be a culprit...but that would be a last resort.

One thing we've forgotten to ask.. Did it ever idle OK after the engine build, or did it have the high idle the first time you started it? If so, it could be a timing issue like Dig suggested over on Code59.

Have you tried taking both main vacuum feeds off of the intake plenum (one at a time), plugging the plenum, and seeing what happens? If it's not a leak in the hose it could be a leak in a manifold gasket, but I think you said you used the starting fluid trick.

Hope you get it worked out... that would suck having to drop it into gear from 1800 rpm.

 

[Aeroking]

Re: High Idle

When i reset the IAC, i can feel/hear it spinning. so i'd imagine its working. also in the time period when the IAC is open, the rpms do fluctuate, not alot, but some.

The TB, plenum, and intake gaskets are brand new, EGR and egr gasket are brand new. All the intake and plenum bolts are torqued to spec. Like you said, i tried the starter fluid spray, with no results. I also tried to isolate the plenum vacuum. I disconnected the fron plenum vac port, and put the gauge there, but i left the rear port, cause thats what feeds my MAP, and fuel press reg. and the system was low on vacuum.

Again, this is a custom cam, 115LSA, and 575 lift. so the vacuum numbers should be low.

Yes the truck didi idle on this IAC valve last summer, and yes, since the rebuild, the truck has had the high idle.

Datamaster read the same vacuum at the MAP sensor, as i did with the gauge at the MAP sensor.

Throttle blades are as closed as they can get.

Havent checked my RPM brake booster hose. but i dont think its leaking, its pretty heavy duty.

I know SBC's idle at 4~6 degrees advance. at my 1800rpm idle, the advance was 22 degrees. But since, i've dropped the advance down to 1 degree, and the idle only dropped to 950, so spark advance might have aided the high idle, but its still pulling air from somewhere.



Maybe the turbo is too big, and i'm Forcing to much Induction. :lol: :lol: :lol:

 

[WyoSyclone]

Re: High Idle

Looks like you've covered most of the bases Brandon... since your AFR's are normal (indicating OK fuel pressure), the only remaining thing I could think of would be a vacuum leak to the inside of the motor (not evident using the starting fluid trick). The only other thing would be if the IAC is working (you can feel it moving), but it's not working correctly (maybe a nick on the pintle valve or something).

When i reset the IAC, i can feel/hear it spinning. so i'd imagine its working. also in the time period when the IAC is open, the rpms do fluctuate, not alot, but some.

The TB, plenum, and intake gaskets are brand new, EGR and egr gasket are brand new. All the intake and plenum bolts are torqued to spec. Like you said, i tried the starter fluid spray, with no results. I also tried to isolate the plenum vacuum. I disconnected the fron plenum vac port, and put the gauge there, but i left the rear port, cause thats what feeds my MAP, and fuel press reg. and the system was low on vacuum.

Again, this is a custom cam, 115LSA, and 575 lift. so the vacuum numbers should be low.

Yes the truck didi idle on this IAC valve last summer, and yes, since the rebuild, the truck has had the high idle.

Datamaster read the same vacuum at the MAP sensor, as i did with the gauge at the MAP sensor.

Throttle blades are as closed as they can get.

Havent checked my RPM brake booster hose. but i dont think its leaking, its pretty heavy duty.

I know SBC's idle at 4~6 degrees advance. at my 1800rpm idle, the advance was 22 degrees. But since, i've dropped the advance down to 1 degree, and the idle only dropped to 950, so spark advance might have aided the high idle, but its still pulling air from somewhere.



Maybe the turbo is too big, and i'm Forcing to much Induction. :lol: :lol: :lol:

 

[Aeroking]

Re: High Idle

funny thing is i had a problem very similar to this when i restarted the engine last spring.

the engine was surging when it was idling. Thats when i replaced the IAC, and for good measure, replaced it again at the end of last summer.

but last year it was the RPMs were surging cause of a bad IAC, this year, it is stable. its just at 1800rpms.

 

[sly dvl]

Re: High Idle

the throttle cable doesnt hold the plates closed, the spring on the linkage does.

What I meant was if your throttle cable wasn't adjusted properly it can prevent the plates from closing completely.

Did I miss a post? I thought you had 19"Hg? Now you've got 16"? Is this in gear or in park?

 

[dgoodhue]

Re: High Idle

funny thing is i had a problem very similar to this when i restarted the engine last spring.

the engine was surging when it was idling. Thats when i replaced the IAC, and for good measure, replaced it again at the end of last summer.

but last year it was the RPMs were surging cause of a bad IAC, this year, it is stable. its just at 1800rpms.

IAC rarely go bad. Typically the real problem is clogged IAC passages in the throttle body. Just buy Throttle Body Gasket set (lower, IAC Gasket) and clean away.

I would address the low vacuum issue. If you timing is way off (20 degrees) it will also affect your manifold vacuum.

 

[Aeroking]

Re: High Idle

What I meant was if your throttle cable wasn't adjusted properly it can prevent the plates from closing completely.

Did I miss a post? I thought you had 19"Hg? Now you've got 16"? Is this in gear or in park?

No i had a bad vac gauge. lol, it was reading 3~4in high. bought a new one, and the new one reads vacuum at 16in.

 

[Aeroking]

Re: High Idle

i pulled the IAC and wiped it off, hardly any dirt, it looked brand new. i cleaned the IAC port and the passages, put it all back together, no change.

i thought it might be a leaking brake booster. so i pulled the brake booster line off the intake, and capped the port, no change.

as it sits, its idling at 1675~1700rpms. timing at 22 degrees advanced. after resetting the IAC, it reads 110, then slowly drops from 100 to 0 position with no change in the rpms.

Vacuum gauge at vac ball reads 16inHG. Tunerpro reads 14.43inHG at the MAP sensor. Once again, i isolated the intake plenum, but disconnecting the fuel tank line, disconnected the EGR vacuum system, disconnecting the PCV system. So only the MAP sensor is hooked into the plenum vacuum. and no change.

good thing i have patience, and its raining outside

 

[phoonTy]

Re: High Idle

Sounds like your truck is stuck in ALDL mode. :lol: :lol:

Find any other vacuum leaks?