5.0 EFI engine high idle problem

[broncosbybart]

So far this engine runs well except it has a high idle. Not too bad when cold but very high (probably 1200 rpm) when warm.

The engine is a stock 93 mustang with stock MAF, TB, etc. I have checked and cleaned the following:

-throttle body. it is in nice shape and doesn't hang up.
-MAF using the correct MAF cleaner
-IAB

The engine was just put in. I have the vacuum lines routed as shown in the illustration attached (at least for the PCV). It is hooked into the intake here, not to the TB.

I haven't found any vacuum leaks. The brake booster is new and I can't find any leaks there either. One thing I did note is that the idle air bypass does allow air to flow across it with no voltage applied. The valve isn't fully open, but it isn't fully closed either. I have another valve here and it is the same way. Is that correct or is it flawed? Grasping at straws, I also put a known good computer in the truck and it has the same issue. No codes are being thrown. Thoughts?

 

[Mountain Ram]

How is the IAC mounted? I remember something about how it should not be mounted upside down... Grasping at straws... Electrical connector should be pointed down

I am assuming you checked timing and the idle air screw wasn't messed with...

 

[DirtDonk]

Couple of things that come to mind.

1. Are you running a computer controlled purge valve between the manifold and the charcoal canister? If not, there's a leak.

2. Where is the PCV valve? In the back of the lower intake, like a Mustang? Or into one of the valve covers?

3. Where do you have the PCV return routed? That should connect a valve cover to the intake tract (usually at the throttle body, but not always) in front of the throttle blade. NOT behind it.

You're running some of the emissions stuff still? Good for you. Just make sure they're all plumbed correctly and you should be good.
And presumably all the other unused ports are capped off, but are you using any newer rubber/plastic caps? Seems like only the original equipment stuff will last. Anything that you bought at an auto-parts store is suspect within 3 months.
I'm serious. I've had several literally fall apart and create some pretty big leaks after just a few months. In one case, they deteriorated just sitting there with the engine having never run. So heat, fuel vapor, and UV were not possibly an issue.

Anyway, good luck. It's a royal pain tracking down this kind of stuff when everything looks perfect.

Paul

 

[Rox Crusher]

Take a real close look at the gasket for the IAB.

 

[broncosbybart]

Thanks for the replies so far. That picture isn't mine, I robbed it off of somewhere else. The PCV return is routed after the TB, as was illustrated in that pic. If memory serves correct (truck is a the body shop right now) there is a hose going from the filler neck on the valve cover to the TB. IIRC, that is how the OEM routing is. I don't have any emissions stuff or charcoal canisters. I will double check the rubber caps on the vacuum ports. They were new as well, but have seen what you desribe with them falling apart quickly.

The engine is pretty much stock. PCV is new and is in the lower intake. Factory Mustang EFI valve covers. It sucks because this was a complete resto. I would think that everything should be perfect, but nothing is ever easy...

Main thing I am not sure of is that if the IAB will allow air to pass across it with no voltage applied, or if it should be completely closed. Can't find a good answer to this anywhere. It is mounted correctly and the gasket is brand new.

I do still need to check TPS voltage at idle. I did look at the fuel screw. It is backed further out than it needs to be. It doesn't even contact the throttle blade at idle.

 

[chuzie]

What voltage is your TPS @ when the engine is off?

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[KyleQ]

Both of my fuel injected engines I've had in my Bronco's had no emmisions, not a single vaccum line to anywhere, and my 351W has no PCV and have none of those issues.

Test your IAC, they CAN be mounted backwards and cause a high idle - I'm betting that is where your problems lie.

You should have voltage KOEO and voltage for KOER

 

[surfer-b]

[QUOTEMain thing I am not sure of is that if the IAB will allow air to pass across it with no voltage applied, or if it should be completely closed. Can't find a good answer to this anywhere. It is mounted correctly and the gasket is brand new.
][/QUOTE]
Yes it will allow air to pass without current applied, when looking at it it will be partially open. Make sure its mounted in the proper direction, ie: its not mounted 180* out, this will cause it to not work proper, it doesn't matter if its upside down just has to be in the right direction.
The next thing I would do is check the TPS, it should read good all the way through the butterfly opening, should start out at ~.98 and climb steady to ~4.8 volts on the sig wire. Then I would put a smoke machine on the engine and check for leaks this way, a lot of times a vacum leak will not show until its smoked. I would be looking at the IAB and vacum leaks.

 

[chuzie]

Is the solenoid plug on the IAC high or low? The correct answer is low.

For my 351 EFI build, I ran a PCV and TB supply line to the valve covers.

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[Viperwolf1]

Warm it up and pull the IAC connector. Now reset the base idle speed with the stop screw on the throttle body. You want it to idle at a slightly lower rpm than the desired 700-800 rpm the ECM tries to keep it at. This allows the IAC to do its job throughout its entire range of operation. Don't adjust it so much that the throttle blade sticks in the bore though. Turn it off an reconnect the IAC.

If you can't get the idle speed down you have a vacuum leak somewhere. Junk in the IAC can cause a vacuum leak.