Ok so I just wrapped up my rebuilt long block install and I'm having a high idle issue. Engine was built by an awesome local shop and and runs like a champ. So far I have done the recommended 20-25 min 2k RPM break in and driven it about 5 miles around the neighborhood.

Anyway during my drive I noticed the idle was around 1500 RPM. My first guess was a vacuum leak which I found. I fixed the leak and RPM's lowered to around 920, which still seems high. I again checked for leaks and found none.

I had set my timing to 10* while i had the vacuum leak so I rechecked the timing to make sure it was still good. i noticed that when i started the truck with the SPOUT removed my idle was around 720-750, which is what seems right to me. I made sure the timing was still at 10*, shut truck off, re-installed SPOUT, and started it back up....idle is still right around 900-1000 RPM's.

On a side note the CEL is not on but my rear anti-lock light is on which Im assuming may be vacuum related? Any help would be very much appreciated!

 

Another thing while I was assembling the intake system I removed the throttle body plates from the throttle body to clean them and now I'm worried maybe I messed up their coating? I cleaned them non chlorinated brake cleaner and an SOS pad. I didn't notice any damage or scratches but now I'm worried. Any advice would be awesome! The IAC valve and TPS are new by the way.

 

Is your truck manual or automatic trans? I think 900 is correct for manual trans.

However, if you removed the throttle plates, it is very likely 1 or both are misaligned, letting more air past than Ford intended.

An SOS pad? That's a new one. Yeah, I would say you removed or at least damaged the coating, but it's too late now. Just run it. The only bad thing that will happen is they will gum up easier going forward.

 

Ha ya the old motor caused a lot of caked on black oily carbon to build up on the back side of the plates so I took them off and had to scrub pretty good to get them clean. They look centered in the throttle body with a tiny amount of gap at the top and bottom of the plates when the throttle is closed.

 

It's a manual transmission

 

Ok so truck cooled down and i started it up. RPM stayed right around 1k and when i disconnected the IAC wiring the idle stayed the exact same. Is my new IAC bad?

 

Same test with the engine warmed up and no change in idle RPM with the IAC disconnected

 

did you clean your iac?


its one of two things; the iac is gummed so its stuck in a position that happens to give you 1000 rpm idle, so unplugging doenst change anything.


OR


you have too much air getting past the throttle blades, or somewhere else (vac leak), resulting in your 1000 rpm idle.


use a can of carb cleaner or ether to spray around your intake and vac lines to see if idle changes. if it does, you have vac leak.


plugging the egr will prevent the caked soot from coming back.


note: a common "mod" for 460 owners is to plug the holes in the throttle bodies because their idle is too high as a result of said holes.

 

Ok so I replaced the IAC and truck still idles between 950-1K RPM. My repair manual says the idle should be around 750 RPM but I haven't been able verify that. I have been looking at my throttle body and I do not have the small hole on the throttle plates that other years have. The plates are open slightly and it looks like the set screw has be messed with. My family has owned the truck since new but after the original motor let go we had a local shop rebuild it and I am learning that some corners were cut. I am thinking they messed with the throttle set screw. This is the 3rd engine in this truck.

 

It sure seems to me that your idle speed is outside the ability of the IAC to control because 1) you have a vacuum leak or 2) your throttle plates are letting too much air past. I vote for #2. First thing is try to back off the throttle screw with the IAC disconnected and see if you can get the idle speed down. You would want that speed, with the IAC disconnected, to be 400-500 RPM.

 

I appreciate everyone's help on this! Ok so I noticed my oil pressure sensor was leaking a little so I decided to pull the upper intake plenum with throttle body so I can tighten the sensor and re torque the intake manifold.

When I pulled the upper intake plenum I noticed that the gasket was pretty wet and smells like fuel. There is also what looks like a dried stream of fluid running off the back of the intake manifold. Could a vacuum leak cause this? Or do I have a bigger problem?

 

 

Here is a picture of how it looked when i pulled it off. The carbon is some left over from the old engine...i'll clean it up better before I put it back on.

 

Is the dried stream on the outside of the intake? If it's inside, it could just be cleaner that got shot in there, or from when the machine shop tanked it (if you gave it to them).

I've had leaky or stuck open injectors over the years, and my intake gasket got soaked with fuel. If the injector is spraying and has nowhere to go, it will go back up into the intake and can even puddle....

This leads to the next point.....leaky injectors can also cause poor idle quality.

This is not necessarily cause to abandon diagnosing the easy stuff....because leaky injectors my not even be an issue.

There are two ways to easily check injectors, however, and would be worth the time even if you weren't having problems.

1) leak down test; start the engine with fuel pressure gauge attached, shut off. If there's a leaky inj, the pressure will drop noticeably. It's supposed to hold pressure for a few minutes at the very least.

2) road test; downshift to ~2500-3000 rpm, preferably on a hill where you won't slow rapidly. At 0% throttle, and rpm above idle, there should be no fuel getting to the engine. If it stumbles while in downshift you have a leaky injector(s).

 

I forgot to mention I replaced all injectors with brand new ones when I put in the new motor. So they have maybe an hour or two of use tops. If my intake plenum gasket was leaking it would be sucking right? So somthing leaking out the back shouldn't happen. Maybe it was sucking the break clean in through the back but slowly enough it didn't increase the idle? Smells like gas though but I have a hard time telling the difference. The fluid doesn't evaporate fast and feels oily