I have a 1997 F150 4.6L with port injection. I recently had a vavle job done on both heads (heads were resurfaced). After I put it back together the truck has a low idle (600 RPM in park & 500 RPMS in gear). The idle is a little rough at 600 RPMs but is really rough at 500 RPMs. If the idle is brought up to 800 RPMs it is smooth (by using the gas pedal). When the truck first starts (when it is warm) it will stay at 800 RPMs and then gradually moves down to 600 RPMs (within a minute). I have done the following:

1) replaced IAC motor
2) replaced the MASS airflow sensor
3) check for vacuum leacks by spraying WD40 around the intake manfold gaskets and all vacuum lines going to the intake hoping to find a vacuum leak.

Is there anything else that can cause a low idle?

Thanks!

 

Hi kjester and welcome to FTE

Did you disconnect the battery terminal for a while so the PCM can relearn the idle speed?
If you unplug the IAC with the engine idling does the RPM change? It should die or come close to it if it is working right.
Maybe try checking for vacuum leaks with some Carb Medic instead of WD40....I think it works better.

 

Thanks Racerguy!

I will try this over the weekend to see what happens.

 

I pulled the battery cable but it did not help. So, I decided to run a compression check and found that cylinders 1-4 were 125 PSI and cylinders 5-8 were 155 PSI. What would happen if I got the CAM off a tooth or two on cylinders 1-4?

1) Is it possible that the valve timing could cause all 4 cylinders to be lower (valve starts to open before full compression).

2) Could it cause the low/rough idle?

3) Is there any way to check the CAM timing without taking the timing cover off?

 

With the compression lower on the 1 bank it would seem like you got the cam timing off a bit
It would only take a tooth or 2 to cause your problems.
It's very easy to get the timing off from where it should be.
Dont ask me how I know
And no I dont think you can check the timing without pulling the front cover. Did you have the special tools to line up and hold the cams when you did the job?

 

I did not have the special tools to hold the cams. I marked the links and the cams (since Ford did not make this easy) and I put it back together with the marks of the chain and cam together. This was okay but when I spun the motor over a few times without the valve cover son ( to make sure that I didn't screw up and had the valves hit the piston) I noticed that one of the chain oilers wasn't working. So in the process of fixing the oiler, the chain jumped a tooth but I thought I got it back on correctly ( the marks were gone by now). So I guess it is time to take the cover off again. Is there any way of checking the alignment with out the special tools (since the cam marks don't line up in a logical manner)? If I remember correctly, there are the same amount of links going from the crankgear marking to each cam gear marking.

 

There are 2 different procedures depending on whether your engine is a Romeo or a Windsor. You can tell by the 8th digit in the truck's serial number. If it is a W your engine is a Romeo, if it is a 6 it is a Windsor.
Let me know which it is and I'll try to post the procedure for setting the timing.

 

It is a 4.6L Windsor motor. Any procedure that you could post would be greatly appreciated!

 

Hopefully the pictures here will make some sense. I've never tried doing this without the special tools. Maybe you can improvise.

Mark the chains like this.....

hmm that didnt work.....I'm trying to get pics in here.

 

mark the chains like this......lay the chain out flat.

 

Install the LH timing chain onto the crankshaft sprocket, aligning the one copper (marked) link on the timing chain with the slot on the crankshaft sprocket.

 

NOTE: Crankshaft sprockets are identical. They may only be installed one way. Refer to the following illustration for correct crankshaft sprocket installation.

If removed, install the LH and RH crankshaft sprockets.

 

Position the LH timing chain on the camshaft sprocket with the two copper (marked) chain links and the camshaft sprocket timing mark aligned.

 

Position the RH (outer) timing chain on the crankshaft sprocket, aligning the copper (marked) link with the timing mark on the sprocket.

 

Position the RH timing chain/belt on the camshaft sprocket. Make sure the two copper-colored links align with the camshaft sprocket timing mark.