Hola!
I'm new in this forum and I do have 3 Ford vehicles (1998 Navigator, 2002 Focus and a 2000 Sable) all have a comun problem they do have rough idle under the same or close conditions.
Here is what happens:
Start engine and rpm's go high and start the ramp down to idle (I see this normal).
After some time and engine temperature starts to go up, and you drive the vehicle for some distance (couple of miles) and make a stop, idle looks ok but, sudenly there is a step down in rpms and is when problem starts, the engine starts to behave as a diesel engine (engine rpms go up an down for about 100rpms, vehicle in drive and vss = 0 the vehicle starts to surge and if you gas a little the rpms go high (1400rpms) and slowly go down and agian, some time after the rpms drop low and the surge goes back again.
In the Sable I looked in to the IAC and had to block the by pass and set the mechanical rpms so the engine will not stall and the surge was dramatically reduce, but I think that this is not the way to solve the problem, sure is something else wrong.
I looked in to the 3 vehicles and they behave exactly the same, has somehone in the forum experienced the same problem? is this a common problem in Ford vehicles?
Any help will be appreciated.

Yes I looked in to the spark plugs and are new in all, fuel pressure at least 35psi, fuel quality (seams to be OK I had refuel in different gas stations) no MIL, O2 Sensors trigger up and down normally (exept when in idle they read 0.05v), in general all the sensors look to work ok (MAF, IAT, CMS,CRS,...

Looking forward for any comment and/or suggestion.
Regards.

 

Hola, one done!

Here I'm back again, and through these days I did some searching in the WEB and found that must of the information founded say that there must be a air leak in the intake. Well I took the vehicle (Navigator) and used (as many of us know) Carbuclean and spray it looking for an none controled intake air leak, it was not easy but it started to provide some info. not accurated because it look initially that the TB gasket was not sealing correctly but was because the spray was passing close to the TB and getting behind the engine and was asspirated by a PCV hose that was damage (with a hole) in the rear of the engine. I noticed till I remove the complete TB and the duct and as I said it was to the very end of the TB base it was between the Engine and the firewall.
I replaced the hose connector being evident that was the source of the air leak, also replaced the TB, IAC, and uper intake manifold gaskets and the idle came back to be steady and normal operation.
Is evident that an air leak can cause a bad behaven in an engine represented with rough idle (at least in this case).

I will look in to the other two vehicles for the same and will report if there were air leaks too.

Have a nice day!