Stumble at idle
In the last couple weeks my 00 2.5L auto has had a slight to bad stumble at idle and more recently a stumble cruising at 50-60mph under light to moderate load. When it's happening the only common factor is the engine is running and it's at operating temp. Doesn't matter if the AC compressor is on or off and it also does it with the hvac switch turned to any position. I did a major tune up about 2 months ago which included wires, plugs, cleaned MAF, both O2 sensors, pcv, fuel filter, iac and I cleaned the entire intake and tb. I did all of that in one day and have put over 3000 miles on it since then without a problem. All the replacement parts were Motorcraft.
When it's stumbling the CEL doesn't even flicker and the transmission feels fine. I have checked all of the vacuum hoses for leaks as well. A couple of the hoses are cracked but don't seem to have any leaks.
It's never consistent (other than it only happens when the engine is warm) and mostly happens at idle. It can be just a hickup or start rockin like it's got a monster cam in it. Then engine never dies.
I unplugged the iac while it idling very rough and it seemed to even out pretty well, I also gave it some gas and the revs went back to idle fairly well but it never dies.
I have cleaned the MAF and IAT sensor with no change in the condition.
I put a fuel pressure gauge on it and it held rock steady at 68psi while idling fine and while it was idling rough.
Suggestions? I have never touched the EGR system or the coil packs. This truck has 134,000 miles on it as well, could this problem be a sign that the timing belt is about to break?