So when I start my truck it runs pretty rough... not the usual I'm a diesel and it's early in the morning rough, but the idle is all over. It'll be at standard idle then it'll drop 75-100 rpms then jump up 150 just to fall back to regular idle. It does this until it warms up a bit.

Then, I get it on the road and the SES comes on. Sometimes if I drive it for a while, it'll go off but there is not really any power loss. At stop lights it idles a little rough and if the SES is on sometimes it will surge... just once... a jump of 75-100 rpms and back and the SES will go off.

I'm thinking IDM, or the harness under the valve cover...

 

You need to hook it up to a scanner and see what's happening when it does all of this. Could be many different things.

 

there's a couple places where the main harness can rub,one is where it crosses the valve cover,the other i believe where it crosses over the shock tower.I would check that first to see maybe if it is shorting out

 

Any mods to the truck not listed in your sig? How many miles on the engine oil and is it full? Air and fuel filters in good shape?

How quickly does the engine RPM romp up and down? Any other useful info like cold temps or thin air we should consider?

 

Sorry guys I got busy at work. All the mods that I have done are in my sig except the 4inch exhaust.

How quickly does the engine RPM romp up and down?

Well the scenario I described above would all happen in the space of 3-8 seconds.

How many miles on the engine oil and is it full? Air and fuel filters in good shape?

About 3k on the oil. Level is good. All filters replaced when I did the oil.

I live at 7100 feet, but I've been here forever. I took the truck on a 2000 mile round trip, and it ran perfectly. Two days after I got home, it started this stuff. And lets throw a new symptom in. Yesterday it died when I was pulling up to a stop light, just like it did a few times with a bad CPS. It started right up, no hesitation, no complaint.

 

X2. It would really help if you could post the codes.

 

Maybe water in the fuel! If it was under the valve cover harness it would run rough right away all the time. JMOP

 

Clogged filter/pick up screen???

 

Gonna go and pull codes tomorrow I'll post 'em up here.

water in the fuel
Clogged filter/pick up screen

Would these trip the SES?

 

Water in fuel will trip the "water in fuel" light.

Clogged filter / pick up screen "MIGHT" throw a SES light, but will most likely result in the truck falling flat on it's face when you try to accelerate for a while before the light trips.

 

Not showing any codes at all.

 

Where did you get the codes read? Places like autozone won't work. If the check engine light came on you should have codes stored. I have a tuner that can read codes but in the ABQ area. If you come down this way let me know.

 

Which CPS do you have? Is it possible that you got a bad one and it is going out again? My replacement went out after about 1000 miles (and 6 months time since I do not drive very much). I believe that a bad CPS will sometimes throw phantom codes, though. Just a thought.

 

CPS was installed by Ford last year.

Thanks guys, I'll keep plugging away at it.

 

You are describing exactly what happened to mine (except I had codes) when my ICP sensor went out.