Rough Idle, tried everything...need help.

[Motorco]

I bought a 2004 3.0 Edge 70KMiles about 3 weeks ago, I have read through everything and tried everything I know of to get rid of this rough idle. At first it threw the code saying Cyl # 4 was missing....So I bought brand new coil pak, wires, and spark plugs, Fuel injector for that cyl., ...and also both upstream O2 Sensors just to be safe. Nothing helped, reset the code and it runs fine for a week but still with a rough idle...as soon as the Check Engine light comes on, it will spit and spudder from idle and then finally kick in and drive fine.

My mileage has stayed good even with this problem...19 to 20mpg....Then I checked all the cylinder compression....highest 160 psi lowest 135psi which is in the #4 missing cyl. Replaced the MAF with a new one, replace fuel filter....checked under hood in pitch black to see if any cross firing..and there is none. Cleaned the other sensors on throttle body and intake....checked every vacuum hose on the engine.

I am at a loss on what to do next....only thing left are the cats and downstream O2 sensors....What do you think?

 

[Rearanger]

Then I checked all the cylinder compression....highest 160 psi lowest 135psi which is in the #4 missing cyl.

This is more than likely the cause. Missing is picked up by crank position sensor detecting changes in rpm. Electrical missing can also throw a rich code as too much unburned fuel goes into the exhaust. Higher rpms will cover for rpm speed changes.

it will spit and spudder from idle and then finally kick in and drive fine

I think you need to further check for vacuum leak. Check PCV connections, hoses and valve.

I am at a loss on what to do next....only thing left are the cats and downstream O2 sensors....What do you think?

CAT would not be a problem at low rpm, especially with 74k miles. Downstream O2 sensors monitor efficiency of CATs.

There have been complaints here about premature valve wear. Did it do this when you test drove it? Maybe this is why previous owner sold it?

 

[06RangerXLT]

that lack of compression may do that. sounds like bad rings or a valve not seating. drop a little oil into that cylinder and re-test immediately. if the psi goes up, you have bad rings. if it doesnt you have a stuck valve, or bad head gasket.

id check your oil for any discoloration that looks milky. thats a sign of coolant in the cylinder and a blown head gasket.

id go to the dealer and say WTF. you should have at least a 30 day warranty.

 

[Motorco]

Thats what I was afraid of....it didnt do it on the test, only a couple days later when the check engine came on did it spit and spudder off the line, It on does that when the check engine comes on....

I'm mechanical, so I just thought it would be any easy fix. I am sure this is why the previous owner traded it in. This was from a ford dealership and sold "As Is" like all their other used cars. No warranty.

I thought that the lowest compression could be was 75% below the highest reading....which would be 120psi....

 

[Rearanger]

it didnt do it on the test, only a couple days later when the check engine came on did it spit and spudder off the line, It on does that when the check engine comes on....

Can't say why the check engine would trigger this
The problem would trigger the CEL not the other way around. Clearing the code would allow it to sell without a CEL then as you drive the necessary cycle the CEL would show. Is it any better on cold start-up than on hot running? When the code is cleared the computer runs the engine on factory pre-sets until it learns from sensors how to adjust fuel trim, etc. That may explain the not until CEL problem???

Some states have more legal protections when buying through dealers so you may want to research the attorney general's web site for possible warranty.

 

[Motorco]

No real difference from hot to cold...and it is sporadic. sometimes you start it up and it idles fine...then drive 10 miles or so and hit a red light and it idles rough again, but always takes off fine...Until the CEL is on, then its a 50/50 on whether it will accelerate normally. And I am still getting great MPG thats what I dont understand.

I just went through all the vacuum lines and removed stock clips and put regular hose clamps on them....with the PCV hose un-hooked it would not even start, dont know if that is correct or not. Well I say that, but this PCV valve had electric wires running to it...never have seen that before. when those where unplugged it would not start.

 

[Rearanger]

Well your A/F ratio seems to holding. If it were way off you'd get a lean or rich CEL.

with the PCV hose un-hooked it would not even start, dont know if that is correct or not.

Yes, that is correct. The PCV hose is large diameter by design so that leak/disconnection will stall engine.

this PCV valve had electric wires running to it...never have seen that before. when those where unplugged it would not start.

Also by design. Either the wires are for sensor and/or for heater. My PCV has hose connection to coolant.

If you've got some extra bucks it may not hurt to take it to a Ford dealer for full analysis. Too bad on a new purchase but it looks as if it was unloaded on you.

 

[Motorco]

Thx for the comments everyone... I think I will take it to my friends repair shop and either have the head gasket replaced and/or a valve job done. The truck is in brand new condition other than this problem, so its worth it to me.

 

[Cleanster]

Had the dreaded #4 MISFIRE turned out the head was cracked. The rest of the motor went south quickly after.

 

[Motorco]

Had the dreaded #4 MISFIRE turned out the head was cracked. The rest of the motor went south quickly after.

Ugh! Dont tell me that! Throwing it on the diagnostic next couple days and see exactly what wrong...