Mikeaycock, can you describe the KIND of rough idle you have.

Does the engine lope when it idles, or does it feel like a miss fire, or maybe feels more like too low on rpm???

Is your ride auto or manual tranny???

If it's auto, how does the idle feel if you shift to "N"????

How many miles on this vehicle???

If you can be a little more descriptive, we can probably point you to something more specific to check, as different idle "feel" causes you to want to look at different things.

 

Truck is manual with 100,000 mi.

For about the first minute the truck is on and running the idle is fine. At about 1 minute it gradualy slows its idle until at about 3 minutes it begins to cut out in 3-4 second intervals. It never completely dies.

Other question. Ever had an experience where an oxygen senor is bad but check engine lite is not on?

Thanks pawpaw

 

Well on the oxy sensor question.
The only expierence I've had was on my 94 Taurus 3.8L after the dealer replaced the head gaskets, he apparently missed some coolant in the exhaust manafold, when they buttoned her up.
After I drove it about 4 miles I got a intermittent check engine light. I drove it on a 100 mile trip that weekend, on the first 50 miles the CEL was still intermittent, on & off, by the time I got home it remained on.
Dealer got it back that monday, found the left bank oxy sensors lazy, he replace them, all ok. That was at about 36K miles. They don't like even a whiff of coolant.

At 100K miles it's iffy IMHO. But If they were causing the computer to not be able to compensate enough for ageing, such that the emissions were out of range, it would probably set a CEL.

If your thinking it's part of your rough idle poblem, I wouldn't rule it out or in at this point.

If your gas mileage has gone kaput lately, maybe you should begin to suspect them/it. Don't know how many the 4 banger has, maybe 2, one before & maybe one after the cat converter ???

On the rough idle, is it rough after a cold start, or after a warm restart??

 

The symptons I described occur after a warm restart. I have two O2 senors. They are easy enough to get to, but they cost 30 bucks a pop, and I already put down $40 for a IAC that did no good.

 

Ok Mikeaycock, good input, this will help us think this one through, I believe.

If I understand correctly, the problem ISN"T present when the engine is COLD & ideling.

If so, this tells us, when the engine is operating in "Closed Loop" (with the canned operating parameters programmed into the computer) controling things & the engine runs ok, then items, like the plugs, wires, injectors, are probably ok.

It also tells us the oxy sensors aren't in this closed loop, as they aren't yet warmed up.

After the engine is above about 113 deg F, the EGR begins to be told to do it's thing. Too much EGR will cause a rough idle, or rough running engine during off throttle opertation. (deceleration)

After the engine is fully warmed up it's operating in "OPEN LOOP", the computer is now getting all it's operating input from ALL it's external sensors, including the oxygen sensor/s.

So it sounds to me like it's an "open loop" operating problem.

Something is temporarily lying too & confusing the computer, when the engine is warmed up & at idle.

That something, seems to me is messing with the air/fuel ratio on an intermittent 3-4 second cycle.

A number of things can cause this.

Dirty, sticking, EGR valve. Has it EVER been cleaned???

Dirty, sticking PCV valve. Has it EVER been replaced???

Mechanically loose or, noisey electrical contact in the throttle position sensor, such that it lies to the computer about the throttle's true position.

Damaged wiring, bad elecrical connector, corroded contacts to the oxy sensors, or bad oxy sensors. Has your gas mileage gone down lately?

Well you get the idea, there needs to be some testing of sensors & a good bit of detective work done here.

The likes of a good service manual would be a good inestment IMHO.

Also a good scanner that can do real time data scans of the various sensors & do a freeze frame, when the problem begins, so you can scan all the sensors data to see who is misbehaving.

The Actron CP9145 scanner & others will do this.

I believe AdvanceAuto & AutoZone use them & could probably take a look at the KOER parameters & pehaps give you a good idea on just where to look.

With this much mileage, I would clean the EGR & replace the PCV valve if you haven't already done so, then proceed fom there.

 

Thanks.

I have just changed the PCV but have never touched the EGR. Gas milage is around 22 mpg with half highway/half stop & go driving. Which is not too far off original which was around 24 mpg. I will look into finding a scanner and cleaning or replacing the EGR in the next week. I have a Chilton Service manual I am using for most of my references at this point.

Thanks again.

 

Mikeaycock, good show on the PCV valve, thats one more we can eliminate.

Make shure you've done ALL the scheduled maintenance called for, previous to & at this mileage.

Yah @ 100K miles, the EGR is likely to need a good cleaning. If it's sticking or clogged up, it could screw things up.

Are you feeling ANY off throttle roughness, say when getting off the big road, decellerating down the off ramp???

While your there, be shure to check for any loose or damaged vacuum hoses or pipeing or mounting bolts.

If you find any hard, dried out, cracked, split, dryrotted, rubber lines, DON'T just cut the bad end off & reuse it, as it'll likely just fall apart again, replace the entire length of hose.

Yah we haven't even begun to look at vacuum leaks & faulty temp sensors yet, but it seems to me it's likely simpler than that, like a stickey EGR valve or a oxy sensor acting out or the like.

Did the problem begin suddenly, after some event, or repair, or drive cycle, or did it begin slowly over some period of time???

Yah Santa brought pawpaw a Actron CP9150 kit, which has the CP9145 scanner.
This kit has all the cables to enable me to test Ford, DC & GM vehicles, which the family ownes, so you can guess who the trouble shooter gets to be.

Anyway it's enabled me to rundown & fix two pesky problems with my 94 Taurus.

An intermittent speed sensor & evap cannister vacuum purge solenoid.

Used correctly, a scanner can take a lot of guess work out of trouble shooting.

It's saved me 2/3 it's cost so far, in running down those two problems.

On your origional problem, getting the fuel filter lines disconnected.

I found the nice cast metal tool, I purchased at Advance Auto just WOULDN"T WORK.
It wasn't long enough to fully release that pesky spring.

So all tools aren't the same it appears.
Bet the your local Motorcraft distributor has one that will work great.

Well, keep us posted on your findings, remember to look for something simple FIRST.