What can I look at to see why my truck is running rough at idle/low speeds, yet no CEL comes on. I mean running rough by a random hesitation for lack of a better description. like when running a lawn mower and it is ready to run out of gas (but I'm not running out of gas though), if that makes sense. Let me try to explain my problem as best as possible:

'98 5.4L 4x4, about 95,000 miles

December 2003: replaced all spark plugs, and also Coil-on-Plug in cylinders 1 and 4.

March 2004: replaced Coil-on-Plug in cylinder 3.

August 2004: sat in driveway for 2 weeks while on vacation. on the way home from work after driving it again, starts running rough at idle, and speeds from 35-70 mph. I replaced PCV valve and fuel filter while waiting to see if problem disappears or CEL comes on. CEL finally comes on a week after initial problem. replaced fuel injectors in cylinders 1, 3 and 4. After getting it back, now it only runs rough after first starting it, sometimes it won't run rough at all, sometimes it goes away and comes back after driving it for an hour. If driving at 55-70 mph, problem does not appear to occur at all. What bothers me most, is sometimes when going up a slight hill, it will start to really labor trying to get up the hill, and CEL starts flashing for 1-3 minutes, then goes off. If I plug in a scanner, no code is pulled. Because of the time frame, and relative gas mileage, several tanks of gas have been run through in the mean time.

I don't know if all of the info above matters, but it is everything I have fixed in the last year.

 

After the Check Engine light has been on there should be a code stored in memory. Some parts stores that do scans might not be able to help you with that so you may have to take it to a real shop.
Have you checked for vacuum leaks?

 

Sorry I forgot to mention that all of the work was done by a mechanic with the exception of the PCV/fuel filter. Mechanic cleared all codes before I got it back. I have a cheap code reader I got from Harbor Freight to help me determine the problem. Mechanic still charges me a diag fee, but I have peace of mind knowing what it's going in for before he starts to look at it.

I haven't checked for vacuum leaks. I guess I need to learn about what and where they are before I can check for them. I appreciate you pointing me in some direction. The good part of all this is the truck is driveable, it's more of a nuisance than anything right now. However I'd like this nuisance to go away, but who wouldn't?

 

Ok, it seems that I am getting somewhere. A friend on another board who is also a mechanic specializing in the F-Series, tells me that a vacuum leak will allow un-metered air to enter the engine and cause it to run lean. In the old (pre-fuel injection) days a vacuum leak would cause the engine to idle high, but with a fuel injected engine it will cause the engine to lean out and run rough. It also wouldn't set a check engine light in most cases.

So now I will concentrate my efforts on a possible vacuum leak and report back.

 

If you search on here you'll find some posts about vacuum leaks. The most common place that most people find a leak is the PVC tube where it attaches to the back of the intake manifold.

 

At 95000 miles i put on new bosch plugs and motorcraft plug wires. Over the last month or so the engine had a cough at idle but without CEL so this weekend i put on new motorcraft platinum plugs (truck now has 174,000 miles). the new plugs eliminated the idle miss and truck has never run smoother so from now on i am using only the motorcraft platinum plugs.

i wonder if you dont have 2 problems. First being plugs and/or plug wires causing the idle miss, and secondly maybe bad DPFE sensor causing the 35 to 50mph issue.

Assume you have clean air filter, have cleaned MAF, and the IAC.

 

Racerguy, thanks for the advice, I will look into that.

At this point I am not ruling out the plugs even though they're less than a year old. No plug wires due to Coil-on-Plug system. Don't know what a DPFE sensor is, but I can search for that as well.

Clean air filter - Yes, K&N in fact. Never cleaned MAF or the IAC. I guess I have my work cut out for me.

 

2mrchio, sorry about the plug wire commetn. Didnt throughly read your post.

 

No problem. I learned enough about the engine in the last year, where I'm even surprised I have learned what I have learned.

 

BULLSEYE!!!!!

I only recently took my truck back to my mechanic, partly because the last time he fixed it, replacing the fuel injectors, he was so busy that he had it for a day and a half, and I lost a day and a half's worth of wages, not to mention I had to pay him. So needless to say, since it is running, I can live with it.

Only lately it has been losing gas mileage, from a shade over 15 to a shade over 13.5. Mechanic checked the fuel pump, it is pumping out within normal range. He then noticed a vacuum leak under the vehicle. What specifically, I will find out when I go get it.

To quote Jack Black in the movie School of Rock, you get an A+ and 50 gold stars!

 

To update, mechanic reported that the connection to the air filter was disconnected.