Suddenly when I start my Ranger, 4.0 when the engine's cold it idles very rough. You'd think it was going to stall. Yet when I put it in gear and drive 50 yards it goes away. I was thinking maybe a vacuum line but if it was a line it should run rough all the time. Any ideas??

 

Welcome to FTE.

Acting out IAC maybe.

Look in the "Tech Info" thread atop this forum for some trouble shooting steps & let us know what you find.

If you come to suspect the IAC, you could try cleaning it with a plastic safe cleaner & see how it goes. Cleaning them often doesn't last, it didn't for mine, but was worth the effort as a trouble shooting step.

Some thoughts for pondering.

 

Engine temp sensor might be sending incorrect signal to ECM.

 

I cleaned the IAC as suggested and the problem seems to have stopped...
Thank you for the advice...

 

OK, good feedback & to hear it seems to have fixed your problem.

As I've suggested, don't be surprised if the problem returns, it did for me. I guess thats why Ford recommends replacement rather than cleaning the IAC, so I finally just replaced mine, when the problem returned.

Cleaning was easy & a good trouble shooting step for me though, so it was worth the effort, as I knew what to replace when the problem returned.

 

Well after cleaning the IAC the problems stopped....then it came back...so I cleaned it again and it stopped again...so I replaced the IAC . It ran good for 2 days and now the problem is back at both idle and high speed...idles rough and has a 'hobble' or a miss at high speed...for example this a.m. on the highway I had to hold the pedal to the floor going up a small grade and the fastest it would go was 70mph and that is holding the pedal to the floor. this is a most of the time problem...on a few accosions it seened to run smooth like it should

 

any codes even pending code? change the fuel filter. has it had a tuneup? air filter? plugs etc?

 

Could be a fuel pump issue, what's your fuel pressure?

 

With the WOT trouble, you may have more than one problem, so lets think about this some more.

If the idle problem goes away for several days after messing with, then replacing the IAC, maybe you have an electrical problem with the IAC wiring, or it's electrical connetors pins/sockets????

Like corossion, bent pins, spread sockets, loose fitting pin/socket, broken wire under the insulaton.

Or maybe the computer is getting corrupt info from another sensor, like the TPS at the idle position & thats causing the computer to send the IAC corrupt idle pulse commands, ect, ect.

I like the idea to have the computer scanned for pending, or set trouble codes. Most autoparts stores will do it at no cost if your having problems & the CEL isn't lit, as you could have pending codes that haven't turned the CEL on yet.

If you have it scanned post All of the code Numbers, as they could provide good trouble shooting clues.

A TPS problem can cause idle & wot problems too, so if the code scan turns up naught, review the "Tech Info" thread atop this forum, for some "how too info" on component & fuel pressure testing, ect.

Or if you have or can come by a scantool that'll read PID's, that'll make your trouble shoot Much easier, faster & more accurate!!!!

Let us know what you find.

 

New plugs, wires, fuel filter about 4 months ago.at that time fuel pressure was good also..No check engine light but I'll try to find some time to get over to autozone to see if they can get a code. I've been a FORD man all my life but this Ranger has been a real pisser...I bought it new.then it started.....upper and lower ball joints at 60K, Blend door control breaks so the A/C stopped working until I finally figured out a fix for it, MAF, intake manifold gasket. It vibrates when running between 45 and 70 (tried a different driveshaft but no change) I wanted the full size P/U with a 6 foot bed when I bought this piece of crap...and like a fool I let my wife overrule me....."honey it has to be small so I can drive it" She never drives it cause the damned thing always has something wrong with it. Oh Well...thank you for your input I do appreciate it....I'll post something if I ever figure out what's wrong.

 

Did you have this problem before the new plugs & wires????

We have a "waste spark" ignition system, so did you replace the wires & plugs with at least OEM quality & were the plugs of the specified double platinum type & of the proper heat range????

The 4.0L sure is capable of a lot more than 70mph at wot, so it sounds like a fuel delivery, or fuel trim problem of some sort at speed, so another fuel pressure check is probably in order.

Sure would make it easier if you could come by a scantool to have a look at the short & long term fuel trim & other parameters while your driving & it's acting out.
Do you have a friend that owns a scantool, or maybe an auto club, high school, or collage vocational shop, ect????

Is the air filter ok & the MAF sensor clean???? Are you using an oiled foam, or cotton gauze type air filter????

 

do you have 4wd? xcab or reg cab?

 

Local shop did the tune-up and a new fuel filter went in at that time...he's tuned it before and it's run just fine...and it ran fine after this tune up. this problem started about a week ago. It's a 98 4.0 4wd, xcab

 

I agree with some others posts. it might be time to do a fuel pressure test again. koeo, koer and see how far the psi drops in 5 min after the truck is turned off.

When they changed the driveshaft did they pull the slipyoke apart and put the "special grease" on it?

 

OK good feedback, so you have a 98 4.0L bought new, thats been tuned up by a shop & it ran ok afterward, but the problem/s began about a weak ago.

So which problem began after what event????

Right now it seems that you've described two running problems, a rough cold start up idle & low power output at wot, maybe they have some relation, maybe not.

Cleaning & replacing the IAC has gotten some short term positive results on the rough cold idle, but the problem has returned for some reason.
So have you investigated the IAC electrical contacts & wiring, maybe wiggle the connector, or wires to it, when it's in one of it's rough cold idle fits & see what happens.

Have you done a TPS ohms resistance sweep test, to make sure the TPS isn't sending the computer corrupted input about true throttle position???

Did the low power out at wot come about at the same time as the rough cold start up idle problem began????

Have you done another fuel pressure & flow/volume vs time test????
If you don't have a fuel pressure gauge, most autoparts stores have one in their "Loan-A-Tool" program, for a refundable deposit.

Why was the MAF sensor replaced????

After we get to the bottom of your engine operational problems, we can probably offer up some trouble shooting ideas on the vibration problem too.

Sounds like your frustrated with your Ranger right now, but you should be able to address these problems with a little time & careful trouble shooting.

FWIW I had to replace the upper & lower ball joints on my 99 at about 60K miles too.

The rubber dust boots had dryrotted & let dirt & moisture in. It looked like they used recycled rubber on the boots, a they began to show signs of surface cracking at three years!!!!

Hang in there & keep giving us good feedback & you'll likely get your problems solved & when you do, maybe those bad feelings will go away, as a good running 4.0L Ranger sure is fun to drive!!!!!