View Full Version : 1998 4.0L auto Ford Ranger irregular idle
08-12-2007, 02:08 PM
Hi guys, i have a crazy problem with my 1998 ford ranger 4.0l auto. It idles wierd, inconsistent. If you let it idle you can hear it ramp up and down very evenly. Its very annoying but not severe.. i was told to clean out the MAF sensor so i spent a couple hours tearing it all apart and it looked brand new! None the less i bought some CRF MAF sensor cleaner and gave it a gentle cleaning.. No change in the idle at all. I even unplugged the MAF to let the computer run at a default value. Still no change it just idles high low ect.
If you guy have any suggestions please post away :-)
The truck only has 38k on it!
I would really like to try and fix this without spending hundreds of $$ at the dealership..
08-12-2007, 03:12 PM
Might wanna clean the IAC valve, and I kno the 4.0 Rangers from around that year have problems w/ the intake gaskets getting holes in them might be somthing w/ that but i dunno
08-12-2007, 04:38 PM
is your check engine light on? if so pull the codes. there are many causes for loping idle and diagnostic codes will help narrow the search down.
vacuum leaks, sticky IAC, bad TPS.....the list goes on.
08-12-2007, 04:49 PM
Nope, No check engine light... I cleaned the MAS this morning and i even uplugged it(still inconsistent idle) I figured with 38k on the truck it should not be having these problems but i guess i was wrong! Anyhow, it looks pretty clean under the hood and the idle never gets too low(does not stall) just kind of alternates by about 100-200rpms up and down consistently and forever... At stop lights you can feel it a little pulsing. I reved it up a little and held it there and it seems to also idle up and down when the gas it pressed..
I pretty stumped on this one.
Also when you press the gas at a light i can feel what seems like the engine shift a little? like a little thud, is this a normal ranger thing or are my engine mounts shot? My buddy's truck does the same thing but who knows maybe his mounts are shot too..
08-12-2007, 04:53 PM
i had a problem with a "tick" sound that came from under the hood any time i put torque to the wheels. i thought my engine mounts were shot untill i finally crawled in there to check and discovered the passenger side mount had worked its way loose, allowing the engine to rock back and forth. tightened the bolts down and it cured the problem.
as for your idle...it sure sounds like a bad TPS to me. but i would start by cleaning the IAC, since you can do that for free and it may help.
08-12-2007, 04:56 PM
Is there a way to test the TPS?
08-12-2007, 04:57 PM
Also i have looked for directions to clean the IAC but have come up with bad links.. Do you have one handy?
Will check the bolts on the mounts.
08-12-2007, 05:06 PM
according to my chiltons, to test the TPS:
1. unplug the wiring harness that goes to the sensor.
2. with a DVOM set to OHM reading, probe the sensor pins corrosponding to the brown/white and grey/white wires on the harness.
3. slowly open and close the throttle while monitoring the DVOM. resistance should slowly increase and decrease in relation to throttle position. any sudden jumps or irregularities means the sensor is shot.
to clean your IAC, just pull it off the engine and spray it down with carb cleaner (i prefer seafoam deep creep). if it has a lot of built-up residue, you might want to remove the electrical solenoid and soak the valve in a tub of cleaner over night.
08-12-2007, 05:36 PM
Just went out and tested the TPS it seems to be okay it registered 3.5k ohms and slowly went down to about 2kohms.. I honest dont think the iac is too dirty and i dont want to mess up the gasket around it.. Is there a way to test that?? i also heard a rumor of a bad intake manifold gasket or look bolts.. i checked a couple bolts though and they seemed tight. I cant beleive the intake manifold is plastic on these things!
08-12-2007, 05:54 PM
the only test for the IAC is visual inspection. you can re-use the gasket.
you could very well be looking at a vacuum leak problem. usually this will register on the computer as lean codes, but not always. spray carb cleaner around the intake manifold gaskets and listen for an increase or decrease in RPM.
most people are shocked to learn that most newer vehicles run plastic intake manifolds. the manifolds are plastic (actually composite) for 3 primary reasons: composite is cheaper and easier to manufacture then aluminum. composite is lighter. and composite is a better insulator for heat (your air charge arrives at the engine cooler, thus more power is produced).
08-12-2007, 08:14 PM
That makes sense for the plastic intake manifold.
I just took it for a drive and it idles high/low like it normally does but when it was warm it did it but at a longer interval.
So for example it goes high/low ever half second and when it gets warm its ever 1 sec... Maybe its just a fluke but its so wierd it does it so evenly.
Anyhow when i get some carb cleaner i can take off the IAC.. and clean it. With only 38k i cant imagin it being too dirty. The Maf was perfectly clean.
Another side note, the engine makes a quite clacking.. Its not too loud but it that also normal for a ford? Later i can record the sound to give you a better idea.
08-12-2007, 08:39 PM
use the rest of the carb cleaner to check for vacuum leaks after you clean the IAC.
clacking is never a good sound. you need to poke around under the hood and figure out more precisely where its coming from. use a long screw driver held to the ear and place it on each valve cover, oil pan, engine block, injectors, belt driven accessories...ect.
when it starts doing this, watch the AC compressor and see if the idle differences corespond with the AC Clutch cycleing the compressor ...... start with the simple things first. if you don't have AC then ignore what I said.
Mileage does not matter to electronics, only age. your sensors (assuming they have not been replaced) are alomst 10 years old. 98 did have problems with the IAC .... used to be a tech article on it in the tech library. I would lean towards the IAC but I also know that the symptom you describe coresponds with an AC system that has a bad clutch or needs to be charged (the lower the charge the faster it cycles) and Ford AC compressors cycle wether they are on or off (or used to and it does on my motorhome ..... 1995 E-350 with a 460) to keep the oil curculated in the compressor.
08-12-2007, 08:56 PM
or used to
they are supposed to cycle in "defrost" and "ac on" settings..none of the other settings.
08-12-2007, 09:01 PM
I do have AC but i was watching carefully and the compressor never came on when idling, if i turn the AC or defroster on it cycles. I will go ahead and buy carb cleaner and clean out the IAC and let you know how it does-hopefully well :-)
Just to be sure on this, i just unbolt the IAC, and spray a bunch of carb cleaner in the holes? let it drain of course and dry but thats it?
I will also find the longest screwdriver i can find to figure out where that noise it comming from.
I priced out some cheap parts from schucks i think the TPS was 30$ and the IAC was only $60-70.. i could replace them both, i just hate it when i waste a bunch of money on things that work. I am sure you know what i am talking about, i have done it before.
08-12-2007, 09:41 PM
i wouldnt start replacing parts untill we identify the problem.
for the IAC, its best if you can soak it for a little while...but spraying it down is better then nothing. ive also heard that a drop of oil on the shaft of the valve can help.
08-12-2007, 09:51 PM
The 4.0L are notorious for having a ticking noise come from the valve train. They don't get as oiled as well as they should. I'd check the motor with a mechanics stethoscope or a screw driver but if it sounds like its coming from the valve covers it's a good chance its the normal 4.0L tick.
08-13-2007, 08:05 PM
i have a 94 exploder that was doing the same thing so i looked into it and found in alldata that the IAC can cause that wah-uuh-wah-uuh. the rpms rose and dropped very evenly in amount and duration. i pulled the IAC cleaned it as best i could.i didnt see enough soot to make it jam but it did make a difference.i had to drive it for a while before it quite surging 100% of the time.if you decide to just throw parts at it i would go for the new IAC.ive not heard that the tps sensor could do this also as it doesnt regulate the idle though it does have input on spark and fuel.
08-14-2007, 12:19 AM
adjusting spark advance and injector pulse width is more than enough to cause a +/-200 change in RPM. it wouldnt be the first time a faulty TPS has caused "seeking" idle.
08-14-2007, 04:40 PM
well with the hi/low idle i would just check all of the vacuums caps... just to make sure b/c i had the same prob with my old 2.9 and it come to find out that the one cap split and you couldent even tell it was until you took it all the way off.. But would also consider an intake
09-20-2007, 07:18 PM
i had the same problem on my 96 super 4.0 , it turned out to be the egr valve was rusted out on the underside of it,where you coundnt see it. causing air leak.
09-23-2007, 05:36 PM
Sorry it has taken so long to get back to you guys but you wont beleive what was wrong with this truck!! Apparently at the dealer when it was new they installed the AC in the truck. Its not really aftermarket, its a ford AC kit that looks just like the factory kit. Well, the kit has its own fuse and relay (in a wire harness) some joker decided to put the factory fuse and relay (in the fuse box) it somehow glitched with the aftermarket system and caused it to idle low/high. I pulled out the factory fust and relay and it runs perfect!! And the ac works just fine. This is the craziest thing ever but its fixed now.
I will check on the mounts & ticking later..
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