PLEASE HELP.....can't figure out why I have an intermittent rough idle!

Hardwareman · Jul 28, 2008

I just can't figure it out guys. I just tested my IAC sensor connector and according to the book I'm ok as far as voltage goes. When I disconnect the IAC while the engine is running the engine rpm's do not change.

I'm thinking this is my problem but I'm not sure. The engine will idle great, and then all of a sudden the rpm's will drop and it will sound like it's misfiring. Then it will go back to idling good again, only to drop back down again. Typically when I've had an IAC go out the rpm's will stay accelerated for a while but that's not the case here. Can an IAC go out in the way I described?

Please, please, please, tell me where to look! I've already checked coil resistance but I'm stumped right now. Thank you!!

Allen

Hardwareman · Jul 28, 2008

With all the new parts I have in this truck I'm starting to think that my PCM is bad? What do you think?

Allen

KELLY88 · Jul 28, 2008

Hardwareman said:
When I disconnect the IAC while the engine is running the engine rpm's do not change.

Your IAC just failed it's test. If you unplug the IAC while the engine is running the engine should bog down. If it doesn't then that means the IAC is bad. Try cleaning it with throttle body cleaner before replacing it.

Hardwareman · Jul 28, 2008

Thanks for the reply Kelly.

Could that also make the engine run rough at idle with the symptoms I have? I tried tapping the solenoid. I thought they usually made the engine rpm's go high when they went bad?

Allen

KELLY88 · Jul 28, 2008

Ya it sounds like that's probably your central cause of it. It could be other small issues like dirty air filter or dirty throttle body plate. I had a small rough idle and everything tested ok. I pulled the throttle body off and spent about half an hour cleaning the crap out of it. Ran like a champ after that. Clean things just work better.

Hardwareman · Jul 28, 2008

Ok, I guess with all the work I've already done to this thing lately I might as well spend a little more time and remove the throttle body and clean it up. It may just be that because before I replaced the 2.3 engine with this 2.5 a couple weeks ago the old motor was blowing oil all into the intake and throttle body.

Thanks Kelly, I'll post up the results.

Allen

Hardwareman · Jul 28, 2008

Well does'nt seem like it was the throttle body/IAC. It was dirty and I cleaned the hell out of it. I also sprayed some carb cleaner down the intake and it started smoking like hell. I just shut it off for now. I'll drive it for a bit in about an hour or so and see if it clears up or not.

I'm really leaning towards the PCM right now.

Allen

BobbyB2300 · Jul 28, 2008

I had this idle problem too along with bucking and jerking. I replace the DPFE and the EGR selonoid...did the trick. I misdiagnosed the IAC, replaced it for not. I think it was a symptom of too much exahust going into the intake, controlled by these two components and the EGR valve itself.

The DPFE is prone to failure in these puppies and you have the right symtoms. You might want to start with that and inspect/replace the two hoses attached to the DPFE, the heat takes a toll on the hoses. One of them is silicon based.

I put a silicon bead along the seam of the DPFE to make it more water resistant. I've heard water will trash these.

Ya, DPFE failure.

Hardwareman · Jul 28, 2008

Thank you so much guys for helping me out, my Ranger is driving me mad!

Where is this DPFE sensor thing at on my Ranger? Is it the gray box that's bolted on the passenger side of the engine and has two hoses going into the EGR tube?

I just did a 2.5 swap about 2 weeks ago and if you've read any of my other posts, you will see I've had issues. My old 2.3 had the same problem right before I did the swap and everything carried over to the new engine. The misfire was a lot worse and actually was there during driving, but after I swapped out the alternator it's only been there at idle now. Sooooooooo, it has to be something in my wiring or computer that's causing it because it carried over you know? GOD, this is driving me mad!!!!

This is what I've done so far: replaced plugs & wires, fuel injectors, tore out wiring harness and went through it all, new alternator, different cam and crank sensors, new catalytic converter and exhaust, checked for vaccum leaks, cleaned throttle body, and probably tons of other things I can't think of right now.

The DPFE (whatever that is) is probably worth changing out at this point because this thing's got the better of me right now. Where can I get one at and does it have to be brand new cause I heard they're expensive?

BobbyB2300, thanks you've given me more hope buddy. Thank you very much for that info, I'm starting to feel lost.....haha

Edit: My Ranger is a 95 BTW so does it still have a DPFE?

Allen

Hardwareman · Jul 28, 2008

Hey you may be right?

I just got done reading through my Haynes manual and it says it's a "Differential Pressure Feedback Sensor". There's a couple checks I can perform on it to tell wether it's that or the PCM. There's new hope for me now......YAY!!!

I had an extra EGR and and DPFE sensor yesterday and I sold them to a guy who bought some extra 2.5 parts I did'nt need from my swap.....doh! I don't know where to get them at now. Maybe I'll call him back and see if I can go get them.....haha.

Allen

Bryan22 · Jul 29, 2008

how do you test the dpfe? I've been having a sort of rough idle on mine and it'll buck sometimes when i floor it, then there's the crappy mileage. one of the hoses wasn't on it when i got it. So if you could tell me when you did to test it and how it turns out for you i'd apperciate it.

Hardwareman · Jul 29, 2008

Bryan22 said:
how do you test the dpfe? I've been having a sort of rough idle on mine and it'll buck sometimes when i floor it, then there's the crappy mileage. one of the hoses wasn't on it when i got it. So if you could tell me when you did to test it and how it turns out for you i'd apperciate it.

Well according to the manual you first test to see if you've got between 4.0 & 6.0 volts power supply from the wiring harness going into the sensor. This rules out the PCM, which in my case it did. I'm right at 5 volts and after doing some reading online I'm perfect in that department.

Now........in order to test the sensor itself you can use jumper wires on the pins of the sensor. Use one jumper for battery voltage and the other pin for ground. While the jumper wires are connected you blow through the sensor. If you cannot blow through it then it needs to be replaced. At least that's what the manual states anyways. I was unable to blow through mine so in essence I guess it's bad.

I'll be replacing it tomorrow and I'll let you know how it goes.

Allen

Hardwareman · Jul 29, 2008

Ok well the new DPFE sensor is in! I started her up and let her idle for about 2 minutes or so. She was still kind of rough but did seem to get smoother the longer I let 'er run, so maybe I need to drive her for a bit as I did disconnect the battery too.

I hope this does the trick man cause I'm tired of this mess. Later.

Allen

mhagedon · Jul 29, 2008

The IAC went bad in my t bird. Idled at 3000 RPM or so. Pulled the motor and put a cardboard gasket between it and the intake and set the idle with the set screw on the throttle body. It might not be right, but it works.

Hardwareman · Jul 29, 2008

mhagedon said:
The IAC went bad in my t bird. Idled at 3000 RPM or so. Pulled the motor and put a cardboard gasket between it and the intake and set the idle with the set screw on the throttle body. It might not be right, but it works.

That's pretty much what I've had to do for now until I feel like getting into it further.....haha

It does run pretty good right now though if I may say so. My neighbor said it sounds perfect to him and that he can't hear what I'm talking about. When I get a chance though I'll get back under the hood and check voltage and "yada yada yada" with that IAC valve to see if there's anything I can do to fix it.

Allen

PLEASE HELP.....can't figure out why I have an intermittent rough idle!

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