I have a 2002 ford f150 xl 4.2l v6 5spd.

So truck runs great except at warm idle. Only sometimes it will drop about 300revs and the whole truck vibrates this will last about 2 or 3 seconds then the idle picks up and it runs good til it dose it again. If u tap the gas itl run jus fine. I cleaned out the throtle body aswell as the aic valve, the problem went away for a few days then came back. How do i check the aic valve? Could a vaccum hose be bad? Also when the idle drops i have no power steering and its worse if i push the break. I know the issue is it idles to slow at times just not sure why... Any help is appreciated

 

Best $79 I ever spent at Harbor Freight was getting one of their OBD-II scanners. It's helped me get my truck running like new again.

I would almost guarantee that your PCM is switching between closed loop and open loop fault when the idle changes. This can happen for a variety of reasons, and the OBD-II scanner would help you figure it out.

My truck was idling too fast (opposite problem) and running lean. There were multiple problems, plugs, wires, coil, a vacuum leak in the intake manifold plenum, a clogged fuel filter, a crusty o-ring on a pcv valve, brittle pcv hoses, dirty MAF sensor, dirty throttle body, plugged EGR ports... to list a few.

By having the OBD-II scanner I was able to get familiar with how it WAS running and could see the improvements that I was making with each repair. So I'd highly recommend that, if you're looking for a diagnostic tool.

To check your IAC valve you will need a multimeter. With Key on, engine off, unplug the IAC connector and check the harness side for 10.5 to 12.5 volts. Then measure the resistance between the terminals of the IAC valve itself. There should be between 6 and 13 ohms. Then check the valve for a short. Measure the resistance from either terminal to the IAC body bolts... there should be 10,000 ohms or greater to show it's not grounding internally.

You may also have a problem with your Power steering pressure switch which will cause timing to retard at idle which will affect idle quality. My Chilton's says this should be repaired by a dealer service department, but you might research it online first.

 

Will the obd-ii scaner do me any good if my check engine light is off?? Ive heard all they are good for is telling you why the lights on and mine is off. Also can i check the egr the same way as the iac? And what would the ohms on the egr valve need to be? Il resurch the power steering pressure switch if i cant figure anything else out. But like i said in the post when i cleaned my iac and my throtle body it made the truck run better for a little while i plan on changing air and fuel filters soon when i change the oil il probably do the plugs and wires at the same time hopefully itl work out for me

 

IMO, yes it will. Some codes may be thrown, but they might be temporary, in which case your check engine light doesn't turn on. An occasional misfire at startup, for example. You can see the voltage readouts from many of the parts that directly tell the PCM how the engine should run. True, if there is something seriously wrong, the PCM should send an error code, but it doesn't always work that way. For instance, my MAF sensor was FILTHY because I didn't know anything about it at the time. But the PCM never threw a P0102 or P0103 because technically it was still working. After cleaning it, you could SEE the difference in the OBD-II scanner readings. Things have to be pretty seriously wrong for the check engine light to turn on. This prevents false positives and wild goose chases.

The same could be said for looking at your SHRTFT1 and SHRTFT2 (short term fuel trim). Both should usually bounce around as the computer feeds gas or takes gas away... but should always be pretty close to zero when the PCM is operating on CL or closed loop. If one bank is higher than the other... you know you have a problem of some type on those cylinders. So, let's say your fuel trims are both 10% or higher at idle. That would tell me the PCM thinks the engine is starved for fuel, so it's adding more. Using that as a clue, first thing I'd check is the fuel pressure, and if it's low (below 30 in-lbs), replace the fuel filter, then next suspect the fuel pressure regulator or the fuel pump. You could probably borrow a fuel pressure gauge from a parts store.

The same could be said for the oxygen sensors. They usually oscillate between .2V and .8V because .43V is supposed to be the average they maintain. If the average is below .43, your engine is running lean. If the average is above .43, the engine is running rich. If they aren't oscillating... well there is a problem. And it doesn't necessarily mean the O2 sensor is to blame.

But the scanner just helps you SEE a lot of things that would otherwise be invisible and it provides clues that can sometimes help you figure out what's wrong. It also gives helpful things to check when it does throw a code. Then you can narrow them down by further testing.

Back to your problem, it could certainly be the fuel filter... that's something else I hadn't changed because I didn't know any better and thought it was difficult (its not) but you do need the proper tool to disconnect your fuel lines. Mine had dark rusty bits that fell out the back side, which was surely restricting the gas flow and possibly the pressure. If fuel pressure is too low, your injectors won't spray correctly, they may dribble gas leaking to poor performance.

EGR is a bit more complicated to explain. Unless it's stuck open, (making engine run very rough at idle) it's usually not something that adversely affects your performance. You might borrow a hand vacuum from a parts store (they lend tools) and attach it to the top of your egr vacuum port and at idle, apply a little vacuum (5 to 6 in-Hg) and see what happens. At idle, applying vacuum should make your engine run poorly, or even quit. If applying vacuum, then releasing that vacuum makes no difference in how the engine idles, I'd say your EGR may be stuck open. Do NOT spray anything on the EGR to clean it, as it will destroy it.

There are also EGR controls, the DPFE and the EVR, but I don't think that's something I can explain here. I will say that with the ignition key on, the EVR (EGR Vacuum regulator) should have between 30 and 70 ohms if you unplug the connector and test the resistance between the 2 terminals of the EVR itself (not the harness side). The harness side should show 12V. However this part is hard to get to, its on the back left side of the upper intake plenum. Google it if you need a picture, or go to a parts store website, enter your model and look at a picture.

You can unplug the DPFE sensor and check the harness side for voltage. There are 3 terminals... at key on, the bottom and middle terminals should show 4 to 6 V.

If you plug it back in, backprobe the top 2 wires. (straighten a small paperclip and carefully slide it all the way into the back side of the connector, next to the wire.) Don't let these 2 paperclips touch. With the engine cold and at idle, you should show between .2 and .7V, showing no EGR at cold idle. As the engine starts to warm, that voltage should increase. At operating temps, it should be between 4 and 6V.

Replacing things unnecessarily is a real waste... the best thing to do is to try and find the real problem and solve it. Your plugs, wires and coil might all be fine. The coil can easily be tested. If its not broke, why fix it. With more information and some testing you can usually find the real culprit.

Hope this helps. Keep in mind that I'm an amateur, not a full time mechanic. I just recently have done a lot of work to my 2002 4.2L so a lot of this is still fresh in my mind and I have the 97-03 Chilton's repair manual to refer to, which is where most of this info is coming from.