4.6L (1996-2004 Modular) MustangTechnical discussions on 1996-2004 4.6 Liter Modular Motors (2V and 4V) within. Sponsored by Cruizin Concepts
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Hi everyone, this problem has plagued my car since I bought it from the previous owner last year. Last year, whenever the car was warmed up, I could not get it to idle below 1100 rpm's. So, I did some research to figure out how to fix this and I came across some guys who said they put something in the IAC to restrict air into the engine at idle. I did this along with replacing the IAC valve, checking the TPS voltage and adjusting it, and also moved the throttle set screw to help set the idle. This did (and still does) help to keep the idle down, but it's still not perfect.
But the really annoying thing is that whenever I have the AC on, I cannot get it to idle smooth. I have tried various things to fix this as well, but have yet to make a difference!
So far, I have Re-charged AC and pressure-checked for leaks with a professional recharge station, and reset the computer's memory many times.
have you checked for vacuum leaks? any codes? It has been my experience that fixing a hi idle issue simply by mechanically lowering the idle, will cause the rolling idle you are experiencing when put under a load (ie. turning on the a/c or putting it in gear (if it's auto))
2002 GT: 4.10s, AM CAI, UPR o/r X-Pipe, MGW STS, Flowmaster 40's, more to come
The TPS on the EECV ('99 through '04) GTs is not adjustable, as it does not need to be adjusted (though some have actually filed the mounting holes to make it adjustable--really fouling things up). The PCM constantly reads the TPS voltage and saves the lowest observed value as indicting closed throttle--this is called a "ratch" (reverse latch) function. As the engine warms the ratch processing slows.
The TPS voltage with the engine hot, at idle, should be between 0.59 and 1.27 V.
You may wish to check the throttle stop screw setting, here is how to do so. It might have messed with, by someone thinking it was an idle speed adjustment.
I have checked for leaks, and came up empty handed, and the only codes that come up on a scan is the rear O2 sensors (which have also been replaced) indicating a lean/rich condition (most likely because it's a cat-less exhaust system).
And in response to cliffyk, I have actually moved the throttle set screw myself in order to see if it would adjust the idle speed along with tilting the tps just a tiny bit (did not file the mounting holes though!), and it seemed to improve the idle a little. So, I figured I would leave it be for a while and see if it would get better as the PCM saved the new values, and it did, but as you see, it still has the AC problem...
Also, have you ever heard of this being a problem with the PCM??? (I took it to my Uncle a while back and he looked it over and said that the only thing he could think of would be that the computer is not working correctly and needs to be reprogrammed by a dealer...)
Last edited by Fordman131; 05-16-2013 at 11:08 PM.
I doubt it is a problem with the PCM/tune--they are very stable unless something goes wrong while flashing a tune.
You should first use the procedure described in the link I provided above to make sure the throttle stop screw is properly set. As explained in that article if it is not the PCM cannot properly control the IAC, and idle speed problems will be the result.
Ok, I reset the set screw today, and took it out for a ride. The rolling idle with AC on seems to have gone away. But now when the AC's on, it won't idle below 1100rpm's. Otherwise, with it off, it idles great right around 750! This is really confusing!
If it were a leak or something to do with the IAC or the MAF, etc, wouldn't the problems be more consistent between AC off and on? Correct me if I'm wrong, but could that mean there would be something wrong within the AC system itself?? Keep in mind that it blows ice cold air and has been recharged with no difference in the idle issues.
Also, I have looked at many other posts on forums, but no one seems to have this exact problem, or if they do, the post stopped before they got it fixed! haha!
I did end up taking the air restrictor out when I adjusted the set screw back to factory specs, so that's outta the picture now. As far as checking for leaks on the hoses and the EVAP, I will try to get to that sometime this week... I'll post another response after I get it done stating my findings!
Again, thanks for the responses! I appreciate your time and thoughts, and hopefully we'll get this figured out so I can better enjoy my Stang!
Well, I looked over the hoses, and sprayed carb cleaner all around the hoses and around the intake, and none got sucked in. As far as the EVAP, I have not gotten to that yet, but hope to sometime this weekend.
Another thing I did today was scan the computer with our scanner, and came up with a couple things that stood out while in datastream. I took a video of this so you can interpret it yourself, as it would be easier to show you than to explain it. Keep an eye on the fuel trims, and the oxygen sensor outputs. It seems like they are not where they are supposed to be IMO... How bout in your opinion, and what would this point to?? Here's the video -
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