Hi, I have my autometer boost/vacuum gauge hooked up on my powerdyne charged f-150 and I was wondering if it is normal for the vacuum to drop when the a/c is turned on? With out the a/c on, my truck idles in drive at 20 in/hg. Then when I turn the a/c on it idles in drive at 17.5 in/hg. Is this normal? Thanks.

 

Yes, this is normal, when the ac comes on the idle drops a little and the engine load is slightly higher. Vacuum is a product of engine load so this is what should be expected. I have a powerdyne on my 4.6 and get the exact same readings, approx 20 inches in drive and slightly less when the AC comes on, and slightly more in neutral.

 

Ok, just making sure. I am trying to diagnose a wierd problem I've been having with the idle. It will drop below where it usually sits, at 650rpm, when comming to a stop at a traffic light or slowing down to make a turn. It will drop to about 500rpm. Once it went all the way to about 400rpm for a couple sec, then it jumped back up to 650. Problem is it dosen't happen all the time, only every now and then. I figured it might be the IAC valve so I went ahead and replaced it, but that wasen't it.

 

Are you on stock tune or do you have a custom tune and larger injectors?? You may want to clean the throttle body and plate sometimes you will have an accumulation around the throttle plate that does not allow enough air to bypass the throttle body and the idle air control cannot compensate. I have a similar problem on a hot restart but it is something in my custom tune, It only does it at start up and then settles down, I have learned to live with it. I did read on a mustang forum where they open the throttle blade a little more at idle and this seems to help, I have been going to try that but haven't had the time or urge to get at it. Mine only acts up in hot weather and under certain conditions. Mine started with my 90mm mass air tune, I never had a problem until then, there must be a glitch in the tune that makes the idle hunt at start up. I also tried 2 new idle air valves and a new DPFE sensor It is much better when you clear the computers memory and then as time goes on it seems to learn bad habits. You may try clearing the memory and see what happens.

 

Yes, I have 42# injectors, 90mm Maf, and a custom tune by Mike Troyer. Come to think of it, it really never happened till this recent revision of my tune. Mike was trying to fix rich codes that the computer was throwing on banks 1 and 2, so he made a change to the tune. The check engine light is off and I do not have those codes anymore, but now I have the idle problem when comming to a stop. I think you may be right, that it could be the tune. I'm going to try and load back in the previous tune and see if it still does it. Thanks.

 

My issues never started until we changed to the 90mm mass air, when i had the 42# injectors and the first set of tunes it was perfect Mike did some changes and still OK then the truck was making serious power and pegging the mass air and Mike suggested the 90mm , he sent me one and the tunes and it did it upon start up and ever after. I have talked to Mike several times about this, and he is adamant that it is a truck issue and not a tune issue, WLF had the same problem and since has gone back to stock injectors and FMU and also has had no issues. I like the way my truck operates and runs except for the starting issues, and I am a little rich at wide open throttle, so for now I have just decided to stop throwing money at it and leave it alone, some day I may re tune who knows . As I have said the starting and idle hunt are my main issues. I maight add that sometimes it will not start when hot unless you hold the throttle about 1/4 then it will start and you maintain 1500 to 2000 rpm for 20 to 30 seconds and all is well until the next hot start. I have never said much on the forums about the problem as I don't want to be perceived as bashing Mike's tunes, as I said the overall performance is excellent he just needs to find out why these perticular truck seem to do this, every one I have talked to seems to have trouble has a 4.6 maybe the 4.6 computer can't understand the 90mm mass air strategy??