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High and rough idle and vents only blow out defrost vent

posted May 08, 2016 16:29:55 by Brian Martin
I have a 1998 ford ranger 4.0 liter v6 xlt 4x4.I have 2 problems one is there is a high idle at start up goes as high as 2.5 rpms then settles down after about a minute of running but then it still idles rough I ran the codes got a p0455, p0171, p0174 and the other problem is when I turn on the ac or heat air only comes out of the defrost vents the blower works on all settings but no air goes on the other settings like the floor or the front vents only blows up at the windshield but I still feel some heat coming out of the other vents but not hard enough  it seems like it's stuck on heat setting because the cab inside gets hot and I need to put the windows down because it gets hot the air conditioning doesn't work I look and see the compressor clutch is not spinning I will work on getting the air conditioning working later after the air goes in all the vents
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3 replies
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Tyler said May 08, 2016 18:14:54
Hey Brian!

Given the various symptoms you have, I'm tempted to tell you to ignore the P0455 (Evap Large Leak) for now. The P0171 and P0174 indicate a lean condition, and may be associated with the HVAC symptoms you're experiencing.

ScannerDanner has several excellent videos on diagnosing these very codes, if you haven't seen them already:

https://youtu.be/9gJoH5_Thaw

https://youtu.be/nYN-RoFVwAA

https://youtu.be/K-on2hbYUCc

The high idle, rough idle, inoperative HVAC mode door and code all potentially point to a large vacuum leak. I don't know to a certainty if your truck uses them, but vacuum door actuators were very common in the 90's, so a lack of vacuum to the HVAC system could easily explain why some functions aren't working.

If you can provide us with some scan data/test results, or even freeze frame data from the P0171/P0174 codes, we can help you further.
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cheryl hartkorn said May 08, 2016 19:36:30
agree with tyler. sounds like an intake leak. very common on the explorers with the 4.0 as well
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Noah said May 08, 2016 23:52:53
The P0171 and P0174 indicate a lean condition, and may be associated with the HVAC symptoms you're experiencing.


I believe there's a vacuum block near the heater core box under the hood on the passengers side. Could even be the vacuum source for the purge valve as well as the HVAC system. I could see that being central to all of your symptoms.

I would be interested in freeze frame data as Tyler has already mentioned. Also fuel trims at idle as compared to trims at 3000rpm, and also IAC counts/position at idle.
Massachusetts, USA
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