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| Devon-Rover Member Since: 22 Jan 2015 Location: South Devon Posts: 923   
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The Idle system is fairly basic on the 3.9 and changes to idle can usually be associated to a variation on airflow into the engine.
The engine works on the basic principal that it see's X airflow through the MAF sensor it knows the throttle position on the intake butterfly and it knows that certain amount is sent through the bypass hose and past the IACV (the stepper motor valve on the back of the plenum) this gives a Y airflow amount and it injects Z amount of fuel to hit the determined engine speed 700 ish RPM ~ 50 So I would check that there isn't any air leaks but TBH they usually cause uneven idle rather than a flat steady abeit lower one. On cold startup the revs should climb and you should hear it step down in stages down to idle when warmed. The IACV will move in and out to vary the amount of air and thus the idle speed. You could just reset the Base idle and assume all is well but to me they don't just change because they feel like it. It is a long days work but if you don't know the history of the engine then maybe it'll be a good idea to go through all know associates to check all is in order. Soo: Engine breathers clean. Bypass hose is sealed and not gone hard so possible leaks. Vac hose to FPR is sealing and not gone hard. IACV works freely Distributor vac advance working fine Distributor static timing correct. Finding something amiss means really rectifying it and start from the top with getting all the settings right and the base Idle set. I could write it out but this link spells it out well. http://www.roverparts.com/TechTips/base_id...cedure.cfm Also can be found on Fb, Ytube, Insta & Twitter @4WDSouthwest |
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22nd Jan 2018 9:20pm |
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| Romadog Member Since: 07 Jul 2011 Location: Powys Posts: 1750 |
As well as the above make sure that your battery is fully charged and holding its charge as this can affect the sensors working correctly.
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23rd Jan 2018 8:17pm |
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| big ives Member Since: 24 Dec 2013 Location: Northern Ireland Posts: 125
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Romadog
U wouldn't believe this hadn't read your post until tonight however today after checking what Devon_rover suggested l decided to swap my battery as the deep cycle oddessy I had in her seemed to be down in volts I know it not the alternator as its only been replaced last month 
Anyway another oddessy battery which was fully charged fitted and she's running sweet Checked for battery drain fully charged battery getting 0.03 amps on the meter Can't believe a low battery would make her run low rpm  2016 Discovery 4
1989 90 CSW V8 (fully rebuilt and running like a dream) 1996 defender tray back fully rebuilt/loaded for challenge events |
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23rd Jan 2018 11:32pm |
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