Weird Idle

I have no codes...everything seems to be fine...but when i start the car or when idle, the rpm seems to be at 1000-1200, then after a while they suddenly drop to 700 or 800...at one point waiting at the redlight it was idling at around 1100 rpm then it suddenly dropped the rpm so low that the car turned off...just turned it back on and everything was fine again...i had already fixed a rough idle before with a purge valve and the iac..so what can this be?

What year is your Taurus and which engine does it have?

-Rod

sorry i forgot to post that...it is a 96 mercury sable GS 3.0 v6. im not sure if its the DOHC or the other engine option though.

If it's the DOHC engine there is probably a badge on the front fenders that says something like "24V DOHC".

You could have a vacuum leak which might be due to a faulty DPFE sensor or sticking Intake Manifold Runner Control. I'm trying to remember if either of these apply to the 1996 DOHC....

-Rod

yeah, i use to get two different engine codes before..and they were the dpfe sensor and the purge valve..i fixed those....and changed the iac valve while i was at it...now no engine codes....it feels a lot better..but it started to do the idle rpm high low thing again like i explained above...but this time with out engine codes....so even though theres no engine codes it could still be the dpfe sensor? or should i start with the sticking Intake Manifold Runner Control?

If it were my car I'd start by carefully inspecting all the vacuum lines, then checking for any other air leaks after the MAF sensor. A co-worker once had a crack in the accordion portion of the intake air tube that caused an air leak only when under hard acceleration, so a careful inspection of the intake air tube would be worthwhile as well.

And to answer your question regarding if a sensor can be bad without a code, yes. Codes are only set when readings are outside some range of expected values. You can have symptoms due to failing sensors without any codes being set. If the DPFE is pretty new I wouldn't jump to that yet though, but I would suggest carefully checking the vacuum lines to it.

-Rod