single digit fuel trims are completely normal, it'll fluctuate based on a bunch of things - temperature, moisture in the air etc.
1996 Taurus GL intermittent low idle (not stumbling)
I've been scratching my head at this for a while now. From what I've been reading here in the forums, it should be idling around 900 in P/N, which it does most of the time no problem, but every now and again it'll just decide it wants to idle at 600-700 in P/N (what it should do in R/D and does do), even after a restart or several drive cycles, sometimes lasting an entire day, seemingly randomly comes and goes really. It doesn't affect anything negatively that I've seen, still drives fine and makes all it's power
Some background: over a period of time I had my bank 1 O2 sensor go bad and eventually my DPFE (p1131 and p0402, codes came up once and never again) so I replaced both upstream O2 sensors with Bosch units and the DPFE with a Standard Motor Products (which I now realize I should have gotten a motorcraft one, but it's well too late for that, this one seems to be behaving) and that allowed me to pass smog. Sometime after the DPFE code though it started to idle low intermittently like described. I had assumed it was due to running really rich since disconnecting the egr vacuum line doesn't affect the idle (passes the suck test though) at any point. It was actually so bad it would idle at 500 until I replaced both O2 and dpfe and now it only goes as low as 6-700 (typically 700). It didn't then and doesn't now stumble or anything, it's quite smooth actually. Anyway replacing those sensors largely fixed my rich condition and I get really close to the mpg I used to (got as bad as 12 before I replaced anything, now it's around 18-19). But the idle issue remained, so I did all the research I could on my own to figure out what it might be. Cleaned MAF and I don't suspect it to be an issue, went ahead and replaced the IAC recently (Motorcraft, as per forum recommendations) and that seemingly helped but just today it decided to again idle down to 700. It almost feels like the PCM is intentionally targeting the RPM because all else is normal. Unplugging TPS while running doesn't change anything and that's been replaced a year ago due to the original one causing a high rpm condition.
Spark plugs are also new as of 10,000 miles ago (6 months ago) they're a bit excessive being Bosch double iridium's but the car came with 3 brand new ones in the trunk when I bought it so I bought 3 more lol.
Knowing this car I'm not ready to completely rule out the EGR, although the behavior seems so deliberate that I'm not certain of it being that either. TPS is on my maybe list though since it's not Motorcraft (duralast...). My scanner says it's at 17.6% at closed but today I noticed it jumping between 17.6 and 18%. From what I understand 17.6 is the normal closed value but I could be wrong.
Another thing it does is vibrate like hell in drive and at a stop, and only a dead stop. Let the car move even a little and it goes away. Doesn't do it in reverse or P/N. I'm not convinced this is related though since I do have a collapsed rear motor mount (although it didn't do this either when I got it in good running condition with front and rear collapsed and trans mount separated. Yes I actually bought it like that and I absolutely consider myself dumb for the conditions I bought it under, but that was 2 years ago now and 35,000 miles ago, front and trans mounts since replaced)
I do apologize for how rambly this post is, and it's possible I'm overlooking something obvious or it's so complex I should just take it in for a proper diagnosis. But I'm curious if anyone here might have any ideas what it could be.
Oh ha I also forgot to mention that in drive at current, if the fan kicks on the RPMs would dip to 500 and climb back up, new IAC seems to have fixed this though. Reacts just fine to turning on and off AC
Edit: forgot to add I have been looking at my fuel trims and with my admittedly limited knowledge they look alright to me. I could log a drive cycle with my fuel trims if anyone wants me to (I don't recall the exact numbers when driving) but at idle I get something like this: (Approx)
LTFT
B1 1-4%
B2 2-6%
STFT
B1 -3-3%
B2 -2-4%
I'm approximating from memory but that's around where they hover once warmed up and settled in, at 900 or 700 rpm in P/N AC off
Some background: over a period of time I had my bank 1 O2 sensor go bad and eventually my DPFE (p1131 and p0402, codes came up once and never again) so I replaced both upstream O2 sensors with Bosch units and the DPFE with a Standard Motor Products (which I now realize I should have gotten a motorcraft one, but it's well too late for that, this one seems to be behaving) and that allowed me to pass smog. Sometime after the DPFE code though it started to idle low intermittently like described. I had assumed it was due to running really rich since disconnecting the egr vacuum line doesn't affect the idle (passes the suck test though) at any point. It was actually so bad it would idle at 500 until I replaced both O2 and dpfe and now it only goes as low as 6-700 (typically 700). It didn't then and doesn't now stumble or anything, it's quite smooth actually. Anyway replacing those sensors largely fixed my rich condition and I get really close to the mpg I used to (got as bad as 12 before I replaced anything, now it's around 18-19). But the idle issue remained, so I did all the research I could on my own to figure out what it might be. Cleaned MAF and I don't suspect it to be an issue, went ahead and replaced the IAC recently (Motorcraft, as per forum recommendations) and that seemingly helped but just today it decided to again idle down to 700. It almost feels like the PCM is intentionally targeting the RPM because all else is normal. Unplugging TPS while running doesn't change anything and that's been replaced a year ago due to the original one causing a high rpm condition.
Spark plugs are also new as of 10,000 miles ago (6 months ago) they're a bit excessive being Bosch double iridium's but the car came with 3 brand new ones in the trunk when I bought it so I bought 3 more lol.
Knowing this car I'm not ready to completely rule out the EGR, although the behavior seems so deliberate that I'm not certain of it being that either. TPS is on my maybe list though since it's not Motorcraft (duralast...). My scanner says it's at 17.6% at closed but today I noticed it jumping between 17.6 and 18%. From what I understand 17.6 is the normal closed value but I could be wrong.
Another thing it does is vibrate like hell in drive and at a stop, and only a dead stop. Let the car move even a little and it goes away. Doesn't do it in reverse or P/N. I'm not convinced this is related though since I do have a collapsed rear motor mount (although it didn't do this either when I got it in good running condition with front and rear collapsed and trans mount separated. Yes I actually bought it like that and I absolutely consider myself dumb for the conditions I bought it under, but that was 2 years ago now and 35,000 miles ago, front and trans mounts since replaced)
I do apologize for how rambly this post is, and it's possible I'm overlooking something obvious or it's so complex I should just take it in for a proper diagnosis. But I'm curious if anyone here might have any ideas what it could be.
Oh ha I also forgot to mention that in drive at current, if the fan kicks on the RPMs would dip to 500 and climb back up, new IAC seems to have fixed this though. Reacts just fine to turning on and off AC
Edit: forgot to add I have been looking at my fuel trims and with my admittedly limited knowledge they look alright to me. I could log a drive cycle with my fuel trims if anyone wants me to (I don't recall the exact numbers when driving) but at idle I get something like this: (Approx)
LTFT
B1 1-4%
B2 2-6%
STFT
B1 -3-3%
B2 -2-4%
I'm approximating from memory but that's around where they hover once warmed up and settled in, at 900 or 700 rpm in P/N AC off
1 - 6 of 6 Posts
600 rpm to 700 rpm is where your idle should be fully warmed up.
Interesting that you lose rpm when the fan kicks in. Fans require a decent amount of current so perhaps something up with your belt / pulley / tensioner / alternator / battery / wiring.
I would log rpm desired and rpm actual and see if and where they deviate. Could log speed, transmission gear, coolant temperature, transmission temperature as well to see if there is any pattern to deviations.
Interesting that you lose rpm when the fan kicks in. Fans require a decent amount of current so perhaps something up with your belt / pulley / tensioner / alternator / battery / wiring.
I would log rpm desired and rpm actual and see if and where they deviate. Could log speed, transmission gear, coolant temperature, transmission temperature as well to see if there is any pattern to deviations.
Outside shot. I had a problem like this and it turned out to be a bearing going bad on an idler pulley. It would drag down the idle just a little and car felt just a little robbed for acceleration.
Question is: is this documented anywhere in the Workshop Manual?From what I've been reading here in the forums, it should be idling around 900 in P/N, which it does most of the time no problem, but every now and again it'll just decide it wants to idle at 600-700 in P/N (what it should do in R/D and does do), even after a restart or several drive cycles, sometimes lasting an entire day, seemingly randomly comes and goes really.
I just checked this on my fully warmed up 2000 Vulcan.
The idle RPM is either 642/min or 706/min. Apparently either the PCM or my OBDII monitor only uses 1 byte for the RPM with the maximum at 16,384/min, which means 1 bit = 64/min.
In my case the RPM does not depend on the gear, A/C ventilator, lights or brake lights.
So I think rjacket is right. If there is anything out of the ordinary it is the 900/min RPM. I vaguely remember that the PCM raises the RPM during parking to correct for the additional burden the power steering puts on an almost idling engine. Also in models after 2002 the PCM can dynamically adapt the alternator functioning. Perhaps it is raising the RPM to charge the battery.
Could be a failing a/c clutch. Perhaps try a start, keep it under 5 minutes, with serp belt off?
1 - 6 of 6 Posts
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