P0172 with rough/ low idle (1 Viewer)

Dirt Ferguson

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I was taking my 97 LX (251K miles) over the hill to get a new windshield this afternoon. 10 minutes into the drive the CEL came on, but it was driving fine. Once I climbed the hill (1000 ft of elev gain) and dropped back down into town, while I was stopped at some road work, I noticed while idling, my RPMs dipped below 600, then below 500. I put it in N and kept the RPMs up to keep the engine from running rough. I got my windshield switched out and driving it home I had to keep my RPMs up in city traffic so the truck would not dip below 500 at every stoplight, causing the rig to shake and feel rough.

Once the truck was up to speed, it drove great. I powered up and over the hill without issue. When I got home I plugged in my OBDII scanner and it came back with P0172.

I’ve read the other P0172 threads and I’ll try cleaning the MAF area and look at the exhaust, but no one else seems to have issues with low and rough idle. Am I missing anything in this diagnosis?
 
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Woodland Hills, Ca
I was taking my 97 LX (251K miles) over the hill to get a new windshield this afternoon. 10 minutes into the drive the CEL came on, but it was driving fine. Once I climbed the hill (1000 ft of elev gain) and dropped back down into town, while I was stopped at some road work, I noticed while idling, my RPMs dipped below 600, then below 500. I put it in N and kept the RPMs up to keep the engine from running rough. I got my windshield switched out and driving it home I had to keep my RPMs up in city traffic so the truck would not dip below 500 at every stoplight, causing the rig to shake and feel rough.

Once the truck was up to speed, it drove great. I powered up and over the hill without issue. When I got home I plugged in my OBDII scanner and it came back with P0172.

I’ve read the other P0172 threads and I’ll try cleaning the MAF area and look at the exhaust, but no one else seems to have issues with low and rough idle. Am I missing anything in this diagnosis?
I have a 96 and my 0172 code just came up a couple of days ago and I too am getting rough idle when stopped. It can be a whole list of things other than cleaning your MAF.
Faulty Front Heated Oxygen Sensor
Faulty Spark Plugs
Faulty Mass Air Flow (MAF) sensor
Loose vacuum hose(s)
Exhaust Gas Leaks
Ignition Misfiring
Incorrect Fuel Pressure
Faulty Fuel Injectors
Timing


I'm going through this list one by one. For me, I've been having a P0420 come up last 2 years. It's the front Cat. By now, my cat has likely getting so bad that's it's now affected the oxygen sensor and running to rich and now throwing the 0172 code. I'm replacing my front cat tomorrow and see what happens. If I still get the 0172 code I'll replace the front 02 sensor.

**Update: May 14: My front cat was fried-literally. Replaced and welded in. We'll see if the gas mileage improves, 0172 and 0420 codes don't return.

IMG_5699.JPG
 
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Dirt Ferguson

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I’m glad that I’m not alone in this. Thanks for the list of possible triggers. Here are a couple more data points from my situation:

— The CEL came on about 10 minutes after I filled the tank with 91.
— After I ran the code at home, I cleared it, came inside, ate dinner and let the engine cool down. 30 minutes after first coming in, I fired the LX back up and took it for about a 30 minute drive.
— It was super sluggish to get up to speed at first. I practically had the pedal floored and it was barely gaining speed. That lasted for the first 5 minutes and the rest of the drive was pretty much as normal. I even stopped on the side of a quiet road, left it in D and with my foot on the brake sat there for about 3 minutes and the RPMs stayed roughly around 700. They would dip below 600, but only for a split second and then bounce back up to around 700.
— It’s in the driveway for the night. I’ll see what comes with tomorrow.

Thanks again for your input, Gil.
 

Dirt Ferguson

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To update on my situation. I took the LX to the store yesterday, after work. It fired up great, acceleration out of the driveway and down the road wasn’t a problem, got up to 45 mph as it normally does, and it drove the 10 minutes to the store without issue.

On the drive home, the CEL came back on (p0172) and the idle was all over the place at stop signs. I had to rev the engine to keep theRPMs up around 600. Once I got home I sat in the driveway with my foot on the brake, in D, and watched the RPMs dip, dip some more and then the rig shut down.,

Any thoughts on why it would drive fine for 20+ miles after I cleared the code?

And, to address the real problem, do you think this is fuel pump / fuel pressure regulator related?
 

landtank

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P0172 indicates a rich system condition. It could be a faulty O2 sensor and just as likely a bad wiring harness causing an injector to over fire.
 

Dirt Ferguson

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Thanks Rick, that’s definitely something I’ve considered. Right now I have new plugs, and a fuel pressure regulator on order. I’ll add the upstream O2 sensor. Wiring harness will be last (most expensive) resort, but it would sure give me great peace of mind moving forward for all things electrical on the engine.
 

Dirt Ferguson

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97 LX450

An update on this issue: Last week I replaced my upstream 02 sensor, my fuel pressure regulator, and cleaned the MAF.

I cleared the code and the truck idled perfectly at 650, and it has been driving flawlessly since then. My wife and I are actually on a little road trip right now and I've driven the truck about 600 miles and it's been fantastic. We got into town yesterday afternoon and there's no CEL, but I'm back to dealing with a bad low/ rough idle. I'm confused as to why the. truck would drive so.well for 600 miles and then revert back to the low/ rough idle.

With no code I'm flying a little blind. All the vacuum hoses I can see are in good shape.

Any insight would be appreciated.
 
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Any updates? My vehicle just started having low idle issues, sputtering while stopped at a light & a bit upon acceleration. But it's inconsistent. Sometimes I'll be at a light and while it low idles, it won't periodically drop (what I am calling sputtering while stopped) and so if it doesn't do that, it doesn't have any issues while accelerating. It only just started last night and I only drove about 15 miles, so I'm not sure of any pattern yet. For years the CEL has turned on and off, apparently at leisure, with no idle issues. When it first started, years ago, don't recall how long ago anymore, I had the O2 sensors replaced, but it came back on right away. Mechanic I was using at the time told me I probably had a short, and we let it alone.
 

Dirt Ferguson

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@KO KO someone who knew the PO of my rig reached out to him after reading my road trip obvs thread, the PO is also a MUD member and he texted me to offer some guidance. He dealt with the same 172 in the past, and it was a loose MAF connector. I did recently unclip the MAF connector when I cleaned the sensor a few weeks ago.

The PO encouraged me to re-attach the MAF connector and make sure it was seated evenly with a secure connection. That was the problem for him in the past and he said he was 90% sure it would remedy my issues. The thoughtful advice of people on this board came through yet again, for now......

The LX is driving perfectly right now. Many people in this thread and in my road trip obvs thread commented the condition could be wiring related/ electrical, so my fingers are crossed it's as simple as the connector.
 

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