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Old 01-13-2011, 09:34 PM   #1
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Default '98 F-150 idles rough, dies at idle

This has been going on for some time, but it's starting to happen more often. During cold weather, my '98 F-150 idles rough. Now it's starting to die sometimes, especially when I'm coasting to a stop at a red light or something. Once I take my foot off the gas, the engine will slow down dramatically and sometimes just stop. It restarts instantly with either the key or by "push" starting while in motion.

Trouble codes are 171 and 174. I've been told by three different automotive people that "it sounds like an oxygen sensor". I purchased two today to replace the ones closest to the engine. There are two more closer to the rear axle than they are the engine.

I was told that a bad mass air flow sensor would kill oxygen sensors repeatedly. This was at Autozone, so I went outside and took a voltmeter to the MAF sensor. After that they told me it was the fuel pump.

The MAF sensor has 6 volts DC coming to it from the battery, and the output is 1.0 volts at idle speed. It increases to about 1.6 throughout the RPM range I use while driving, and if I rev it extremely high, the output is 2.0 volts.
I doubt it's the fuel pump, since when I shut down the pump at the inertia switch, the engine dies within seconds and will not restart. The vehicle does need a new fuel filter (it's that time again), but this has been going to through several times in the past I've had the fuel filter changed.

AZ's computer printout says the fault could be on "bank 1, sensor 1" and "bank 2, sensor 1", the MAF, the fuel pump, or the "engine mechanical condition".

The engine is a 4.2L Essex V6, mated to a 5 speed manual tranny, and it has a K&N air filter in there with that MAF. It has a tick under 200,000 miles on it. Dunno if it matters, but I run non-ethanol gas all. the. time. period.

I think I need to check and maybe replace all four O2 sensors, but I may be wrong. Any ideas what I need to do next?
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Old 01-14-2011, 01:48 PM   #2
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Default had a similar problem with my v8

I had a very similar problem where mine would start and stall out instantly or idle so low it would kill the engine coming to a stop. a friend that had an explorer had the same problem and he replaced (i did too) the idle air control valve. it had become gummed up with gunk and was causing the butterfly valve that regulated airflow into the manifold to not work properly. the problem was exacerbated with cold or very humid temperatures. I could start mine sometimes 5 or 6 times before the engine would stay running in the mornings. If it was dry or warmer sometimes that problem would not even manifest.

hope that helps
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Old 01-15-2011, 01:41 PM   #3
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I was actually thinking about that myself since it only acts up at idle speed. Once I even touch the gas, the problem goes away. It also goes away during the summer, for the most part. When it's really hot out and I have the AC going, the truck will lurch forward when the compressor kicks in, provided I'm moving less than about 5 MPH. I think the IAC opens a little when the compressor kicks on, then closes when it turns off.
Did your and your frien'ds Explorer give those same codes?

EDIT: Looking at Autozone's site, it says that one could have high or low idle if this goes bad. Now that I think of it, I do get high idle sometimes in the summer, but hardly ever in the winter. Sometimes it'll idle really fast when it's hot out.
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Old 01-15-2011, 07:51 PM   #4
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Default I remember the codes but...

I can't remember if bank one bank two and that pair of 170 something codes were the problem you describe or codes that were thrown due to a vacuum leak i had that was solved by just replacing a simple hose that had become overheated or worn somewhere on the intake manifold. Either way those codes sound familiar to me. Check any short tubes that are near the engine going into the intake manifold and see that they haven't become distorted by heat and have opened up. The tube I am remembering is small and short and it ran too close to something that got it hot. The combination of heat and vacuum caused the pipe to collapse and create a rupture that was kind of hard to see unless you pull the pipe out and examine it carefully.
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Old 01-26-2011, 01:26 AM   #5
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I had a problem like that on my 97 4.6 V8 a few years ago. It would always have a very low rpm at idle and it would often stall when I slowed down or turned while going slow. It even stalled on the highway going 65 mph, but it would stay running as long as I had my foot on the gas so I went to Auto Zone to get it checked. An O2 sensor went out so I replaced that and I was told that the airflow sensor might need to be cleaned but it wasn't dirty. Then I replaced my fuel filter and all of this barely did any improvement. Eventually I was told that I should check my throttle body because I had about 190,000 miles and hadn't ever checked it. Turns out that all I had to do was clean the gunk buildup off of my butterfly and that did the trick. Part of it might also have been gunk in the idle control sensor which had a small hole coming out by the butterfly (I think). I just used plain ole throttle body cleaner and a rag to wipe off the gunk.

I hope this helps you out if the other things posted already aren't working
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Old 01-26-2011, 08:34 AM   #6
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i think you need to look for a mechanical failure rather than an electrical component. Use choke spray and check for leaks around the intake system while the engine is idling, RPM will rise if the spray is pulled into the intake system.

If your engine has the black plastic plenium chamber between the throttle body and the engine itself, there is a TSB covering the attachment bolts and isolators. Am not sure if the 98 was within TSB coverage.
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Old 01-26-2011, 12:23 PM   #7
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Choke spray works, we used the ol' Burnsomatic torch, turn the flow on low and run it around all your joints. If you have a leak it will smooth out real nice.
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Old 02-22-2011, 07:03 AM   #8
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I am having a simular problem with my '94 when its cold it doesn't want to hold an idle, but when it warms up it doesn't want to idle under 2k. I already replaced the IAC any other thoughts on this one? (sorry didn't me to highjack this thread)
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Old 02-23-2011, 10:47 PM   #9
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I was having the same problem with my 2000 5.4l. After checking for vacuum leaks (no leaks) and replacing my maf sensor (stopped getting some trouble codes) it was still quitting when i came to a stop. Then I changed my throttle position sensor. When I removed it you could see it was worn at the location where it interfaces with the throttle body. Now no problems.
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Old 02-28-2011, 05:34 PM   #10
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Default Same low RPM issue, chug, stall

My '98 is doing the exact same thing as the first post described. Thanks for sharing the rest of the posts too. I found I had a vacuum leak and fixed that but it didnt solve the problem. Now I'm told the AC compressor is bad. It makes a lot of noise and will stall the truck if I turn it on at park or idle, but the original problem of the engine dying happens even if the AC or defrost is engaged. Can these all be related?

And to the talk about air flow and mass flow sensors, did that fix your problem of the engine running lean? Any good effects after? I dont know if the sensors just tell you how the engine is running, or if good sensors will tell the engine HOW to run, and be more effective. Thanks for the advice!
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