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Old 03-23-2012, 11:46 PM  
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Question SPI Rough Idle Engine Light p171

Ok I'm at a loss as to what this could be causing my rough idle and check engine light. This is a common issue and I've tried my best to research before posting but I've checked out most of what I've seen. I can't get this engine light to stay off and I have to complete emissions testing by April.

I drive an 02' sedan SOHC SPI Engine with about 155,000 miles on it.

What's happening at idle is the engine is surging slightly. Over all the idle is slightly rough but nothing too bad. But if I leave it idling for say 3-4 minutes it will stumble pretty hard and then sort of recover, only to stumble again every couple minutes. Engine is smooth otherwise, still has decent power.

Originally I had a p0171 code (about 2 months ago.) So I figured it was a vacuum leak, or the PVC elbow problem, but I had the elbow replaced about 2 years ago and it is in good condition. So next I suspected the injectors needed cleaning so I got a home flush kit from Berryman, cleaned the throttle body, ran treatment in the tank, and a flush through a main vacuum line (PCV line). I also changed the fuel filter. I didn't really see much of an improvement, though there was a TON of grey smoke out of the tail pipe when I was running the flush through the vacuum line.

Having no luck with that I noticed a large crack in the MAF sensor housing (in between the air box and intake tube.) I figured that was definitely the issue. I got a new housing at a scrap yard cleaned my MAF sensor with cleaner, and installed it in the new housing, but again nothing changed and the light was still on and I noticed no improvement at idle. That's when I took it in for a diagnostic.

There they did a smoke test and found no vacuum leaks. They pulled codes p1131 and p0401 which I thought was strange since it's always been a p0171. Anyway, upon their recommendation I installed new O2 sensors and a new DPFE sensor, they said they tested my EGR valve and it was fine. Did the O2 sensors yesterday and the DPFE today (had to wait for a new sensor connector to wire as my old connection is apparently outdated.) Anyway if anything connecting the new DPFE sensor made the car idle worse. I was driving around with it disconnected and I really felt like the idle was better than having it connected. That could be in my head though. I was told to replace the two hoses coming from the DPFE to the EGR but I took them off and they seem fine. Should I do this anyway?

Overall I'm at a loss as to what the issue is. Ive done a tune up in the past 2 years. Changed the plugs to Autolight Iridiums, the wires are Autolight 8MM, the coil pack was changed, PCV still clicks when shaken so it seems good. Not sure if this is relevant, but I've noticed on the three bolts atop the valve cover, there is a small leak on the two outside bolts and oil is slowly seeping out. Not sure if this is a huge issue or not, or even related.

Any help is greatly appreciated, sorry for the length but I didn't want to leave anything out. Thanks again.


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Old 03-24-2012, 10:18 AM  
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I wouldn't assume parts are still good simply because they were replaced recently or look OK. With these types of problems, you have to be really methodical and re-examine and test everything.
Thoroughly test that your EGR system is working properly: http://easyautodiagnostics.com/ford_...gr_valve_1.php
If the EGR checks out and you're confident there are no intake leaks after the MAF I would suggest turning your attention to the IAC valve. Especially on higher mileage cars, it could be sticking or need cleaning. A rough preliminary check is to unplug it and see if the idle situation (the loping) improves.
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Old 03-24-2012, 12:27 PM  
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Thanks for the link, very good information there! I'll have to buy a multimeter and a vacuum pump to do the EGR system tests but worth it I think (still building my tools and def need a multimeter...) For today I will look to the IACV, I didn't think of that! It is connected or near the throttle body correct, like a cylinder type shape?
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Old 03-25-2012, 07:53 AM  
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On the SPI, the IACV is just below the TPS on the left side of the throttle body, looking at it front the front of the car. You can remove it from the top, 2 8mm bolts and watch the gasket in there too.
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