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2005 STS V8
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165 Posts
Discussion Starter #1
OK this has been annoying me for months and now that it's warming up it's time to hunt it down.

'05 STS V8, under 60K miles, runs fine drives fine starts and idles fine. Gets a P0171 and sometimes also P0174 trouble code, at seemingly random and infrequent times. Have tightened the intake manifold bolts to spec, checked all hoses and intake fittings, did the carb cleaner spray test and haven't identified any leaks. I do suspect the MAF a bit as I had to clean it once before when it acted up (also used that opportunity to clean the throttle body).

Any other ideas? I've been searching for tips on what else to check, but haven't come across an "aha!" answer. Thanks!
 

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2005 STS N*
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125 Posts
It sounds like you did everything I did, and then I had an Aha! moment myself. These codes had me flustered for a long time until I replaced the MAF. I found an AC Delco one on Amazon for $70 and then boom. Problem solved. Just make sure your air filter is also an OEM or it will ruin the MAF!

I would also recommend replacing the fuel filter as well.
 

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2005 STS
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33 Posts
I know Cadillac's can be finicky about the parts you use in them, and I've read the different threads about filters, AC Delco vs others. I'm having a hard time understanding how a Fram or Purolator air filter could cause any big problems. I'm going to change my spark plugs in the coming weeks and will use AC Delco plugs and will probably do the brakes before it has to be inspected in June and again I'll go with AC Delco pads. Not every parts store out there sells AC Delco parts and I don't want to have to order all my parts online and I won't pay dealer prices for them either.
 

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Premium Member
2006 STS V8 AWD, '95 Ford Ranger
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29,061 Posts
I know Cadillac's can be finicky about the parts you use in them, and I've read the different threads about filters, AC Delco vs others. I'm having a hard time understanding how a Fram or Purolator air filter could cause any big problems. I'm going to change my spark plugs in the coming weeks and will use AC Delco plugs and will probably do the brakes before it has to be inspected in June and again I'll go with AC Delco pads. Not every parts store out there sells AC Delco parts and I don't want to have to order all my parts online and I won't pay dealer prices for them either.
The problem air filters are those with the elements that can be cleaned and reoiled such as the K&N. The oil contaminates the MAF.
 

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2006 STS 1SF N* RWD NAV F55
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705 Posts
Thanks Guys, I will try to clean the MAF, sensor, but expect to replace it soon.
I have about 89,684 Miles.2006 Northstar.
My CEL was on for a long time, I let it be there until i did
another repair, and reset it, and it has been OFF for months now,
Let Firestone do a Diagnostic, with Smoke, and NOTHING showed up
Happy Motoring. :):stirpot::)
 

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2005 STS V8
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215 Posts
Have you checked the vacuum line that goes from the valve cover to the back of the engine on top? Mine was cracked where it bends to go into the back of the intake and was hard to find.
 

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2005 STS V8
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165 Posts
Discussion Starter #7 (Edited)
I did check that line awhile back -- removed it (kinda hard to get it off back there) and inspected it closely, I will do that again though.

The CEL came on again a couple days ago but today I plugged in the code reader and there were no codes stored...

I hate throwing parts at a problem, but I think I will go ahead and replace the MAF sensor considering it is 10 years old. I remember awhile back, a friend with a Crown Vic had recurring CEL issues, we cleaned his MAF a couple times and it would come and go, but the problem was only solved when he bought a new MAF.

Does the N* throttle body have a serviceable IAC valve on it? Thanks for the feedback!
 

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2005 STS V8
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165 Posts
Discussion Starter #8 (Edited)
I believe I've solved my issue. I replaced the rubber ends of those two PCV lines on top of the engine, although only a couple of them showed a tiny hint of cracking. Generic 3/8" fuel line & elbows from the auto parts store work great for these. I also went ahead and replaced the MAF sensor as well as the throttle body, although the TB was unnecessary; but it's a surprisingly inexpensive part. The "aha" moment came when I made the problem worse instead of better -- turns out the larger rubber hose that goes to the brake booster was loose, and I inadvertently knocked it off when I was monkeying around with the vacuum line that runs behind the intake manifold. The engine ran like hell and would not idle correctly until I figured this out. Popped the brake booster hose firmly back in place, cleared the trouble codes, and it's running good as new :) So if you are getting random System Too Lean codes, that hose back there is another one to check.
 

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2006 STS 1SG & 2007 STS 1SE
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7 Posts
Your first post did provide the clue. If the trouble isn't loose intake manifold bolts, where else is that air getting in?

Some Mod ought to update the 'Sticky' on intake manifold bolts...
 
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