Intermittent P0171
 

So, I purchased a used MT RX-8, even after researching it to death. Know it may have issues, but the car drove very well in the test drive, looked great, still smelled of new leather even. Weeee!

2 days later, out driving in 95 degree weather, all is good. Stop to get some groceries, start the car, and WHAMO! CEL comes on. Autozone reports P0171, bank 1 lean.

Great. Lists possible causes as (1) fuel pump, (2) vacuum leak, (3) MAF dirty/malf or (4) engine go boom.

So, the cheapest route was to clean the MAF... it wasn't dirty, but cleaned it anyways. Put it back on, pumped the brake 20 times... light still on. Let the car cool off, take it out the next day... the CEL is gone.

At this point, I'm thinking, well, either it was the MAF, or a gas cap wasn't on (I had filled up that day). Panic subsides.

Take the car to gets its emissions test before I can register . No CEL on when I park the car at the shop. CEL on when they start it. I had driven 30 miles from work to the shop near my house... so engine was plenty warm. Oddly, the first time the CEL came on, I had been running errands in warm weather.

Pissed off again. Made appointment for next week with Mazda. CEL on this mornign when I started up. Stopped, got gas, started back up, CEL still on. Car runs great still, pulls well... a little rough on the idle.. but seems somewhat normal and plugs/wires on the way and possibly coils if that doesn't shape it up.

Go out to lunch, CEL still on. 5 hours later, leave work (it's 90+ today outside)... and CEL is GONE! wtf.

Dealership I bought it from finally returned my call... and said "I talked to a mechnic buddy at Mazda... says it probably an O2 sensor."

Any thoughts on the probably cause? Without the CEL on, I'm afraid the mazda folks will charge a ton looking at random things... only for me to go fail emissions again. Emission check, btw, reported ok on the cat, the 02 sensors, but just the damn P0171. Too bad I have to prove I spent money to fix something, or I'd just drive it back to the shop, let it cool off, then let them emission test it again :P Either way, I'd rahter get this fixed than blow a cat.

Is this an intermittent O2? An intermittent MAF? Or, can a fuel pump that is failing be effected by temperature changes? Random, hard to find vacuum leak?

Again, runs great (best I can tell). looking under the hood.. only one odd thing:

there are two little "knobs" near the intake manifold(? I think... large plastic looking housing leading from the airbox, to side of engine.. but on the metal part..)... I think this is where compression test thingys are plugged in. One is capped with a black rubber cap. The other (the one closer to the rear) has no cap. is this normal? Can this cause an intermittent vacuum leak?

Here's the capped vs uncapped -- crudly circled in white. Left is uncapped (obviously). Attached pics too in case the flickr link fails.

http://farm3.static.flickr.com/2647/...b4a00ebccc.jpg

Nope. need a cap on both.

Haha.. think it can cause the intermittent P0171?

Yes! you are sucking in air.

P0171 = system too lean.

Went to dealership, they gave me the cap for free, plus an extra.

Other than bending my arm in unnatural ways to squeeze my hand in there, I put it on.

Did the brake pump and odometer reset (it idled worse after installing the cap... coincidence?) and driving it about 60 miles, idle seems to have smoothed a bit and all seems good.

No check engine light yet, so we'll see. Taking it back in for emissions -- so if it's going to come back on, it will do it right as I pull into their parking lot. Let's hope irony is napping.

This is exactly the problem I'm having i think. I've been doing a lot of research, cleaned my MAF, and used fuel injector cleaner all with no success. I don't notice any symptoms other than MAYBE the slight smell of fuel on some startups and a little bucking at very low rpms in first gear. I'm not sure if these are related but I'm no mechanic. I've done a lot of reading on this forum and your issue seems to match mine most closely. Intermittent P0171, especially during warmer temps. Car idles fine and I don't notice anything else. Where are these knobs you found specifically? Or if someone else knows, I'd really like some help.