Thread: P0171 and P0300 Codes, Mildly Rough Idle, Rough on starts, Slightly Elevated Idle

Ok guys, break out your reading glasses this is going to be a novel...

So I bought a 2003 GTP with a blown motor (no big deal) and rebuilt it with all new rings and bearings and new heads and got it all together and once I fixed an injector wiring plug it ran like a champ. Driven it for about 1200 miles not and not a hiccup until about 2 days ago. I noticed that one day when I went to start it it struggled, it fired off fine but struggled to get to idle RPM. It would bounce between like 500-1200 rpm for about 2-3 seconds wanting to die and then would idle a little high and a little rough. Not bad at all, but knowing how a motor should run its ever so slightly rough. It does this almost everytime I start it (wanting to die) sometimes to lesser degrees, but always has a higher idle. Car drove fine - so I thought. More on that in a min.

Then yesterday I was waiting for a buddy of mine and was just sitting in the car listening to music and then I went to start the car and it popped to life, then about 1-2 seconds later died, tried to start, fired off then died, did this once more before I started it and gave it some gas and then it got going and stayed running (idle about 1000-1200 rpm or so). Kinda thought this was a little odd. well on my way home about an hour after this it through a CEL. Got home and checked Codes and had a P0171 (bank 1 lean) so I started looking for vac leaks and basic sensors (MAF and TPS). TPS seems to be working fine (scanner readings) unplugged the map, and plugged it back in and the car did all kinds of odd things for a split second then returned to its usual. So I atleast know the MAF is working. I swapped out the front O2, no change, covered the motor in starter fluid trying to find a vac leak and nothing...

By this point i'm getting a little annoyed seeing as how I should've been in bed since I get up so early so I ran down to a buddies house to get some advice (dead end) and then on my way home I was cruising about 35 and punched it to see what would happen. (maybe 50-60% throttle?) and the car fell on its face. didnt die, but just sounded plugged up. going up hills or using any torque around the 1500 rpm mark the car bucks real hard too until it down shifts to 3rd then it gets its **** together. By this point the CEL is now flashing (must mean good things right?) so get home and pull codes again, and now there is a P0300. Last ditch effort I throw a fuel pressure gauge on it and key on 45psi, running at idle 41psi. I know this is SLIGHTLY low but cant imagine it would make THAT much a diff?

Going to replace fuel filter tonight, but other than that I just done know....

(oh the car is a 2003 GTP supercharged)

This all being said I'm now at a loss.... Please help. lol

Thanks fellas!

[QUOTE=Scottydoggs;1310587]

Originally Posted by nskyline34

I will check the fuse. cant imagine it needs a tune up. Motor is 1200 miles/8 days old. new cam and crank sensors and plugs/wire/coils. Any way I can test the MAF? and yes, shortly there after it started flashing. it was def caused by giving it a little too much go-juice. since this motor is new that was the first time i gave it any skinny petal really. i've just driven it like a gramma since its my commuter. but i figured for the sake of diagnosis it could help to see what it does.

thats how to test the cat. your on the right path there. how many miles are on the car, and the cat?

Car has 119k on it. I'll give that a test tonight (cat) and then I'll also toss in the new fuel filter and see where that gets me... from there I'm going to get a new MAF (and check fuse 21) and then I'll start really examining FP while driving and considering the possibilities of a FPR going bad. but I cannot imagine 4psi too low of FP being this much of a hassle...

Originally Posted by BillBoost37

To throw 171 aka lean bank 1 you need to have been out of the acceptable range 3 consecutive time running. This means you are over 16.4% fuelling being added by the pcm. So it's telling you.

Flashing CEL means that the PCM sees a potential catalyst damaging condition aka you had misfires based on the code.

From what you are saying.. it sounds like you have a vacuum leak. You did the right thing, you checked for a vacuum leak. Except, there's one smaller leak that you can never find with starter fluid. Remove the pipe from the EGR to LIM and pull back the retaining piece, then look where the retainer would hold the lip for cracks. Likely...it's cracked, 90% of them are cracked.

Now that's not giving you enough to go on for a true fix. Let's jump in deeper and talk about fuelling. 41 at idle is fine in my experience for the average SC motor. Does the pressure increase properly when you remove the vacuum from the FPR? What does the fueling do when you are driving? (I made an extension pressure hose).

Keep in mind that the swivelling plastic piece in the hose at the brake booster is also a check valve. Shouldn't be an issue, I have seen one or two cause a problem though.

Good information thank you. so is the car over fueling itself right now or running too lean? I'm just worried about burning a valve from lean, but I'm also not a fan of driving my Z06 everyday either... lol First world issues I guess? I'll check that EGR piece out. def could be the issue, all the symptoms point to a vac leak for sure. its just so odd tho when i gave it a lot of throttle though how it really fell on its face. its like it drown almost.

I'll have to check the fuel stuff later on tonight when I get home. going to be a late night again. but any night in the shop is still fun i guess.

What do you mean "Now that's not giving you enough to go on for a true fix." you think there is more than a vac leak? I guess youre right, a tiny vac leak shouldnt cause all this?

When I pull the vac line off the FPR what shoudl the pressure rise too? I will do this as well.

Ok I will check the battery, if memory serves its clean but couldnt hurt to check! I'm going to grab a MAF on the way home just in case and toss it in. Couldnt hurt right? Worst case I just take it back. Really wish I had a really good scanner to go through this... But I'm only $2830 into this car so far, so I have some money I can throw at it. I'll check the EGR and I'm thinking the booster could have a leak in it. I have to press the brakes a little harder than I think I should to get adequate brake performance.

Maybe another thing I should do is check for exhaust leaks? I tightened up a couple of joints a little... But not a lot. mainly just crushed the gaskets further because I used an impact gun thsi time. lol But I'll check the manifolds and make sure they're still torqued... Never had a car leave me scratching my head like this before. so odd...

Are there any ways to test a MAF? Or just swap it out real fast?

Ok, well i dont hear a distinct leak so I'll leave well enough alone. prolly just paranoid for some reason. I was just going to get a MAF from o'reillys? Hopefully that works. Really curious about FP. any tests I should do in particular?

If you get a MAF from a parts store they may not let you return it once you try it since it's an electrical component. It's happened a few times. Just a heads up.

Originally Posted by BillBoost37

Exhaust is fine.. don't worry about it unless you seriously hear it.
Maf.. be careful. it's better to get an stocker from a JY than a reman. For some reason our cars are very fussy on MAF's. I know...cheap is actually better in this case..lol

I'm talking the cables and wire themselves. Might be the issue...but it's more common on the power hog Bonneville.

One questoin I do have is if the car is running rich right now or lean. Does the car think its lean so its throwing extra fuel at it? I just want to know if I can drive it still without worrying about burning a valve or screwing something up.

No its just a DD car. Sees maybe 2500 rpm when merging on the freeway, other than that its 100 miles of freeway a day and about 3-5 miles of city. haha LOOONG commute.

Ok, so Update!!

Checked fuse 21 - was fine and hand continuity, replaced fuel filter (old one was dated nov of 2013), Fuel pressure at idle was still 41psi, revved car up and stayed at 41 and then hopped to 45psi at like 3k rpm I think (going off sound). Unplugged vac line from FPR on fuel rail and pressure spiked right to 50psi and never moved from there, when I did this and caused a vac leak the idle really had no audible increase. Put a hose clamps on the main vac line coming from the booster to SC and also pulled the 90* elbow/valve and when I pulled it out of the booster (car off) it definitely had a lot of vac behind it. so figured it was just fine.

Cleared the codes on it to see if that did anything and went out for a drive. idles at about 700-800 now on the code reader and went for a drive and the P0171 code did NOT come back, however when I put my foot in it it really struggles and will not really rev past 4k or so. its odd, sounds choked and it shutters/hesitates too. When i did this the CEL flashed for a while and then went out. Through a P0300 code (no surprise). Came back to the house and then checked cat, temp going in (idle) was about 350*, cat was about 250* and then after the cat it was about 420* or so. Then I started pulling data from the O2's since those temp arent good and the front O2 bounces around between 0.020 and like 0.8 or so while the rear one also jumps around a lot (basically same range) when rev'd to 2500 rpm and held while reading #'s.

So at this point I'm thinking its a clogged cat. which would makse sense consitering when I bought the car it had a burnt valve in cyl #4 so bad that it had 15psi compression in it. yes, 15. so I'm assuming that is what has caused it to really clog up and then when the motor runs well (since I rebuilt it) now it needs the flow of the cat.

Thoughts?

Thanks fellas... Still not 100% sure though if this is the cat. need to rev car to about 2500rpm then check cat temps tonight before i take it down for a new weld in universal cat. (my car lift is in use right now).

one thing I should add is the MAF seems to be working fine based on live data i pulled with my code reader so i didnt bother swapping it out.

Yea that's what I'm thinking. Any other methods for testing before I pull out my wallet for a new one?