[FIXED] 05 Vibe P0171

I'm working on this vibe and thought it was going to be an easy vacuum leak since the data stream showed a text book vacuum leak, but I cannot find it for my life. At idle LTFT is at 30 and STFT is at 10, when I give it some throttle, LTFT goes to 10ish and STFT goes to 0. They instantly rise when I let off.

I have smoked it, tried water on the intake, propane while watching STFT and I am not finding anything.

Freezeframe shows the code setting at idle, 185*, CL, LTFT 30.47%, STFT 12.5%, 0 mph, absolute throttle position 16%, calculated load 10.98%, MAF 1.43g/s.

The car already has a new MAF and the data seems very clearly pointing to a vacuum leak. The last shop they brought it to said it was the intake gasket but I don't know how they confirmed that. I just smoked it again when it cooled down and found nothing. I hate to put a gasket in there if I can't confirm it.

They trims also don't seem to change at all when it warms up, like when an intake gasket sort of seals itself a bit, but I guess it might not always do that.

Any other ideas or tests you recommend?

Hey Tutti57! I chased my tail round and round on a similar Vibe... Have you tried pinching off the vacuum booster hose? Obviously, if you see an improvement, then you know you're on the right track.

Hello tutti I would recommend pulling the intake to check the gasket I have found tons of these with the intake gasket pinched(from the factory) that didn't show up with a smoke test in fact the last 1 only threw the cel in the winter ! Moral of the story it's a common failure and the intake isn't hard to remove so I would definitely look there first

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Thanks for the speedy replies! I just pinched off the brake booster with no positive results, so I guess I'm going to pull the intake off now. I'll post back with my findings!

Dang. :lol: If I remember right, I think you can swap these gaskets by just unbolting the intake and pulling it away from the head?

Yep, it comes off easily enough. Unfortunately, the old one looked fine and I put a new one on for the hell of it, but still have the problem. Screaming lean at idle. Double checked and I do have all of the hoses back on that I took off. Now what?

There are a bunch of TSBs for dirty injectors but I wouldn't think that it would only be an issue at idle if they are clogged.I have an ultrasonic cleaner and flow tester but want to see if there are any more ideas out there before I pull any injectors. I really don't think that's the issue based on the data.

purge valve?

Just tried that. Pulled it off and sucked on it hard and nothing came through.

Any other ideas what I can test?

Any tears in the air intake cleaner boot that maybe aren't so obvious?
Does it have the factory intake box and a good (correct) clean filter?
Sometimes those "cold air" intakes and cone filters will throw a wrench into the works.

I was thinking about fuel quality, E85 specifically, but then why do the trims get better off idle? You'd think they'd get worse. Same thing with the injectors and fuel pressure.

If you have a injector bench tester, then I think that's where I'd go next. The symptoms don't match, but it IS a common problem on these engines. Or, if you have a injector pulse tester, you could do a balance test on the car. Might just be easier to pull the rail at this point?

What kind of PCV system does this have? I've seen stuck open valves suck air through seals and gaskets, but there's always an idle control problem associated with them.

Any kind of aftermarket air filtration going on? Something that might disrupt the laminar flow of air across the MAF.

Yeah, I'm getting desperate. :lol:

I'll closely inspect that tomorrow. A weird thing that I just remember was that about a year ago I replaced the maf sensor because the opposite was happening, the fuel trims were increasing with rpm. After I changed it, I think I remember it going into this state. I still have that sensor so I think I'm going to put that back on and see what it does. Maybe the newer aftermarket one is bad?

Can get the scope out and figure out how to test the maf sensors and check out the PVC system. Anyone have a good way to test the PVC? I feel like I remember reading in Paul's book to block off something on the intake and pull a certain amount of vacuum through the dipstick hole?

You're right,you pulling the rail would be easier on this thing since there is no valve to tap in for a fuel pressure gauge. I hate rigging up those set ups!

Got it! I swapped the old MAF in for the heck of it and the STFT was at -20%, so I gave it a minute and the LTFT pulled down to 7%. I put the other MAF back in and the idle trims started climbing immediately.

So, the car is fixed and the replaced MAF was faulty. Thanks for all of your help!

Wow. :blush: That's really surprising, did NOT expect a MAF to misreport that way.

Dunno how much help we were! :silly: But I'm glad to hear it's fixed. Aftermarket parts strike again.

I'm genuinely interested to hear if anyone has a suggestion on how we could have caught onto this issue sooner.

Seriously! It's like it was calibrated for a car that required more fuel at idle.

I'm lucky I happened to hang on to that old part. I'll be honest this is the first time I've run into a real aftermarket part issue. This one will stick with me to be cautious.

I wonder if it's just a quality control issue where maybe 1 of 5 might be junk?

I'm just now learning scope diagnostics and pulled it out last night to play around but I wonder if that would help even helped here? Couldn't it be the same wave form and I would have dismissed that as the problem? Like if the meter wasn't really defective, just not calibrated right or for the wrong car or something.

What were your maf readings at idle with old maf compared to new maf on this engine fully warm at idle we expect to see approx 2gs

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Tutti57 wrote: I'm just now learning scope diagnostics and pulled it out last night to play around but I wonder if that would help even helped here? Couldn't it be the same wave form and I would have dismissed that as the problem? Like if the meter wasn't really defective, just not calibrated right or for the wrong car or something.

Agreed, I don't think the scope would have shown us anything useful. I'm betting it's peak airflow reading was probably accurate, but wasn't reading correctly around the idle airflow rate. You could do some throttle snaps with the scope, or WOT runs with the scanner with both sensors?

I was thinking along the same lines as Ben with the idle MAF reading. I think you noted around 1.4 g/s with the lean MAF sensor? Definitely seems low in retrospect. :lol: An underestimated reading like that would definitely cause a lean condition.

I wonder if the technique in this video from Wells VE would have helped at all?