Correlation between EVAP system and running lean??

[Mike]

Wanted to pose a question, first-the situation:
2006 Trailblazer I6 engine.
Previous owner told me check engine light was on and it was difficult to fill with gas. Bought it anyway as it ran well and I got a reasonably good price.
So, I did a little research, found that the 0455 code( EVAP large leak) is rather common with this vehicle. I replaced the vent solenoid by the engine block and cleared the code. The P0455 came back about 3 days later, however I never had any problems putting gas in after that.
Got sidetracked with house stuff, kid/school/sports stuff…life for a few months. Finally in July, I replaced the purge solenoid (by the gas tank). Cleared the code again.
About two days later, the check engine light comes on again. I check it, expecting to see P0455 again but it’s not…it’s P0171 (Bank 1 too lean). This code sets, then goes away on it’s own…then returns apparently randomly.
We’re now at almost four weeks since I resolved the EVAP issue-no more leak codes since then.
I have another thread going about troubleshooting the 0171, but my question for this post is this:
Is it just coincidence that I never got a P0171 code until AFTER I finally resolved the P0455? Or could it be that now that the EVAP system is sealing and purging like it should that I now have a lean condition?
Thanks for reading…several people on this board have already been very helpful and I appreciate it!

Topic

[college man]

here are a few things to check.

http://www.obd-codes.com/p0171

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

[EricTheCarGuy]

This sounds very familiar. Do you have another post on the forum for this issue? If so, please keep your posts together to avoid confusion.

I doubt the codes are related. The difficulty filling the tank may have something to do with the EVAP however. Often difficulty filling with fuel is the result of a faulty roll over valve in the fuel tank. This can also sometimes set EVAP codes. You might want to look into that. As for the lean code, you might start by checking for vacuum leaks and exhaust leaks.

More info here.

http://www.ericthecarguy.com/faq/solving-automotive-performance-issues

[Sam]

You need to replace vent solenoid and evaporate canister for slow fueling. That engine is unlined so you only have one bank. I am fairly sure the purge valve is on the intake. It could be sticking however in my tine at gnome I did quite a few intake gaskets on those for lean conditions.

[Mike]

yeah, I did replace both solenoids…didn’t do the canister though. EVAP and fueling issues are fixed, at least they appear to be for the moment.

I have not done the intake gasket though I did check the bolts as they are known to be found loose at times on these trucks.
Question though…when I check vacuum , I get a solid needle at 20 in/Hg. If the intake gasket was bad, wouldn’t the vacuum gauge read lower…or not? thanks

[Sam]

It should read lower if a leak is present. However vacuum leaks are (in my experience) more prone to a cold engine and will seal up after engine warms. You may also have a H02 sensor reading erractic or possible MAF issues. If you can read the fuel trims post them. Does it run rough? Also the exhaust manifolds are prone to loose nuts on those, but it sounds like you checked it.

[Mike]

LT fuel trims are usually around 12-14% been as high as 20% but only briefly. Got as low as 5% but again didn’t stay that way for long.
ST fuel trims stay pretty close to zero..bouncing between maybe -1 up to 3.4.

I had cleaned the MAF about 6 months ago…could still be an issue though. Replaced the upstream O2 sensor tonight and drove it around 10 miles or so, still LT FTRM hanging out around 14%

as for running rough…maybe a little sometimes at idle and I think (could be in my mind…) that it feels a little lacking in power, but other times takes right off like a rocket.

[EricTheCarGuy]

Based on your description and fuel trims, I think it’s worth checking for vacuum leaks again if you haven’t already.

It might also be a fuel delivery issue. I’d check for vacuum leaks first however.

Keep us posted.

[Mike]

I’ve checked with the gauge a few times, but always warmed up; from Sam’s suggestion, I want to check it after it’s cooled down…can’t hurt. I think I’m going to try your spray method next.
Couple of other questions come to mind.
1. could an erratic check valve on the brake booster cause the lean condition?
2. as far as using a vacuum gauge is the spot on the manifold the only place I can plug one in?
3. I did catch a code for EVAP small leak the other night (went away on it’s own). The only component left I haven’t r/r’d is the vapor canister…maybe time to check that and/or get the whole thing smoke tested? could that be making it run lean?
thanks -Mike

[Mike]

for giggles, I pulled the resonator off tonight and did a little looking around. found the bolts holding the throttle body kind of loose, so I tightened back down. I had unplugged the neg on the battery also. put everything back together and cranked it up.
After letting it warm up about five minutes, I put the scanner on. Idling at 600rpm, the LT trim was around 14%. At 1500 rpm, LT trim dropped to about 4.5%. Looks alot better…now was it because I tightened up the throttle body bolts…or just temporary due to the battery being unhooked? …we’ll see.

[Mike]

still hunting for leaks…I grabbed the freeze data from when the code set today…

[Mike]

Checked fuel pressure…key on/engine off I show 52 psi. with the key on, the needle goes to around 60 but flutters between about 58-62.
Looks like it’s in the right area for pressure, but I’ve never seen a gauge flutter like that. (I’ve only checked pressure on one other vehicle ever so not much experience either). Looking around the net…looks like maybe that’s normal behavior, or is it?

[EricTheCarGuy]

I don’t check for vacuum leaks with a gauge like that. I do it this way.

[Mike]

…how about fuel pressure?…is that flutter normal?

[John Hugon]

2006 uses a pulse-width modulated fuel pump to regulate fuel pressure that’s the reason for needle flutter.

That vehicle uses an electric pump to pump additional air to the catalytic converter during warm up. I had a few that the pump control solenoid valve was bad and turned the pump on intermittently after warm up and did not set a PO410 or PO411,but the PCM set a PO171….. I lost a lot of hair on the first one of those….

If I remember the Solenoid/check valve is in front of air intake box (right front of engine) and the air pump is under the vehicle around the driver’s seat area.

[Mike]

thanks much, I did not understand that about the fuel pump before.
I did replace that check valve (on mine it’s below the air intake trunk, passenger side) $100…still didn’t fix the issue but I figure it will need replacing sooner or later anyway.

thanks again
-Mike

[Author]

Replies