Advise please, idle issue and code P0502

[ScottS]

Hi all, 06 Envoy SLT, 4.2l.

As the weather got colder last month, started to notice a surge at idle... truck would get into a cycle of dropping down slightly below 500rpm (enough to dim lights and slow fan motor) before surging back up above 500. Sometimes stopping and starting vehcile again would stop it for a while. At one point it gave me a "low idle speed" code.

Last couple of weeks the surging has basically stopped, but our weather has been considerably warmer. I had been waiting for it do surge again so I could take it to the shop. Yesterday check engine light came on again, this time code P0502, (VSS).

Couple of questions. Is there any possible relation between engine surging and a VSS low voltage code? If not, any ideas on where to start if surging starts again? Also, is the VSS relatively easy to access if I want to try and change myself? (I'd check connection and pigtail first)

Thanks

 

[HARDTRAILZ]

Couple things I would try is cleaning the throttle body...only a couple bucks and it will help overall and get your battery checked. Most places will do it free.

 

[Wooluf1952]

Next time it happens, turn the HVAC off completely. (Including the Defroster)

If that stops the up/down revving, it is a 99% (or more) sure sign of a dirty throttle body.

 

[Black_tb]

 

[jrSS]

Its definately gotta be the throttle body. My lights would dim at a complete stop and idle would be up and down. Cleaned the tb and it idles much smoother and lights don't dim at a stop.

 

[Wooluf1952]

This will help: GMTNation - How to Clean an 4.2 Inline 6 Throttle Body

 

[ScottS]

Thanks guys.

A bit of an update that is literally about 10 minutes old - just got in after driving it home from work. The check engine light came on again (it came on last week and when I put my scan gauge on it it told me P0502... a few days later, the check engine light went off on the dash, and stayed off until today).

I just scanned it, and it shows two codes.

P0411 - Airpump?!
P0506 - Idle speed (again)

Interesting, or alarmingly, the P0502 code no longer shows on the scangauge (I know I didn't clear it)

I think this might be beyond what I can diagnose...

 

[jrSS]

How bout just doing the tb and seeing if ur prob goes away. And if it doesn't at least u could cross that off ur list. BUT its ur truck do what u deem necessary.

 

[ScottS]

How bout just doing the tb and seeing if ur prob goes away. And if it doesn't at least u could cross that off ur list. BUT its ur truck do what u deem necessary.

Not a bad idea... I'll do it tomorrow, cheap easy way to start. Thanks.

 

[ScottS]

Update: thank you for the suggestions and very easy to follow DIY for the TB cleaning. It was FILTHY, crazy dirty. Cleaned it all up, replaced the air filter (I have a K&N, it could use a cleaning, but just put a Fram in for remainder of winter) and cleared the codes, we'll see what happens.

I can report that it idled much better, a rock solid 700 and seemed a little more responsive. I have my scangauge hooked up, one unit I have it reporting is GPH. I wish I would have been paying more attention, but I'm quite sure I never saw it below .48 at idle before cleaning, I now have consistently .36 and a low of .33 GPH. Hardly scientific given I'm not really sure of the before measurement, but I know for a fact that I never saw it in the .3's.

Cheers and thanks again... great community!

 

[HARDTRAILZ]

Glad it helped. You may want to read the threads on k/n on here. They let in way way more dirt and don't actually flow better.

 

[ScottS]

So the truck is driving great, no surging, but this morning the code 0411 (airpump?) came back on again... looks like I might have to take it in after all...

 

[ScottS]

I cleared the 0411 code this morning, this evening driving home I got ANOTHER code, P0171 (lean condition on one bank)? Good grief. Does anyone have any ideas?

 

[ScottS]

Found that I had left the short vacuum hose off on the bottom of the resonator, air leak hence the lean code. Seems all is right now. Got the feeling I'm talking to myself at this point, but thanks again for the help!

 

[Wooluf1952]

Found that I had left the short vacuum hose off on the bottom of the resonator, air leak hence the lean code. Seems all is right now. Got the feeling I'm talking to myself at this point, but thanks again for the help!

We're listening, but you keep finding and fixing the problems.

If you you get stuck, we're here.

 

[ScottS]

We're listening, but you keep finding and fixing the problems.

If you you get stuck, we're here.

lol, well thanks... and I'm stuck again. Truck lit a 0411 code (again) when I got home tonight... feeling like a gong show at this point!

 

[ScottS]

Zombie thread! I am dealing with this issue after a friend and tech scanned it and poked around determining it was the valve. Going to replace it now, but locally only dealer has it and it's nearly $300. Are the ones off eBay worth a look for $71 or is there another trusted aftermarket source?

 

[The_Roadie]

Many, many people succeed in removing the check valve and cleaning it up and then it works. Have you tried that yet?

Otherwise Amazon has legitimate ACDelco 214-2151 Air Check Valves for $91. Cheapest on Ebay was over $100. Sure you're not accidentally looking at the 2003-2004 version valve, which was cheaper because they didn't have airflow sensors to confirm the valve was successfully opening?

 

[ScottS]

Thanks... it's very corroded and we're pretty sure we wouldn't get the bolts off without shearing them to take the valve apart... on eBay the 2006-up valve lists for $98 (my bad, not $71), but not sure if it's genuine or not (my sense is it's not), but the price is sure right!

Air Check Valve Chevy Trailblazer 2006-2008 214-1683 | eBay