Great Site...Been a lurker for a long time. Not very technically talented.

98 Expy 5.4, 71K. Last week I filled up at a discount club, about 30 miles later, developed bad idle. Jumps to 1500...drops to 500 nearly stalls out. Feels like a bad IAC, based on description and it happened before. About 15K ago ( 3 + years because the truck is infrequently used.) I drove it this way for about a third of the tank..no CEL's. Added some drygas, and just topped it off with 89 from a different station. Within 10 miles, CEL came on. I am planning on an AutoZone visit. Do they check this for free?

Bottom line, curiosity. Can bad fuel cause this? of course. but will it trigger a CEL? Should I be chasing the discount store? or just coincidence?

Thanks for opinions.

 

I went to autoZone and they pulled P0171 and P0174. They told me I needed 4 O2 sensors because their tester wasn't more specific, and that they were a required replacement at 30K miles. I know just enough to be dangerous after reading this board, but I keep about the IAC problem, and vacuum leaks that I read so much about.

 

Those code just says it is lean on both banks not nesscarly bad o2 sensors sounds like they want to sell you a $200 + worth of 02 sensors. Could be the cheap fuel plugged the injectors or you have pick up a vacuum leak. The iac sounds like it is working since it is trying to adjust the idle. Remeber the Iac is a controlled vacuum leak so it is possible it is having issue. Take it off and clean it.

 

Vacuum leaks will cause similar symptoms, I am guessing the gas is a coincidence. My first O2 went out around 120K but it didn't cause any problems, it was also the downstream one that just makes sure the cats are working. O2's are more of a 60-90K maintenance IMO, 30K is a bit ridiculous. One trick on vacuum leaks is to take propane or some other small gas bottle and air it out around the vacuum lines, if it finds a leak the engine will rev up from the gas entering the system. You should also most likely hear it hissing if it is leaking very bad.

If you floor the truck will it stumble or is it only a problem at idle?

 

Thanks for the replies....It only seems to happen at idle. Under acceleration it seems fine.

 

That should eliminate any fuel issues then.

 

A leaking PCV hose elbow (usually the one on the back of the intake manifold) and or a dirty MAF are the usual causes for these codes.