I made a thread on here before about the idle problem on my truck ("Wandering Idle"). I've replaced the PCV, TPS (which was actually broke), IAC, and the O2.

I've noticed from driving that there is a difference from day to day on how the truck drives. Most days it runs lean, but some days it runs normal or slightly rich. It doesn't seem to be affected by weather.

The MAP sensor is working at the proper voltage. When I had the codes read on the truck, one of them was "lean exhaust bank one." The garage said this could be due to a leak in the intake manifold gasket. Is it possible that a bad intake gasket could cause a change in the pressure within the intake and mess with the MAP's voltage when running?

Any thoughts on this would be appreciated.

 

Yes, if it is a 5.0, or 5.8, the plenum gasket will tear resulting in a vacuum leak causing it to surge at idle.

 

Thanks. It's nice to be able to finally make sense out of the problem. I'm going to upgrade to 1.7 rocker arms in a few weeks, so I'll replace the gasket when I do that job.

 

You would have to anyway. The fun part is, you probably wont be able to buy the plenum gasket by itself. You will have to buy an entire gasket set.

 

http://shop.advanceautoparts.com/web...S_966090698___

You were right. But at 6 bucks, I suppose there are worse things that could happen.

 

Great, but the MAP on your truck uses a varying frequency as its output signal, so testing voltage alone isn't enough to diagnose it. Either swap in a known-good MAP, or use a tachometer or true-RMS multimeter to test your original MAP. Haynes describes how in detail.A leak like that wouldn't have that effect ALONE. If it were big enough to throw an oxygen sensor code, the truck wouldn't idle, or even run very well. There would be MANY other symptoms.

Top of my list for lean HEGO codes is always exhaust leaks between the heads & cats. To find one, I stuff a rag into the tailpipe & then listen for a noticeable hiss at idle.

A close second is 2ndry air system failure (of any kind). Stuck solenoid valves, crossed vacuum lines, bad check valves, & cracked crossover tube are the most common culprits IME.

 

"A close second is 2ndry air system failure (of any kind). Stuck solenoid valves, crossed vacuum lines, bad check valves, & cracked crossover tube are the most common culprits IME."

When it comes to the secondary air system I have some reading to do; I'm not that familiar with it. I've been thinking this whole time that the problem had to deal with something that led back to the ECM because the idle problem changes when you turn off the truck and turn it back on. It will run lean after that, but the idle is closer to "normal." I figured this had something to do with the ECM reseting or compensating for a bad signal in some way. Am I off base on this one?

Thanks for the suggestions. I've got the Haynes manual open as I type, trying to figure out what in the heck these things are, what they do, and where they are.

 

Click that thumbnail diagram at the bottom of my previous post - its caption explains the 2ndry air system, and the NEXT few show more details. If you don't recognize a term or abbreviation, look it up here:
Automotive Terms & Abbreviations