Check engine code "System Too Lean, Bank Two" in '99 4.0 SOHC

[roboman]

It's thrown the code 5 times since January. That first time, I decided to put off taking it in since I noticed absolutely nothing wrong with the truck. Then it came back 2 months later. Then 3 weeks. Then 1 week. Then another week. An Advance Auto parts technician said it could be something like an O2 sensor or something bad with the spark plug wires, so a tuneup would probably do the trick. I took it to a local mechanic, and he raised an eyebrow about a tuneup solving it. He said sys too lean is usually vacuum related. He could hear pretty clearly that air was sneaking in somewhere when he popped the hood. The vac lines were healthy, but he said the intake manifold gasket, which should have rounded edges, was flat as a pancake. He said he could stick his finger through where the gasket should've been. I was relieved when he said it was only a gasket since he said another possible fault could be the (not sure the whole name so bare with me) plenum which could have a hairline crack - and those cost $600.

Fast forward to today, a week after the gasket was replaced, and the damned engine light comes on again with the same code. I'm scared $hitless at this point if there's a crack in the plenum. What else could go wrong though? The mechanic said a dealership is better suited to diagnose a cracked plenum, since they can do a smoke test. Logic is telling me that if there was a crack (unless it's getting larger as we speak) then it wouldn't have set off the light 2 months after the first time, then 3 weeks, then weekly - point being, the condition started off mild and has gotten quite worse.

What else could've signaled the engine light? Or does your gut instinct tell you that it IS the plenum that has a hairline crack? I'm told a system too lean code is pretty vague, especially when it's only on one of the banks. Any ideas?

Thanks for any advice.

 

[eazyasone23]

mine was actually caused from a bad o2 sensor the wire actually melted. it was only affected when it rained outside. hope that helps if your problem is similar

 

[roboman]

It was sunny and 80 today for me... so rain hasn't been an issue. I'll check that out though.

Now that I think about it... would a loosely connected IAC valve cause this? I took it off for cleaning coincidentally sometime in December I'm pretty sure. I'll check it tomorrow just in case, since it's easy to get to.

Still... I'm open to other suggestions.

 

[shoalcreek5]

Mine threw the same code for both banks when I had all of my O2 sensors fail nearly simultaneously. Funny thing is most mechanics would almost immediately say that both banks being lean is sure sign of gasket failure or other vacuum leak.

A vacuum leak will usually cause both banks to report too lean, not just one bank. O2 sensor failure on one side will report one bank lean, but not both banks. It's very unusual for the upper intake manifold to develop a leak except through the gaskets or through connected vacuum hoses. A gasket kit can cost about $10-$30 depending on brand and where you buy it.

Also, a tuneup does not usually include replacing the O2 sensors, so a tuneup wouldn't fix it. Because you have just one bank reporting lean, I would first replace the O2 sensors on the bank that is reporting lean. If that doesn't fix it, then I would buy a gasket kit and replace the gaskets (you want an upper intake manifold gasket kit). There are threads on this forum that tell how to do both jobs--neither job is that difficult to do yourself.