there has got to be someone with both banks running lean!!

[2004explguy]

my truck is a 2004 4x4 4.0 SOHC and i cannot figure out for the life of me what is bringing up these dumb codes and need to pass emissions test in july and wont pass with SES light on both codes r 171, 174 i have checked all vacuum hoses twice and found no leaks replaced both upstream o2 sensors MAF sensor air filter and intake gasket and still bringing up same 2 codes and cannot figure out what is the problem and gotta be someone out there with this same problem.....

 

[2004explguy]

also the trcuk runs absolutlely fine no misses no stalling i drive it every day to work and around town....

 

[EasyRhino]

How did you check the vacuum hoses? Although I am skeptical that a vacuum leak big enough to trip a CEL would not result in running roughly/poorly, I would still want to eliminate this possibility first, and it's easy to check.

I use an UNLIT propane hand torch run carefully over all the bundle of snakes of hoses on the engine topside while the engine is idling. You must be careful propane ignition hazard and moving/rotating parts; however, I know of no better way to unearth the pesky hard to find vacuum leaks, and they can be a bugger to find.

In other news, are you running out of time to pass you sniff test? If not, I would counsel patience as you are not likely to actually cause any damage from running in this condition, so you have time.

Just pay attention to the engine indicators so that lack of a CEL for a significant problem (e.g. overheating) does not get masked and ignored.

 

[2004explguy]

i have untill july before its gotta pass the test so i have time its just becoming a frustrating problem and am trying to eliminate all the cheap things first and all ive heard is either a pcv vacuum leak or intake gasket... and its very puzzling because it runs great but doesnt take too long for the light to come back on after erasing it ( within 30-50 mi.) and on the 04's it just has 3 vacuum lines (one is split into four) i have pulled all off twice and pressure tested them with no leaks visually inspected for cracks, wear exc. i have a generic code reader and basically check after replacing any parts and or when the light comes back on with the exception of last night after replacing the intake gasket the codes come up pending codes an eventually mature and before they jus matured right away...

 

[2000StreetRod]

No real experience

I have no real experience with the 2004 SOHC V6 but it sounds like you have tried all of the usual things to correct both banks lean with no success. Have you checked the seal where the EGR tube enters the intake manifold? This is a common place for air infiltration on the OHV V6.

Since you have a code reader you might try to determine if the lean codes are temperature dependent? The intake gasket leaks on my Sport were much worse on cold morning starts than warm afternoon starts because the gaskets would shrink more when cold. It seems unlikely that a leak from a cracked hose would be temperature dependent. You could clear your codes before starting the engine in the morning and notice (engine temperature and time) when they reoccur. Then clear the codes after the engine is warmed and see how long again. The lean condition source should be something that affects both banks.

 

[EasyRhino]

I have no real experience with the 2004 SOHC V6 but it sounds like you have tried all of the usual things to correct both banks lean with no success. Have you checked the seal where the EGR tube enters the intake manifold? This is a common place for air infiltration on the OHV V6.

This is the next thing that came to my mind.

I would still do the propane check for vacuum leaks as they can exist where you least expect them.

 

[edwx]

Just my .02. I had similar codes on my Gen. II X, and for the life of me I couldn't figure out what it was. I did tons of diagnostics and came up with nothing. I got as far as trying to check the upper intake manifold gasket. I got most of the intake manifold bolts out, but eventually gave up. My truck wasn't running fine though. It wouldn't idle on cold mornings. I eventually sold it. It had 169K miles on it.

 

[2Trux]

I had the same codes on my 97 Expy 5.4. Mine turned out to be the MAF. I was wondering if low fuel pressure could cause this. It might let less fuel be injected and look like a lean run condition. If it's an air leak it should be between the MAF and the head.
Also mine ran perfectly and had good MPG, it must not take much to set the code. It only took me 3 years to figure mine out. ( I didn't care until I failed emissions)