CEL 136/173 - O2 Indicates Lean but with Bad Mileage? WDF!?

[fiftyfordfloored]

Ok, I'm having weird problem... The truck runs and drives great, very smooth since I addressed the KOER 116 code by replacing the ECT sensor. However when I drive it on the highway now or for a somewhat long period of time "city" after a while the CEL comes on and remains on until I restart the truck. I checked the codes KOER and came up with:

136 - HEGO sensor circuit indicates system lean (left side)
173 - HEGO sensor circuit indicates system lean (right side)

How a single O2 sensor on the right side can come up with that idea, I don't know . Engine idles smoothly but it idle seeks from say 700 to 1000. I also have really bad mileage, dropped from the 15.4 I had before to maybe 12 or less... and my foot hasn't gotten any heavier. Both the idle seek and CEL/bad mileage all started at the same time. I found a vague mention of the FPR causing this in my search, but there is no fuel in the vacuum line or anything like that seems to be related.

So as of right now, I have the mileage of a truck with 35's on it, but without the benefit of actually having 35's! This is really starting to suck... Any ideas?

Thank you in advance!

 

[JDraper]

Intake manifold leak?

 

[fiftyfordfloored]

Manifold leak? I know I checked it for vacuum leaks and there weren't any... Or do you mean like in between the upper and lower half of the intake manifold?

 

[JDraper]

Either one. What you're describing would point me to some sort of vacuum leak somewhere in the system. If it were the fpr, you probably would have gone dead rich, not lean.

Have you checked for restrictions in the intake? Dirty filter or debris?

Other standard questions would be what condition are your plugs, fuel filter, MAF, IAC etc. in?

 

[fiftyfordfloored]

It does sound like vacuum... No weird whistles or anything though, and like I said, I checked all the lines.

No restrictions in the intake or filter or anything like that. Plugs are somewhat recent (platinums under 16K), wires are fine, MAF was cleaned like 700 miles ago, IAC was a little dirty last time I checked but it was working fine. No idea on the fuel filter but the thing runs great at speed and above 1000 RPMs. Runs smooth at idle too, just idle seeks.

 

[triggerzz]

Same problem

i have a '92 Explorer and it's doing the same thing. And the 173 code is a HEGO fault/rich code. I had a fuel line leakage right at the gas tank. so i had no choice but to replace the pump and everything. I had a garage install it only to find out that my gas gauge wasn't working right and that the fuel pump was actually faulty. (gas gauge would only go as far as half a tank even tho the tank was full). My gas guage was so messed up that when it said it was at 1/4 of a tank it was actually telling me is was empty. So i blew that fuel pump the garage put in. (Luckily it was still on warranty). Had this new one put in once again and this is when i started having the same problem. So along with having a new pump, I've changed the fuel line filter, oil, and oil filter. I was hoping someone would shed some light into this, before i change the other 2 sensors. I'm not a mechanic or mechanically knowledgable, but I do have family members that are. I just need to find a solution so they can help me fix this instead of bringing it into a garage.

Thanx in advance. triggerzz