Alright, father in law reports his truck has really bad fuel mileage. Supposedly went from around 18 to 12. Been like this for a while. So I ask, does it have a CEL? Yea? Take it to the auto store and get the codes.

 

Its a 99 Chevy Silverado K2500 with the 6.0L gas engine.

 

Codes:

 

P0171

P0174

 

Fuel trim system lean bank 1, 2. Anyone have ideas? I have not had to work on a gasser in years. I was going to go out and start sprayin ether in different areas to see if I get an audible change in RPM.

Could also be an oxygen sensor, or even the MAF sensor. I would also start checking around the intake for leaks though. Dont forget to check the PCV hose to make sure its not cracked anywhere. The problem is on the newer gas engines there are so many vacuum lines and plastic fittings to break, it makes it hard to track down a leak. I wonder how hard it would be to get a vacuum diagram for that beast.

 

On second though, maybe its just being a chevrolet product 

Well, if I had a smoke machine that would be the way to go. Little spendy for one diagnosis though... maybe I can find a poor mans version of doing it...

A code P0171 may mean that one or more of the following has happened: The MAF (Mass Air Flow) Sensor is dirty or faulty Note: The use of "oiled" air filters may cause the MAF to become dirty if the filter is over-oiled. There is also an issue with some vehicles where the MAF sensors leak the silicone potting material used to protect the circuitry. There could be a vacuum leak downstream of the MAF sensor Possible cracked vacuum or PCV line/connection Faulty or stuck open PCV valve Failed or faulty oxygen sensor (bank 1, sensor 1) Sticking/plugged or failed fuel injector Low fuel pressure (possible plugged/dirty fuel filter!) Exhaust leak between engine and first oxygen sensor

Read more at: http://www.obd-codes.com/p0171
Copyright © OBD-Codes.com

 

PO174:

In the vast majority of cases, simply cleaning the MAF sensor does the trick. Consult your service manual for it's location if you need help. I find it's best to take it off and spray it with electronics cleaner or brake cleaner. Make sure you are careful not to damage the MAF sensor, and make sure it's dry before reinstalling Inspect all vacuum and PCV hoses, replace if necessary Check for a dirty fuel filter and proper fuel pressure

Read more at: http://www.obd-codes.com/p0174
Copyright © OBD-Codes.com

My first thoughts were o2 sensor and maybe the exhaust gasket ... Also when was the last time the plugs were changed??

They had a recent tune up that included a fuel filter change, spark plugs and air filter.

 

Would the wrong type of spark plug such as gap or makeup/design throw such a code?

He must REALLY baby that truck, every 6.0 I've ever driven was doing good to get 12 mpg.

Haha, but like I said, supposedly...

 

They did not even know how to calculate fuel mileage until they met me...

They had a recent tune up that included a fuel filter change, spark plugs and air filter.

 

Would the wrong type of spark plug such as gap or makeup/design throw such a code?

 

I would take a step farther and test the fuel pressure of the rail and see where it sits too. Too much or too little fuel pressure can create mixture issues. Some vehicles use a vacuum regulator to regulate fuel pressure in the rail might want to check.

 

Spark plug gap would cause more of a misfire code P0300 to P0308 codes.

 

Haha, but like I said, supposedly...

 

They did not even know how to calculate fuel mileage until they met me...

 

Maybe you should get them a ScanGauge II and calibrate it for them...

They had a recent tune up that included a fuel filter change, spark plugs and air filter.

 

Would the wrong type of spark plug such as gap or makeup/design throw such a code?

Probably not. The first thig to check and clean is the MAF sensor. If that doesn't do it oxygen sensors are known to cause the lean readings. Check the simple things first.

Drivers side rear o2 sensor is bad. The computer thinks its running lean so its over fueling causing the poor mileage. Dont over think it

Well, my FIL decided to go out and tinker on his truck... Went and bought everything required for a typical tune up... He changed the air filter and said that it looked like the sahara desert! Then he did the plugs and wires and broke the last plug on the back of the engine. Still has the 6-point on it to receive the socket so I should be able to get it out for him. Hes broken, literally, so he cannot do it physically. I'm really surprised he got as much done as he did.

 

So, I'm gonna go out there and finish it for him. Just need to double check his work and get that plug changed and do the fuel filter. Apparently the CEL is not on, but I told them that after this, they need to go back to the auto store and have it checked for codes and clear them to see if it comes back.

 

That way, I can no for sure if this is still an on-going problem or if it was just an old code.

 

I guess the place they got this tuned up at really screwed them over. The guy only did half his job and used the wrong plugs with a HUGE gap.

 

I was thinking it was some sort of emissions thing as well. But since they already bought parts, I'm gonna start from there...

The huge gap on the plugs you see may not have been put in that way. The gap will grow as the plug gets used. The wrong plugs may have contributed to the large gap, but I have seen them go in with .030 gap and come out a .100.

Also check for proper gaps. I've seen some GM products with gaps as large as .065 inch. Even the 96 Dodge I think is .040 IIRC.

Gaps would cause miss fire and rich codes

The plugs have only been in the truck since last August. So its only been like 9 months. But if the plugs were the problem, the codes would have been thrown a long time ago...

 

I guess for his truck the gap is .045".

 

Hes going to do the fuel filter today and run it to the auto store to check codes again. More to come...