Causes of "Too Lean" Code

1996 K1500 5.7L V8

My aftermarket fuel pump (replaced about a year ago) is LOUD and seems to be running constantly. It may need replaced, I just havent got around to it yet. Just replaced the fuel filter.

Whenever I drive more than 30 miles or so at a time, I get a check engine code of "System Too Lean." Usually the code goes away once I get back home, but yesterday I drove about 150 miles and had trouble getting power to pass on the highway. Eventually it started knocking and lost power completely. I pulled over and after a few minutes of not being able to drive above 50 mph, the engine got enough power to drive 65mph the rest of the way. The check engine light is still on and the truck definitely lacks power.

My question is what types of problems can cause this code? The fuel pump was my first guess, but even though it is loud, it is practically brand new, so I'm not sure where to start diagnosing the problem.

Any suggestions on how to diagnose/fix this problem would be greatly appreciated.

Need to see what the fuel pressure is doing and if it is within spec especially when the check engine light is on.

//2000CAYukon

Need to see what the fuel pressure is doing and if it is within spec especially when the check engine light is on.

//2000CAYukon

this is correct as this code [LEAN?] is one of the many causes..for this...loud pump means it won't be long...

I just read on the OBD-II code site that the MAF sensor could be dirty? Any thoughts?

Is the fuel pump causing the lack of power, or are they two seperate issues?

I'll post back the pressure ASAP.


Thanks for the replies!

I just read on the OBD-II code site that the MAF sensor could be dirty? Any thoughts?

Is the fuel pump causing the lack of power, or are they two seperate issues?

I'll post back the pressure ASAP.


Thanks for the replies!

Lean means not enough fuel for the amount of air entering the engine. If the MAF is dirty/bad it could report the wrong amount of air and it can cause a lean condition.

A vacuum leak after the MAF can also cause a lean condition.

Low fuel pressure can also cause this. Since the pump is making noise, you really should check the fuel pressure. If the fuel filter has not been replaced, it should also be checked.

An exhaust leak can also cause a lean condition.

//2000CAYukon

Since I only have driveability problems on longer trips, would checking the fuel pressure be diagnostic at this point? When the engine is running at short distances, there are no problems, so I'm pretty sure the pressure will be normal.

I have an exhaust leak at the gasket directly downstream from the exhaust manifold - assuming the fuel pressure is adequate, is there a way to differentiate between this leak vs. the MAF vs. the fuel pump?

Thanks for all the help!

Since I only have driveability problems on longer trips, would checking the fuel pressure be diagnostic at this point? When the engine is running at short distances, there are no problems, so I'm pretty sure the pressure will be normal.

I have an exhaust leak at the gasket directly downstream from the exhaust manifold - assuming the fuel pressure is adequate, is there a way to differentiate between this leak vs. the MAF vs. the fuel pump?

Thanks for all the help!

these problems all must be repaired before any DTC troubleshooting is to be done...


you are most likely unaware that an exhaust leak at this point is extremely dangerous as CO will enter the operator area and you will get poisioned with CO...

I just had the truck inspected and the mechanic told me that he was aware of the leak and that he was not concerned. I don't think he'd send me home with a massive leak, but it's possible that the leak got worse since then, I dont know.

Is there anything I can do to tell if the leak is significant or not? The truck is almost 13 years old and I don't want to have the gasket replaced if the leak is small. Doing it myself is risky and having a mechanic do it is expensive. In the meantime, I'll check my cab and make sure there are no holes.

I want to pressure test the fuel pump, but I don't know where to put the gauge. The CSFI section in Chiltons doesn't indicate where to test the pressure. Any thoughts?

I need a better idea of where to start since any of the above problems could be causing my problem.

Thanks again!

Not sure how to explain this, but!!

An exhaust leak upstream of the o2 sensors will actually suck "clean air" into the system and thereby tell the o2 sensors that the mixture is lean rather than reading pure exhaust gases.

Hope that helps and it would be the 1st thing I would correct and see what happens after that.

Talk with your mechanic about doing a fuel pressure test

I just had the truck inspected and the mechanic told me that he was aware of the leak and that he was not concerned. I don't think he'd send me home with a massive leak, but it's possible that the leak got worse since then, I dont know.

Is there anything I can do to tell if the leak is significant or not? The truck is almost 13 years old and I don't want to have the gasket replaced if the leak is small. Doing it myself is risky and having a mechanic do it is expensive. In the meantime, I'll check my cab and make sure there are no holes.

I want to pressure test the fuel pump, but I don't know where to put the gauge. The CSFI section in Chiltons doesn't indicate where to test the pressure. Any thoughts?

I need a better idea of where to start since any of the above problems could be causing my problem.

Thanks again!


With all that has been stated by you with these truck issues, and the 13years .............I would sell/trade in this worn out vehicle and get something thats smaller and easier to afford...

why put money in a vehicle this messed up ....next week it will be something else..


this vehicle is for a young person that enjoys working/fixing his vehicle every weekend..

Thanks for all of the help everyone!
I cleaned the MAF sensor, but the code had already been reset prior to cleaning it, so only time will tell if the MAF was the problem.

Jcat you took the words right out of my mouth, and if I wasnt a medical student I'd agree completely. I enjoy working on my truck whenever I have time, and its a lot of fun to learn about since its easy to do a lot of the repairs myself, but right now I have no choice but to keep the wheels turning. It's pretty stressful trying to work 60-80 hrs a week and then come home and have to deal with all of these vehicle problems so that I can get to work the next day. I want a smalller, more reliable car that gets better gas mileage, but it's just not possible at this point.

So again, thanks for all of your help!!!