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2003 4.8 Lean Codes


thearizonacowboy

Question

Trying to track down the cause of a couple lean codes on my 2003 4.8 Silverado 1500 (P0171 and P0174).

 

So far I've replaced the fuel pump, fuel filter, spark plugs, intake gasket, intake tube and all rubber, and the MAF.

 

Cold start it knocks like crazy and feels like its going to stall out. Once it warms up after 15-20 minutes on the highway it runs significantly better.

 

After unplugging the MAF it runs perfectly. Smooth idle at cold start and has plenty of power...

 

Any help would be appreciated!

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MAF should read roughly the engine displacement in grams per second - I.E. 4.8 - at idle.  If it's way off from that, you probably have a bad new MAF.

 

The running joke in automotive repair circles these days is, NEW = Never Ever Worked. Parts regardless of brand are a total crapshoot today - some are better than others, and some are worse.

 

I'd be re-checking everything you replaced. First, I'd take a look at the live scan data and see what's going on - is the computer taking away(-), or adding(+) fuel (should be adding if truly lean)? Get a soda bottle (or whatever container you have on hand) full of water, poke a good sized hole in the cap, and squirt it along the intake manifold while watching scan data - any change in fuel trims indicates a gasket issue. Many professional techs have made mistakes here before, so it happens. Fuel pressure & volume test is next up.

 

In addition to bad new parts, there could be a wiring problem as well ... but the fact the engine ran correctly with the MAF unplugged makes the MAF highly suspect. GMs normally run on a default strategy when the MAF is unplugged.

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Just now, Jsdirt said:

MAF should read roughly the engine displacement in grams per second - I.E. 4.8 - at idle.  If it's way off from that, you probably have a bad new MAF.

 

The running joke in automotive repair circles these days is, NEW = Never Ever Worked. Parts regardless of brand are a total crapshoot today - some are better than others, and some are worse.

 

I'd be re-checking everything you replaced. First, I'd take a look at the live scan data and see what's going on - is the computer taking away(-), or adding(+) fuel (should be adding if truly lean)? Get a soda bottle (or whatever container you have on hand) full of water, poke a good sized hole in the cap, and squirt it along the intake manifold while watching scan data - any change in fuel trims indicates a gasket issue. Many professional techs have made mistakes here before, so it happens. Fuel pressure & volume test is next up.

 

In addition to bad new parts, there could be a wiring problem as well ... but the fact the engine ran correctly with the MAF unplugged makes the MAF highly suspect. GMs normally run on a default strategy when the MAF is unplugged.

Good info and advice! I'll check all this out and report back. Thank you!

 

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No problem! :) 

 

Easiest way to figure out for sure, would be to find a buddy with the same truck, and borrow his MAF for a minute - if it runs normally with another known-good MAF, you're done - it needs a MAF. 

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Not sure if I am reading this right, are you saying that when you unplug the MAF sensor, you can drive the truck and it runs perfectly, or are you saying when you unplug it the idle returns to a normal state?  If it is just the idle that is corrected, then you will want to verify that the new MAF sensor is the correct one for your vehicle.

 

When you got the new one, assuming you are only installing new parts, where did you buy it, and how did they determine which one your truck took?  Are you sure you have the 4.8 engine?  How are you sure you do?

 

Have you gone around the various vacuum locations with some form of alternate fuel or even water to see if you can locate a spot where the idle changes, either up or down.  Check around the brake booster as well.  You could also check the booster by seeing if it holds vacuum after shutting engine off.  You should have one assisted brake application a minute or so after shutting the engine off.  It is pretty obvious if you don't have that assist when parked, the pedal gets very hard to press.  If you are losing brake assist on this test, then unplug the booster's vacuum line and plug the line.  Start the truck and see if the idle is good.

 

The knocking noise on cold start up maybe piston slap, which is somewhat common, usually on the larger engines (6 litre) though.  It may also be valve train noise, what oil are you using, weight/type I mean, not brand

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6 minutes ago, Doug_Scott said:

Not sure if I am reading this right, are you saying that when you unplug the MAF sensor, you can drive the truck and it runs perfectly, or are you saying when you unplug it the idle returns to a normal state?  If it is just the idle that is corrected, then you will want to verify that the new MAF sensor is the correct one for your vehicle. When I unplug the MAF it idles and runs perfectly either cold or warm.

 

When you got the new one, assuming you are only installing new parts, where did you buy it, and how did they determine which one your truck took?  Are you sure you have the 4.8 engine?  How are you sure you do?  Bought all the parts online as genuine OEM parts while using my VIN to verify the part number. Made sure to confirm the part numbers when the parts arrived. Yes I'm sure it's a 4.8, bought it from a friend and original owner. 

 

Have you gone around the various vacuum locations with some form of alternate fuel or even water to see if you can locate a spot where the idle changes, either up or down.  Check around the brake booster as well.  You could also check the booster by seeing if it holds vacuum after shutting engine off.  You should have one assisted brake application a minute or so after shutting the engine off.  It is pretty obvious if you don't have that assist when parked, the pedal gets very hard to press.  If you are losing brake assist on this test, then unplug the booster's vacuum line and plug the line.  Start the truck and see if the idle is good. I've sprayed starter fluid through the engine compartment but I didn't focus on vacuum lines so I'll give that another shot along with the brake booster idea.

 

The knocking noise on cold start up maybe piston slap, which is somewhat common, usually on the larger engines (6 litre) though.  It may also be valve train noise, what oil are you using, weight/type I mean, not brand Cold start with the MAF and it knocks. Cold start without the MAF and it idles smooth without issue or knocking. I only use Mobil 1 full synthetic.

 

Thanks for the questions and advice. My answers are in red.

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