My 4.8 is dying!

Derek Elmer · January 8, 2007

The vehicle: 2001 GMC Sierra 4.8L Automatic

Trouble codes: P0300- Multiple misfire

P0171- Lean bank 1

p0174 - Lean bank 2

Symptoms: Stalls at idle, Black soot out the tailpipe, Stumbles and misfires under load. Has burned a quart of oil since last oil change.

I have replaced the PCV valve, fuel pressure is fine, cleaned the EGR and replaced the intake manifold gaskets (did not affect anything). Also replaced the spark plugs, fuel filter cleaned the MAF.

These symptoms have been intermittent over the last couple months but now it is worse than ever and is not going away.

If I unplug the MAP it quits. If I unplug the EGR nothing changes.

All of these symptoms are more noticable after warm up. If I unplug the Oxygen sensors it runs fine. It also will run fine after I reconnect the Oxygen sensors for a short time (couple of days) and then the problems come back.

I really do not want to take it to a dealer after all of the trouble shooting that I have done because they will not trust me and charge me for their time doing what I have already done.

The Oxygen sensors see that the engine is running lean and they are dumping a bunch of fuel in to compensate which then makes it rich and exhaust black soot.

I think I can hear a vacuum leak but the propane bottle does not change idle anywhere on the intake.

Any ideas?

Wingnut · January 8, 2007

Cylinder leak down test?

Derek Elmer · January 8, 2007

I have not looked into mechanical failure, I figured it would be a sensor or a vacuum leak somewhere. The vacuum reading is 20 in. so I thought the motor seemed healthy enough. Also it runs great without oxygen sensors. There is a funny smell outside of the truck when hot and I shut it off. Could it be the catalatic converters being melted down? I am not sure what that would smell like.

Thanks for the reply

slowpokebanshee · January 9, 2007

what is your fuel pressure, at idle, and key on engine off?

TonyMazz · January 10, 2007

Start with fuel pressure, key on engine off.

Have you changed the O2 sensors ever ?

Do you know if your CAT is plugged ?

What have you done (if anything) since it ran good before (if it did)?

You may have some plugged injectors, stick valves, or sensor problems...

Have you changed your plug wires ever ?

Plugs ?

Tell us when and under what conditions the engine began to crap out...?

Derek Elmer · January 10, 2007

I do not have the equipment to test the fuel pressure myself. I had a shop do it the first time the truck was acting up back in mid summer. At that time the guy cleaned the EGR and the throttle body. The truck ran good for 2 weeks, I cleaned the EGR myself and it was good for a week. The next time I changed the PCV and it was good for 4 weeks. At this point I felt that the intake manifold gaskets may me the culprit because it said it was running lean so I changed them. It did not make it any better. At the same time I changed the fuel filter and spark plugs. All of this did not make the truck run any better. So I unplugged the Oxygen sensors (upstream) and the truck ran good. I was nervous that it might actually be running lean so I hooked them back up. The truck continued to run good for a couple of days and then the problem returned. I again unhooked the O2's and ran it for a couple days. I made an appointment at the dealers and before I took it in I hooked the O2's back in and when they put it on the scanner it showed the truck running great and they do not believe me.

So it is an intermittent problem that must be a sensor.

Other things that I have noticed. The truck has 3:73 rear end and at 70 MPH it is running at 1800RPM, seems low for those gears. At 40 it is just above 1000RPM and has no power untill it drops down a gear and revs up. Also the truck always gave me 19-20MPG and now it is around 12-13MPG.

Sorry about the novel.

1999 Tahoe 4x4 · January 11, 2007

All I know that if you disconnect the O2 sensor the computer will switch into open loop mode. In short that means that your truck in this case would run according to factory preset values without monitoring emissions and without adjusting according to emissions measured.

Having said that I suspect some sensor problem (btw: NOT necesserily O2 sensor) or at tleast that the problem is caused because some sensor reading not according to what it should be.

However, since the problem is intermittent it is hard to tell if it REALLY runs good with the O2 disconnected, since (to my understanding) it ran good a few times even after it was connected.

Derek Elmer · January 12, 2007

On my previous message I posted that the engine runs at 1000RPM at 40MPH, it is actually at that RPM at 55MPH not 40 (I am canadian and am not used to the conversion.) Also the truck is always dropping into overdrive too early as far as I am concerned. At 65-70 KPH it drops into overdrive. Now if I cruise along at this speed and at 1000 RPM and then want to accelerate, the engine revs up to 1200RPM and then back down to 1000 very quickly and it does not have very much power until I kick it down to 3rd.

I took the truck to my local dealer on Tues. and of course it was running good to them and it has not acted up since. But the mechanic told me that he opened up the EGR Valve at idle to clean the pipe out, this might have helped my cause, but I do not think it will have fixed the problem permantly.

I did notice that the connectors on the O2 sensors are the same on the upstream as the downstream. Are the sensors interchangeable?

hotrodrado · January 20, 2007

i'm having similiar problems with my truck. i have a 2003 5.3 and i'm getting those same engine codes. I changed the two upstream O2 sensors and it helped for a while but the problem came back. By the way the O2 sensors are interchangable. I cleaned the throttle body and maf and that didn't help I have new wires and plugs in it. Just today the check engine light started flashing. So i'm trying to figure out the same problem so keep me posted if you find out anything.

tim73 · January 20, 2007

Is it doing it when you first start up, or after the engine is warmed up? If it is doing it prior to being warmed up, might want to check your engine coolant temp sensor, this is what the computer looks at to see how much fuel to dump in the engine before it warms up, and the O2 sensors take over.

You can check this sensor with a ohm meter.