I know there are a lot of techs in this crowd and I hope you can provide some guidance. I am desperate and really don't trust the shops out there until I have done enough research to intelligently discuss with them.

Here is my issue. I have a 1999 Silverado with the 5.3L. My check engine light has been coming on and off for the past 3 years and I have sunk money into this thing. More recently, my light has been on and has not gone back off.

Here are some of the symptoms: Runs fine at start up and actually runs ok during moderate climate days and cool days. When it is raining outside it will sometimes loose significant power and start sputtering badly. It also does this when it is hot outside 90+ F.

I had the codes read and I might have missed one when they read them off, but here is what I captured:

P0135 Upstream O2 (Left bank), htr performance
P0154 Upstream O2 (Rt bank) circuit, insufficient activity
P0155 Upstream O2 (Rt bank), htr performance
P0300 Engine missfire

The scanner also had some random words that I saw as I looked over the persons shoulder: (Fuel, comp, missfire, EGR)

Over the years I have replaced wires, plugs, O2 sensors, etc... I know that I have been bad and have used the lower octane fuel, so I am sure I have carbon build up.

The truck only has 91,000 miles on it.

This stuff wastes all my money for my chevelles and I hate it! Help.

some of those engines (especially the flex fuel vehicles) have been known to leak/suck air at the intake gaskets. try starting it up when it is cold and then taking a water hose and let it run over the engine along the inside rail of the valve covers along where the intake meets the cylinder head and see if the engine runs rougher or misses. this problem is usually associated with a code p0171 or p0174 but doesnt have to be. if the engine does run rougher then the gaskets are leaking and need to be replaced. there is no coolant the runs through the 5.3/ls motor style intakes only a little that runs through the throttle body.

and just a little tip some parts stores dont sell that great of quality of parts and you can have a part that is new not perform the job that it is supposed to.

Have you tried over at www.fullsizechevy.com I am there as 021500HD, it's a pretty good site too.

Thanks for the tip on the intake leak. I have taken the EGR valve out to inspect and clean, so once I get it back in I will try the water trick. I have been thinking that I have some sort of intake leak because of the performance on rainy days, but the shops have always lead me to believe that it was an O2 sensor problem, so after replacing several of them in the past it must be something else.

Have you tried over at www.fullsizechevy.com I am there as 021500HD, it's a pretty good site too.

Leo,

That is where I should have posted, but when I try to registger I get a message that I forgot to fill out a field. I have tried over and over and I cannot find a field that I miss. I might contact the administrators of that site to see if they can help.

I know that I have been bad and have used the lower octane fuel, so I am sure I have carbon build up.

If the truck is rated to run on low octane fuel, which I'm pretty sure it is, this wont cause the problem. Octane for the most part slows the burn rate down with higher octane and cools the combustion pressures. So by adding higher grade fuel and not changing anything in a motor, performance wise, that will increase spark or compression ratios is a waste of money, again assuming that the truck is rated to run on the lower octane fuel.

Well, I tried the water on the intake trick and the truck did not seem to run bad. I did find a wire exposed on the downstream O2 sensor which was touching the frame. From what I understand though, this would not make the truck run bad since the PCM does not react to this sensor.

I am starting to wonder if maybe my fuel pump is not working properly. I know that the fuel filter is good because I replaced it about 5,000 miles ago. Occaisionally when I get out of the truck with it running, I here a loud whining sound which I believe is the fuel pump. I have replaced this pump in the past, but the one that I replaced it with was a used one that was in working condition.

If perhaps there is not enough fuel pressure then would the fuel vapor pressure which would elevate on hot days cause insufficient fuel delivery resulting in a missfire? That missfire in turn would cause all the O2 sensor faults....

There was a reflash for the computer for those codes. If you give me the VIN I can look it up and let you know.

What does a "reflash" mean? Does it mean that the codes are in error currently and really don't mean anything or could they be affecting the way the truck drives also?

I will PM you the VIN!

Thanks!

Just FWIW, I have the exact same vehicle and have never used anything but the cheapest gasoline so I doubt that that caused any problems.

A used fuel pump is not worth the hassle, get yourself a new GM Delco fuel pump.

What does a "reflash" mean? Does it mean that the codes are in error currently and really don't mean anything or could they be affecting the way the truck drives also?

I will PM you the VIN!

Thanks!

In this case both. It could also be caused by low fuel pressure either from pump starting to fail of a restricted fuel filter. I have an '04 GMC with the 5.3 and have had the O2 codes that was fixed with a reflash.

Hang on, I reread the post. You will have to replace the O2 sensor on the right and left front sensors, (buy OEM sensor's not the universal ones, they are garbage) those codes will not be set by fuel pressure or vacuum leaks, they are sensor failure codes. Still may be reflash to prevent future issues. If it wasa fuel issue or vacuum leak then you would have lean codes for the O2's.

Hang on, I reread the post. You will have to replace the O2 sensor on the right and left front sensors, (buy OEM sensor's not the universal ones, they are garbage) those codes will not be set by fuel pressure or vacuum leaks, they are sensor failure codes. Still may be reflash to prevent future issues. If it wasa fuel issue or vacuum leak then you would have lean codes for the O2's.

Is it common for O2 sensors to fail under 100k? I have already replaced the passenger front O2 sensor at about 70k. Truck has just over 90k now.

You may need to replace the crankshaft trigger sensor,(located behind the starter, held in the block by 1 screw). This sensor is a known source of intermittent rough running and misfires. It reads a 24 tooth reluctor wheel on rear of crankshaft, the ECM uses this signal to fire the injectors and ignition coil packs. A trip to the dealer for an ECM 'Crankshaft Relearn' procedure will probably also be needed.

Hang on, I reread the post. You will have to replace the O2 sensor on the right and left front sensors, (buy OEM sensor's not the universal ones, they are garbage) those codes will not be set by fuel pressure or vacuum leaks, they are sensor failure codes. Still may be reflash to prevent future issues. If it wasa fuel issue or vacuum leak then you would have lean codes for the O2's.

Just wanted to close out this post with good news! I took your advice Mike and bought Bosch O2 sensors for the left and right bank and it solved the problem! I am sure that you have saved me hundreds to put toward my chevelles now!

Thanks again

Thanks for updating us. I wish EVERYONE would do this............... makes for a good database to search against.

Glad your truck's fixed!!!

Happy to have helped.

:thumbsup: