I have 2004 Silverado with a 5.3l in it The CEL came on and got it read.
Misfire and o2 sensor (bank 1/upstream). The problem I have is rough idle. Once I give it gas it smooths out. After tons of research I narrowed it down to fouled plug, bad coil, or vacuum leak. I'm going to start with the vac leak first. I'm not too sure where the vacuum lines and the ERG valve are. Any help or comments would be great.


Thanks,
Tom

 

Here are all the vacuum lines I could find. I don't know what each one does but here's some pictures.

 

Well let me put it this way, if you think it's a vacuum leak, probably the only place that even would is the intake gaskets, or a stuck open purge valve. What does the spark plug look like? It could literally be just a bad o2 sensor and needs a tuneup. I would want to do some scanning with a tech2 before I made any assumptions, but is the missfire a dead miss, or does it smooth out when you give it gas?

So first off, you need to know if the engine is running lean (high fuel trim numbers on the tech 2) or running rich (negative numbers in the fuel trim field on the tech2). Look at the o2 sensor values, and compare them to the down stream values. The down stream values should be pretty steady between 450-800 I'd say, and the upstream should fluctuate between 200-850mv. If the o2 sensor in question looks to be staying pretty steady, then I'd say try replacing it and see how the fuel trim reacts and replace your bad spark plug. Make sure and check your wires for bad connections (corrosion in the connector). Give the wires on the harness side a little tug on each wire - don't go crazy because you CAN break the wires, but if the copper separated inside the insulation, the wire should just pull straight out of the wire loom with little effort.

If things check out, replace the o2 sensor and plug and see where you're at. As for the coil, try unscrewing and moving the coil to another cylinder and see if your'e missfire moves with the coil. Obviously, if the missfire moves, then you've got a bad coil, or a bad wire. Again, swap wires and see if you can make the missfire change.

It's gonna take some testing and equipment to see just exactly what's going on, but chances are if your EGR was stuck open, it would bog down at idle real bad. I don't think it would necessarily cause a missfire on only one cylinder or make the o2 code pop up.

So testing your intake gaskets for vacuum leaks - either use the tech2, or listen carefully to the RPMs of the engine. Take carburetor cleaner and spray right down in the crack where the cylinder head meets the plastic intake. If when you spray it down in there and you hear a sucking sound and the RPM changes, then chances are that the intake gasket is bad. In my experience, any GM intake gasket for these engines that is orange in color, will cause a vacuum leak. The updated gaskets that are green are much better and don't leak, so if you can tell if they're orange or not, that might help you decide if it's a vacuum leak or not.

As for the purge valve, unscrew the 10MM bolt and wiggle it free from the intake. Be careful though, it's just an O ring holding it in. Put your lips on the part where the plastic line was and try and blow through it. Yes that's right, blow through it. It's a little gross at first, but you can wipe it down first if you'd like. If you can blow through it, then that valve is bad. If you can't blow through it, then the valve most likely works.

 

Thanks for the reply. Lots of good advice in there. I think I'm going to replace the spark plugs and plug wires this weekend. I also need to get a code reader, rather than relying on Autozone or Advanced. I'll post my results. Thanks again.

 

I just changed to Bosch iridium spark plugs and Autolite pro wires.


Here are my symptoms: After the truck is warmed up, occasionally the check engine light flashes, the rpms drops to 500 and the whole truck shakes. A flashing CEL means a misfire. With new plugs and wires, what would by me next action? Try and obtain a reader to find out which cyl. is misfiring and move the coils to see if the misfire follows. Anything else I should try?


Tom

 

If your original read of the CEL indicated a problem with the O2 sensor, why haven't you tried addressing this? O2 sensors aren't just for emissions standards; they tell the vehicle how the engine is responding to the fuel flow. Vehicles try to enter what is called "closed loop," in which the fuel consumption is optimized and being burned efficiently (your air-fuel ratio is predicted and maintained to reliably burn up all the fuel). Upon warming up (cars burn hotter at the beginning to speed up the warming process), your truck could be trying to enter closed loop status but fails due to a bad O2 read, causing your drop in RPM. Your truck would then abort the closed loop attempt and return to known static fuel flow calculations that work, but are less efficient.

This is more of a guess than a fact, but my money is on bad O2 sensors, wires, etc.

 

Ok, sounds good, I'll switch out my O2 sensor being that driver side upstream is bad. I have evidence of poor air/fuel mixture. Check out how the plugs look. The first one is from the driver side, all four plugs looked alike. The second picture are plugs from the passenger side and again they all liked alike.

 

O2 sensor is bad!!

 

Exactly. That's soot depositing on the bank 1 plugs. Each side of the engine has it's fuel controlled by feedback from the O2 sensors (mostly the first, pre-cat, O2 sensor). The engine is depositing too much fuel per unit of air, causing an incomplete burn and all the carbon build up on the plug (and the rest of your cyclinder). Honestly, as long as those plugs don't have too much wear, the old ones might still be fine. I'd keep them as backups. The wires, too. Never know when you need to replace them.

You need to get that O2 sensor fixed (or the wire that connects to it, whatever is causing the problem) or you're just going to foul up your new plugs. I'd also give the engine a good cleaning with some injector cleaner and some seafoam to clean out that gunk that is now likely built up from the O2 issue.

 

Well, I replaced the o2 sensor and it seems to be running good. It's hard to believe that a $40 part can cause so many problems. My CEL is still on and the parts store said they can not clear the code. They suggested either take it to an inspection station to have it cleared or disconnect the battery. I did the latter. Let it set for two hours, reconnected and no dice. Light is still on. No biggie after a few miles of driving it with turn itself off. Thanks for all the replies and information.