4.3 Vortec Problems

[ollie]

I'm going to just throw this our here since I am at a loss right now on what to think about it. Has anyone experienced problems with this motor before and if so did you have issues with random misfires? One of the main reasons I bought the truck was because of the simplicity to work on the motor. I had one in an S-10 that I drove the piss out of and I found it to be pretty easy to repair most of things on it myself. Fast forward and now I own one in a Silverado and it is driving me crazy! I know it could be one of MANY different things, but I just can't isolate it. I'm thinking either vacuum leak somewhere or a sensor has gone bad. I am leaning more towards a vacuum leak at this point. Code P0300 is such a random code and it is the devil!  lol Although I am not ruling out other possibilities since I think this will be more of a try this and then try this type of thing. I don't want to dump a bunch of money into it as well. If I have to do that then I will sell the truck. If anyone has had this motor with this type of issue please chime in.

Thanks

[nomolites]

I put 300k on one and never had an issue other than O2 sensors failing.  Have you replaced plug wires?  I’d start there.

Mike

[ollie]

Complete tune up in the last 6 months. New plugs, wires, cap, rotor, and air filter. Cleaned the MAF as well. Doesn't seem to matter at what speed I am going as well. It always happens when the motor is under a heavier load. As in when it down shifts from overdrive to deliver fuel and air. If I am going up hill and that happens the engine light will start flashing and I start losing power. As soon as it levels out the light will stop flashing and go away. That and you can feel it misfiring going down the road. I'm thinking of taking the intake manifold off and checking for leaks there. I have read that is pretty common. I mean it could be in the cap I bought since it was a cheaper one and I know there is a lot of heat getting to it, but at only six months you wouldn't think that.

[fishinwrench]

Bad, or crusted over Oxygen sensor would be my guess. Especially if there's no noticable loss of power, or no sensation of an actual misfire. 

If it is a single exhaust setup then there are 3 (three) O2 sensors.   If dual exhaust then there may be 4 

A leaky head gasket, or intake manifold gasket will allow a bit of coolant into the exhaust and throw an emissions code.    If you get a whiff of antifreeze when you first start the truck in the morning.....then this may be the clue.  That's also the reason that you're supposed to use the orange Dex-Cool instead of regular green anti-freeze.

Next guess would be a fouled spark plug or bad plug wire.  

[Devan S.]

I did a tune up on a minivan one time. Fixed intake manifold gasket leak and while I was there changed out plugs and wires along with filters and fluids. 

After putting it all back together, I was getting a misfire under load. Idle it would run fine but under load it would miss on one cylinder. 

 

Turns out a brand new plug had been cracked along the insulator. I'd double check them before just assuming they were good.

[fishinwrench]

11 minutes ago, Devan S. said:

Turns out a brand new plug had been cracked along the insulator. I'd double check them before just assuming they were good.

If you drop one during the swap process, or it gets dropped on the floor at the parts store, that can sho'nuff happen.

[ollie]

Just sooo many things to check and what it could be! I have had the truck a little over a year now and it has always done this, but not to this extent. You can feel misfires going down the road as well. Now at lower speeds you can't really tell, but once you get it over 40 you can feel it. The truck has never been cared for you can tell when driving it. As far as the motor goes that is. The one thing I have noticed though is when I first got the truck the engine light never came on until I was at higher speeds and over 2k in rpms. After I replaced the air filter and cleaned out the box that had a huge mouse nest in it the truck actually runs at lower rpms. I also cleaned off the MAF, but maybe it is the culprit. I mean the tune up makes the truck run with less stress, but now the engine light comes on more and more.

[ollie]

Ok, so last night I unplugged the MAF sensor and drove around some. Didn't really notice any difference in performance. It might have been actually better. Of course engine light stayed on, but when I went up a hill it did not flash. Plugged the sensor back in and when I went up a hill the engine light started flashing. Beginning to think I need to replace that for sure. Even if it doesn't completely fix the problem, I probably need a new one. I keep forgetting the truck ran for who knows how long without the proper air flow getting into the engine. I am guessing maybe it damaged that wire?

[ollie]

Went and spent $20 at the salvage yard for another MAF sensor just to see what would happen and I was surprised, but not really. The truck does run a lot better, but still has the misfires when under a load. Before I could feel the misfires going down the road over 40, but now that has disappeared. The engine light now only flashes at higher speeds like it did when I first got the truck. Back to the drawing board. 

[fishinwrench]

You can locate a misfire with a timing light.     I'd try that.  

Once you know which cylinder is misfiring then you can attempt to figure out why.