Hmm, Ghost Codes?

[mudseeker]

well, i don't know what to tell ya bud.

[junior3382]

Okay i can axpand on this now a little bit. There are times where I have to do a lot of driving. So I have tested a theory.

When I am on the freeway, I almost always use the cruise control to conserve on gas and to prevent tickets. Unless it is bad weather. Anyways, there is a run I have made a few times recently that while on the freeway goes up a very steady incline. For all but two miles it climbs up through the mountains. The cruise will shift sometimes all the way down to second gear to keep the truck at 65mph where I have it set. the last three times I have monitered the check engine light. Every time about 3/4 of the way up, and right around 3200 RPM's the light comes on. There is no difference in driving abilities any of the times.

Also, every time it happened, I stopped at Advance auto or Autozone, which ever was closer to my exit I take. All three times it is the po300 code indicating a multi-cylinder missfire. Everytime I get it erased and then they put it on their diagnostic computer and runthe truck, it is running perfect, no hicups.

I would assume it is because of the higher stress and load at those moments, maybe the thining air as I climb, or something right there that the computer reads this as off. It has been suggested here, in another forum and even at the parts store that replacing the EGR valve would solve this. According to the guy at the counter it is electronically controlled, unlike most other EGRs so there is a possibility that a new one would help. However, if there is nothing really wrong, and this is just do the road conditons, and trying to keep up with traffic on the incline, do i really have anything to worry about.

The thing that has concerned me the most is that this has happened EVERY time, not just once in a while, but never anywhere else, or under anyother driving conditions. The ECU says there is nothing wrong, and my mechanic and I are not into just randomly replacing parts to see what works and what doesn't, I also do not have money to just throw at this truck like that.

Any suggestions on where to start, and why?

[436'd skylark]

I'd replace the fuel filter. my running theory is at a sustained load the fuel system can't keep up with the demand.

[junior3382]

Originally Posted by 436'd skylark

I'd replace the fuel filter. my running theory is at a sustained load the fuel system can't keep up with the demand.

And that would trip a missfire code? Would never had made that connection.

[mudseeker]

going with skylarks theory, if the fuel filter is dirty then the engine will recieve less gas, but still recieve the same amount of air. your o2 sensors will notice this change and set the dummy light, letting you know you are no longer getting an efficent burn.

[436'd skylark]

I think it would. if you runs until the fuel pressure below spec, the injectors won't fire. if they aren't firing, obviously a mis fire occurs, and then the pump has a split second to catch up, regains enough fuel pressure to fire the injectors for a while, and then the truck runs fine. probably happens so fast you only can't even feel it.

[junior3382]

Originally Posted by 436'd skylark

I think it would. if you runs until the fuel pressure below spec, the injectors won't fire. if they aren't firing, obviously a mis fire occurs, and then the pump has a split second to catch up, regains enough fuel pressure to fire the injectors for a while, and then the truck runs fine. probably happens so fast you only can't even feel it.

So fuel filter will be my first thing to look at, but are you hinting I might need to look at the fuel pump too? I thought those had no real signs of dying, they just quit?

[junior3382]

Okay boys, my favorite friend has come back to visit tonight.... As I have been doing until I get some time, I had the code read and cleared to get rid of the stupid light. However, tonight I was shocked to see something new. It was no longer a Po300 multi missfire code, this time it was a Po303. Number 3 cylinder missfire. I think I may have narrowed it down. It is going in the shop tomorrow. I wish I had time to do this myself, but if it is IN the motor then it is covered under a warranty. Before I bring it in, I would like some ideas of what this could be so I know what to make sure they look for! Please anyone?

[mudseeker]

take a qiuck look at your plug wires. i was working on a grand prix tonight with a multi misfire code. when i hooked the scan tool up, most of the cylinders showed a couple dozen misfire counts except for number 3, which had over 3000! under the hood you could see the plug wire jumping spark to the exhuast manifold. the sad part is, i was told the wires are only weeks old.

i doubt this is your problem, but worth a look.

[junior3382]

Ok, well here it is. I am a big one here on the forum to urge people to post back with the solution to issues for future references. Well, this problem may have been solved for me today.

I spent five hours with the tech at the dealer today. Every diagnostic read perfect, and once again, nothing wrong with the truck. So we began to dig. I took Joe's suggestion first and suggested the fuel filter. We tested the fuel pressure, it was 52psi at idle and 56psi at two grand. So filter was ruled out.

So we moved on to EGR valve. We removed it from the motor and using the remote access link off the lap top, we tested the EGR. vlave opens and closes perfect with no hesitation. There was some carbon build up, but not hateful at all.

So then we moved on to ignition. We soaked the wires and dist. And no change in performance, nor and missfires. I suggested diging a little further. Now when I bought this truck, they told me they gave it a tune up, as they do with everyone, and I was going off the assumption I was told the truth. We started with cylinder #3 and worked around. Low and behold, the plugs were original! AC Delco! The #3 had more gunk on it then the rest but we replaced all the plugs, and followed by all the wires right behind it! re tested and of course nothing changed, still ran perfect.

SOOOOO, I took it out on the freeway after and duplicated the EXACT circumstances that trip that stupid light every time, and alas, it is gone! I have also noticed a definite increase in power, and the motor runs smoother from the get go.

I have been around engines for years, and my first guess was the same as most of everyones elses, tune-up..... however, like I had mentioned I was told it was done less then 10,000 miles ago!! Thats what I get for trusting them, I should have checked that myself. I kinda feel like and idiot here.....

The only good news that came out of this whole thing, is.... the plugs and wires were covered under my warranty, so it did not cost me a dime! So at least that worked out. This was just proof that the computer will notice a problem, it is not fake! AND that most of the time it is just the simplist explanation!

So, basically I have no worries again. Now if I can just find a "free" and simple solution for my heat problem..... hmmmmm.

Anyways, though Y'ALL would want to know.