I just got a 98 Blazer that gets a P0300 code. I have been reading about all the various things that can cause it but the symptoms have me confused.

When driving around town it runs great. No CEL after I reset it. Plenty of power. When warm and on the highway the CEL will start to flash when I go up hill. The truck will loose power. My wife was behind me when this happened and said there was smoke from the tailpipe. The CEL will flash during this. It will go on solid after a while and the truck starts running perfectly again. No problem on hills or rough running. The idle is always smooth.

The only code is the P0300. I am thinking it could be an O2 sensor that is causing the truck to keep adding fuel causing the miss and code. Once the code is set I think the truck goes into Open Loop mode and stops paying attention to bad inputs. Could the misfire problem trigger the code before the computer figures out the O2 sensor is outputting garbage? There are no other codes stored or pending.

I was going to try replacing the 2 upstream sensors to see if that helps. Does that sound like a good idea or am I chasing my tail?

 

Highly doubtful it is a sensor. Most likely fuel or ignition related - that is typical on these Blazers.

Lone misfire codes are best addressed by first verifying a number of things are correct. Start by checking fuel pump pressure per sticky at the top of this forum - all 4 readings - obtain/make up the necessary things to accomplish it exactly as written - just do it! Post your readings and we will help guide you. If all that passes, we will then move on to verification/optimization of the ignition system. There is a little Voodo involved in fixing lone misfire codes, and it may involve replacement of some ignition parts to ensure all is optimal - but it doesn't involve O2 sensors. Once we have fuel and iginition optimized, we can go from there if a problem still exists (usually doesn't).

 

Thanks for reply but it may be fixed. I know there are many possible causes for this issue but it would only show up under specific conditions on the highway and the truck would be running very rich when the code triggered. It never starved for fuel under full throttle or detected a misfire at all anyplace but steep hills on the highway. The multiple misfire was only up long hills at speed under hard throttle. Black smoke and no power with a flashing CEL until it went solid and then no problems. I thought the very high exhaust flow and elevated temp could cause a false lean signal from a failing sensor.

I replaced the upstream sensors and took the truck to all the places that would trigger the problem. Drove for almost 2 hours. All good.

I'll keep driving it and see if the CEL comes back. It looks like the EGR was replaced very recently. The last owner may have been chasing this issue for a while and couldn't get it fixed. The O2 sensors were original and were do for replacement anyway. 116,000 miles on the truck. It feels more responsive now. An O2 failure code would have helped.