For about the last 5 months I have had this misfire problem with our blazer. It has the 4.3 with that fuel pod with tubes coming from it. It started back in December when I thought the fuel pump went out. I checked the pressure at the valve and found the pressure a little low not by much though. After changing the pump the Blazer still would not start. So I gave it a basic tuneup...plugs wires cap rotor. I finally got it started soon after that but have had the check engine light come on not all the time but only when it was cold or damp out. The thing ran real bad from idle till it got up to speed. Sometimes it will act up going down the road at 55-60 you can watch the tach jump from 2000 down to 1500 then back up again with a miss thats drives me nuts. I did notice that when it was running in the driveway it had a rattle coming from the catalystic converter. I though it was just a loose guard. I changed to a GM distributor cap after hearing aftermarket caps on Vortecs are junk. After going through many forums on many sites I found about 10 to 12 reasons for the misfire. One said the tank pressure was bad. Replaced the gas cap and dropped the tank again. I found the connector didnt get fully pushed into the tank pressure sensor. I thought I had it fixed but it was a warm dry day out. About two days later it misfired as soon as you hit the gas. Funny thing about it...it doesnt do it all the time. I finally got to look at the cat and found it was something inside rattling. I just replaced the cat today. The light didnt come on but the miss did. I had it read several times in 5 months and it comes up p0300 random misfire. I disconnected the battery every time I disconnected any sensors thinking it would clear the codes but I hear you have to use a scan tool or just let the computer relearn on its own. I also removed and looked at the crank speed sensor and the cam speed sensor. I removed the distributer and check it for wear. It spun fine and seemed in good shape. I did clean the cam sensor up it had a cast of rust dust on it. I was told to pull the crank sensor from the front cover and check it for a burnt spot. I didnt see a burn but it looked like the tone wheel had been rubbing it. So far it runs better no miss but the rpm dont really go up smoothly its kinda like jumpy. Not real noticable but its not smooth. I also noticed that when I give it gas the rpm drops first then goes up. Im getting about 18-22 mpg which was better than 15 mpg I was getting. Hopefully its fixed but I find out tommorrow morning. Could the coil be getting damp over night? I sprayed it down with water and it didnt change anything. I also checked the vacuum lines and they seem fine.

 

A search for P0300 will turn up a rather lengthy thread as well as the book diagnosis.

One thing that can happen is that the drain screen in the bottom of the distributor can plug up and not allow any moisture that accumulates while the vehicle cools, to escape once it warms up. This can cause a misfire.

You can remove your coil and ICM and have them tested. Most chain autoparts stores should have the proper tester to cycle them. Have them tested quite a number of times in a row to stress them. Most problems do not occur at low temperatures within either device.

 

Thanks Swartlkk. I think the moisture caused the rust dust in the distributor. I m pulling the screens. I have to say this is the best site for blazers and the tech I have found here is the most useful from any site. I have been searching for 4 months for a fix to my problem and the knowledge I have learned here may have saved me a lot of money. I would like to thank all of you for all your input. I think I have mine fixed. Autozone will not read or reset codes without the service engine light on anymore. They got sued for resetting a guys codes that caused the engine to totally mess up. This was the BS line I got at least. I need to buy a scanner and check my codes to make sure there isnt any. Im still going to read the posts here so maybe may input will help others and hopefully I dont have any more problems. Mike

 

Hey guys,

I was reading your conversation and realized I have been having a lot of the same issues with my blazer. The random misfires and rpm jumps, as well as the service engine light coming on at random periods of time and even turning itself back off.

I love my blazer, it's had relatively few problems until this all started up these past months. I recently took over the maintenance as I moved out and got married so I don't have a lot of car experience, but I know my dad took a lot of pride in keeping it up. I guess my question is what, of all the things you did, do you think helped the most? I don't have experience to work on my car on my own and since the issue has started it's been in the shop probably 4-5 times and none of the mechanics have fixed it or really thought anything was wrong.

I'm hoping you can give me some direct guidance from your experience that I can take to the shop this next time.

Thanks