Every now and then my Blazer (4 WD, 190,000 miles) spits out a P0300 misfire code and previously also spit out a P0410 code (the 410 code was fixed, read below).

Another symptom is the engine hesitates (aka "buckles") when doing 65-70 mph and the RPM is higher than normal. The check engine light does not always come on. Normally the RPM is around 2200 but is 2400-2500 when the hesitation occurs. Further, the gas mileage seems to have fallen 15% during the past year when these symptoms started happening.

What was done thus far by my mechanic:

- Full tuneup (plugs, wires, cap, timing belt)
- On the secondary air injection system, verified secondary air injection pump works on a cold startup and the solenoids and check valves work normally
- Replaced oxygen sensors
- Replaced Mass Airflow Sensor
- Replaced air filter
- Verified fuel pressure is constant under heavy load and flows all the way to the manifold where the injectors are located.
- Compression test came back with 190 psi or more on each cylinder.

Aside from the hesitation at 65 mph, she runs around town just fine and acceleration is decent.

Any ideas?

 

Assuming AC Delco cap and rotor.

Had the same issue with mine and when I put the new cap on my distributor the hold down tab for the cap was cracked so I replaced my distributor with a Skip White Performance aluminum one. My old one had all kinds of play when I took it out. Have never had an issue since.

 

Hi i have a 1996 chevy blazer vin W and recently came across a cylinder 3 misfire within the last 2 weeks. The misfire comes and goes. Cold start is good then as driving it starts to misfire but still drivable. After it goes threw a moment 3-4 mins. Of misfiring It goes away then comes back. I have a new reman engine from jasper in it with 5,000 miles on it and was great until now. I used there install kit came with cap rotor plugs wires ect. So far i changed everything again fuel filter, cap rotor plugs wires ign coil with ac delco for the most part. I Changed whole distributor 2 yrs ago with a delphi cause the ball gear was wore and i had a random cyl misfire than but that was with the old engine. Also put a new delphi fuel pressure regulator when i installed the jasper engine 6 months ago. Cat/muffler new walker brand. I done every test i can think of. Cyl #3 compression test @ 175 lbs tested spark tested fuel psi and leak down. Fuel psi key forward = 51 idle = 50/52. Holds pressure with key off for as long as my patience lasted " over 20 mins "
every sensor is new cleaned egr cleaned maf gone threw everything except tearing off plenum to inspect injector or valve cover to check for lift " cylinder compression test passing should rule out bad valve. Please any advise would be greatly appreciated! I been going threw hell with this thing trying to figure it out. Thank you

 

Usually I'd say a misfire on #3 is a bad spark plug. That's the first thing to suspect for that cylinder because quiet often, that plug doesn't get changed with the rest of them. Without the right "flex/swivel" spark plug socket, that one is really hard to get to.

In your case, I'm assuming the spark plugs were new when the engine was installed.

Your fuel pressure is low. Key on, engine off should be 60-66psi for a 1996 VIN "W" engine. Fuel pressure at idle should be 55-64psi. You definitely have a fuel pressure issue somewhere. Usually low fuel pressure related misfires are evenly distributed. It's odd for that to affect only a single cylinder.

How does the spark plug look? Any carbon traces on it indicating arcing in the boot? How is the wire on that plug? Is the wire clear of everything? or is it laying against some metal somewhere? How does the inside of the dizzy cap look at the post for cylinder #3? Is there any arcing visible under the hood if you look on a dark night?

On the fuel pressure issue, If you can hook a hose up to the bleed valve on the gauge, and run that to a glass jar ( a quart jar is best, but in a pinch, a pint jar can work). Jumper the fuel pump, and with the fuel pump running, open the bleed valve. The pump should fill a quart jar in under 20 seconds (a pint jar should fill in under 10 seconds). If the pump doesn't meet that volume, I'd replace the fuel pump with a AC-Delco or Delphi pump assembly.

You can also hook the gauge up directly to the fuel pump at the filter (either end of the filter will work), and check pressure there. You want the pump connected to the gauge instead of the engine. Pressure directly from the pump into the gauge should be >70psi, and it should hold pressure over 60psi after 10 minutes. Again, if it fails, the pump is bad. If the pump tests good for pressure and volume, but your pressure is low at the test port, then the fuel pressure regulator is the most likely culprit. That's inside the plenum, on the fuel injector assembly.

 

Spark plugs are new along with fuel pressure regulator. I figured it out. I pulled the plenum off and the #3 injector was leaking gas a little bit im guessing the biggest problem was that the #3 injector wires that power the injector rubbed itself some how and there was bare wires exposed so i would think it was shorting itself out on and off cause the misfire would come and go. Replaced the spider assembely with a ac delco mpfi unit from amazon for $225. Fuel pressure still low but the truck is running and performing like a champ. The part that sucks is i didnt look at the spider first so i had already went a replaced the plugs/wires cap and rotor and distributor (all ac delco) so i dropped some hard earned cash where it was not needed. I appreciate all the advise though. I been reading forums for years which is how i figured out my random cyl misfire i had 5 years ago that turned out to be the ball gear on the distributor shaft but this is my first time posting on a forum. Awesome to know that there are still plenty of good people out there! Once again thank you for your time

 

i have an 2002 blazer it has a random misfire (P0300) you can feel it more at idle. i have replaced the spider injector, plugs, wire, cap, rotor and the whole distributor, new timing chain, crankshaft sensor, ignition coil. it will go back and fourth from (P0300 random misfire) and (P0301 cylinder 1 misfire). i dont know what to do anymore. i need help