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2000 Chevy Blazer Code P0300

Tiny
BPANNING
  • MEMBER
  • 2000 CHEVROLET BLAZER
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 154,000 MILES
4.3L engine, CMSFI. Code P0300 - Mulitple/Random misfire. Barely idles, runs very rough, smooths out at higher rpm but still is missing. After clearing codes, will repeat consistantly. Have replaced ignition coil, cap, rotor, plugs, plug wires, fuel pressure regulator, fuel filter. Little or no improvement. Checked fuel pressure at shrader valve - will fluctuate wildly between 55 to 60 psi at idle, smooths out to 52 psi at or above 2,000 psi. When replacing the fuel pressure regulator, I noticed that there was a port on the regulator as if there should be a vacuum hose, but none was present. Should there be one? Any ideas? I have a lift and lots of tools, and would consider myself to be an above average mechanic, but this one has me stumped. My daughter has a 1995 Blazer, but it is a CMFI injection, a little different. I have never worked on the CMSFI before.
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Friday, February 27th, 2009 AT 7:24 AM

8 Replies

Tiny
2CARPRO JACK
  • MECHANIC
  • 11,533 POSTS
Those engines had alot of issues with injectors sticking/quitting. If a compression test tells that the engine is good, I would suspect a bad injector, very common to these. The updated A/C Delco injector eliminates one of the check valves that causes the problems. When idling lightly spray water on the exhaust manifolds (ports) to find out which one is coldest. Then pull the plug to see if it looks lean (white) compared to another plug. Also remove and inspect the EGR valve (if equipped) to be sure a piece of carbon isnt holding it slightly open, that will do the same thing
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Friday, February 27th, 2009 AT 7:41 AM
Tiny
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I have not done a compression check yet, but can over the weekend. This problem surfaced "all of a sudden". I did check the EGR valve, but it is working properly - even swapped a known good EGR with no change in performance. The plugs were changed a few months ago, but I replaced them again. All six looked the same - somewhat carbon fouled, no indication of a lean condition. I did replace injector #3 because when I pulled the injectors it had the plastic nose on the squirter end mushroomed out. Does the fuel pressure regulator (inside the air intake)have a vacuum hose or does it just vent to the intake?
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Friday, February 27th, 2009 AT 7:50 AM
Tiny
2CARPRO JACK
  • MECHANIC
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It just vents to the intake plenum, It is very common for these to hang up injectors after the fuel system has been opened up, another small quirk. If you replace one at a time, it is unlikely that updated injectors are being used. Just for laughs see if they offer the complete "spider" for that year, it will most likely have the better injectors. When you see the difference in the two, you will know what I am talking about
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Friday, February 27th, 2009 AT 8:10 AM
Tiny
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  • 5 POSTS
I checked at the Chevy dealership - they don't sell the "spider" unit, you have to buy the injectors individually. They don't know if it's the updated injector. Price is $104; I can get them for $78, but I would like to be sure that is the problem before I spend another $500 on parts. Would it be of any benefit to remove them and have them cleaned?
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Friday, February 27th, 2009 AT 8:30 AM
Tiny
2CARPRO JACK
  • MECHANIC
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I guess you could try, I have never heard of anyone cleaning them before. If it has compression and spark, the injector is just about all that is left. Just need to find out which one
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Friday, February 27th, 2009 AT 9:23 AM
Tiny
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I did a compression check - #3 has zero compression; other five were consistant, between 175 and 200 psi. I pulled the valve cover, but no broken rocker arm or bent pushrod. I turned the engine over by hand and watched #3 valvetrain parts, all seemed to be in good working order. I guess I'll need to pull the cylinder head and see what the problem is. I believe it must be a bent or broken valve - absolutely no compression.

I was told by someone else that 52 psi fuel pressure is not enough for this injection system - should have 58 psi or more. Do you agree?
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Monday, March 2nd, 2009 AT 12:01 PM
Tiny
2CARPRO JACK
  • MECHANIC
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Should be 60-66 psi for that engine, that year.
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Tuesday, March 3rd, 2009 AT 7:27 AM
Tiny
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Pulled the cylinder head, and sure enough, the exhaust valve was bent and not seating properly. It had almost a full 1/4" of a gap on one side. I have taken the head to a machine shop. Should get it back in a week or so, and when I re-assemble the engine, I will let you know how it works out.

The valve did not show any signs of damage (i.E, contact with the piston) nor did the piston. I'm not sure what caused the problem, but it definitely was the root cause of the idling and drivability issues.
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Friday, March 13th, 2009 AT 5:55 AM

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