bil2000

Hello all. My first post.

I have a 2000 Chevy Blazer with 170k miles on it.

It started misfiring recently which caused the SES light to come on. I tried nearly everything and still cannot diagnose the problem.

I get the following codes:

P0301 - most of the time

P0300 - some of the time in lieu of P0301

P0171 and P0174 - some of the time

P0302 - came on once

Obviously, the codes change without any rhyme or reason.

The car runs but shakes a lot around 1800 RPM. It seems that it gets smoother when it's over 2000 RPM.

Gas mileage is around 9MPG.

Some days it runs better than the others.

When under load (usually over 3000RPM), the SES flashes.

The fuel pressure is good, pressure remains the same after shutting the car off as supposed to.

At idle fuel trims are over the roof. STFT is frozen at 50 and LTFT is at 25.

There are no vacuum leaks.

So far I changed spark plugs, wires, distributor cap, rotor. Still the same.

What bothers me the most is the inconsistency with which it switches from P0301 (Cyl#1 misfire) to P0300 (Multiple Cyl misfire).

Anybody had the same symptoms? What could it be?

bil2000

Any suggestions guys?

bil2000

Another thing happened today. I got P0147 (O2 Sensor heater circuit malfunction on bank 1 sensor 3) for the first time. I didn;t come back after I cleared it off.

rfetter

I had the same issue w/my 2000 S10 ZR-2 trk. P0300 MULTIPLE misfire code along w/P0301, P0302, etc & also catalyst below efficiency. All of that misfire, stumble & a P0410 faulty P.O.S. secondary air-injection system code to boot as well (surprise, surprise)! Scan tool showing cylinder misfires on cyl # 3 & # 4 (middle cyl's directly across from each other - coincidence??) Tried the usual - check/inspect: plugs, wires, cap, rotor - all the usual suspects. Good AC Delco plugs (verified correct part # & re-gapped) & ignition parts, no problems there. Pulled crank sensor to inspect/check gap, even tried slipping a thin piece of cardboard in to change air gap w/no difference. Ended up pulling intake to reseal oil/coolant leaks & replace C.P.I. unit, set cam retard - still had the misfire problem! Ended up running a bottle of Sea Foam thru the upper air-intake system & adding a bottle of Chevron Techron fuel-system cleaner to the tank (already verified fuel-pressure & good quality/quantity of fuel). Took the truck on Interstate 81 in # 3 DRIVE, NOT overdrive & hammered to 90+ mph 'til the rev limiter kicked in, did that SEVERAL times (w/Snap-On Modis scan-tool attached). After about the 4th run, misfire counts on scan-tool began DROPPING drastically - right down to ZERO. Idle smoothed out & engine ran great. I suspect some sort of hydrocarbon build-up on one of the valve seats/face/stem or guide preventing proper seal of valve(s) & running the engine @ high speed under load & the addition of cleaner/additive thru fuel & air-intake broke the build up loose. This truck is not used everyday, really only for hauling/bad snow, etc. so that's why I'm thinking some kind of build-up due to intermittent operation/short trips, etc. Engine has been running great since (Dec 2010). On the down side though, running engine that hard while misfiring did cause the catalytic converter to melt-down & plug my exhaust tight. Picked up a universal fit unit from local Advance Auto store & piped it in w/out any problems. Replaced the air-pump yrs ago & M.I.L. is back on again, still have the P0410 code but I know what that is & it's not a problem. I hope this helps or at least gives you some ideas. This truck had approx. 90k on it when the problem occurred, has about 110k on it now.