Misfire Cylinder 3 P0300

[jce98]

98 Tahoe 5.7. Replaced spider injectors with MPFI, new intake gasket, plugs, cap rotor wires. CEL is cleared, and then kicks back on with Cylinder 3 misfire. Swapped plug and plug wires, still showing Cylinder 3 misfire. According to the test light, it does not appear to be getting spark. I also replaced the ICM, but I would not think that would only affect a single cylinder?

 

[eXo0us]

I'm chasing a Cylinder 5 Misfire for the last 2 years. I know the frustration. First - don't trust a new cap - get a full new distributor. These engines eat distributor gears.

I have a hard time seeing sparks with a test lamp even on perfectly running cylinders. Always verify with a good running one.

How is compression looking?

Do you got a diagnostic reader where you can count misfire live?
My misfire is very specific and only happens between 27000 and 3000 RPM. Outside of that band I hardly see any misfire on the counter.

Knowing in which range the misfire happens should help to narrow down the options. I'm down to a weak valve spring or it's something with the head not flowing. Since I got good compression...

 

[east302]

98 Tahoe 5.7. Replaced spider injectors with MPFI, new intake gasket, plugs, cap rotor wires. CEL is cleared, and then kicks back on with Cylinder 3 misfire. Swapped plug and plug wires, still showing Cylinder 3 misfire. According to the test light, it does not appear to be getting spark. I also replaced the ICM, but I would not think that would only affect a single cylinder?

Was the #3 misfire present before all of this work?

New distributor caps are hit or miss. Delco caps seem to be better out of the box.

 

[Schurkey]

Cylinder 3 misfire. Swapped plug and plug wires, still showing Cylinder 3 misfire. According to the test light, it does not appear to be getting spark.

Photo of, or a link to this "test light"?

If you're testing for spark, USE A "REAL" SPARK TESTER calibrated for HEI. Nothing with a light-bulb is going to be appropriate, including junk sold as "spark testers", and timing lights. You need to have the spark jump a calibrated gap, not fire some silly neon or fluorescent bulb.
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

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As said, #3 seems to be a weak spot in the Vortec distributor cap. There's a frightful number of caps coming from Communist China, including crap with fancy, boutique names like MSD.

I also replaced the ICM, but I would not think that would only affect a single cylinder?

Correct. The ignition module (and the ignition coil) has no idea which cylinder is firing. IF (big IF) the cam sensor/crank sensor alignment is within spec, the computer knows. That's how the computer identifies a misfiring cylinder. So if the cam/crank sensor alignment isn't correct, the computer could be confused about which cylinder is the problem child.

 

[jce98]

I'm chasing a Cylinder 5 Misfire for the last 2 years. I know the frustration. First - don't trust a new cap - get a full new distributor. These engines eat distributor gears.

I have a hard time seeing sparks with a test lamp even on perfectly running cylinders. Always verify with a good running one.

How is compression looking?

Do you got a diagnostic reader where you can count misfire live?
My misfire is very specific and only happens between 27000 and 3000 RPM. Outside of that band I hardly see any misfire on the counter.

Knowing in which range the misfire happens should help to narrow down the options. I'm down to a weak valve spring or it's something with the head not flowing. Since I got good compression...

Compression is in 5% of other cylinders. I have used two different caps, but, I' strongly considering the new dizzy route.

 

[jce98]

Was the #3 misfire present before all of this work?

New distributor caps are hit or miss. Delco caps seem to be better out of the box.

No. And the second cap I tried was Carquest premium, I may give the AC Delco a shot. Third time may be the charm.

 

[jce98]

Photo of, or a link to this "test light"?

If you're testing for spark, USE A "REAL" SPARK TESTER calibrated for HEI. Nothing with a light-bulb is going to be appropriate, including junk sold as "spark testers", and timing lights. You need to have the spark jump a calibrated gap, not fire some silly neon or fluorescent bulb.
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=list_c_wl_lv_ov_lig_dp_it

xc_hide_links_from_guests_guests_error_hide_media

As said, #3 seems to be a weak spot in the Vortec distributor cap. There's a frightful number of caps coming from Communist China, including crap with fancy, boutique names like MSD.


Correct. The ignition module (and the ignition coil) has no idea which cylinder is firing. IF (big IF) the cam sensor/crank sensor alignment is within spec, the computer knows. That's how the computer identifies a misfiring cylinder. So if the cam/crank sensor alignment isn't correct, the computer could be confused about which cylinder is the problem child.

Makes sense. I will take a look at the crank sensor and make sure it is buttoned up correctly.

 

[Schurkey]

Makes sense. I will take a look at the crank sensor and make sure it is buttoned up correctly.

It's not likely to be the crank sensor that'd be the problem. More likely to be the cam sensor, since it's "adjustable" by turning the distributor.

What about my other comment regarding your spark tester?

 

[jce98]

It's not likely to be the crank sensor that'd be the problem. More likely to be the cam sensor, since it's "adjustable" by turning the distributor.

What about my other comment regarding your spark tester?

Spark tester ordered, should be in tomorrow. Easy enough to mover the dizzy slightly to see if that has any effect. AC Delco cap should be in this morning.

 

[jce98]

Spark tester in, no spark. Replaced new cap with new AC Delco, same issue. Misfire 3, Is it possible bad cam sensor could cause a misfire on only one cylinder?