Request help: L31 5.7 Cylinder 5 Misfire

[Augusto]

I'd say your intake gasket has a leak, usually it happens on the bottom side of the intake port, or maybe a wiped out cam lobe or bent pushrod, definitelly sounds like a mechanical failure

 

[George Schreffler]

Thanks to all for the additional posts.

Schurkey,

Multi-port injection was installed on the old engine and carried over to the new engine.

Will try observing ignition wires while running in complete darkness.

Used a spark tester - acknowledge that only gives me a visual signal and not a voltage reading.

In addition to swapping the new spark plugs around I did try several of my old plugs and old ignition wires. They did not change the misfire.

I also don’t understand why the P0300 misfire DTC is the only one that sets when the misfire is specific to cylinder 5 only. But the truck set the code. The scanner only reads it. The scanner’s ability to read the misfire counters for every cylinder is how I zeroed in on cylinder 5.

Until I locate and fix the root cause I am not driving the truck - don’t want to damage the new engine.

Will run cranking compression test and cylinder leak down test.

I know someone with an ignition oscilloscope who will probably be thrilled to put it to work.

Orpedcrow,

Borescope was how I found the crack in the original engine. I scoped the new engine, too, but did not find anything noteworthy.

 

[Schurkey]

Used a spark tester - acknowledge that only gives me a visual signal and not a voltage reading.

A spark tester that forces the spark to jump a calibrated (or adjustable) gap is close enough to a peak-voltage reading. My preference is the non-adjustable kind, calibrated for HEI.
www.amazon.com/dp/B003WZXAWK/?coliid=I3S98D7T1J0RLJ&colid=2VLYZKC3HBBDO&psc=1&ref_=lv_ov_lig_dp_it

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A spark tester that has the spark voltage light up a light-bulb is worthless.

I know someone with an ignition oscilloscope who will probably be thrilled to put it to work.

Kinda guessing that this is more air/fuel related than ignition, but that'd be an excellent way to rule out ignition issues. Worthwhile. Provides more info than just "peak" voltage.

I'm still leaning towards intake manifold or manifold gasket defect.

 

[blackdeathmessenger]

Did you ever find the issue? I've since done some research that mentioned something about the timing being out of phase

 

[Schurkey]

Did you ever find the issue? I've since done some research that mentioned something about the timing being out of phase

On a Vortec 5.7L?

The ignition timing is not adjustable. Turning the distributor changes the "phasing" or the alignment of the cam sensor signal in relation to the crank sensor signal. It also affects the alignment of the rotor tip to the distributor cap terminals.

The sensor signals can be WAY off before the engine runs differently, and even then it's not so much due to the sensor signals as it is due to the misalignment of the rotor-tip-to-distributor-cap-terminal.

 

[454cid]

Isn't #5 the one that tends to short internally on the cap?... especially on cheaper caps. (I didn't read the whole thread if it's been mentioned)

 

[blackdeathmessenger]

I'm not fully sure on either of those, was just presenting a possibility I've come across in my research in case I have to deal with the same issue

 

[eXo0us]

I'm currently also going down the Nr. 5 Cylinder misfire. P0305
Got similar results and troubleshoot as the OP.

Still my misfire stays with Cylinder 5 - no changing of spark plugs - wires - coil - distributor etc. did change it.

Then I needed to do the intake gasket - and I also changed the spider injection system with the MPFI.

Now the truck is running better - the misfire is only in mid RPM ranges. I read the miss fire counter with Torque Pro and see that they only begin at around 2700rpm - get really bad around 2800-2900rpm and almost completely disappear at 3200rpm. At WOT - the engine runs fine.

Occasional (every couple weeks) I get a P0300 at idle when warming up - but only when I start the truck and just let it sitting idle - when I start driving right away - no code.