intermittent missing

[Jeff Erwin]

New issue. 71 440 6-pack. Engine has been running like a sewing machine, very smooth. Last weekend on the way home from a car show the engine started running on 7 cylinders, but intermittently. It would run smooth, then feel like it was dropping a cylinder, then smooth again. No predictable cycle. Felt like someone was randomly pulling a spark plug wire then replacing it.

Checked all plug wires, everything tight. Distributor is the MSD electronic that has the ECM builtin, 9 months old. No burning on the cap or rotor. Using a temp thing looks like I have #4 and #6 a bit cooler at the exhaust manifold (headers so easy to measure) which would be consistent with an intermittent miss.

So, any suggestions before I start pulling the top of the engine apart? Why would this start happening all of a sudden and be intermittent? Engine has some clattering sounds when cold coming from the top of the engine, not the valves. Been doing that for a while.

 

[AR67GTX]

I had a plug wire with a weak connection at a plug boot once that would go in and out from vibration. Drove me nuts.

 

[PlymCrazy]

Before I loosened even one valve cover bolt I’d pull the spark plugs from the suspected cylinders. See if they tell you anything…

 

[Bee1971]

Then if suspect on those cylinders based on spark plugs , a compression test would bee next

 

[turbine68rt]

Have you checked the primary and secondary coil resistance?

 

[MWbirdLOVER70]

Was gonna say coil windings grounding and cooling then hot and touching

 

[turbine68rt]

a compression test would bee next

I see what you did there

 

[khryslerkid]

Pull the suspected plugs and see what they look like. Plugs do go bad. Look at the plug wires for chafing or burn marks where they might be arching on the exhaust manifold or holder, grounding out. No sense tearing anything major apart until you eliminate the simple things.

 

[Geoff 2]

[1] Measure the resistance of each plug lead
[2] Remove each spark plug & inspect for cracked porcelain.

 

[Jeff Erwin]

As part of this diagnostic I just installed new plug wires. MSG Street Fire I think. They are measuring 500 Ohms of resistance per foot. The old red MSG wires were measuring about 200 Ohms total per wire.

Whats right? Is a long wire measuring 2000 Ohms a problem?

 

[Geoff 2]

No, it is not.
A lot of BS about low res wires making more HP. With spiral wound wires, you have inductance & capacitance, as well as resistance in the ign cct.
The point of measuring the wire resistance is checking for consistency. It checks for a break in the wire & the connection to the terminals. The res varies with each lead, & it should be proportional to the length.

 

[bret schucker]

my gtx has an electric fuel pump that has had a very similar situation a time or 2 and it stemmed from having extra weight in the back seat and trunk area causing car to squat in the rear somewhat, combined with the fuel pump placement on the car , which made it hard for fuel to be supplied to the pump consistently ,causing a random miss while cruzing down the highway ! Take the people out of the back seat and remove the stuff in trunk and No issue , but then i always get the problems in my car that knowone else has ever heard of happening ! just a thought , but more than likely its an electronics problem , as said already .

 

[Ron H]

Engine and road vibration can obviously create gremlins if having loose wiring connections or damaged wires. When you mention intermittent/unpredictable, nothing noted then, such as within a certain RPM range, throttle, coast, or after running over some road bumps? Does the missing happen if your ride is just sitting and running giving it some throttle? As posted, could be a connection or wire.

I’d add look along the ignition circuitry closely for any evidence of burned wiring or connections. Spotting this can be difficult as I found out. My hassle was different than yours; but frustrating as hell. Only way I did was checking connections with motor running and moving a harness (new engine bay wiring) at the BH just so slightly, caused it to stall out. In my case the BH was crappy anyway and this hassle forced me to replace it when I should have earlier. BUT, dang, the problem cropped up again. Come to find the ignition wire having not been tight for some time at the BH, caused overheating inside the new harness creating a loose connection so replaced that connector and solved. Any skank connections will create excessive heat, lose secure contact and may require new connectors.

May not be related with your hassle, but just tossing it out as a check before having to get more invasive as posted.