Slight misfire?

[Tom944]

Since uncovering my 87 NA this season it has been idling a bit like a v8 with a slight shake. Occasionally it misfires very quietly while idling. It lacks power sometimes and tends to go "chuff chuff chuff" a bit it low revs when accelerating.

In the last couple of years it has had,

Timing belt
Plugs
Wires
O2 sensor
Rotor
Cap
Intake manifold leak fixed
Injectors serviced
All vacuum hoses are ok or replaced

It has 62,000 miles and is my summer daily driver. I did check for dead animals in the air filter housing! Plugs look ok. Rotor and cap look ok. Coil is original.

Any thoughts?

[Tom944]

Just ohm'ed out the coil. 0.8 ohms and about 6k ohms. Probably close enough. My DVM reads 0.4 ohms with the probes connected together!

[Tom944]

Just revved it a bit in the garage (since my wife went out) and it sounds smooth. Pulled each of the plug wires in turn. Pulling the front most plug had little to no impact. Pulling the others dropped the idle for a second. All plug wires are sparking.

[dgcantrell]

Number one plug resistor end is failing. My 928 was doing that when I first got it many moons ago on the number 5 cylinder. I eliminated it with a traditional crimp-on sparkplug end until I got a new set of wires. I have replaced the original wires on my 944 with Magnecor wires to eliminate those resistor ends. Much cheaper and actually better than OEM. I've had them on the car for almost 20 years now with zero sign of degradation of the wires or performance.

[Tom944]

Thanks for the response. The wires are only about a year old and are OEM.

[Tom944]

I also listened to all the injectors with a stethoscope and they all sound the same.

[dgcantrell]

But still if you pulled the wire from the #1 cylinder and noticed no difference in the running of the engine, your culprit is there. The engine will make a noticeable change in running sound on three cylinders. Pull the spark plug after letting it run for a minute or two. If it's wet, it ain't firing..

[Tom944]

Thanks again. I pulled the plug after running it and it was not wet. Seams to run ok and idles without hunting but just pops out the exhaust every 30 seconds or so. Runs ok under load. Runs a touch bouncy when just touching the gas pedal at 2000 rpm. Tried spraying easy start around to find any vacuum leaks but gift no change in rpm.

Plug on front cylinder was a little blacker than the others but looked ok.

[dgcantrell]

Black means it running rich on that cylinder or the fuel isn't atomizing as well as it should. Could be an injector starting to get clogged up. Run some fuel injector treatment (Seafoam or whatever your favorite flavor is) in it for a while and see if it helps improve it.

[Tom944]

Thanks again. I will try that. Foes anyone think that the 29 year old coil could be the issue? It tested okbut I am not sure that means that the spark is always good.

[dgcantrell]

Being that the issue seems to be occurring on just one cylinder, the coil shouldn't be the issue. It fires all the plugs. I'd still run some injector cleaner though it before doing any more physical work on it.

[Tom944]

I just noticed a gas smell after starting a then stopping in traffic a few seconds later. No visible signs. Clearly its not igniting all the mixture properly! Not sure what to try next.

[Tom944]

I do have spark on all plugs when I pull the leads. No idea if its a good spark or not.

[dgcantrell]

Check the plug gap. Should be .28". But it could be an 02 sensor going out. I'd check the gap first.

[Tom944]

I re-gapped them last night. They were a little big, particularly the front one. O2 sensor is new. I ordered a coil since its the only thing in the ignition that is original. It will be here tomorrow.

[Tom944]

My train of thought is that maybe the spark us weak. That, and I can't think of anything else easy to try!

[dgcantrell]

Could be. I had coil issues on one of my motorcycles back when I was a teenager. I pulled my hair out over it. Finally swapped out the coil and, bang. Never had another issue out of it.

I'd still run a can or two of fuel injector cleaner through it.

[Tom944]

Thanks for all the help. I will let you know how it goes. Ran much better just now after it was sat in the sun for a few hours. It was raining last night when I was testing it. Maybe I should just move to somewhere warmer and less rainy.

[Tom944]

I just plugged an old coil to distributor lead into one of the plug lead connectors on the distributor cap. Held the metal end near the engine to see the spark color. No resistor in this lead. However close I put it I only got an orange and occasionally white/orange spark. A slightly shocking test but maybe it is the coil.

[Tom944]

Replaced the coil and it seams to be running fine now. Will really know in the morning since it was always worse when cold.

Here's what my google research taught me - you need a blue spark that can jump about 7/16 inch in air to be able to jump .030 in the compression inside a cylinder.