MinimusMaximus

'95 Civic 5spd. Car has developed an intermittent misfire but only on acceleration.

Intermittent meaning; yesterday on my way home, lots of misfiring here and there throughout the drive. This morning on my way to work it didn't misfire once.
Only on acceleration meaning; the car idles just fine, it will even drive away without issue, the moment I apply any reasonable amount of throttle (nothing aggressive, just normal get-up-to-speed throttle) it will begin to stumble and misfire. It occurs mostly through the 2000-2500 RPM range. If I can stumble through that range and approach 3k RPM the engine will operate normally. Shift gears, drop RPMs, and again... stumble through 2k-2.5k RPM till it clears up. While its actively misfiring I can see hints of white smoke out the rear windshield.
When its not actively misfiring the engine runs smooth as butter. For instance, once I'm finally up to speed on the freeway, its cruise sailing. Apply a bit of throttle to change lanes and make a pass, misfire... stumbles like one of the injectors is unplugged or one cylinder is not firing. Back off the throttle, everything's normal again.

Just did a full tuneup (cap, rotor, plugs, wires, fuel filter) thinking that would fix the issue, and although it cleared up for a little while (a day or so) the misfiring came back.
The only thing I did not do was check the timing because I don't have an inductive light. But the car idles and as mentioned, when its not actively misfiring, accelerates strong and fine.

No CEL either.

Jimi Hondrix

Pull the spark wires out (not the spark plugs) one by one and test them against a the valve cover or better yet an extension to chassis ground and check for arcing/insulation leakage. Spark plug wire insulation can breakdown over the years and cause misfires to ground inside the spark plug tubes.

Here's another test for leakage:

fcm

Never test for spark or "spark leak" by jumping from spark plug lead to ground, it is a dead short and can/will damage the coil and/or the ICM, for "spark leak" the Eric the car guy way works pretty good pulling the leads and inspecting them for cracks also works and checking the impedance with a multimeter is the best way, to check for spark, use only an HEI spark tester. 94

MinimusMaximus

As mentioned plugs and wires were changed and issue still persists. NGK expensive type.
Besides, wouldn't a bad plug wire only be evident if the engine was currently misfiring while the plug was out and being tested? It wont misfire at idle, just driving and then only under acceleration.

It reminds me of the old vacuum advance distributor days where if you had a vacuum leak to the ignition diaphragm the timing wouldn't advance. It was always more noticeable under heavy acceleration when more vacuum signal was needed to make the advance but the leak kept it from responding properly.

No such thing on one of these though...

fcm

Have you checked the TPS or wiring to it?
And as mentioned check the spark with an HEI spark tester at all 4 cylinders, should be bright blue. 94

Jimi Hondrix

Maybe, I should read more thouroughly..Lol.

If the wire/boot itself has an insulation leak it could arc to ground with or without a spark plug connected and cause a minor misfire

Jimi Hondrix

I keep hearing this but, never seen it happen..have you?
Not doubting..just speaking from my own experience and others I've seen test similarly.

fcm

In 35 years of doing auto electoral, yes more then a few times, not that it happens all the time it was the go to way to check spark on older cars, screwdriver in spark plug lead and touch/close to ground, on todays cars they are much more sensitive to that. an HEI spark tester can be had at any auto parts store for $10 or less, so not really worth taking the chance on blowing a coil or ICM, the other issue is spark in a combustion chamber is different then in open air the HEI spark tester compensates for that it is far more accurate in what the spark looks like, [bright blue]. 94

Jimi Hondrix

Okay ..sounds reasonable. I have this tester: https://www.amazon.com/Lisle-20610-I.../dp/B0002STSC6 and like it a lot.

TomCat39

That one is not calibrated for High Energy Ignitions (HEI). That is the inline tester designed around the older V8 muscle cars to replace the old flat tip screw driver method.

You want one like this:
https://www.amazon.com/GearWrench-Ig...I+spark+tester

Jimi Hondrix

Thanks for the link. I just read the 1 star reviews and they are saying item doesn't match picture. Would this one be okay: https://www.amazon.com/OTC-6589-Elec.../dp/B0050SFVO2 I can return my old one and purchase the new OTC at local parts store.

MinimusMaximus

So I believe I solved my problem.

I decided to take the cap and rotor off and upon inspection found that the rubber oring seal had slipped out of place at the bottom causing it to get pinched and ultimately (I'm assuming) left a gap for moisture to get into.
Car drove fine after the new cap and rotor were installed but then we had a monster rain storm. Didn't occur to me at the time but that's when the misfiring started back up. Moisture must've gotten in the cap.

anyway, clean dry dizzy cap, happy car.
Hope that helps.

TomCat39

To be honest, I am not sure as it doesn't indicate being calibrated for HEI systems.

I have the plastic one and it works fine as well as the gap is significantly larger than the old ceramic based model that looks like the OTC one.

I would actually contact OTC and see if that model is the model designed for HEI systems. They may direct you to a different model number OTC tester.

Good deal, glad you isolated the issue and got it corrected.

Water in the dizzy cap will definitely wreck havoc.