tight plug gap required, why?

[RonQ]

'98 Plymouth Voyage, 2.4L, near 95,000 miles
engines misses with ODBII error: "Cylinder 3 missfire". Self help needed since dealer support with diagnostic gives no answer.
Changed coil, plugs, ODBII, tested water temp and MAP sensor with replacements. Nothing found. Since "misfire" symptom could be plug gap, I changed plug gap gadually down to .18. Engine runs ok for now since abt 20,000 miles. But as plug gap gruadually opens up every 8,000 miles, I gap it back down to .18 and engine is mostly happy. Factory sticker says plug gap should be .50. What's up?

 

[Metalguy]

You aren't getting enough voltage to the plug, or the voltage rise time is too long. I'd start by using a fat new plug wire on that cylinder.

*IF* your eng. uses separate coils for each cyl., I'd try a new/good used one.

 

[patprimmer]

Even try alternately swapping coils and leads from cyl to cyl if they will reach. If the miss moves, you have found your problem

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[RonQ]

I have tried those tips, changed plugs to new ones, changed the dual coil pack, tested the plug wires by swapping and with ohmmeter test, swapped the OBDII module. I have looked with my oscilloscope and see the plug voltages on the primary side (from the OBDII module). If I knew what rise times would be good, I'd check for that.
The dealer could not pinpoint the problem either, wanted several hundered dollars to "get started" trouble shooting by swapping parts "till the problem might be resolved". I am not going to give him a blank check.
BTW about 5,000 miles before trouble started, an other dealer had replaced head gasket ok. This problem seems not really connected to that?

 

[RonQ]

BTW at .50 plug gap engine idles ok, but drivability is poor, with cylinder missfire diagnostic at different cylinders. Poor drivability is: acceleration produces misses.

Has any one seen graphs on compression versus plug voltage needed to fire? It feels like, since head gasket change, the compression has gone up and the plug voltage is too low for the now higher compression. Sorry to say, I have no record of what the compression was originally and what it is now; especially under acceleration...

 

[Metalguy]

This still sounds like a secondary voltage-to-the-plug problem. You can't test the plug wire INSULATION with an ohmeter! Sounds like a new, good set of plug wires might cure this. If you're gapping spark plugs at 0.018", that's way too small. Stay with ~0.030-0.035" until you get this problem fixed.

 

[patprimmer]

Try running the engine in the dark with the bonnet up. Look for glow worms running around the high tension side.

It takes a lot more than a head gasket thickness change to require a noticably different plug gap.

Could an injector be playing up, and the missfire is fuel related, and the the improvment when closing of the gap is coincidental

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Warpspeed]

Yes definitely a voltage to the plugs problem, but the causes could be many and varied.

It could be series impedance, such as severely burned distributor cap or rotor button, or the resistance of the plug leads if they are of the carbon resistance type.

Or voltage could be leaking away because of shunt losses due to arcing, corona, or leakage resistance. A good way is to run the engine in total darkness, and look for a blue glow or sparks jumping from plug leads or around the distributor cap.

If in doubt replace the plug leads, cap, and button anyway, they are not expensive.

 

[RonQ]

Engine won't run decent at .030. I understand that .018 is small, but that is the "happy" medium of usefull performance. I tried .015~.014 and that is really too small. 0.018 just hits it right in this case, but have to regap every 5000 to 8000 miles to compensate for wear. I'll get a set of wires tomorrow, but not expecting that it'll fix. Then what?

 

[patprimmer]

Check all connections are clean and tight, especially on the low tension side and on electronic components.

It's not possible I suppose that the wrong wire is on the low tension side of the coil.

Has any part of the loom ever been cut and repaired poorly.

I know these sound far fetched, but you have tried all the normal problems.

Some one else mentioned it, but have you tested the distributor cap if it has one or the coils if it is a coil pack system.

Have you cleaned the coil posts, and checked for cracks, carbon tracks etc. If there is a fairly high resistance to earth from the leads, anywhere, the current will bleed of, weakening the spark.

Does it have capacitors or igniters in the ignition system. One of thewm might be defective.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[RonQ]

Thanks for help to everyone for sticking with me.
It was an hour just before Autozone closes. So I rushed out on 20 mile trip to closest one. Got new set of plugs (champions RC12YC5) and set of wires. Changed plugs with .050 gap checked. Still same problem. Installed new wire set and presto every thing is .

I guess, I lived and learned at 60.

PS. any one care to know what I see, if I slice the old wires; just to see what is going on?

 

[RonQ]

This engine design has the plugs on the bottom of 4 3/4 inch deep hole. The plug wire boot is about 4 1/2 inch long, and the boot covers 3/4 of the porcelain plug insulator.
I found carbon track on the -inside- of the boot. The track was going from the top of the plug, basically by-passing the porcelain insulator, then jumping either to the plug threads or the wall of the plug well. Looks like this might happen a lot - the wire boots must be under more electrical stress than in other engines....

 

[Warpspeed]

Great stuff Ron......

I have been through exactly the same thing myself. If you run the engine in the dark, it is like looking at a firework display down in those deep plug recesses.

I managed to find some PVC electrical conduit that was a nice sliding fit down those evil holes, and it has made a lot of difference.

When you cut your old leads apart, expect to find some grotty looking black carbon string in there. Real metal plug leads are better, but the radio interference might be pretty high if you decide go that way.

 

[Metalguy]

Plug wires with a tight coil of fine monel wire are avail, and the radio will be OK. Get fat 8mm ones. I do not like the "carbonized linen thread" wires that cost the manufactures about 2 cents a foot--or per yard.

I once bought a turbo Toyota Supra that the owner was having trouble selling. It would pull hard for a second or two, then "lay down". I got it fairly cheap, thinking it was a simple fuel filter/supply problem. Turned out to be the plug wires. I adapted a fat new set of Moroso silicon rubber wires-end of problem. Still have the car.

Here's another good one. WAY back in the 1960's I had a '52 Ford with a '55 Buick engine. Did a tuneup one day, installing Autolite resistor plugs. Car immediately developed a weird misfire, but only after being loaded a while. At full throttle, it would only misfire after 3 or 4 seconds, but at ~ half throttle it wouldn't misfire for ~7-8 sec. I was POSITIVE I had a fuel supply problem, esp. after "doing a tuneup".

I tried about a dozen things-nothing helped. Then I took the car to a shop, and an old-timer told me to replace the plugs w/non-resistor ones. Said he'd heard about resistor plugs heating up and having the resistance go way up. I followed his advice, and it worked. I used those resistor plugs for target practice with a 7mm mag!

 

[patprimmer]

Sometimes metal wires can interfer with electronic ignitions and engine managment systems, so it pays to use premium grade suppresion leads on a modern car. I like the genuine MSD leads myself.

Those deep sparkplug wells can hold dirt, oil or water. This will then cause a short which will result in a carbon track.

Sounds like that is what happened to you.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[RonQ]

On my last post I forgot to mention that the plug boots have an integral well seal - so no way to see or hear any arcing/fireworks. With tongue in cheeck: the wires didn't go bad, just one boot. In the 'old days' one could just replace the bad boot; but no such thing here because of the length of the plug boot and the integral well seal.
In retrospect: it appears that 0.018 just out-competes a 1/4 air gap for juice. since 3 times 0.018 is near 0.050 therefore the plug tower is 3/4 to 1 " tall. Think, I aught to gap at .045 to 0.050 and never let the plug gap wear past 0.060. I have seen one wire set(for different car make) come with dielectric gel in a packet to line the inside of plug boot. Maybe this 2.4L Chrysler engine needs this/ could benefit from this gel too?

 

[RonQ]

PS. any one know of a Chrysler/Plymouth knowledge base, where I could deposit this and/or find similar - "known-to-us" - problems and fixes?

 

[Metalguy]

I would set the gaps on all the plugs at 0.040" and forget about them for ~25,000 miles.

 

[JayMaechtlen]

As a side note- my last two Buicks have needed plug wires at 90-110k miles. No big deal, they're cheap. The Aerostar got cap, rotor, and wires at 120K (second tune-up).
cheers
Jay

Jay Maechtlen

 

[Warpspeed]

As far as those rubber sealing boots go, a cynic might suggest they seal in the water AFTER it has leaked past the boot. Finding rusty spark plugs is not that uncommon either,(on other peoples engines).

I prefer to leave the rubber sleeve that fits over the plug, but cut away the top sealing cap. Heat from the cylinder head will keep the plug dry provided the hole is open to the outside air.

During routine maintenance, a quick blast with an air hose makes short work of any dust or grit down there, and some PVC sleeves also assist with electrical insulation.