Random Misfire while cruising

[jalexquijano]

18 hours ago, AK260 said:

OK, that's good.

So, the way I see it is this:

1. It's not oil on the plug itself

2. Your coil is healthy and your mix on that carb is OK as the rest on that bank of 3 are not getting fouled the same way 

3. I am making an assumption without reading up the thread that you have done your valve lash adjustments and that they are consistent across those three cyls.

4. I think I recall you doing a compression test showing no.4 is pretty much inline with the rest in compression, so you don't have compression issues or valves leaking issues.

Assuming this is an electrical issue (which is what we're left with after discounting the above) and also assuming that all three plugs in your photos have the same heat rating, you need to identify why that one isn't getting enough spark / energy for a full burn.

Some thoughts:

a. The spark plug itself:  Have you tried swapping one of the other spark plugs into cyl 4 to prove the plug itself isn't the issue?  Let's prove the spark plug itself doesn't have a hairline crack or other issues. Does no.4 always foul regardless of which plug is in it? Yes. It fouls. Right now i have bp5es on all 6 spark plugs.

b. Ignition leads: Have you tried a different set of ignition leads or swapped, say no.4 lead with no.3 lead (both on the distributor cap and at the spark plug) to ensure the lead isn't at fault? This was done. This is the second set of ngk blue wires. 

- I once had a Magnecore KV85 lead with intermittent issues (after 6 years) that had the same symptoms as you are experiencing. It drove me nuts until I found it. I only discovered the issue after bench testing. In the car, it's very difficult to tell if it's firing ALL the time - our eyes are simply not quick enough to see it. But on the bench with an ohm meter connected to both ends, I could see the resistance changing from tens of KΩ to hundreds of MΩ when I wiggled it. When I raised it with them under their 10 year warranty, the owner at Magnecore was just unbelievably condescending, claiming it's impossible for their leads to fail and it's always the customers' fault for not unplugging the leads regularly!! Gobsmacked, I didn't waste my time and money sending them back and made my own for a fraction of the cost - not only do they fit perfectly but they have worked flawlessly since.

 - Also, does the HT lead "click" as you push it onto the spark plug terminal? Make sure you have a good connection. On that note, inspect the HT lead for signs of corrosion / arcing. When a spark jumps a gap between the lead contact and the spark plug terminal, it leaves a powdery white / greenish residue on the HT lead contact. It's a sure tell-tail sign of spark jumping rather than a good conductive connection. Yes it clicks.

c. Distributor: Have you inspected the inside of the distributor cap for any damage on no.4 or any obvious signs / issues? Have you tried a totally different distributor cap? Is there any wobble in the distributor shaft that may be consistently affecting no.4 (this is unlikely but worth a check)? In fact, what distributor are you using? Are you on points or electronic ignition? If on points, could the dizzy cam be worn for no.4 and causing less "dwell" time? Im using the hitachi single with pertronix ignitor module.

d. Spark plug seat on the head: Have you cleaned the threads on the head? Sometimes, when the heads are painted, you can end up with paint on the spark plug hole threads / seating area that causes resistance, reducing spark energy. This is unlikely but worth eliminating.  Use a very small amount of paint thinners or carb cleaner (carefully) and a small wire brush to ensure that where the spark plug seats itself has a good electrical connection. I say carefully as you don't want thinners getting into the cylinder and contaminating the oil, or small metal particles ending up in the cyl.
 
Good luck fella - we're rooting for you!

Replies above

[AK260]

Ok, so you are eliminating many of the possibilities which is great.

What I’m not getting from your replies is:

1. Have you tried a different cap and rotor?

2. I don’t know what a “hitachi single” means - can you elaborate please? Is that a conversion made to a points distributor to make it electronic?

Do you have access to another distributor to drop in and test out to eliminate your dizzy as a possible culprit?

[AK260]

Actually dude, reading up the thread and thinking about this again, I think you would do well to replace the valve seal on no.4 WITHOUT taking the head off.

One of my friends in the uk just did this on his L28 and it was a piece of cake!!

[jalexquijano]

Thanks man. I really think the seal is broken or damaged. I already checked and swapped 2 distributor caps and spark plug 4 still gets fouled even after replacing it with a hotter one such as bp5es

[jalexquijano]

Should i open the distributor also?

[Mark Maras]

 I wouldn't go into the dist. Concentrate on changing ALL the valve seals without removing the head. We've come to the conclusion that one or both of the valve seals are leaking on #4, likely from improper installation. If one was damaged during installation, there's a good chance others were damaged too.

[AK260]

Should i open the distributor also?

You’re most welcome. I would do exactly what Mark said above and leave the distributor alone for now.

[jalexquijano]

21 hours ago, AK260 said:

 

You’re most welcome. I would do exactly what Mark said above and leave the distributor alone for now.

 

Ok. Ill go for it. Hope i dont mess the engine. I thought that since i bought the whole kit of valve seals taking the head out and replacing them was better.

[siteunseen]

1 hour ago, jalexquijano said:

Ok. Ill go for it. Hope i dont mess the engine. I thought that since i bought the whole kit of valve seals taking the head out and replacing them was better.

The only reason I see removing the head is if you plan on taking it to a machine shop and have the work done. You could drive it to your mechanic and have them replace the seal or seals without removing the head.

If you plan to give it a try yourself the valve spring tool would be much cheaper for head ON replacement. The one I have for head OFF is expensive and harder to find.

Question for the gang as I'm not sure if one of these would work or not. I bought it for something I cannot remember what though but I've pretty much only worked on Zs since the late 80s.

I know the cam style copressor is the easiest but wouldn't one of these $20 compressors work for one seal?

Edited May 12, 2021 by siteunseen

[siteunseen]

Another thing I'm not sure has been mentioned is our straight 6 L motors are pretty much identical to the older Mercedes and BMW six cylinder motors. Surely there's more mechanics besides the Porsche one you've used in the past. As the L motors were used in north America from 1970 to 1984 there has to be a mechanic in Panama that is familiar with the motor.

[AK260]

Ok. Ill go for it. Hope i dont mess the engine. I thought that since i bought the whole kit of valve seals taking the head out and replacing them was better.

You will not mess it up. In fact your risk of having issues will be much higher with the head removed.

I totally get the logic but for now, if I was in your shoes and knowing that only cyl 4 is fouling, I would just focus on that one. You know the others are good as there is no fouling on the other plugs.

Follow the procedure given further up in the thread on doing it with the head on.

[siteunseen]

Has the ring around the bottom  or top of the seal somehow come loose or slid out of it's seat?

I think that cheap compressor would work. Hook the bottom hooks on the lowest ring of the spring and the upper on the retainer. Use a small telescoping magnet to catch the valve retainer clips.

I think it could be done with a big socket on the retainer and rubber hammer, magnet to catch the clips. I've read that before. 

Good luck with it Alex.