Random Misfire while cruising

[siteunseen]

If he has an dark plug at #4 have the plug wires been swapped around as a test. Loose fitting wire caps could be an issue with a lot easier fix. Just saying but I'm sure I'll get flogged. 

Edited May 7, 2021 by siteunseen

[jalexquijano]

1 hour ago, siteunseen said:

If he has an dark plug at #4 have the plug wires been swapped around as a test. Loose fitting wire caps could be an issue with a lot easier fix. Just saying but I'm sure I'll get flogged. 

This was already done

[Johnny280]

22 hours ago, jalexquijano said:

Well since i bought all the complete valve seal set i believe its better to change them all, so ill guess the head must be removed. What kind of hassle. Mechanic here is charging US$200 for removing the head. 

Hi, can you elaborate a little bit why you still want to remove the head to change the valve stem seals? The video I posted (pretty easy to find for everyone) shows exactly what needs to be done. I did it together with my girlfriend and it was pretty easy. You just need a valve spring compressor tool and some rope (and maybe a new valve cover gasket if yours is done). 

[jalexquijano]

15 hours ago, Johnny280 said:

Hi, can you elaborate a little bit why you still want to remove the head to change the valve stem seals? The video I posted (pretty easy to find for everyone) shows exactly what needs to be done. I did it together with my girlfriend and it was pretty easy. You just need a valve spring compressor tool and some rope (and maybe a new valve cover gasket if yours is done). 

Spark plug number 5 and  4 oily. This is a hotter plug bp5es.

[AK260]

Are you sure that is not just carbon on this plug? As if running too rich or not having enough spark?



Here are two of mine for comparison from the old engine that used to drink 1L of oil every 500 miles. That engine was definitely burning oil. Now see the difference to yours: you can see they are shiny and have solid deposits on them. Yours from the photo, looks matt and dry as if it’s carbon.




By all means go take the head off to put your mind at rest but I genuinely think you have another issue here. If you are still convinced it’s the seals then do what has been suggested first - namely replace the seals WITHOUT removing the head.

[jalexquijano]

3 hours ago, AK260 said:

Are you sure that is not just carbon on this plug? As of running too rich or not having enough spark?



Here are two of mine for comparison from the old engine that used to drink 1L of oil every 500 miles. That engine was definitely burning oil. Now see the difference to yours: you can see they are shiny and have solid deposits on them. Yours from the photo, looks matt and dry as if it’s carbon.




By all means go take the head off to put your mind at rest but I genuinely think you have another issue here. If you are still convinced it’s the seals then do what has been suggested first - namely replace the seals WITHOUT removing the head.

Would you recommend a final test drive and take the plugs out after leaving the car idling for more than 20 mins? 

[jalexquijano]

8 hours ago, AK260 said:

Are you sure that is not just carbon on this plug? As if running too rich or not having enough spark?



Here are two of mine for comparison from the old engine that used to drink 1L of oil every 500 miles. That engine was definitely burning oil. Now see the difference to yours: you can see they are shiny and have solid deposits on them. Yours from the photo, looks matt and dry as if it’s carbon.




By all means go take the head off to put your mind at rest but I genuinely think you have another issue here. If you are still convinced it’s the seals then do what has been suggested first - namely replace the seals WITHOUT removing the head.

Okay. I found this picture inside california bill's nissan & datsun ohc engine book as guidance. My 4th plug looks  as C.

[AK260]

Excellent! So now you know it’s not a head / seal issue!

[jalexquijano]

Hard accelerating for 40 mins on highway, parked the car and have the following resultd cyl 4 ,5 and 6. Spark plug on cyl 4 shows a lot of difference.

[AK260]

I’m assuming your other plugs are from the same bank of three - so it seems to me that no.4 is not getting enough spark / electrical energy.

Is it safe to assume they are all gapped the same?

[jalexquijano]

1 hour ago, AK260 said:

I’m assuming your other plugs are from the same bank of three - so it seems to me that no.4 is not getting enough spark / electrical energy.

Is it safe to assume they are all gapped the same?

 

They are all gapped at .031 

[AK260]

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?

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?

- 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.

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?

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!