Oil on Spark Plug threads - Likely Cause??

[Jim Jermain]

I recently replaced the heads on my '00 4.0 OHV due to the infamous crack - leaking coolant. Before I did I noticed the plug for the cylinder I suspected (#3) was difficult to remove and the threads were coated with a hard carbon - oil substance. I've now replaced the heads and my rough idle problem is gone. However I now have a stumble on acceleration at higher speeds. I decided to replace the plugs and wires. I noticed the base and threads of the #3 plug are coated with oil. I don't think it is leaking from the rocker since the gaskets are new. I checked the compression on cylinders 2 & 3 and both are exactly 150 lbs. Is it possible the leaking coolant messed up the rings on the 3rd cylinder. The trunk has 82K miles. If it is bad rings on this one cylinder, does anyone know if it is possible to remove the piston from below without removing the crank or heads?

Thanks,

Jim

 

[kihei777]

did you ever resolve this issue? Let me know. I have the same issue...small oil on spark plug thread. My explorer runs fine but i have to change cylinder 4's spark plug every 2 to 3K miles as it keeps on fouling up.

 

[deweyville65]

did you ever resolve this issue? Let me know. I have the same issue...small oil on spark plug thread. My explorer runs fine but i have to change cylinder 4's spark plug every 2 to 3K miles as it keeps on fouling up.

What is the compression reading of that cylinder? Is there oil on the outside of the head? Start with answers to these two questions, then there can be better diagnosis. There can be valve cover leaks, valve stem seals bad, broken rings.

 

[waskly]

compression only matters when its across all cylinders.

if you think its rings, do a leak down check....

 

[deweyville65]

I understand the compression matters across all cylinders. But if that particular cylinder.is say, 85 psi, are you going to say it doesn't matter?. The post was intended to show if that cylinder is way off, there is good reason to suspect broken rings.

 

[kihei777]

I understand the compression matters across all cylinders. But if that particular cylinder.is say, 85 psi, are you going to say it doesn't matter?. The post was intended to show if that cylinder is way off, there is good reason to suspect broken rings.

how do I perform a leakdown test to see if it's broken rings? Also, when you guys refer to rings, what specifically are you guys talking about?

 

[deweyville65]

The piston rings. They ride on the piston, and seal against the cylinder wall. This is what gives you compression. A leak down test, in my opinion will not help you as much as a compression. test. All engines I have ever had will leak down, some faster than others. There is no airtight seal in the rings. There is a small -.015 approximately- gap in the rings. The compression test will show you more because the action of the piston creating compression is quicker. This should just be used as a diagnostic aid in your case. You really should check compression on all cylinders and compare. I am sure some of your buddies will help you.

 

[waskly]

I understand the compression matters across all cylinders. But if that particular cylinder.is say, 85 psi, are you going to say it doesn't matter?. The post was intended to show if that cylinder is way off, there is good reason to suspect broken rings.

if its 85 and the other others are also 85 yes it does not matter.

 

[deweyville65]

If the compression is 85 on all cylinders, he has more problems than oil on the spark plug threads. Do you really not understand what I was trying to get across?

 

[keleko17]

To heat up this old post, I did a compression test on my 99 OHV. I am trying to find out what is acceptable. I had the following data:

Rear
110 110
120 100
120 90
Front

To confirm cylinder #:
Rear
3 6
2 5
1 4
Front

I'm not sure if I started cranking it longer as I went along. We went 4, 5, 6, 3, 2, 1.

Since I don't have anything that is really far off, is it safe to say that there isn't significant compression lose? Or are all the numbers too low?