1998 Dodge Durango, 140K miles. 5.2L V8.
My engine has a spark plug issue.
The vehicle when the plug is inserted runs great - if I keep it under 50 mph. If I drive it over 60, on the highway, it will shoot out the spark plug after about 20 minutes. Its not a thread issue as it actually breaks the spark plug.

The ceramic part of the plug ejects out of the metal threaded piece. The threaded part is still in the engine. I remove it with a spark plug socket wrench, remove the old plug stem from the end of the wire, put a new one in and I am good to go again. As long as I keep it under 50 mph.

I went to a local shop and was recommended to insert a spacer which screws into the engine and the plug goes into that. Now the plug completely ejects and the spacer remains in the engine. This scenario is a threading issue but it saves the plug. I don't need to keep buying and installing new plugs.

I am fairly comfortable with my auto mechanic skill level but I can't find anyone who has any idea why the plug keeps getting ejected.
Anyone know why this happens and what I can do to fix it?

 

Scope the the piston dome maybe to see if its hitting the plug. A hotter plug than should be used will generally be longer so make sure you are using the right plugs. Honestly I have never heard of what you describe happening. Maybe some sort of pre detonation is damaging the plug?

 

I don't have a way to scope the dome but I understand what you are asking about, if the piston is hitting the plug physically. I don't think that is happening as the plug still ejects with the spacer in it. At first the plug seemed oily when I pulled it out. I was recommended the spacer which screws into the engine and keeps the plug out of the cylinder completely.
It may be running hot though as twice the plug didn't have the hook at the top of the plug anymore as if it were burned off (not crushed).
How do I diagnose a pre-fire issue?

 

Pre-ignition and spark knock make an audible sound. If you go on youtube there are examples of this. Have you checked you plenum gasket? If your plenum is blown it can cause detonation that may be the root of this problem. Its simple to check, shine a light down your throttle body and if its full of oil. If its got a bunch of oil in there your gasket is blown. Which spark plug is blowing out? Some times the gasket blowout is visible from the throttle body, check on the opposite side of the intake from the problem spark plug as air flows through channels across the top of the intake to the cylinder head.

 

I understand your comment and I will try to get a look at the throttle body for oil issues. The failing plug is the second from the front on the drivers side of the engine.
So if this is the case then its most likely a gasket issue. Would that be a head gasket or the intake manifold gasket? Thanks for the advice.

 

Check the intake gasket first visually through the throttle body. I should have clarified the plenum gasket is what usually blows out but it is possible the intake to head gasket could have failed. If it passes the visual check for oil then with the engine cool pull the radiator cap and run the motor. If it bubbles you have a blown head gasket/cracked head. It will be very noticeable. You could also do a cylinder leak down test to test this. Have you been consuming excessive oil/coolant?

 

Thanks for the additional tips. I will see if my mechanic can follow your tips and I will post the results - or I may just get rid of the truck and forget about this for good.
Thanks again for replying.

 

I'll have to give it some more time to be sure, but so far the truck is running better and getting better fuel economy (particularly cruising on the highway) following the PCM reset and new MAP sensor installation. I'll try to report back in a few weeks to update if the improvement has been permanent.

Thanks for the input everyone.

 

Compression check that cylinder. I'd really like to know the number.