OK, so I have had an intermittent misfire on my 3.9 that I have been trying to track down. I just did a compression check, and got the following readings:

1 - 140psi 2- 130psi

3- 135psi 4- 100psi

5- 130psi 6- 100psi

The compression gauge is from Harbor Freight, and some reviews state that it reads consistently low by 40 psi, so take that into account. I tried renting gauges from Oreilley auto parts, and got two in a row that wouldn't hold pressure, so I got fed up and bought this one.

From these readings, I have a possible compression leak between #4 and #6.

The 2001 FSM states that a leak between cylinders would be shown by a 50%-70% loss in compression from the other cylinders.. That means I should be getting only 70psi max from these two cylinders if it were a blown gasket between the cylinders.

I have never had a code for any misfire, but it is there. I notice it at idle (light) and upon revving the engine. It seems to go away under constant higher rpms.. All signs of a head gasket leak between cyls.

There is an audible ticking that you can only hear from the sides, and underneath the truck. Can't hear it at all from above the engine. It sounds like an exhaust leak off the bottom of the manifold, but I haven't been able to test for it yet. It also only ticks when warm, starting off light, then getting louder as it gets hotter.

Oil is ok, no water present. Also coolant is fine with no level loss noticed.

I haven't noticed any drop in MPG, though I haven't driven it much.

Is there anything else it could be? What are my next steps diagnostically?

EDIT: forgot to mention all spark plugs looked good with a light ashy powder on the ground electrode. Autolite 3923s

Also found this page that states the 3.9's should have 100psi compression... WTF? http://dodgeram.org/tech/gas/specs/3_9v6.htm

EDIT2: just confirmed in the 2001 FSM the compression should be 100psi. So what is causing the higher compression readings on the other cyls? carbon buildup? Is it Seafoam time?

 

You can run a chemical test on the HG. Rent the unit from any auto parts store. It sniffs the coolant for exhaust vapors. If blue liquid turns green or yellow, you have a leak.

 

I don't have any coolant loss. If the HG were leaking into a coolant jacket, it should have coolant loss and overheating issues. Even with the outside temps hitting 115 lately, it has been running cool as a cucumber.

I re-checked cylinders 4 and 6 later that day and came up with 110 psi each, still more than 15% lower than the others.

But after that, I threw it all back together and fired it up.. No more misfire.

It will eventually come back and drive me nuts again.

 

May just be a leak between the cylinders, and not get into the cooling system at all. Only way to tell though, would be to pull the head, and have a look.

 

So I just tested for exhaust leaks and found one off the driver's side manifold to y-pipe connection. I didn't see any leaks around the manifold to head on either side. I used the shop vac with the hose on the exhaust to blow air up into the tailpipe, then I sprayed soapy water all around it and looked for suds to form.

I fired it back up and still no misfire (YAY) but it is still ticking like a timebomb. It is loud underneath the engine, and you almost can't hear it when above the engine. It's also louder on the passenger side. I am wondering if a collapsed lifter is the culprit? It is definitely a tick, not a knock.

I made a video:


Sorry for the shaky cam, I have a condition similar to Parkinson's and I can't help it.