2004 E250 intermittent compression loss

2004 E250 5.4L

Customer states check engine light is on and vehicle runs rough.

P0301 stored ( along with a startup misfire code I forget which code it is exactly ).

I pull the vehicle in and the first thing I do is an audible relative compression test. Everything sounds even so I move on to watching the data list. No misfires are counting at idle. I snap the throttle and let it idle down. Misfires develop over the course of about two or three seconds. Now there's a dead hole, or holes because there's only a bulk misfire counter not a cylinder specific one in the scan tool.

I perform the audible relative compression test again and can clearly hear uneven cranking. Out comes the lab scope. Repeated relative compression test. Here's what I got: Channel A is cranking current. Channel B is primary ignition current cylinder 1.



What?! Looks normal! That's right. And it sounds even during cranking, too.

So, I start the engine, snap the throttle and let it idle. As soon as it misfires, which only takes a few seconds, I shut it off and repeat the relative compression test. Here's what I ended up with.



Cylinder 6 and cylinder 7 have low compression. Now, I repeat the relative compression test and the results are normal again!

I removed the spark plug from cylinder 7 and installed a compression gauge and repeated the relative compression test. This is what I found: Same results except now only cylinder 6 has low compression. The only thing I changed was putting a compression gauge in cylinder 7.

I'm suspecting sticking valves but what's weird is that installing a compression gauge seems to have corrected the low compression on cylinder 7. How would removing combustion from the cylinder correct the loss of compression?

Do you still have the compression gauge in #7? I'd be tempted to remove the Schrader valve and see how running compression behaves when you the snap throttle -> idle down thing. I bet you'll be able to get the compression to disappear. :lol:

Valve spring failure isn't uncommon on 5.4's, so pulling the valve covers for a visual might not be a bad idea before selling a head job.

Here's the problem. I used my pressure transducer but never lost compression! 60 psi running compression. Valve timing appears correct. Exhaust pressure stayed near zero. No dip after decompression. This thing only acts up when there's a spark plug in the hole.

EricGoodrich wrote: Here's the problem. I used my pressure transducer but never lost compression! 60 psi running compression. Valve timing appears correct. Exhaust pressure stayed near zero. No dip after decompression. This thing only acts up when there's a spark plug in the hole.

Crap! :lol: How about a vacuum gauge on the intake manifold? That'd give you more evidence of a mechanical issue.

Perhaps it's the heat of combustion that's causing the problem? :huh: