Hey guys. New to the forum but I should be around for a while now since I recently picked up an expedition as a second truck. I'm an active member on another Cadillac forum as well. Here's the situation: I bought this truck very cheap because it was in great cosmetic shape but didn't run. Guy said it lost compression in one cylinder and it's been sitting now over a year. I did a full rebuild and there were no signs of lost compression as far as I could tell except for the timing chain guides which were both eaten all the way down and even ground down the tips of both tensioner pistons. Pretty crazy so maybe it jumped timing? I should have done my own compression test but I figured nah since I'm doing the rebuild anyway. So I get it put back together with all new plugs, coils, and injectors. It fires right up but it has an obvious miss. Drives 90% smooth until it idles. I test each cylinder by unplugging their injector and all give an obvious rpm drop except number 4. When it's unplugged, there is no change at all. This is where it gets sticky....after I did all this work of rebuilding I go to put the new plugs in and number 4 doesn't seat right on its beveled stopping point. It squeezes past that but is still pretty snug. So I leave it. I know the cylinder is getting fuel and spark. But it doesn't appear to be producing any power at all. I even removed the coil and stuffed a ball of paper towel down the hole to see if enough compression was getting past the plug to pop it out but nothing...there is no sign of compression loss out of the spark plug hole. But the cylinder still seems to be producing no power. Valves have been resurfaced with valve grinding compound, it has new rings, new HG, timing was set exactly by the marks on the gears and the chain. Both cams were in good shape as well as the rocker arms, and it has all new lash adjusters. What in the world is going on? What am I missing?

Also forgot to mention it has no misfire code or code of any kind.

 

You seem to be knowledgeable and know what you're doing so I'm kind of scratching my head and wondering why you haven't done a compression test on the #4 cylinder, or swapped it's injector, plug, coil with other cylinders?

 

Sorry that's what I was inferring when I said I'm getting spark and fuel. I've switched both the coil and the injector as well as did a spark test grounding the plug and I could see the live spark as I cranked it. As for the compression test, I'm still unsure about that one. Like I described, the spark plug hole is a little out of wack so when I got a reading of just 30 psi, I figured it must not have been sealed in the hole right. I tried another test this morning with a small washer added so I could really tighten it in there and no compression showed up after about 4 or 5 engine cycles.

Now I'm thinking maybe a rocker arm fell out in the process of flipping it over to do the oil pan. Hopefully that's it.

Thanks for the input though. I'll come back with an update later.

 

OK so I took valve cover off and rocker arms are in place. I guess the real problem is that spark plug hole. I need to repair that before I go any further. I just still find it hard to believe that I couldn't get any compression reading at all. It still tightened down in the hole just like all 7 other holes. I can't imagine all of that compressed air is passing by the tester and I can't hear it or feel it. I even stuffed a tissue down in the hole around the hose of the compression tester while I cranked it and it didn't even pop that out?? Where is all that compressed air going? When I stuffed a paper towel down the hole by itself it popped out right away. If anyone has any other explanation please offer it!!! Thanks

 

Time for a cylinder leak-down test.

 

I was just sitting down to tell you guys my results of that very test...I had to rig up an air fitting to do it but I finally got it and the verdict is in. With valves both completely closed, the applied air comes right out the intake! Dang! That's about 100 psi directly to the top of the piston and it bleeds out in about 2 seconds. I don't have the energy to do any more to it today. I'll be taking the head off tomorrow. Should have done more close inspection while I had it apart, but spark plug port inspection never crossed my mind...lesson learned. Taking the rest of the day off. Be back with an update. Thanks again guys.

 

If the air is coming out of the intake then it's not the spark plug hole, it's a cracked head or the intake valve isn't seating/sealing. Of course do fix the spark plug hole while you have the head off. BTW, there is a known issue with 2-valve engines where the spark plugs blow out because there are only 3 spark plug threads in the head. There are plenty of discussions around here concerning this problem.

 

Yeah I know...that's why I decided the head had to come off. I figured there must be a crack somewhere. If I was just fixing the spark plug hole I could have left it on. But I was wrong in my assumption again anyway because it was neither. The spark plug port already had a helicoil in it! That's why it didn't stop when it should have because the insert kept spinning. It was about three threads into the cylinder when I got the head off. The compression was lost due to a very slightly bent intake valve. It's so slight I never noticed it while doing the compounding and looking through the intake port you can't even see light coming through except if you really get a light right on it. I blew through the intake with my mouth and felt it coming right out.

Much closer inspection next time around. I really should have known especially since I didn't know the exact history of this engine. Picked up another head yesterday. Let's see if we can get it right this time! Be back with the results tomorrow. Thanks again guys!

 

It sucks that you had to pull the head twice. Once in their lifetime is usually enough for most people.

 

Well I didn't plan on finishing today but it went pretty fast once I got the head back in. Started it up and took it for a test drive at about 10pm. Running great I'm glad to say. My $500 non-running truck is now road worthy again! Thanks again for the help guys.

 

Wow that was fast!


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