bad_idea Posted August 17, 2019 Share Posted August 17, 2019 Hi All!I have a 90 MJ with 2.5L and a 5 speed. I paid $400 for it and drove it on the trailer. Previous owner says the truck has sat since 2016 when the radiator leaked out and overheated.I started tearing into it today. Swapped a good radiator into it. Runs a bit rough but runs, drove it up and down the road - has decent power. Blows smoke out of the valve cover. The CCV system is fully operational. Did a compression test - #1 - 120, #2 - 150, #3 - 165, #4 - 30. I did the compression test with the engine cold. I added a bit of oil to #4 cylinder and still got 30 PSI. I'm guessing something is catastrophically wrong with cylinder #4. What says you all?I would like to fix it if I can cheap. If not then I'll let the truck sit until I can find a decent SBC to swap into it.Thanks for the help. Link to comment Share on other sites More sharing options...
omega_rugal Posted August 17, 2019 Share Posted August 17, 2019 30 psi? whoa that´s toooo low, something is really wrong there time to swap a 4.0 there Link to comment Share on other sites More sharing options...
Minuit Posted August 17, 2019 Share Posted August 17, 2019 My next step would be a leakdown test. With compression like that, my thought would be a valve not sealing. Either that, or severe damage to the piston, rings, or cylinder wall. Start with a leakdown test and find out where the compression is going. At the least, my guess is you're probably going to be in it for a head job. If you'll be swapping a V8 soon, may or may not be worth it to you. Link to comment Share on other sites More sharing options...
eaglescout526 Posted August 17, 2019 Share Posted August 17, 2019 You could do a tear down and check the head, head gasket and piston head. Chances are the head gasket could be blown if it still is the original AMC gasket. Valves could be stuck partially open causing a loss in compression there, aw Minuit beat me to it. Link to comment Share on other sites More sharing options...
Minuit Posted August 17, 2019 Share Posted August 17, 2019 1 minute ago, eaglescout526 said: You could do a tear down and check the head, head gasket and piston head. Chances are the head gasket could be blown if it still is the original AMC gasket. Valves could be stuck partially open causing a loss in compression there, aw Minuit beat me to it. Link to comment Share on other sites More sharing options...
eaglescout526 Posted August 17, 2019 Share Posted August 17, 2019 8 minutes ago, Minuit said: Lol. Link to comment Share on other sites More sharing options...
Eagle Posted August 18, 2019 Share Posted August 18, 2019 Start by just removing the valve cover and starting the engine. See if either valve for cylinder #4 doesn't appear to be going through its full range of motion. Link to comment Share on other sites More sharing options...
mnkyboy Posted August 18, 2019 Share Posted August 18, 2019 It's not uncommon for one or more valves to stick on an engine that has sat a while, many times I've worked the valves with a mallet and got the compression back. I would be looking at the valves first as Eagle suggested. Link to comment Share on other sites More sharing options...
bad_idea Posted August 18, 2019 Author Share Posted August 18, 2019 If I get lucky and it is just a valve sticking - could that explain the large amount of smoke coming from the crank case? Link to comment Share on other sites More sharing options...
omega_rugal Posted August 18, 2019 Share Posted August 18, 2019 3 hours ago, bad_idea said: could that explain the large amount of smoke coming from the crank case? what color? white, blue? Link to comment Share on other sites More sharing options...
Jeep Driver Posted August 18, 2019 Share Posted August 18, 2019 4 hours ago, bad_idea said: If I get lucky and it is just a valve sticking - could that explain the large amount of smoke coming from the crank case? If the valves on #4 are stuck open, you won't get 30psi.......you'll get 0psi. If the valves are stuck closed............something is broken, a valve will move from closed to open, if not.........bent push rod at the least. If the valves are stuck closed and all you got was 30psi........something is broken. If a valve is stuck open or partially open.......you'll get some clacking when running. You can read your plugs. You can read your oil. You can read your coolant. This will tell you a lot in just a couple of minutes. Something is broken, the head is coming off, pretty sure about that. If it's in the valve train/head.............$500 for valve job, gaskets, bolts, fluids, cleaning supplies......etc......... Inspect the bores, if the bottom is good......just replace the head. Link to comment Share on other sites More sharing options...
omega_rugal Posted August 18, 2019 Share Posted August 18, 2019 remove the radiator cap and turn it on, see if there are bubbles or worse, the coolant gushes out, if so the cylinder 4 is leaking to the coolant system through the head gasket Link to comment Share on other sites More sharing options...
bad_idea Posted August 18, 2019 Author Share Posted August 18, 2019 The smoke out of the valve cover looks white to me. After driving the Jeep about a mile I opened the radiator and found it was half a gallon low. Added water to top it back off and oil floated to the top. I think it is safe to say the head gasket is done. Can a toasted head gasket cause the smoke out of the valve cover? I am willing to throw $40 worth of head gasket and other gaskets at it with minimal R&R on the head/block. Not willing to re-ring the bottom end. Again, just looking to get another 5k-10k miles out of this engine if I can. Not too worried if I can't. Definitely not my daily driver, I have three vehicles ahead of this one before it is. Link to comment Share on other sites More sharing options...
eaglescout526 Posted August 18, 2019 Share Posted August 18, 2019 The head gasket could do that as it is steam escaping through what could be failing valve seals. Which means you will need a head rebuild. The valve seals can be had for cheap and you could replace them on you own and completely skip the professional rebuild step. But I wouldn't recommend just replacing the seals if the head has never been serviced Link to comment Share on other sites More sharing options...
Jeep Driver Posted August 18, 2019 Share Posted August 18, 2019 1 hour ago, bad_idea said: The smoke out of the valve cover looks white to me. After driving the Jeep about a mile I opened the radiator and found it was half a gallon low. Added water to top it back off and oil floated to the top. I think it is safe to say the head gasket is done. Can a toasted head gasket cause the smoke out of the valve cover? I am willing to throw $40 worth of head gasket and other gaskets at it with minimal R&R on the head/block. Not willing to re-ring the bottom end. Again, just looking to get another 5k-10k miles out of this engine if I can. Not too worried if I can't. Definitely not my daily driver, I have three vehicles ahead of this one before it is. The white smoke/steam is coolant being burnt off. If you have oil in your coolant you have coolant in your oil. It's over. Link to comment Share on other sites More sharing options...
omega_rugal Posted August 19, 2019 Share Posted August 19, 2019 4 hours ago, bad_idea said: Can a toasted head gasket cause the smoke out of the valve cover? yes, stop driving it and get it rebuilt... or put a 4.0, it is time... Link to comment Share on other sites More sharing options...
gogmorgo Posted August 19, 2019 Share Posted August 19, 2019 Yeah, wished I saw this yesterday. As soon as I saw "overheated" then 30 psi... Way too familiar. Bought my 2.5l with "an overheating problem". Radiator was crusty as all hell so I stabbed a new one in, did the water pump and thermostat at the same time. With new parts in, it started blowing out coolant within minutes of starting cold. 15 minutes running had more water in the crankcase than oil. I ordered a head gasket set, pulled the head off, and found a hole in piston #4. Same as you, 25 psi compression in hole #4 before pulling the head. Funny thing, it ran totally fine, made decent power, and didn't shake or make any bad noises either, at least not till it started overheating. Pull your head to inspect. Hopefully it's just a bad gasket. If it was left long enough there may be damage to the block and head, it's worth checking them both with a straight edge. If the bad spot in the gasket has burnt out part of the block or head, you might get away with filling it with JB weld or similar if you don't plan on doing a full rebuild right away, just to get a few thousand more miles out of it. It's also worth putting a feeler out for decent running 2.5Ls. I got two supposedly good ones for $300, stabbed one in, and it runs alright. Eventually I might even get around to rebuilding one of them. Link to comment Share on other sites More sharing options...
bad_idea Posted August 19, 2019 Author Share Posted August 19, 2019 I'm headed out of town for 5 weeks in about two weeks. When I get back (early October) I'll see about tearing it down to assess the carnage. Like I said earlier, I'll throw $50-$100 at this engine. Much more than that and I'll put that money towards a SBC swap. I bought the truck with the intention of a V8 swap and slamming it - already have a lifted, capable XJ. Thanks for the help diagnosing it you all. Link to comment Share on other sites More sharing options...
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