XXLMJ Posted October 28, 2009 Share Posted October 28, 2009 I just bought an 88 MJ 4.0, auto 2wd. The oil pan was removed cause the PO said he put new main bearings in. I have talked to him several time since i bought it, and he swears he did put in new ones. The truck is knocking pretty bad, sounds like its coming from the lower end. I didn't check anything really before i put the oil pan back on. My bad i believed someone. Any ideas on what the problem may be, or some stuff to check? Is there anything i can do before i remove the pan again? Link to comment Share on other sites More sharing options...
Geonovast Posted October 28, 2009 Share Posted October 28, 2009 Generally a knocking sound would be coming from a bad rod bearing or wrist pin. Usually it's a good idea to replace the rod bearings with the mains. Link to comment Share on other sites More sharing options...
XXLMJ Posted October 28, 2009 Author Share Posted October 28, 2009 Ok i just emailed the guy again to see which bearings he actually did change. He told me the mains before. Can a guy get to these others from under the truck? Link to comment Share on other sites More sharing options...
Geonovast Posted October 28, 2009 Share Posted October 28, 2009 You can do the rod bearings from under the truck, and they're going to be much easier than the mains. Gonna be a lot more work if you have a bad wrist pin though. Link to comment Share on other sites More sharing options...
Eagle Posted October 28, 2009 Share Posted October 28, 2009 If it's an automatic, a cracked flex plate can make a knocking sound that's similar to a bad bearing. The test is to run it at idle. If it knocks in neutral, but it in gear with the brakes on and step on the bas just a little. If the knock disappears, there's a high probability of a cracked flex plate. Link to comment Share on other sites More sharing options...
XXLMJ Posted October 28, 2009 Author Share Posted October 28, 2009 He just got back to me and said that he changed the rod bearings. Is it possible they aren't getting oil? The valve cover and oil pan had a ton of sludge in them. Could some of the oil ports to the bearings be plugged? Or is that to easy? If it's an automatic, a cracked flex plate can make a knocking sound that's similar to a bad bearing. I will have to give the flex plate a try tomorrow morning. Link to comment Share on other sites More sharing options...
Geonovast Posted October 28, 2009 Share Posted October 28, 2009 ... he DID put the oil pump back on, right? Link to comment Share on other sites More sharing options...
sinkrun Posted October 28, 2009 Share Posted October 28, 2009 If he put new bearings on a really bad or out of round crank (did he say if he had the crank ground) it may very well knock with new bearings. If he did its not going to last long. Link to comment Share on other sites More sharing options...
XXLMJ Posted October 28, 2009 Author Share Posted October 28, 2009 Yes there is a oil pump, he says he micrometer the crank and everything was in check. But with the odometer reading just under 230k i was thinking that thing could need a grinding too. Link to comment Share on other sites More sharing options...
Eagle Posted October 28, 2009 Share Posted October 28, 2009 Yes there is a oil pump, he says he micrometer the crank and everything was in check. But with the odometer reading just under 230k i was thinking that thing could need a grinding too. Why? My '88 XJ has 287,000 miles on the clock and doesn't knock. It is very common to just throw new bearings into a 4.0L engine to resolve oil pressure issues. They rarely need the crank turned, unless the owner waited much too long before deciding to do the bearings. Link to comment Share on other sites More sharing options...
Geonovast Posted October 28, 2009 Share Posted October 28, 2009 My next question would be... did it knock before he changed the bearings? Link to comment Share on other sites More sharing options...
terrawombat Posted October 28, 2009 Share Posted October 28, 2009 Sounds to me like he replaced the bearings hoping the knock would go away, but the bearings weren't the problem... Link to comment Share on other sites More sharing options...
Eagle Posted October 28, 2009 Share Posted October 28, 2009 Yeah, or he was a doofus and only replaced the main bearings. If you run 20W50 oil, does the knock go away when the engine speed is over about 1500 RPM? If the vehicle has gauges, what kind of oil pressure does the gauge think you have, at idle and at 1500 or 2000 RPM? Link to comment Share on other sites More sharing options...
XXLMJ Posted October 28, 2009 Author Share Posted October 28, 2009 it has an aftermarket oil gauge, the reads 55-60psi all the time. The knock seem to get quieter the higher the rpms, but it may be because the over all motor is drowning it out. My buddy came over and swears it sounds like a lifter, of piston slap. Link to comment Share on other sites More sharing options...
Geonovast Posted October 28, 2009 Share Posted October 28, 2009 While it's running, pull the oil fill cap out and watch the rocker moving. If the sound seems to be happening once every time the rocker moves, it's a valvetrain knock. If it seems to be happening twice as fast, it'll probably be coming from the bottom end. Link to comment Share on other sites More sharing options...
XXLMJ Posted October 28, 2009 Author Share Posted October 28, 2009 I had the whole valve cover off with it running and it seems like its in the valves but now the trick is figuring out which one its coming from? They all seem tight, all the push rods are pumping oil out. My buddy wants me to just beat on it an maybe the lifter will come loose. I don't think this is a good idea. Link to comment Share on other sites More sharing options...
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