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Compression test passed, losing oil, oil clean. 4.0HO


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I have a 1991 4.0HO and ran a compression test last night.

1: 140

2: 141

3: 140

4: 135

5: 138

6: 140

 

I'm getting very light weeping oil out in front of the distributor, but not enough to show a noticeable oil loss after 500-1000 miles. I tore a majority of the engine down already to replace the head gasket because of the weeping. Once I pull the head I will check for a crown where the pistons top out on the cylinder. And if the walls look scored or glazed I will pull the pistons and hone them. I might as well just change the rings too then, right? If the walls look good, I would prefer to leave the pistons as is. Is compression the only tell tail of bad rings or does burning oil tell me something too? The engine has 175,000 miles on it, so it's definitely tired. But it was very very well taken care of and I've been surprised before by the amount of love this Jeep was given. Once the head is pulled I plan to get it re-milled, cleaned, and completely rebuilt. So new valves should help the situation I think too. I got quoted for $300-$400, is that a lot?

 

Now, I personally haven't noticed any smoke from the tail pipe. That is except for starting it the other day after sitting since October! There was a lot of blue/white smoke out of the pipe, but granted October is a very long time to sit. Wouldn't any engine do that?

 

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175k on these engines is not bad. They can go much longer. Oil lose can be seals. There is one under the distributor. As for the smoke issue, you might see about replacing the injectors. I put some Ford ones in both my '98 XJs. Didn't get any more power or MPG. Both engines seemed to run smoother. The smoke could be oil getting past the valve stems. I don't know about prices on head work. I thought mine was about $100. Still, it might be what type of work and also if your valves are being replaced. Also, what grade of valves.

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55 minutes ago, 75sv1 said:

175k on these engines is not bad. They can go much longer. Oil lose can be seals. There is one under the distributor. As for the smoke issue, you might see about replacing the injectors. I put some Ford ones in both my '98 XJs. Didn't get any more power or MPG. Both engines seemed to run smoother. The smoke could be oil getting past the valve stems. I don't know about prices on head work. I thought mine was about $100. Still, it might be what type of work and also if your valves are being replaced. Also, what grade of valves.

Thanks, yeah I did extensive research before replacing my injectors last winter and ordered a set from a reputable company. They could be the issue but I might rule them out. The injector part number I got was: 0280155703

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     If you replaced you injectors, I'd look elsewhere for problems. If having 'Blowby' as DC mentioned, I note that yours is an HO. A bit better valve cover than the RENIX. Still, the latter style is suppose to be better. I had a '95 valve cover on my '88. A bit of splicing in the rear tube. Yours should be a bit more direct. If you still want an aluminum VC, then I'd go with a '93-95 Grand Cherokee (ZJ) one.It has the latter style CCV grommets. I did. I did do the mod that Cruiser54 list. This is on a '98 XJ.

      On my '88 with the RENIX motor in it, I found that getting a good seal on the valve cover worked for me. The cork gaskets didn't hold up, for any length of time. So, RTV sealant or FelPro makes a rubber gasket for it. They are about $50, though.

            Have you replaced your O2 sensor?  I am suspecting that oil is getting past your valves. Possibly your valve seals are bad. A lot of guessing. I'd try other stuff first though. $300-$400 and then gaskets, I think a few other things to try first. Also, squirt some oil in the cylinders and see if your compression goes up.

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I have a 91 XJ with HO 4.0 as well, blowby the where most of the oil is lost now. Newer valve cover should solve that problem.

 

On our 91 it leaked bloody everywhere, I’ve replaced distributor with a new seal or gasket, o rings for the oil filter adapter, crankshaft seal in the timing cover, timing cover the gasket around the oil pan and timing cover, valve cover gasket,

 

Oil pan gasket didn’t leak except by the timing cover, which I finally redid last year.

Probably still have valve seal leaks , the dipstick and blowby in the air filter. Might have some slight oil seepage from the headgasket or it might be something else.

 

Our HO XJ has 300k on it, compression is lower than what the OP posted, actually that compression on a 4.0 wouldn’t bother me.

 

I’ve replaced the injectors several times as well.

 

Right now that motor is not using or leaking much in the way of oil, kind of freaks me out, used to be like clock work after so many miles add so much oil. Now it steady for the first time in 16 years.

 

Over the years I replaced all the seals mentioned but still had various leaks like rear main, bought some snake oil, a seal restore stocked at Walmart, and a few oil changes later a can of Engine Restore.

The oil leaks mostly stoped by the second oil change, I know the seal restore can do some awful crap swelling the seals, but I wanted to try it and figured with 300k miles what the hell. Already have a JY motor to swap it out. Still I wouldn’t necessarily recommend it.

 

The engine restore is snake oil that can work to restore compression, seen it proven. Bad enough problem it does nothing but is certain problems it’s effective. I don’t know if it helped out XJ or not, never bothered to test it again and it never felt any different from usual.

that engine has never run weak, with 300k I know it must be tired but it never lets you feel it when driving.

 

I don’t know it it’s worth trying to redo the rings on the pistons, for one I don’t think the compression is bad, also the way the engines worn together with the piston rings it’s probably better to leave it be unless the cylinder is going to be machined and bored out, so everything new is perfectly circular and size matched.

 

 

 

 

Sent from my iPhone using Tapatalk Pro

 

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