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Unread 04-11-2010, 08:42 AM   #1
grogan5
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Blow-by, but clean plugs, no smoke

I have an 1989 YJ with 70,000 miles, with 258 w/rebuilt carter. I bought it at 65,000. Mileage confirmed with maintenance records. Been nuttered and team rushed. All new vacuum lines, well the ones that are left anyway. Had no oil leaks and very little blow-by.

The first winter we had it, we could never get enough heat to warm the cab. I assumed bad thermostat. So I pick one up and tear into it. To my surprise, there was no thermostat intalled. So put the new one in and now glorious heat. Been fighting oil leaks every since. Blow-by has increased significantly. Added a catch can to the vent side of the valve cover, and just lived with it for a while.

Had some spare time yesterday, so I thought I would check to see how bad the plugs were fouled. The plugs looked great. Clean and tan just like they should. I was expecting them to be encrusted with oil and carbon. Don't have a way to check compression. Also, not getting any smoke from the exhaust, but getting smoke from oil fill hole.

So what do you think is causing the blow-by? Rings, valves, head gasket???
Considering the low mileage, could I get by with a valve job?

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Unread 04-12-2010, 04:09 PM   #2
grogan5
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Anyone have this problem? I searched but did not find anything like this.
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Unread 04-12-2010, 04:55 PM   #3
jay-h
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If your plugs are clean, maybe you just need a good flush or two. Lots of people recommend Seafoam.
Have you taken off the valve cover?
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Unread 04-12-2010, 06:24 PM   #4
Cajun90YJ
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You could have a hole in a piston, bad ring, blown head gasket? I saw one piston that was melted off on one side just letting a little pressure by causing blow by. A compression ring that has failed or a hole in a piston,will allow compression into the crankcase. Bad oil rings cause the motor to smoke. 3 rings total on the piston 2 compression and 1 oil ring set.
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Unread 04-12-2010, 06:38 PM   #5
Karma
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HI,
Quit being so cheap and buy a compression gauge. They are not expensive. Do a compression test. It should give good information about the basic state of your engine. Then come back with questions. Right now we are just gusssing as are you.

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Unread 04-12-2010, 07:58 PM   #6
yellow2kgt
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Also, check the vacuum going to the PCV valve. From the factory there is a restriction in the hose that likes to get clogged. If it is clogged the pressure is going to build up and blow oil out of the weakest link.
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Unread 04-12-2010, 08:28 PM   #7
UnFocused
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Hi yellow, there was an MCU controlled electric solenoid in the PCV line between the PCV valve and the carb which closed the PCV line at idle and was later eliminated by a factory recall. However on my 4.2 there was a second splice which was necked down internally to about 1/8 diameter. I'm just running straight through unrestricted from the valve to the carb now, but are you saying that the 1/8 inch restrictor in the line was factory spec? I figured it was something that one of the PO's had done to help pass emissions.
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Unread 04-12-2010, 08:44 PM   #8
88blkyj
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Mine is somewhat like this too...doing a check in a few days but getting blowby out of my back breather hole. no significant power loss. just blowing oil up that back hole. cylinder 5 is a little weak and missing at idle but just idle. not sure what my blow by is....also sorry to highjack your thread but recently my oil pressure gauge has been jumping all over. Hopefully a bad sending unit but who knows, sometimes, its even down to around 15 psi at idle=[
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Unread 04-12-2010, 09:25 PM   #9
88Husker
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I get the same blow-by and my "expert" told me " Its an old jeep dont worry about it!". As long as my jeep is rolling down the road I am happy
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Unread 04-13-2010, 03:41 AM   #10
grogan5
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Thanks for the comments. Will try to get a compression gage soon. Other than the mess, the thing runs great above idle. Plenty of power. Still have a miss at idle. The rpms are irradically bouncing between 670 and 690 at idle.
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Unread 04-13-2010, 05:13 AM   #11
SkullTripper
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what is a good compression gauge??
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Unread 04-13-2010, 05:30 AM   #12
Louie4
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You could rent one from Autozone. It's one of those tools I only used every several years or so. Even if you buy one I think they are only $35 or so. Either way you win. You can never have too many tools.
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Unread 04-13-2010, 07:26 AM   #13
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Quote:
Originally Posted by SkullTripper View Post
what is a good compression gauge??
HI,
I bought mine at Sears. I don't know which is best. You might check NAPA too.

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