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Blow-by with good compression?

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Old 03-12-2011, 03:13 AM   #1
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Default Blow-by with good compression?

`89 Cherokee, 4.0 I6, Just did a compression check. Cylinders were between 138 and 150 psi. Which seems quite good to me! When I pull the PCV valve, or take off the Oil fill cap, I have puffing blow-by! Could this be bad valves? I`m not seeing any water in the oil, and am not losing any coolant. I`m new to these Jeeps, so I have lots to learn. Also, Oil pressure when cold runs around 45 psi, after warm up at an idle it drops to around 15psi. Is this enough oil pressure at an idle? I know it should be around 37psi at 1600 rpm, warm. Any and all help will be appreciated. . Thanks.
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Old 03-12-2011, 05:48 AM   #2
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*Are you actually using oil (IE: having to add?)

*A little puffing with the oil filler cap off is not unusual

*Your compression is good (120-150 with no more than a 30 psi variation between cylinders)

*Oil pressure of 15 psi at HOT idle is fine. The spec is no less than 13 psi at HOT idle.

*Do you have any evidence of oil in the air filter box? That is very common with the 87-90. It can often be managed or eliminated by replacing the grommets and hoses that are installed on the valve cover.

Here is a pretty good thread that explains the CCV (crank case ventilation) system and its weak points. There is no PCV valve on the 4.0 Jeep.

http://www.cherokeetalk.com/forum/f8...d-blowby-5870/
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Old 03-12-2011, 06:13 PM   #3
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Hey TJ, Thanks for the info and the link to the CCV system! I`m headed out to check the smaller line that goes to the intake manifold. Make sure it`s not plugged. The other line that goes to the airbox, does have a pcv valve in it now, and it still will accumulate oil and have a small drip towards the airbox. Should this fitting just be an open 90deg. fitting going to the airbox, so that air can be sucked into the valve cover?
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Old 03-12-2011, 06:48 PM   #4
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hould this fitting just be an open 90deg. fitting going to the airbox, so that air can be sucked into the valve cover?



yes
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Old 03-13-2011, 03:19 PM   #5
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Cleaned out the small line to the Intake manifold, it was plugged solid! Now I have NO visible blow-by, and also got rid of the PCV valve that someone had installed. Engine seems to be running better, with possibly fewer oil leaks, due to a reduction in inside block pressure. Thanks again for the info TJ ! Still need to change out the Rear Main Seal and pan gasket.
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Old 03-13-2011, 05:32 PM   #6
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Quote:
Originally Posted by Swampy77 View Post
Cleaned out the small line to the Intake manifold, it was plugged solid! Now I have NO visible blow-by, and also got rid of the PCV valve that someone had installed. Engine seems to be running better, with possibly fewer oil leaks, due to a reduction in inside block pressure. Thanks again for the info TJ ! Still need to change out the Rear Main Seal and pan gasket.
Glad I could help. The CCV system is a weak link, especially with the RENIX vintage 4.0

Replacing all of the CCV hardware and a thorough cleaning of the valve cover is always a good idea here. It isn't that expensive ($45 or so for the grommets and hoses) and isn't a tough job to perform.

Now get on that rear main seal and pan gasket!
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Old 03-13-2011, 05:50 PM   #7
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Quote:
Originally Posted by Swampy77 View Post
Cleaned out the small line to the Intake manifold, it was plugged solid! Now I have NO visible blow-by, and also got rid of the PCV valve that someone had installed. Engine seems to be running better, with possibly fewer oil leaks, due to a reduction in inside block pressure. Thanks again for the info TJ ! Still need to change out the Rear Main Seal and pan gasket.
Very often the oil filter adaptor O-rings leak and the oil ends up dripping from the bottom of the engine - transmission area leading many people to wrongly assume the rear main seal (and/or pan gasket) is leaking.
Search oil filter adaptor
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Old 03-14-2011, 01:37 AM   #8
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I`ll check the oil filter adapter first. I bought the O ring already when I picked up the Rear Main Seal and Pan gasket. I was thinking that might be contributing to the oil leakage. Thanks for all the help TJ and DaddyCat.
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Old 03-14-2011, 07:36 AM   #9
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Originally Posted by Swampy77 View Post
I`ll check the oil filter adapter first. I bought the O ring already when I picked up the Rear Main Seal and Pan gasket. I was thinking that might be contributing to the oil leakage. Thanks for all the help TJ and DaddyCat.
The oil filter adaptor uses 3 o-rings but most of the time it's the one large o-ring that leaks, might be all you need. If the smaller o-rings need to be replace be aware some of the aftermarket kits do not have the correct o-rings. OEM Jeep o-rings or bring your old o-rings to the parts store to confirm sizes.

Good Luck and you're welcome
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