Off-Road Forums & Discussion Groups

Off-Road Forums & Discussion Groups (
-   Jeep-Short Wheelbase (
-   -   low compression in #2 only showing 89 lbs (

sailit2000 11-09-2002 11:12 AM

low compression in #2 only showing 89 lbs
Totally rebuilt 258 with 4.0l head. Did a general compression test yesterday. All clylinders reported 170 bs, except #2 which showed only 89 lbs. On a recheck the same results. Took the oil can and added a little oil, ran the test again. Exactly the same pressure. About 10,000 miles on the engine. What could be the cause?
Thank you

GeeAea 11-09-2002 11:48 AM

Re: low compression in #2 only showing 89 lbs
Typically with the oil squirt tends to point to a valve. This is somewhat odd given the short length of time that the head was on the engine.

Can we assume that the head was "serviced" before installation? Have you pulled the valve cover to look at the rocker action?

sailit2000 11-09-2002 12:00 PM

Re: low compression in #2 only showing 89 lbs
The head was a brand new 4.0L head directly from Clifford Performance. It has Chevy S.S 1.92/1.50 valves installed. I left the commpression gauge hooked up for a while and the pressure stayed at 89 lbs. I am getting alot of blow-by from the crankcase. Oil is evident around the breather, the distributor shaft, and other opening around the engine. Would a leak down test be the way to solve this mystery?
Thank you!

RRich 11-09-2002 12:15 PM

Re: low compression in #2 only showing 89 lbs
Uh Oh, I hope not - but sometimes when you put a new or good
head on an old engine that happens.

The head works so good holding compression that it blows the
old tired weak compression rings out.

Yes, a leak down test would be in order now before taking
it apart.

LEVE 11-09-2002 01:10 PM

Re: low compression in #2 only showing 89 lbs
My guess.. the problem's in the head, I'd:

1. Pull the valve cover.
2. Rotate the engine by hand.
3. Watch the action of the valves and rockers with the compression guage attached.
4. Compare this another cylinder.

You may find a valve not closing, bent stem, etc..

sailit2000 11-09-2002 01:53 PM

Re: low compression in #2 only showing 89 lbs
It is a brand new motor. Overbored and honed. All new stuff top to bottom. It runs great. Tremendous power all the way though to range. Just got this weak cylinder. I hope it is in the head.
Thank you

CJ7Taz 11-09-2002 04:13 PM

Re: low compression in #2 only showing 89 lbs
Well, we have used a modified sparkplug shell to put compressed air into the cylinder. If the air comes out the intake - intake valve, same for exhaust. Air out the oil cap - usually rings, air out the side or the radiator, head gasket. Air out the oil cap can be head gasket. Needless to say, you need to start with on the compression stroke with the valves closed. The air pressure will usually push the piston to the bottom.

RRich 11-09-2002 05:38 PM

Re: low compression in #2 only showing 89 lbs
As LEVE suggested, pop the top off and look.
You may see something in the valve action - like bent pushrod,
pushrod fell off, loose rocker etc.
Or, you may find the cam's gone flat - hopefully not that,
it's more work!

Believe it or not, I've seen cylinders with no pushrod on
the intake valve at all still give some compression.
The suction action of the piston can lift the valve enough
to get a small gulp of air at cranking.
I wouldn't believe it if I hadn't seen it.

Bent pushrod? Make sure the valve still operates freely.
One trouble lately is the MTBE in gasoline forms a soft carbon,
and makes valves stick, bending the pushrod.

Saw it recently on a 454 with less than 250 miles on it.
3 pushrods bent like pretzels.
No it wasn't running too rich either, TBI. I think, but
not sure, since the truck sat for about 6+ months or so until
the engine was built, the MTBE in the near full tank chemically
changed into something that caused the excessive carbon.
The carbon was strange too. Usually carbon is a hard substance,
this was a crumbly, almost soft, stickey stuff. It felt soft,
porous, but was hard enough to scratch glass. I've heard
about it before, but this was the first time I saw it.
We had to pull both heads and completely redo them, guides and
all. Guides looked like you could drive a truck through them.

Let that be a lesson for all, if you store something, get
that "environmentally friendly" cancer causing MTBE laden
gas out of the tank. It goes bad fast.

If everything looks good, do what Taz suggested, pressurize
that cylinder and see where the air goes. A leak down tester
is almost the same thing, just has a gauge to see how bad is
bad - you know it's bad, you just need to know why.

sailit2000 11-10-2002 09:18 AM

Re: low compression in #2 only showing 89 lbs
Where can I buy the test equipment for a bleeddown test and is it a difficult operation to accomplish.
Thank you

ozarkjeep 11-10-2002 09:48 AM

Re: low compression in #2 only showing 89 lbs
you can use a $25 compression guage kit, just take off the gauge, and hook your air compressor to teh hose that screws into teh sparkplug hole, make sure the cylinder your testing is at TDC, and the vavels are closed, and also start at a low air pressure ( abotu 20psi) then slowly increase until you can hear it hissing, mke sure its in Nuetral, and teh wheels are chocked!

I did it on a suzuki once and it was enough pressure to roll teh car forward.

All times are GMT -5. The time now is 03:08 AM.

Powered by vBulletin® Copyright ©2000 - 2021, Jelsoft Enterprises Ltd.

User Alert System provided by Advanced User Tagging (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
vBulletin Security provided by vBSecurity v2.2.2 (Pro) - vBulletin Mods & Addons Copyright © 2021 DragonByte Technologies Ltd.
Copyright 1994-2009, VerticalScope Inc. // Off Road forums & discussion groups sitemap
side by side | atv | dirtbike | snowmobile | sandsport | competition | land use | Jeep | Toyota | Ford | GM

For the best viewing experience please update your browser to Google Chrome