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Compresssion test gone awry!!!

ParadiseXJ

ParadiseXJ

NAXJA Forum User
Location
Paradise, CA 95969
In the quest to solve my overheating problem I’m doing a compression test. WTF am I doing wrong??
It’s been awhile since I’ve used a compression gauge…I had a handheld tester that you just hold up to the plug hole…the tip was rubber…you just jammed it in there and have someone crank the engine a couple times. Boom, instant result. Accurate too. The tester I “borrowed” from AZone is the screw in variety which everyone uses.
The instructions say “remove all spark plugs, set at WOT. Remove the coil wire, screw in the tester nozzle, crank engine. I did just that. The results I get say exactly 90 psi across the board…all 6, no variation. Since the truck runs and idles fine, doesn’t smoke, and has power…I’m thinking the results are in error.
What am I doing wrong or is my truck so f***ed up that for some reason ALL my cylinders are running at 90 psi??
 
souske said:
Try again only pull one plug at a time
i didnt set.mine to wot either
Click to expand...

x2, i have only used the screw in kind, do one at a time, pull the fuel pump fuse or relay. I usually pull the coil to the ignition too.
 
mako my day said:
What wrong with 90?
Comp test is mainly to.determine a variation between cylinders. If one is 10 off, then get a new cylinder.
Click to expand...

Generally speaking, for gasoline engines in automobiles, when you are below 100 psi you are on your way to problems in idling and performance.

I doubt he has 90 psi in ANY of his cylinders, based on his performance description--smooth idle and power.

Is there an o-ring on that screw-in tester? Did you clean the cylinder head around the spark plug holes?

Consider the source of the tool, it is a LOANER and the valve maybe damaged or bad.
 
Last edited:
joe_peters said:
Is there an o-ring on that screw-in tester? Did you clean the cylinder head around the spark plug holes?
Click to expand...

Yes there's an o-ring and I got it screwed in all the way.
the head is very clean, all the plugs look clean, I have no real blowby or smoke problems.

IIRC I never pulled all the plugs at the same time...I don't know why it would matter though...?? Since I'm doing one at a time. After the first cylinder I just about soiled myself!!!

The ONLY problem I'm having is overheating.

EDIT: I'm going to get about 80% buzzed right now and attack it again when I get some more info. tomorrow!! Thanks.
 
Last edited:
Ok.

Why pull all of the plugs: It lets the engine crank over freely, saving battery power and allowing the best and consistent reading for all cylinders.

WOT enables the engine to pull in the most air. This was also important on carb vehicles to prevent pulling fuel from the bowls through the idle circuits.

Upon further review of the officials on the field: the most likely issue is the valve stem in the tester.
 
Last edited:
the correct way is what the instructions say to do-- in this case (and every other compression tester I've seen) it is to remove all the plugs, disable fuel and spark, and block the throttle wide open.

the open throttle allows the best airflow into the engine (fullest cylinder possible)

the plugs removed allows the fastest cranking (best possible build of compression before bleed off)

no fuel and no spark= no flamethrowers

As a side note, I've always seen that an engine at operating temp gives you the truest readings, but I'm too much of a wuss to do everything on a hot engine unless there's a real need for me to:looser:
 
joe_peters said:
Ok.

WOT enables the engine to pull in the most air. This was also important on carb vehicles to prevent pulling fuel from the bowls through the idle circuits.
Click to expand...
True and correct for the carburated engines.

For the electronically injected, I thought WOT while cranking, shuts off the injectors, to clear a flood condition... and cranking compression test.
 
Hypoid said:
True and correct for the carburated engines.

For the electronically injected, I thought WOT while cranking, shuts off the injectors, to clear a flood condition... and cranking compression test.
Click to expand...

You'll have the system disabled. I disconnect the CPS to make sure NOTHING in the way of spark or fuel is happening.
 
Yeah...er...I pressed "post" too soon.

Got another gauge...

a little more encouraging news...cold engine...WOT...dry...rad cap off. Cps disconnected.

1 - 118
2 - 121
3 - 120
4 - 119
5 - 120
6 - 121

Those look like good numbers to me, and more what I expected. 1988 4.0 with 190K on the clock. But it still leaves me in a quandary as to why my cooling system is (seems to be) over pressurizing. Any ideas?

I'll do another test later with the engine at temp...with a drop or two of oil in each hole. Should I expect the numbers to increase or decrease...or change drastically
 
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