Compression Test Gone Awry!!!

[ParadiseMJ]

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, repeat for all cylinders.  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 exactly 90 psi??

[nitroxsteve]

I think that sounds good for an older engine what were you hoping for? And the instructions sound correct to me. Also how long did you crank for each cylinder?

[jimoshel]

Equal pressure in all cylinders is not impossible but it is extremely unlikely.Should be at least 2-5 PSI variation. Follow the instructions as given. Use a remote starter button and you won't need another person. Watch the needle and crank until needle quits moving. You DID release the pressure from previous reading BEFORE testing new cylinder didn't you?

[ParadiseMJ]

 You DID release the pressure from previous reading BEFORE testing new cylinder didn't you?

 

Yep!

 

..as I remember the old school tester had a Schrader valve thingy to release/reset the pressure. 

[cruiser54]

I'll bet the gauge is wonky. Vatozone, you know.

[onlyinajeep726]

Yea, I agree with cruiser, gauge is garbage.  AZ has a habit of renting out garbage tools.  They're nice if you're one of the first 5 people to use it when it's brand new, but after a few uses and the people before not giving two $#!&s about messing up the tool, they wind up being garbage that the next renter has to deal with.  Try another loaner tool, preferably not from the same AZ.

[cruiser54]

http://www.amazon.com/s/?ie=UTF8&keywords=compression+testers&tag=googhydr-20&index=automotive&hvadid=1144960541&hvpos=1t1&hvexid=&hvnetw=g&hvrand=8941910265002084&hvpone=&hvptwo=&hvqmt=b&hvdev=c&ref=pd_sl_8mg4qjl79j_b

[64 Cheyenne]

Put a squirt of oil in the cylinder before ea test, should cause pressures to raise and might see more differences.

[ParadiseMJ]

Put a squirt of oil in the cylinder before ea test, should cause pressures to raise and might see more differences.

 

That was my plan this afternoon... if the numbers don't change at all I'll be fairly sure that the tester is at fault.  Then it's back to the drawing board.

[Eagle]

I've only been doing this auto hobby stuff for a bit over fifty years, but I've never heard of "setting" the throttle wide open to run a compression check. Sure enough, though, the 2000 XJ FSM says to do that. Dunno why.

 

2000 XJ FSM says compression should be between 120 and 150, so 90 is VERY low. It would be enough to run, but not to run well. A compression gauge isn't all that expensive. I'd buy a new one so you know what you have.

[jimoshel]

I've only been doing this auto hobby stuff for a bit over fifty years, but I've never heard of "setting" the throttle wide open to run a compression check. Sure enough, though, the 2000 XJ FSM says to do that. Dunno why.

 

2000 XJ FSM says compression should be between 120 and 150, so 90 is VERY low. It would be enough to run, but not to run well. A compression gauge isn't all that expensive. I'd buy a new one so you know what you have.

Fifty Years? Late comer. The reason for WOT is less restriction to air flow. Supposedly higher and more accurate readings. But why they emphasize it now I don't know. Main thing is have everything in same condition for all readings. Don't take a couple readings in one condition and then change something and take another reading. 10-4 on the purchase of a new gauge. In my humble opinion a vacuum and a compression gauge are two of the most useful diagnostic aids in any mechanics tool box. And like anything else, they're only as good as the person using them. .02c

[HOrnbrod]

Yes, invest in a new gauge. Sounds like the meter movement is sticking on the AZ one.

[airspeed]

I would think a leak-down gauge be a much more accurate tool to use, it would give you more information.    Here is a good write up on compression verses leak-down.

http://www.dsmtuners.com/forums/articles-engine-fuel/338152-compression-leak-down-testing.html

[87manche1]

Airspeed I agree do a leak down test plus u could see bubbles in the antifreeze if ur head gasket is leaking

[ParadiseMJ]

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 ?

[cruiser54]

Describe over-pressurizing.

[ParadiseMJ]

It's been overheating and  blowing coolant out the overflow, and although I did test the cap, it was somewhat inconclusive.  The adapter for the tester was (looked) identical on either end.  When I tested it one way it held solid pressure for 5+ minutes or so, when I flipped it around, it lost pressure immediately, down to about 10 psi.  When I hooked pressure tester to the radiator and ran the engine, the pressure began rising up 20-25 +.  I had asked if the cap generally relieves the pressure a bit at a time, but it seems like it was just continually blowing coolant one it got past 210...and up.

 

If the system is over pressurizing...why?  If it's just a faulty cap, I get it.  Where do I find a decent cap?  This one is a month old...the one before that was 3-4 months old.  The SAME make cap on my XJ has worked fine for 2+ years.

 

It's a Stant from NAPA..not a Chinese special from AZone

[nitroxsteve]

Did the pressure rise very fast to 20-25 psi or did it take a little time? If the pressure rises as soon as you start the engine this indicates compression from the cylinders getting into the water.

[Pete M]

 

I've only been doing this auto hobby stuff for a bit over fifty years, but I've never heard of "setting" the throttle wide open to run a compression check. Sure enough, though, the 2000 XJ FSM says to do that. Dunno why.

 

2000 XJ FSM says compression should be between 120 and 150, so 90 is VERY low. It would be enough to run, but not to run well. A compression gauge isn't all that expensive. I'd buy a new one so you know what you have.

The reason for WOT is less restriction to air flow. Supposedly higher and more accurate readings. But why they emphasize it now I don't know. Main thing is have everything in same condition for all readings.

 

 

might be carb vs throttle body? 

[ParadiseMJ]

Did the pressure rise very fast to 20-25 psi or did it take a little time? If the pressure rises as soon as you start the engine this indicates compression from the cylinders getting into the water.

 

The pressure built slowly, but since I didn't have a pressure cap on it (I had the tester on it) it kept building.  I get that part.  The tester instructions said to turn the engine off before the pressure got to 30 psi.  So, if the cap let go at 16, it would have never gotten to 25-30...it'd just blow the pressure...and coolant into the overflow...right !!?

[cruiser54]

From the Snap-On compression tester gauge instructions:

 

• Open throttle plates and choke plate to allow the
engine to breathe. Be sure air is not restricted from
entering into the cylinders.

[nitroxsteve]

 

Did the pressure rise very fast to 20-25 psi or did it take a little time? If the pressure rises as soon as you start the engine this indicates compression from the cylinders getting into the water.

 

The pressure built slowly, but since I didn't have a pressure cap on it (I had the tester on it) it kept building.  I get that part.  The tester instructions said to turn the engine off before the pressure got to 30 psi.  So, if the cap let go at 16, it would have never gotten to 25-30...it'd just blow the pressure...and coolant into the overflow...right !!?

Thats correct it will raise normally with temperature and the cap will allow the pressure to release. I see in your signature you have an 88 4.0 is this the truck your working on? Is this the factory cooling system or have you converted to the later cooling system?

[mpace6a]

90, while low, isn't exactlty a game changer in a 4.0. My friends 88 xj ranged from 87psi to 114, with 4 cylinders below 95, and that thing would still $#!& n git

[cruiser54]

 

 

Did the pressure rise very fast to 20-25 psi or did it take a little time? If the pressure rises as soon as you start the engine this indicates compression from the cylinders getting into the water.

 

The pressure built slowly, but since I didn't have a pressure cap on it (I had the tester on it) it kept building.  I get that part.  The tester instructions said to turn the engine off before the pressure got to 30 psi.  So, if the cap let go at 16, it would have never gotten to 25-30...it'd just blow the pressure...and coolant into the overflow...right !!?

Thats correct it will raise normally with temperature and the cap will allow the pressure to release. I see in your signature you have an 88 4.0 is this the truck your working on? Is this the factory cooling system or have you converted to the later cooling system?

He's now got an open system.