compression test

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shealyp38

New Member
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111
Location
birmingham
just done a compression test on my 99 thor 4.0 engine
and the results were shocking
50-51psi on all 8 cylinders now i think thats low
test was done with a cheapish test kit
any thoughts?
 
depends how you did the test,and kit used, but the fact they are all the same tells you something ,not least that your most likely dont have the real pressures, a worn engine tends to have more varied readings and that low would be very noticeable when starting and running
 
Ignition (spark) immobilised. All plugs removed. Throttle wide open. Turn engine for same period of time for each cylinder. I doubt it would run at 50 psi if it did it would sound very sick.
 
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If you haven't got an assistant to hold the throttle open, take a screwdriver to the two jubilee clips and remove the throttle body, then try again. Compression tests (on a petrol) should always be performed with the throttle wide open or low readings can result.
 
I always use a screw in type with a one way valve, you have to press the valve to release the pressure and reset the gauge....

The push in types, if not held in firmly, an leak past the bung giving false readings!
 
shealyp38 said:
just done a compression test on my 99 thor 4.0 engine
and the results were shocking
50-51psi on all 8 cylinders now i think thats low
test was done with a cheapish test kit
any thoughts?
Click to expand...

I doubt it would run with such low compression. The 3.9 efi (basically the same block and heads) should be 170 - 180psi. The low compression engine runs at 150 -160psi.

Manual says if pressures down by 10% then suspect piston rings or valves. Did you do the test wet and dry or just dry?

As already said, check compression tester and also with these values it would either not run or sound like a bag of spanners.
 
I did a dry compression test a while ago on my 4.0 GEMS and the lowest I got was 170 on one cylinder and 175 on the rest.

My compression tester was only a cheap one I got from Screwfix though.
 
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jacckk said:
I did a dry compression test a while ago on my 4.0 GEMS and the lowest I got was 170 on one cylinder and 175 on the rest.

My compression tester was only a cheap one I got from Screwfix though.
Click to expand...

As long as they are within 10 psi there is no problem.
 
test was done dry
car does start perfect ticksover smooth
there is a problem when trying to overtake or on kickdown where the car seems to struggle and is quite jerky almost as if there is a misfire
just put in new plugs and ive got the leads and coilpacks to change
 
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wammers said:
Ignition (spark) immobilised. All plugs removed. Throttle wide open. Turn engine for same period of time for each cylinder. I doubt it would run at 50 psi if it did it would sound very sick.
Click to expand...

Hi Wammers - what would you advise is the best way to immobilse the spark? Detach the coils or take a relay out?
Sorry for basic question.
 
WatchcamP38 said:
Thanks, will see if I can pick one up. Cheers!
Click to expand...

The price that bloke is asking for a S/H unit is way over the top. Look through eBay you will find one at a reasonable price. Have had mine for years used mainly for flicking engine over to do tappets. Now most are hydraulic, so not used a lot. But ideal for compression tests. If you have problems getting to solenoid, remove relay 16 and couple switch from positive to pin 5 on it's socket in fuse box. Or across pins 3 and 5.
 
Last edited:
wammers said:
The price that bloke is asking for a S/H unit is way over the top. Look through eBay you will find one at a reasonable price. Have had mine for years used mainly for flicking engine over to do tappets. Now most are hydraulic, so not used a lot. But ideal for compression tests. If you have problems getting to solenoid, remove relay 16 and couple switch from positive to pin 5 on it's socket in fuse box. Or across pins 3 and 5.
Click to expand...

Thanks, I have bought another I found. Useful bit of kit, thanks for highlighting!
Won't be here for a few days so if I pull fuel relay and coli pack in the short term that will work?
 
WatchcamP38 said:
Thanks, I have bought another I found. Useful bit of kit, thanks for highlighting!
Won't be here for a few days so if I pull fuel relay and coli pack in the short term that will work?
Click to expand...

I suppose but why not wait until you get your switch? You don't need ignition at all then.
 
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