Compression testing...

[bacardincoke 42 1 Cars]

Likely a fundamental question to many, however I'm still learning the basics as I'm going along.

 

Read various tutorials on how to do compression testing and understand what's required, however some mention removing either the fuel pump fuse and / or the relay.

 

Ideally i'd like to just remove the fuse as the relay is not only less accessible, as it's not causing any issues and the potentially fragile original I'd prefer not to disturb it.

 

Question is - will removing the fuse alone (on a 1989 1.9 GTI) suffice to cut off the fuel supply and will turning the engine over with relay still in place cause any issues?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[welshpug 1,661]

pulling the fuse will do the job to stop the pump firing but the injectors will still open and inject any residual pressure, easier to just unplug the injectors and ignition coil, you dont want any stray sparks

Edited November 24, 2020 by welshpug

[petert 588]

I'd pull the fuse and let it run dry, if you're concerned. Then there's no chance of it firing. Pull all the plugs out and remove +12V off the coil. It's then dead as a dodo.

[bacardincoke 42 1 Cars]

Spot on... Thanks.

[DamirGTI 342]

Disconnect distributor signal cable @ the yellow 3pin plug , is what i usually do prior compression test .

 

 

D

[bacardincoke 42 1 Cars]

Had a go today and all very straight forward...

 

Was using a Kennedy Tools compression tester which I believe is the Gunson one re-branded (or maybe it's vice versa?) so not quite an eBay special.

 

Did the test twice... in the same order with engine previously up to temperature and the accelerator fully depressed while cranking the same number of times, readings were more or less the same each test cycle for each cylinder.

 

Left to right was as follows -

 

185 psi

195 psi

195 psi

205 psi

 

Compression went up evenly and incrementally during the first 3 to 5 cranks to just below what was to become the max. with that last 10% or so achieved in the final 4 cranks - this happened across all cylinders and through both the testing cycles.

 

Probably should have done the wet test on the 185 psi cylinder while things were set up, but that's for another time.

 

Difference between 1 and 4 is borderline 10% if i've worked it out right.

 

For every post or article you read you get a different answer, so I'm basing things on a combination of opinions - The engine's just turned 120K, I think these readings seem to be within the range of what I should expect?

 

 

 

 

[DamirGTI 342]

There's not much deviation between the cylinders and the pressure is in range , to be expected between 12 to 14 Bar for healthy engine ..

 

However , in some cases , it can also be an utter "crapped out" daily oil user engine with 14 Bar's measured across all the 4 cylinders ! (because the engine oil is already present inside each cylinder fooling the compression gauge readings)

 

Leak down test is preferred , as more precise test .. if you suspect internal engine problem best to do both , compression and leak down test .

Tricky part is when the top and bottom compression rings are good with just the oil control rings being bad .. common scenario nowadays on newer "low emission"/"eco"/"green" engines with tiny oil control rings or/and bad piston design . 

 

Something also kinda interesting , each individual cylinder filling/flow will vary a bit depending on the inlet manifold design and position of the TB/inlet (inlet center or inlet sideways)  , with the TB/inlet  on the side of the manifold cylinders closest to the TB/inlet tend to receive less air and the cylinders furthest from the TB/inlet receive more .

 

D

[bacardincoke 42 1 Cars]

Novice at all this but plan to make it a regular check now that I know what I'm doing!

 

Engines been running perfectly and its history seems to show it's been looked after throughout its life... it was just curiosity and wanting to do basic maintenance myself that prompted the test, thankfully nothing else.

 

Cheers for the feedback - It's good to know there's nothing too major to be read into those figures, but even so will be keeping an eye on that first cylinder.

 

 

 

 

 

 

[DamirGTI 342]

Just regular service oil/filters/timing belt/plugs , taking care not to overheat it , and these old engines will flip few rounds to the moon and back ! unlike the most of the new engines unfortunately .

 

From my experience , the XU's are good/reliable with the compression and piston ring wear ...

 

D

[bacardincoke 42 1 Cars]

My own daily's at the age I don't have to worry about warranties and such so can tackle some of the basics like oil and filters myself - car's probably been over serviced if there's such a thing.

 

Wouldn't be doing bigger jobs though... don't have either the knowledge or the equipment, so it still occassionally goes to a local guy instead.

 

Some of the horror stories he tells about failing parts (and the prices to replace them) on relatively modern cars is a real eye opener.

 

Lack of maintenance is his pet hate. He's cars a few years old coming in were even the basics haven't been done since new, then the owners wonder why they need a mechanic!

 

He's the age that he remembers well working on 205's when they were still common on the roads, nothing but praise for them.

 

Both cars were getting a good clean recently and seeing them side by side just brought it home how ugly modern engines are, little wonder they put massive plastic covers on them.

 

Take off the lid and the daily looks like someone randomly tipped in a box of pipes and parts, wee Peugeot's is a work of art in comparison.

 

 

 

 

 

 

[welshpug 1,661]

those small differences could easily be caused by valve clearances, worth checking they are in spec.

[DamirGTI 342]

10 hours ago, bacardincoke said:

Lack of maintenance is his pet hate. He's cars a few years old coming in were even the basics haven't been done since new, then the owners wonder why they need a mechanic!

 

 

True , and it's the same story all over the world with the basic/regular car maintenance (the lack of it !) .

 

In essence there isn't really such thing as an "bad car" by make or model , if well looked after and serviced on time before anything actually breaks form the first day , most of them will be trouble free transport .. with a few exceptions of course , as some of the cars where made new from the factory with certain defects and no brand make or model is "immune" to that .

 

It's also true that new modern cars are , kinda like everything else , "programed" to start to go bad once the warranties expire .. things just aren't made like they used to back in the days material and build quality wise . Guess part of the blame is on us people , getting bored with the old and keep wanting "new shiny things" . 

 

 

D

 

[bacardincoke 42 1 Cars]

1 hour ago, welshpug said:

those small differences could easily be caused by valve clearances, worth checking they are in spec.

Just looked through Haynes... a bit above my own abilities (and tools) at the minute unfortunately.

 

SORN'd the car tonight, so it's going nowhere for a while anyway... will add getting the clearances checked as part of readying it for the better weather again next year. 

 

Hopefully things will be looking up by then and can maybe help out at the garage to get the confidence to do it myself.