Compression testing

[sezicoolcat]

Was told by MOT chap that my engine is "weak".

Had new clutch put in - van now goes just fine I reckon - but still backfires or missfires (I don't know which!). Also stalls everytime I slow down when cold, so I'm pretty used to "coasting" and trying to start her again lol (Ok when warmed up).

Mr Claus gave me a compression tester last crimbo, and I thought, well, lets see whats wrong with the engine. Had a read of the wiki - seems pretty straight forward, but I am a complete novice when it comes to cars. I need help with a term on there that states:

"...crank engine on starter..."

Does this mean go in the cab and start the engine? Or have I got to do something at the engine end with just the key in the ignition?

Sorry folks - learning all this stuff. Interested in cars, but don't know all the terms yet...

Sarah

[Mr Bean]

Was told by MOT chap that my engine is "weak".

Had new clutch put in - van now goes just fine I reckon - but still backfires or missfires (I don't know which!). Also stalls everytime I slow down when cold, so I'm pretty used to "coasting" and trying to start her again lol (Ok when warmed up).

Mr Claus gave me a compression tester last crimbo, and I thought, well, lets see whats wrong with the engine. Had a read of the wiki - seems pretty straight forward, but I am a complete novice when it comes to cars. I need help with a term on there that states:

"...crank engine on starter..."

Does this mean go in the cab and start the engine? Or have I got to do something at the engine end with just the key in the ignition?

Sorry folks - learning all this stuff. Interested in cars, but don't know all the terms yet...

Sarah

Yes just get someone to switch on and spin the motor over for a few turns on the key. But do something about the loose spark plug lead/s. For one electronic ignition is a real killer if you get a belt and second you can damage the electronics if it don't have a load. So find out how to isolate the sparks system. Some one will tell us. In the death ( I wish I wouldn't use that term) fit the plugs to the leads and wedge them against a metal part of the engine. It can make a difference if the throttle is open but I have never heard any specific recomendation. Don't be too upset if you get slightly different readings but gross differences need investigationg. Compression testers are a bit like oil pressure guages. Sometimes what the eyes don't see the heart don't grieve. Good luck
Ken

[Aidan]

take out all the plugs, disconnetct the ht lead from dizzzy and earth it to chassis (croc clips and wire or jump leads sort of arrangement but keep everything loose away from fan belt), turn over usually 5/6 turns will give you constant reading, but note the first instant reading as well as the stable one. do twice on each cylinder to check that you are getting a consistant result. The compression tester will have a reset button .
Post your results and Andy Simpson will tell you what they mean !
This was mine
#1 7.5 bar/9.5 bar
#2 9bar/10.5 bar
#3 8 bar/10 bar
#4 9bar/11 bar

within spec, a bit worn, #1 always lowest as runs hottest, as furthest from water pump, #4 best, next to pump.

engine runs well and uses no oil so despite no history I'm happy to keep running it till it starts to leak, at present just the usual dampness about the oil pump and a couple of pushrod seals but the mud soaks it up, though it may have a pinhole in one of the rocker covers which I'll sort this week when it gets an oil change

[Dubstar]

You can also do a wet test. On mine the instructions were to put a cap full of engine oil down the cylinder bore through the spark plug hole and then do the compression test. One cylinder at a time! So, do the dry test first and then the wet test. From memory, as I threw the instrcutions away, it's supposed to give an indication of how worn the bores are, depending on the difference between the 2 readings. On my old CU the differences on 3 cylinders were ok, but on the 4th the compression readings wet and dry were both the same, and low, which confirmed what I thought and that is it was down on one cylinder. The readings on my Aircooled CU were 138,140,121 and 110 dry, and 152,152,160 and 110 wet, going from left rear to left front, right front and right rear. That was on my old engine. I'll be doing my latest engine tomorrow if it ever stops raining.

[Essex Nige]

Failing all that, get a mobile tuning fella like i did Thursday, told me loads, asked loads, was an education mine was backfiring, poppin and farting... how (once i have put unsplit exhaust on) runs like a dream..

[HarryMann]

Dubstar...

Then that's a leaking/burnt valve seat problem on right rear, as they didn't change with oil..

or a valve sticking being held open problem.

With a ring problem on right front (121 > 160)

PS. As Aidan says, go round engine once, and then go around again and check they are the same, any doubt repeat that cylinder.

Write them down as you go... and make any notes*

Then do the oil test if you need to... (low pressure on some or all cylinders)

Always remove all plugs before starting so that cranking speed will be considtent and battery not become drained, speed can make a difference to a bad cylinder..

*A good cylinder will also pump up almost immediately (within 2 or 3 compressions)... so as Aidan says, 'watch' the gauge as you crank the engine (use a helper or remote solenoid switch)

==========================

Then there's the leak-down test... checking rate at which they leak back from compression, comparing them, and listening in exhaust or inlet for valve seat leaks...

[fairwynds]

Sezi (and Dubstar too?)

same symptoms of backfiring I had, and compression tests showed a low reading on Cylinder 1. During the refurb I took out valves and found this on cyl 1:-

[IMG:640:480]http://i98.photobucket.com/albums/l261/ ... MG1792.jpg[/img]

the seat of the valve itself was pitted and allowing the gases to burn in the guide, causing all the black crud on the stem. New valve and seat sorted it.

Sezi, your cold starting/running probs - maybe choke issue? Just a thought. If you think so - look up my thread re choke refurb (mines a 1900 dg engine wit pierburg carb - you dont say what your engine is...)

Good luck with it....
FW

[maxstu]

Hi Ian & Sarah,

Take your van to Sanspeeds in Bexleyheath. Ask for a tune up and compression test.
These guys are true experts and well worth using. Even Vauxhall used them to investigare carb problems on Belmonts/Astras (do they use a similar carb to T25's?) back in the late 1980's.

Sanspeed found & rectified the fault.

Read their feedback here.....

http://www.goodgaragescheme.co.uk/feedb ... hp?id=2965

[HarryMann]

fairwynds that an exhaust valve ?

[sezicoolcat]

Thanks maxstu - we will be booking Edla in once back from Fox and Goose meet.

SArah

[maxstu]

Hi Sarah,

Sorry but I can't remember if your van is a hi or pop top?

Best check with Sanspeed if hi top as I know the entrance to their premises has height restrictions. Give them a call to confirm.

Mine went in okay. Poppit's a pop top though.

Good luck

Stu

[sezicoolcat]

Edla is that Nato-Military-green-hand-rollered monstrosity - but I love her heaps!
She's a pop top, so should be OK - thanks for the tip!

Sarah

[maxstu]

DOH! How could I forget??

[Dubstar]

I took the easy way out and, er, bought another engine
Second hand of course - thanks Meggles by the way. I thought it may be easier and cheaper than trying to get the head off to inspect and replace any broken bits on my 2ltr CU, which I'm told has to come out anyway to take the head off. And once the engine was out I would then have had nothing to pick the new engine up with.

[Dubstar]

Ok, just done a dry test on mine, and the pressures are from left rear going clockwise 120,120,111,100. I can find the minimum they should be according to my Autodata manual - 73 - but does anybody know what they should be if new? All in PSI by the way. I'll do the wet test tomorrow.