Compression test readings???

[standfirmrasta]

Hi

Am going to do a compression test on my KAMD 43 P this w/e as I have the injectors out.

Anyone know the correct reading that I should expect given all is well?

My manual has no technical information.

Andy

 

[andy59]

I would read this thread - http://www.ybw.com/forums/showthread.php?t=299149 - and trust everything that Latestarter and Volvopaul post on here .

 

[neale]

I would read this thread - http://www.ybw.com/forums/showthread.php?t=299149 - and trust everything that Latestarter and Volvopaul post on here .

I would agree with this statement. I particularly liked Latestarters 'Low Earth Orbit' test

With regards to compression testing I was always told that it is not so much the actual reading from each cylinder, although I guess they should be within certain parameters, it is more that each cylinder is similar (within 10%?) to each other.

If the engine is simply worn out a blow by test is better, however for diagnosing head, head gasket or valve problems a compression test is still useful IMHO.

I compression tested a poorly Range Rover diesel and found low compression on cylinder 4. When the head was removed there was a hairline crack between the inlet and exhaust valves.

 

[Latestarter1]

Hi

Am going to do a compression test on my KAMD 43 P this w/e as I have the injectors out.

Anyone know the correct reading that I should expect given all is well?

My manual has no technical information.

Andy

I swim against the tide on this subject.

Back in the 1980's I was with major manufacturer involved looking closely at service and trouble shooting procedures in anticipation of the introduction of electronic engines.

Following a heap of reserch we concluded we concluded that compression testing was a pretty worthless tool in diesel engine diagnostics for a number of reasons.

Using different starter motors 12 Vs 24 on the same engine gave us a different spread of #'s

Small quantity of fuel slobber left in a cylinder would give a whacky #

Compression testing a healthy engine Vs one with excessive blowby and requiring overhaual did not give a huge range of #'s This appeared to the due to the fact that diesel engines use pressure balanced ring packs. Totally unlike a gasoline motor where you can graph cranking compression pressure against cylinder wear giving pretty much linear results. Diesel engines give you nothing like the same linear performance as gasoline motors.

We concluded that the only reliable test of engine health was by using an orifice attached to the engine breather and either a water manometer or Magnaheilic gauge. Blow by can be plotted as ring pack beds in when engine new, remains stable during the life of the engine, until onset of wear out, when blow by gradually rises.

Cat and Cummins & Mack agreed with truck maintainance council, comression test #'s would no longer be published, blow-by #'s only. However we counted without marine engine surveyors. Surveyors padded their bills by performing compression tests, all part of petrol head thinking, and nice little earner! Surveyors still at it in the U.S. However more reports of screwed up engines due to fighting with injectors as well as putting incorrect copper washers back in. I was involved in case where engine ran clean prior to survey, then after Mr Smarty had ripped into the motor smoked like hell. All became real hot after the yard brains trust tried fixing the issue without success leading to a big bill.

Eventaully turned out AFTER motor had been pulled that Mr Smarty had put the wrong thickness of copper injector washer in. A thicker washer moves the spray pattern relative to the centre of combustion chamber. All ended up in court and I was called to give evidence. Result surveyors insurance company took a big hit! In another case not using the correct tool to remove an injector led to copper injector sleeve leaking, another big bill.

Electronic engines have another reason why they have no need of compression testing, diagnostic tool can perform a power balance test by shutting down individual injectors and testing measuring rpm drop, which measures individual cylinder contribution.

When the Volvo D12 came out must have thrown marine survey numpties into a real tizz, cos it took 7 hours at Standard Repair Time to change just one injector or 8 hours for all six. Over 35 hours to do compression check would have any potential purchaser climbing up the wall unless surveyor has access to diagnostic tool. .