Compression test

[semtex73]

I've seen plenty of references to the blow-by test, and the fact that anything coming in at 40+ is a no. However, where do you draw the line (or is it not that simple)? I.e. would 20-30 be OK and over 30 getting into dangerous territory?

Apologies if this has been answered elsewhere, but had a quick trawl and couldn't find anything obvious.

[lanciachris]

Dont know about jaguars grading, but generally if one cylinder varies more than 10-15% from other cylinders, then its time to find out why.

[semtex73]

Thanks Chris. I have found something on the net that confirms your comment re. 10 - 15, but again not Jag specific.

[Angelis]

I had a compression test done the other day and the reading was 18 so according to the dealer everything was ok.

However, according to another garage, my car still has serious engine wear due to the nikasil problem and so needs a new engine.

>> Edited by Angelis on Tuesday 15th February 11:52

[semtex73]

Were they both Jaguar main dealers? I'm in the process of buying an XKR but have got a main dealer service and blow test as a pre-condition of the purchase. I'm probably best getting the details of the Jag dealer that carries out the service (not buying it from them) and ask them for their comments rather than simply seeing the stats on a piece of paper.

[groomi]

Angelis said:

I had a compression test done the other day and the reading was 18 so according to the dealer everything was ok. However, according to another garage, my car still has serious engine wear due to the nikasil problem and so needs a new engine. >> Edited by Angelis on Tuesday 15th February 11:52

Is a compression test the same thing as a blow-by test then?

[Angelis]

groomi said:

Is a compression test the same thing as a blow-by test then?

I'm not very technically minde, but I think they are the same.

The tests were done by an independent and a main dealer. Each one did a different type of test. One is done cold, the other when warm. Don't know which is which though.The compression test at the main delaer cost £55.

I'll find out late next week hopefully if it is confirmed that there is engine wear.

[John Millar]

A compression test measures the amount of pressure created in an individual cylinder, (8 individual tests in our case), and gives a result in pounds per square inch. There will be only small differences between the results of each cylinder in a healthy engine.

A blow by test measures the total amount of air/fuel mixture that leaks past the piston rings on all cylinders combined, and gives a result in litres per minute.

The blow by test gives a more accurate indication of the condition of the AJV8 Nikasil lined engine.

[semtex73]

Thanks for the explanation on the difference between compression test and blow-by test - I was assuming the terms were inter-changable. That'll teach me for trying to pretend I know what I'm talking about!!

Anyway, to answer my original question (which I now realise was actually refering to a blow-by test), having just spoken to a Jag dealer, apparently 35+ is considered the danger zone.