Cylinder compression

Mark Gordon · Post by **Mark Gordon** » Fri Jul 10, 2009 5:53 pm

I need to check the compression on my 4.2 engine. I can't find the factory specification, but it seems to me that it is supposed to be around 140 psi. Can anyone enlighten me?

Post by **Heuer** » Sat Jul 11, 2009 9:34 am

About 150psi would be good on a 9:1 compression ratio. My car has a 9.5:1 head and shows 175psi.

Mark Gordon · Post by **Mark Gordon** » Sat Jul 11, 2009 1:11 pm

Thanks, David. I'll see what mine reads.
Mark

pdcross · Post by **pdcross** » Mon Mar 21, 2011 7:11 pm

David, I have a 4.2 litre with 8:1 compression ratio. I am getting around 150 to 154 on a compression test with the engine warm (although one is 125 and I am investigating this). I can't find any info about what compression should be. How do my figures seem to you, apart from the rogue one, of course?

Peter

Post by **Heuer** » Mon Mar 21, 2011 7:13 pm

Peter

About right but maybe on the high side for an 8:1 head. Not a problem as the head may have been skimmed sometime in its life.

pdcross · Post by **pdcross** » Mon Mar 21, 2011 8:08 pm

Thank you, David,
It's reassuring to know that the compression figures seem ok.

Peter

pdcross · Post by **pdcross** » Tue Mar 29, 2011 5:08 pm

I have checked my compression figures again after correcting the tappet clearances that were wildly out. On 2 cylinders I couldn't get a 002" feeler under the cam lobes although the tappet buckets still moved, just. All cylinders are around 150 + or - a tad now. This might be useful for anyone else with a similar problem of low compression.

However, along the principle of one step forward and one back, I have just discovered a slight suggestion of oil in the coolant header tank. If I can borrow a compressor I may invest in a blow by test gauge and see if I get bubbles in the tank. However, I can't face taking the head off so, if the gasket is going, it's a job for someone who knows what they are doing.

If anyone has experience of using such a gauge I'd be interested in any tips.

Peter

christopher storey · Post by **christopher storey** » Wed Mar 30, 2011 8:55 am

I should find some test strips for the coolant which ( rather like litmus paper) test for combustion products in the coolant . This is the surest sign of a blown gasket. However, if you have even compressions, as you now do , a blown gasket seems highly unlikely. Incidentally, the most usual gasket failure is between two adjacent cylinders, so that the surest sign is 2 cylinders being down on readings

pdcross · Post by **pdcross** » Wed Mar 30, 2011 2:55 pm

Thanks, Christopher. I shall certainly get the testing kit.

Today, I did another compression test and surprised myself by getting higher readings. Maybe this was because I ran the engine for a little longer than before and so the unit was hotter. The new results were, 157, 162, 161, 158, 158 & 152, cylinders 1 to 6 respectively. (Before I had adjusted the tappet clearances, the readings had been 150, 154, 154, 125, 150 &142.)

I was surprised that cylinder number 6 was the lowest as I had expected number 4 to be the low one as it had been lowest before the adjustment. Do you think that my new readings seem ok, or indicate a gasket problem?

Kind regards,

Peter

christopher storey · Post by **christopher storey** » Thu Mar 31, 2011 7:03 am

Peter : those figures look OK to me. The test method tends to affect results . I would suggest that

1.The engine needs to be properly hot before starting.

2.All the plugs need to be removed

3.Full throttle should be used

4.The engine should be turned over through 10 compressions on the tested cylinder before taking the reading

andy303 · Post by **andy303** » Wed Apr 06, 2011 1:11 am

Shouldn't one also take into account one's elevation above sea level?

Probably not a big consideration for most of the UK, but significant in some areas. For example, the gauge pressure at 6,000 ft (say Denver, Colorado) is about 80% of sea level. The compression results should correspond.

Link: http://www.jeepforum.com/forum/f8/atmos ... ng-722504/

Link: http://www.engineeringtoolbox.com/air-a ... d_462.html

christopher storey · Post by **christopher storey** » Wed Apr 06, 2011 9:08 am

andy303 wrote:Shouldn't one also take into account one's elevation above sea level?

Very definitely so. The cylinder filling in a normally aspirated engine is more or less directly proportional to atmospheric pressure . If pressure is 1013.2 mb (29.92 inches mercury) at Sea Level, it will be about 820 mb ( purists now say hPa) at 6000 feet AMSL and in broad terms I would expect compression readings of about 110 - 120 lbs/sq.in at that altitude

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