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View Full Version : compression for NA GQ TD42 (288k km) - confused and need help please



depree
9th May 2015, 09:28 PM
Hello :)

In case you haven't seen my introduction, my name is Craig, I hail from Hamilton over in NZ. Im just looking at purchasing my first 4wd and ive decided it will be a GQ LWB, manual - would be great to get factory turbo, however these seem to be exceptionally rare in NZ. I am currently looking at one, a 1988 safari with 288k on the clock (see trademe link below) - in very good condition inside and out, I had it checked out (since it is in the south island, and im in the North), and the condition has been independently verified. For peace of mind, I also had compression test done, and it yielded the following:

1 – 280, 2 – 290, 3 – 310, 4 – 320, 5 – 310, 6 - 300 psi

The mechanic indicated these were pretty good, and confirmed the vehicle is pretty much tip top - starts/drives very well and is very good condition cosmetically and mechanically.It was only afterwards when I search typical value that I found these are well below the minimum of 356 psi, let alone the standard of 427psi :( however, they are within the maximum difference of 43 psi.

I have the garage that inspect the truck, the owner and another mechanic that does the owners servicing all saying the truck is in very good condition (ie starts first time every time, even during Christchurch winters) , uses no oil / water , air filter dry (ie no oil) - so am I placing too much emphasis on the absolute values of the compression test ? Owner has service book of 5000 km filter oil changes going back the 17 years he has owned it . in addition, I spoke another mechanic he said if the compression were 280 psi - the vehicle would be a right dog and smoke on start up (he basically said it would be f*&ked (his language) if those were true compression values).

so this is where I ask for help - are the values wrong (is it common for compression to be read low? - he asks optimistically), ie with those compressions it is not possible for a TD42 to run nicely as people are telling me? is the important thing that the relative agreement across the 6 cylinders is within 'spec' ? should I retest? or cut my losses and walk away -already spent about $250 checking this out :( if I retest, is compression all I need to do - I assume leak down is when trying to track source of problem ? in which case, I would walk away, ie I don't want to buy something that has issues from the get go.

I just want to do the right things to ensure I get the right truck - and at the same time, not walk away from a good truck on account of some potentially misleading compressions.


if interested, vehicle link on trademe.co.nz

http://www.trademe.co.nz/Browse/Listing.aspx?id=868416850

thanking you advance for any help / advice / wisdom you might be able to offer this newbie (but appreciative) New Zealander :)

Craig

MudRunnerTD
9th May 2015, 09:32 PM
Hi Craig, head over to the intro section and post an intro thread mate, we like folks to say Gday please mate.

*** Edit- sorry bud I have seen your intro, because your new and embedded a link in this thread it was moderated automatically and showed you as a single post count. I have approved your thread.

Good luck with your car bud.

Winnie
9th May 2015, 09:34 PM
Hi Craig, head over to the intro section and post an intro thread mate, we like folks to say Gday please mate.
He already had Darren!

depree
9th May 2015, 09:45 PM
Hi Craig, head over to the intro section and post an intro thread mate, we like folks to say Gday please mate.

*** Edit- sorry bud I have seen your intro, because your new and embedded a link in this thread it was moderated automatically and showed you as a single post count. I have approved your thread.

Good luck with your car bud.

Thanks very much , I did go back and ad a little more to my intro as I was guilty of being a little brief :)

Rossco
9th May 2015, 10:08 PM
Hi mate, no expert but to me I would think the readings are incorrect. I would imagine it would run rough and smokey on a cold startup this is always a good test for the condition of a diesel. You don't know it's full history however it does not seem consistent with overall condition and mileage of the car either. Again no expert but believe compression tests can be unreliable at times. Also if the mechanic thought those figures were good (which they weren't) maybe he's not the best.

Low k good condition GQs are hard to come by so definatly worth pursuing IMO.

4bye4
9th May 2015, 11:15 PM
G'day mate - I'm no expert on this particular motor, but I do use leak down testing a lot. As you indicate a leak down is more for diagnostics. I don't know what figures you would look for.
The compression values seem a little low but they are not too badly balanced. The problem is you can get variations in a compression test ie. hot motor, cold motor, wet or dry, battery condition and the number of compression strokes the tester was held on for and the order in which the test was done.
For example if you go from font to back with your tester, I would expect to get a slightly higher reading as in 6 than 1, because as you test 1, oil is being spreading in 2 through 6 and this may give a slightly better reading. If your battery is a little down, you may get decreasing values as the motor cranking speed slows slightly.
I agree with Rossco, I think the readings (numbers) are incorrect but all the other evidence ( starts and runs ok with no smoke ) would indicate its all OK.

dom14
11th May 2015, 03:05 PM
Where there is excessive carbon buildup inside the bore due bad timing, etc(around valve, etc), it can give a good or even abnormally high compression reading. Not directly relevant to the OPs concern about his engine. But, I thought it might emphasize the idea that a compression test it not always accurate. I would rely on a leak down test for serious diagnostics.
I have no idea about the reason for OPs engine's low compression readings.
Engine is running good with no smoke, no loss of power, yet the compression readings are below the average.
I would have it leak down tested as it's 10k+ price tag 4WD.
Diesel motors are pretty expensive to rebuild, apparently.

threedogs
11th May 2015, 03:13 PM
Put a set of vacuum gauges on it ?