Compression Test Results.

[jules]

Hi All,

Given Malibu is just about to tick over the big 300,000k, I thought I would do a compression test.
The results were way better than expected, making me think my cheapo Ebay testing kit may be flattering me.
Stock is supposed to be around 192psi. What do you guys think of the below results?

Regardless everyone who has known me over the years knows I am totally anal about this car and never treat it badly especially when cold.

This is the test kit I bought -> https://www.ebay.com.au/itm/Petrol-Engine-Compression-Tester-Kit-Tool-Set-For-Automotives-Motorcycle-AU-SHIP/262525463116?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649


The results of the engine warmed up are as follows.

Compression Test
298,000kms

1 2 3 4

FIRST STROKE 95 85 95 100

DRY 205 205 205 205

WET 275 275 265 280

[ralt]

Hi.
Did you have the throttle fully open to do the test. The figures seem a little!!! on the high side for the age of your engine.

[jules]

Yes, fully open held with a cable tie.
I'm suspecting possible carbon buildup

[greenMachine]

If that is accurate, that is one for the record books! But is it accurate - I'd suggest borrowing another gauge to check, seems too good to be true given age/mileage.

Is it using oil? How is performance? If it isn't broken, don't try and fix it ... just do the big belt service, and keep on doing whatever it is you have been doing

[jules]

Hi,

Yeah, I agree. I have read similar compression results on Miatas with either shaved heads or bad carbon buildup.
That's why I presume it could be the latter.
It doesn't like anything other than 98 octane in Summer or it pings, which also makes me think carbon.
Timing was previously at 14 deg, but I backed it off to 10 deg after it started pinging in hot conditions.
With 98 it's happy as a piggy in mud.

It blows a little smoke when cold. Not sure about hot.
Performance is great, car idles well and accelerates as smoothly as ever.
After a month or so I do get a lovely sooty look on the bumper above the exhaust (which is a Loch Stewart).

It's got a couple of oil leaks in the usual places (rear main seal, front seal or sump) which I will get done when I do timing belt or clutch.
It will typically need a litre of oil to top up every month or two.

It's at that stage where I will need to make a decision about timing belt and clutch vs if the rings are degraded replacement/rebuild of engine.
If the rings are worn there would be no point in my mind doing the above maintenance items at 300,000kms.

The standard clutch was put in at 84,000kms and it just starting to slip ever so lightly under hard acceleration.
214,000 kms is great life from a standard clutch and probably says more about my mechanical sympathy for the car than the standard clutch.

I'm probably more interested in the difference between wet and dry tests as I think that might point to rings?
Clearly the consistency across the cylinders is awesome.

It's booked in for its rego check on Friday, so I'll have the mechanic do another compression test then.

[manga_blue]

They were my thoughts. Get a second test on a known gauge. Then at least you'll have an accuracy test on yours. There's no point in worrying about anything until that's done.

[jules]

Results are in.
The mechanic compression tested the car and said it's fine, just a bit low on cylinders 3 and 4, but no issues with head gasket.

He suggested using a slightly heavier viscosity oil and there was traces of oil evident in the cylinders.
I usually use Magnatec 10W-40, and he suggested going 15W-40.

1- 160
2- 162
3- 140
4 -145

[bruce]

Methinks that ebay comp tester is a piece of junk.

[greenMachine]

Good result! Well done!

Now see if you can get your money back on that piece of crap from fleabay!!

[bruce]

Yes, on specific ebay returns they do cover you for the cost of postage (I think if paid via Paypal - look at the transaction in your Paypal acct and you should see Return Postage).

[Mr Morlock]

- at the price on the link it was certain it was a piece of junk.

[manga_blue]

Those results are not that bad considering the age of the car. It's common to get poorer readings on the back two. I assume that's because of poor cooling after Mazda botched the conversion from east-west to north-south. You don't have a coolant reroute on yours, do you?

[jules]

No coolant reroute, just a big aluminium radiator.
That's interesting to hear about the change of config making the rear two cylinders sometimes wear a bit faster.

I've ordered a new PCV valve and grommet because it has never been changed.
Hopefully that, some new seals and a change of oil will help with the morning smoke and oil consumption.

I'll probably hit it with some Subaru upper engine cleaner as well. Haven't done that for about 4 years.

If those compression figures say the engine is fine I'll also look at doing the timing belt and Exedy HD clutch in the next few months.
The car only does at most 100 kms a week, so no rush.

Happy car. Happy life lol