Poor compression - cause? Updated

Post by: **pharriso** » Sun Jul 05, 2020 3:02 pm

I've been getting a lot of blowby the last couple of years & burning a fair bit of oil, so finally decided to run a compression test (engine hot, throttle wide open):
Cyl 1 Cyl 2 Cyl 3 Cyl 4
110 118 122 118 psi

*** See later tests - these results are from a inaccurate Harbor Fright Compression Tester ***

With a cap of Oil in each cylinder:
Cyl 1 Cyl 2 Cyl 3 Cyl 4
130 130 132 135 psi

*** See later tests - these results are from a inaccurate Harbor Fright Compression Tester ***

All these numbers are low; engine has 92,000 miles on it, unknown if it has been rebuilt. Do these numbers just confirm that it's worn & needs a rebuild?

Post by: **nmauduit** » Sun Jul 05, 2020 3:34 pm

pharriso wrote:Do these numbers just confirm that it's worn & needs a rebuild?

I would say so, all symptoms of a well deserved rebuild...

Post by: **alan.barker** » Sun Jul 05, 2020 3:56 pm

Imho with 92,000 miles it had a refresh to some degree. Maybe new rings and Valves lapped in.
If it hasn't been rebuilt i would be surprised and suggest it is the time to do so before damage happens.

On my Sprint i did a refresh at about 70,000 miles, new Pistons, bearings, Oil Pump. The Ring Grooves were badly worn and 2 or 3 broken Rings but no scoring.

If the Cylinders are OK you might get away with fitting a set of standard original bore Pistons like i did (very cheap everyone's already had a rebore).
Alan

Post by: **pharriso** » Thu Jul 09, 2020 10:27 pm

I had trouble believing the Oiled results (too low), so I purchased a Lisle compression tester off eBay (much better quality than harbor freight) & re-ran the tests with much different results:

Cyl 1 Cyl 2 Cyl 3 Cyl 4
162, 168 170 165 psi

With a cap of Oil in each cylinder:
Cyl 1 Cyl 2 Cyl 3 Cyl 4
195 195 201 197 psi

I think these results with a bit of oil down the plugs are more what I would expect.

I'm losing approx 30 psi between wet & dry results, I assume this means the rings and/or bores are worn & I need new rings and/or a re-bore?

p.s. The Harbor Freight Tester is going back! :roll:

Post by: **sprintsoft** » Fri Jul 10, 2020 12:28 am

Hi Phil,

I think maybe the blow-by and high oil consumption suggests a worn bottom end, but did you consider running a leak down test?

It would allow you to determine more accurately where you are losing compression.

Useful.explanation here:
https://www.mobil.com/en/lubricants/for-personal-vehicles/auto-care/vehicle-maintenance/how-to-do-a-leakdown-test

Iain

Post by: **2cams70** » Fri Jul 10, 2020 10:10 am

Another test you could consider is a "power balance" test. Basically this entails hooking up an accurate low RPM range tachometer to the engine and noting the idle speed. Once stabilized short each spark plug in turn (slide the boot up on the HT lead at the spark plug and short to ground with a long handled insulated screwdriver) and note the drop in speed when each cylinder is independently isolated. The one that has the least RPM drop will be the most problematic cylinder worthy of further investigation. Could be further good backup information for what the compression test reveals. Your compression readings are fairly even however. Usually for a worn engine some are worse than others.

Post by: **alan.barker** » Fri Jul 10, 2020 10:31 am

Hi Phil,
you know you want to so just remove the Engine and get stuck in.
Better now before damage.
Like i said i did that on my Sprint at around 70,000 miles and don't regret it.
If your'e lucky on standard bores, deglaze and new set of original Piston(old stock) for less than £100.
Polish Crank and fit new Vandervall Bearings + new Oil Pump + Full original type Gasket set. Job done.
On my Sprint the wear on the width of the Piston Ring Grooves was about 0.025" just just in time. With 2 or 3 Top Rings broken
Have fun
Alan

Post by: **nmauduit** » Fri Jul 10, 2020 12:25 pm

If you decide to look into it, I would not skip measuring the bore wear (getting it done at the machine shop if need be) : reboring is cheap if you are to change the pistons anyway, and you'd start on a much better base with straight bores of the proper diameter fitting the pistons...

The absolute minimum (changing rings only) is not really a long lasting fix imho.

Post by: **alan.barker** » Fri Jul 10, 2020 12:49 pm

+1
Imho just to change Rings is a waste of time because the width of the Ring Grooves in the Pistons wear.
The big question is what do you want to do with your Sprint or how do you want to drive it.
Your answer will decide what you need to do.
Alan

Post by: **pharriso** » Fri Jul 10, 2020 2:33 pm

alan.barker wrote:The big question is what do you want to do with your Sprint or how do you want to drive it.
Your answer will decide what you need to do.
Alan

Well, keep it forever & drive it in a spirited manner on the road

Looks like an engine rebuild is in order next winter, but I will do a leakdown test & post the results.

Post by: **alan.barker** » Fri Jul 10, 2020 4:43 pm

Phil,
if you buy a Stick Mic/ Internal Mic it's very easy to measure the wear for a road car, it's not a F1.
Alan

Post by: **Spyder fan** » Fri Jul 10, 2020 5:36 pm

Phil,
Shame you can’t take a rebuilt engine home as hold baggage next time you visit the UK ( next year?). I have the Q420 spec engine sitting in its cradle doing nothing. Not for general sale, but open to negotiation to friends in need

Post by: **pharriso** » Fri Jul 10, 2020 5:50 pm

Spyder fan wrote:Phil,
Shame you can’t take a rebuilt engine home as hold baggage next time you visit the UK ( next year?). I have the Q420 spec engine sitting in its cradle doing nothing.

Now there's a challenge!!