Compression Test While Disassembled

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robert.juric

Active member
May 16, 2017
39
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North Alabama
I have my DiscoII engine tore down to just the heads. The rocker assemblies are removed along with the tappets and pushrods.

I tried to run a compression test by manually turning the engine by hand, but this appeared to be futile.

Is there any issues with reconnecting the battery and bumping the starter to test compression? If so how long is it ok to let it try to turnover?

I wanted to verify my piston rings are good before I put this thing back together.
 
4Runner

4Runner

Well-known member
May 24, 2007
663
111
Boise Idaho
You can do a leak down test. You need to have the valvetrain working for an accurate compression test with a gauge and use the starter.
 
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robert.juric

Active member
May 16, 2017
39
0
North Alabama
You need the rocker assembly installed and the valve train functioning so that the intake valve can open? Is that correct? What about the rest of the coolant system or even the valve covers?
 
jafir

jafir

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May 4, 2011
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Northwest Arkansas
jymmiejamz said:
You need the valves closed for a leak down test.
Click to expand...

Yep. That's how we did them on SOHC Hondas when they'd break timing belts. We'd pull the rocker assembly and do a leak down test. Sometimes people got lucky. Sometimes they didn't. The only other way was to install a new timing belt and do a compression test.
 
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robert.juric

Active member
May 16, 2017
39
0
North Alabama
I don't have a leakdown test kit, just a guage for compression testing. I was really trying to validate the piston rings. So is the rocker assembly needed for compression testing? Just curious how does air not eacape the exhaust valve? Can you bump the starter with the valve covers off?
 
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discostew

Well-known member
Sep 14, 2010
7,733
1,023
Northern Illinois
Since you have the rocker shaft off, all the valves will be closed.

Since you don't have a leakdown tester you might be able to do a similar test by modifying the hose off your compression guage. The theory behind a leakdown test is to pump compressed air into the cylinder, and see where it's leaking out. If it's out an exhaust valve you'll hear it in the tailpipe. Intake valve it will come out the open ports in the cylinder heads. Best to have the piston near the top, and you need to hold it there. The air will turn the engine.
 
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discostew

Well-known member
Sep 14, 2010
7,733
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Northern Illinois
I forgot to mention rings since your concerned about them. If you had a piston ring broken enough to cause a misfire, it would have been blowing a lot of smoke out the tailpipe.
 
R

robert.juric

Active member
May 16, 2017
39
0
North Alabama
So I got my rocker assemblies all cleaned up and re-installed. I ran a compression test on all cylinders last night with the plugs out and the intake manifolds off. The results:

8-115 7-125
6-130 5-125
4-125 3-120
2-125 1-110

#8 was the cylinder I was constantly getting misfires on. Are these numbers of any concern or is there anything else I should investigate while I still have it tore down?
 
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JohnnoK

Well-known member
Mar 19, 2017
193
19
Cape Town, South Africa
As a rule of thumb, you want no more than 5% difference between highest and lowest.

The actual figures are not that important because they are variables, dependant on whether the throttle was wide open for all tests, battery state between first and last test etc.
 
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robert.juric

Active member
May 16, 2017
39
0
North Alabama
So if there is a greater than 5% difference (and by my math I'm looking at 15%), then I should follow-up with a wet test and leak down test? To see if its a valve or ring problem? I'm just trying to wrap my head around how to interpret the results.

If I have to buy another test set my wife is likely to string me up :)
 
squirt

squirt

Well-known member
Nov 13, 2008
824
13
Los Angeles
While I don't disagree with what JohnnoK said, I think your numbers are OK for a high-mileage engine, and nothing that would indicate a consistent misfire on cyl #8. You can re-test with oil in the cylinders to see if things normalize, but it's not really going to help you solve your misfire issue.
 
4Runner

4Runner

Well-known member
May 24, 2007
663
111
Boise Idaho
If your intake was off, I would take the opportunity to measure your cam lobes. Squirt is probably right that your problem is elsewhere. Coil, wires, fuel injectors vacuum leak etc. might want to do the head gasket / valve job thing anyway while your there.
 
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robert.juric

Active member
May 16, 2017
39
0
North Alabama
I just did the head gasket/valve job, which is when the misfire started. I THOUGHT I tested all the electrical/fuel things and replaced most everything (plugs, wires, coils, injectors, front O2) without solving it. So I tore it back open to verify my rocker assemblies, valves, etc.

I've gotten some conflicting info on the cam lobes. I've been told it can cause misfires and I've been told it would have to be practically flat to misfire. I would assume if it were that flat I would also have compression issues? I started to measure the cam lobes but was having a heck of a time so I figured compression test would be a quicker indicator.

So I'm kind of stuck. I can try harder to test that camshaft, or just throw $200 at it for a new camshaft and lifters. Or I can button it back up and start from the beginning verifying everything.
 
4Runner

4Runner

Well-known member
May 24, 2007
663
111
Boise Idaho
Sorry, I missed the back story on how it all started. Yes if the cam was open and sitting there, I would take the time to measure it. You are right about the info the compression test provided. That's not your issue. Perhaps a vacuum leak specific to the #8 cylinder? Intake gasket problem on at runner? O ring on the fuel injector base? Just a couple of thoughts. Was the #8 misfire your only issue after you put it all back together?
 
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robert.juric

Active member
May 16, 2017
39
0
North Alabama
I've wanted to go back and verify everything that I changed originally. I replaced the HGs, water pump, timing chain and oil pump gears. A machine shop surfaced the heads and did the valve job. So far I haven't found signs of a coolant leak, none of the cylinders are pressure washed and I haven't found any coolant in the oil. So I believe the HG itself is is ok.

I'm hoping the compression test numbers validate the valve job done by the machine shop. I took a picture with a straight edge on the timing gears to ensure they were inline, so that would validate timing chain replacement? I haven't had any oil pressure problems so the oil pump gears are ok.

That should clear up all the lower stuff, right? I'm definitely not a mechanic so I'm just trying to work through each thing. I'll try and measure the cam again before putting it back together and starting from the top.
 
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robert.juric

Active member
May 16, 2017
39
0
North Alabama
Ok, instead of pulling the tappets and putting a dial gauge on the cam lobes themselves I left everything assembled and measured the lift of the tappet. I grabbed measurements from a few tappets to compare and they all came out to roughly .25" even the #8 I've had a misfire on.

I think I'll press on with re-assembly and start my troubleshooting all over.
 
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