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Second engine rebuild - compression readings

795 Views 3 Replies 3 Participants Last post by  95classiclwb
Ok so for those that remember, I recovered a low mileage 1983 original Texas import Range Rover (3.5 Carb). It has not run since 1997 when the PO removed one head and it had been sitting since (mostly in a closed garage). PO bought plenty of spare parts and I bought the lot for cheap from the Junk Yard who had put the car aside as it was too nice to scrap.

Anyway that was in January and I since overhauled the engine, but (silly me) honed the bores but did not change the rings. After about 600miles the engine developed a misfire and I recorded low compression and 0 compression on cyl 7. I took the engine apart again, deglazed and installed with new rings. The engine has done 41k miles and looks fantastic. I change the bearing shells and had the heads overhauled with all new parts from the stash (one exhaust valve had burned out). Some bores have marks on them from when the engine was standing which I could not hone out, but the pistons moved smoothly in the bores.

Anyway all went together fine and has been running great since (400 miles so far). I have an TBI (Holley Sniper EFI) installation and have done some tuning getting about 17-18 USMPG. The 3.5 is not particular powerful but accelerates smoothly and cruises nicely at 75mph if you want to. It uses no significant amount of oil as far as I tell (on 2nd change - mineral oil for run in) no other fluid losses. Idle vacuum is about MAP 45kpa which is about 13.2 inHG

I checked the spark plugs and all fine with good colour and no oil residues. Is did a compression test again and the results are a little odd:

1 - 56psi (going up to 60 with some oil in the bore)
2 - 60
3 - 60
4 - 58
5 - 55
6 - 56
7 - 58
9 - 58

It takes about 5 strokes to top out. The engine should be 9:34 - 1 compression, but I fitted composite head gaskets which will drop the compression slightly (although I did have to skim the heads a little as well).

So I am confused. Is my compression tester bad?
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I would agree with you – bad gauge. It seems to consistently read 100 PSI low.
Found the problem:

Using a compressor I put 90psi on the gauge through the spark plug fittings. Gauge would not move. Removed the fittings and it would read 90 (same as compressor gauge, so gauge was good).

Checked the fittings, there is one schrader valve in there to keep the pressure in the gauge and one to bleed it off. A long time ago I removed one of the valves to use the fitting for a leakdown test, lost the valve and refitted a generic schrader valve. I removed both valves and noticed the original valve had much lighter movement with hardly any spring pressure compared to the generic valve. Turns out that generic schrader valves rely on whatever fitting you put on it to push the little pin, so it has a strong spring and you need a lot of pressure (100psi as it turns out) to pass air if you don't push in that pin.

Swapped the valves over and now the gauge read 90 psi and stayed there when pressure was removed.

OK back to the car and just pulled plug 1, fitted gauge, cranked and.... 155PSI. So I can add 100 psi to each reading, excellent! Book says 135 minimum so my new rings are working just fine in the (slightly) marked bores.
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great detective work, in the end man made convenience man made errors.
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