Andrew Murray wrote:
> I have always had some oil leakage past the head gasket
> (forward and rearward), but no real worries.
Actually this is not so uncommon. But if it bothers you, a head
rebuild might be in order. But FIRST check your valve clearances.
They should be checked cold and should be at .010 If these are WAY
out, that could be *some* of the cause of your low compression.
> recently performed a compression test and recorded the results (F to
> B) as follows:
>
> 120-115-102-105-89-113
OK, now perform a 'wet' compression test. Be sure the engine is warm,
and take an oil can that you can squirt some oil (about two large
squirts) into the chamber through the spark plug hole. You should see
an increase of 10-15 psi on all of the cylinders, equally. If you see
a LARGE increase, then it's likely that you have ring problems on that
cylinder. A large jump in compression means that those rings were not
sealing. Be sure you add the same amount of oil to each cylinder.
Assuming that you get even compression with the wet test, it's likely
that you have valve problems OR you might find that you have a flat
lobe on your cam. Less compression in #5 could be due to the intake
valve having less lift than expected, but IMO that is not very likely.
If you have access to compressed air, a leak down test would enable
you to tell if the valves are leaking or if it was the rings as well.
Simply put, a leak down tester allows you to put pressurized air into
the cylinder and then listen for leaks. Air from the crankcase = bad
rings. Air from the exhaust = bad exhaust valves. Air from the carbs
= bad intake valves. It also measures the difference of the air going
in, and the air escaping which tells you the severity of the leak. If
you don't have access to this test, don't worry about it. :-)
You can check your cam lift without removing the head. Basically,
you'll need a dial indicator, and some sort of magnetic base. These
can be had for about $40 from mail order parts places like JC
Whitney, etc. Sears probably has them too, but not sure of the cost.
To check cam lift, remove the valve cover, and the rocker shaft (with
rockers) and then measure each lobe of the cam by placing the dial
indicator in the top of the push rod. As you rotate the engine by
hand, you'll see the push rods move up and down. Measure this lift by
moving the engine till the push rod is at it's lowest point, set the
dial indicator to zero, and then measure the lift. You should see
about .240 of lift depending on what cam you have installed. (Stock
for your car is .240, I think, although it was .220 on earlier TR6s)
The point is, all the intake lobes should have identical lift, and all
the exhaust should have equal lift. ON a stock cam, all the lobes
have equal lift.
If the cam checks out it's time to pull the head off your engine,
you can expect to perform these minimum things:
1. mill the head to create a flat mating surface to the block
2. lap the valves
3. replace the manifold gaskets, and the head gasket
In addition, I would
4. replace the valve springs.
weak springs can cause lots of problems, such as trashing
cam followers, and floating valves, which will allow carbon
to build up on the seats.
5. replace the valve guides. (You can have this done at a machine shop)
6. replace valve seals (if it didn't have any, put them on)
the above two steps will reduce oil consumption and reduce emissions.
If you take the head to a machine shop, and had them do all of the
above work, I would guess you'd be looking at $250, approx.
You might need to replace a valve or two, but they are likely to
be fine. 89 PSI is not *that* low really, and I suspect that it's
due to poorly seating valves, since you told us that oil consumption
was low.
Once you are in the engine, you should also check the cam lifters.
They can be pulled out from the block, BE SURE TO KEEP THEM IN ORDER.
(actually, keep the push rods in order too.) You can simply pull them
out one by one, to ensure they stay in order. If the cam lifters show
any sign of pitting, they should be replaced. If the lifters look
hammered (REALLY BAD PITS), then you should pull the cam to inspect
the lobes. Pulling the cam is possible with the engine in the car,
but is not trivial. Be sure you remove all the lifters first, as they
will drop into the oil pan, and that would be a bummer. :-) (Keep the
lifters in order)
If the cam needs to be replaced all the lifters need to be replaced.
Here's the bottom line ... only rebuild it if you want to. It's your
car. If you're happy with it, and it passes emissions, then keep
driving it! I don't think you're doing any significant damage, but
that's a judgement call.
Another consideration you may want to think about is that by
rebuilding the head, you will create further stress on the piston
rings. It's quite possible that a head rebuild would start you down
the road of burning oil. Right now, the older rings can handle the
compression you are giving them. If you solve your compression
problems, it might be more than the older rings can take, and you'll
start burning some oil. Probably not significant, but it's a possible
outcome of your rebuild. Based on the compression on your other
cylinders, and the fact you aren't burning any oil, I would suspect
that you'd be fine here. But you should be aware of the possible
outcome.
> Now for the strange part. When I remove the oil filler cap the
> engine bogs down - almost stalls. Now, I admit that I'm no ace
> mechanic but his seems strange. Can someone explain all this.
This is normal as you have created a vacuum leak. In fact, this is a
very good sign that you don't have other big vacuum leaks lurking
about.
Cheers,
-Scotty '75TR6 (head is off, and currently rebuilding)
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