Throttle stuck and now the engine won't stay running.

[Gary in NJ]

A differential compression test (aka leak-down test) compares the pressure retained in a cylinder to a reference pressure. The tool used is one like this:

http://www.harborfreight.com/automotive-motorcycle/diagnostics/cylinder-leak-down-tester-94190.html

Here's how it works:

1. With a warm engine, place the cylinder to be tested on TDC and mechanically hold that position (car in gear, e-brake engaged, wheels chocked). Disconnect the coil and hold the throttle at WOT (I use a block of wood);

2. Fill the cylinder with a reference pressure (80 psi is typical)

3. Read the cylinder pressure. My gauge shows the actual pressure, most show a percentage.

100% cylinder pressure simply isn't possible. A healthy cylinder will show 80 to 90%. Below 70 to 75% you should investigate the source of the leak. If the air is escaping from:

a. the exhaust valves, you have a bent valve, bad valve seat or guide (remove valve cover to investigate further)

b. the intake/carb, you have a bent valve, bad valve seat or guide (remove valve cover to investigate further)

c. dip-stick or breather, your rings are worn or you have a hole in a piston

d. from around the base of the head, your head gasket is blown

All six cylinders should have a similar reading.

This test provides useful information whereas a simple compression test only tells you the max pressure that a cylinder can make.

[Zed Head]

Thanks for the details, sorry that I misinterpreted your comment.

Shouldn't it be called a differential "pressure" test though, since there's no compression involved? Pressure from an air compressor (air compressor compression doesn't count does it?) instead of piston movement. Just sayin'...

But cylinder pressure testers are pretty common and inexpensive and the easiest if you don't have an air compressor. Based on the signs, he'll probably get some zeroes or teens and have to pull the head anyway.

[Gary in NJ]

I didn't name the test It's a common name in the aviation industry and I wouldn't be surprised to discover that it was actually a "differential pressure compression test". The only difference between a typical aviation test unit is the test value is given in actual pressure. So a result is recorded as 72/80 (said 72 over 80) as opposed to a 90% value recorded on the leak-down test. It's also easier to run the test on an aircraft as the prop makes for a good place to hold the cylinder at TDC (easier to find too).

[LeonV]

This test provides useful information whereas a simple compression test only tells you the max pressure that a cylinder can make.

A compression test can tell you many things, not just "max (cylinder) pressure". In fact, it's a poor way of determining max cylinder pressure as it's more for comparative purposes than absolute measurements. It will diagnose a blown head gasket and/or leaky valves quickly and easily. A leak-down test will narrow the results further. Both should be done, especially if compression results are unhealthy.

With that said, I'd probably pull the valve cover before doing anything else. Overly high engine speeds can cause rockers to fly out of place.

Edited August 1, 2012 by LeonV

[M.Gwizdek]

Well I had expected the results of the compression test to be different, but:

1. 150 psi

2. 150

3. 150

4. 150

5. 145

6. 150

I then pulled the valve cover. Everything looked good except for the oil. It is very light brown in color. I assume this means coolant has gotten in there? If so I would have thought the compression test would have shown something different? Any ideas?

Thanks,

Mike

[Zed Head]

If the oil is clear, not milky, then light brown is normal. Water in the oil gets frothed up and creates an opaque milky emulsion.

Looks like your problem might have nothing to do with cylinder, pistons, rings or valves. Your pressure numbers look very good. Maybe you just bent your throttle linkage in your haste to get the RPM back down. Have you tried adjusting the idle to see if things get back to normal?

In retro, reading back, you've only described a low idle RPM as the problem. How does it run otherwise?

[madkaw]

What did the spark plugs look like? Did they look any different then the last time you checked them-maybe one plug is a different color?

[M.Gwizdek]

If the oil is clear, not milky, then light brown is normal. Water in the oil gets frothed up and creates an opaque milky emulsion.

Looks like your problem might have nothing to do with cylinder, pistons, rings or valves. Your pressure numbers look very good. Maybe you just bent your throttle linkage in your haste to get the RPM back down. Have you tried adjusting the idle to see if things get back to normal?

In retro, reading back, you've only described a low idle RPM as the problem. How does it run otherwise?

Maybe light brown is not the appropriate description, more like a tan color. In really isn't clear either. I tried to take a picture; it actually didn't turn out too bad.

As for how it runs, I took it for a drive and it seems to run well at higher RPM, except the temperature gauge slowly crept higher than it usually runs. I took a picture to show the needle position. Typically the needle is at about 1/2.

Incidentally when I started the engine I did not see any smoke from the tailpipe.

What did the spark plugs look like? Did they look any different than the last time you checked them-maybe one plug is a different color?

I checked the plugs this morning and #2-#6 are black with a lot of build-up but #1 was closer to the tan color you would except (or hope) to see. What could this indicate?

I'm new to diagnosing engine problems, so I appreciate all of your help on this.

Thanks,

Mike

Edited August 2, 2012 by M.Gwizdek

[SteveJ]

That, sir, is milky. IMHO, stop running it and get the head off. The oil doesn't lubricate well when mixed with coolant.

[Gary in NJ]

I'd say it looks like coffee, light & sweet. That, coupled with the hi engine temps makes me think head gasket. Can you spot any blow-by at the base of the head?

Not to sound like a broken record, but a leak down test would have given you the answer to the basic question; "whats wrong?".

[M.Gwizdek]

I'd say it looks like coffee, light & sweet. That, coupled with the hi engine temps makes me think head gasket. Can you spot any blow-by at the base of the head?

Not to sound like a broken record, but a leak down test would have given you the answer to the basic question; "whats wrong?".

I don't see any blow-by at the base of the head. Sounds like my next step is to pull the head and see what's going on. I certainly appreciate your suggestion of performing a leak down test, but at this point do you think it would help? Seems like the head has to come off either way.

Can a head gasket be blown and still show relatively good compression?

Thanks,

Mike

[SteveJ]

Can a head gasket be blown and still show relatively good compression?

Yes, the damage to the head gasket could be worse between an oil passage & a water passage.