Dead cylinder, now what?

So for those keeping track at home the Monster Miata is a project I picked up over the winter for cheap.  The old motor had broken and the previous owner got tired of paying to fix things.  His mechanic had already sourced a 302 short block from a 2000 Explorer and installed the cam, heads, and custom oil pan.  I put everything else together and have a running and driving car.

But.  It doesn't run right and has a dead miss.  Compression test shows zero compression on number 4 (yay).  Very experienced guy who did said compression test said that he firmly believes it's the valvetrain not a bad piston (backfiring and popping but not burning oil). Last night I pulled the valve cover hoping to find a broken valve spring, rocker arm, or push rod.  No such luck.

So do we agree that bent valve is the likely culprit?  I suppose my next step is to pull the head which isn't an insignificant amount of work but at least now I know how this thing goes together.  Suggestions, ideas, sarcastic jabs?

Did you crank it with the valve cover off and watch the rocker arms off the offending cylinder? If the valve spring, rocker arm and pushrod are intact and the rocker arm doesn't move, then you might have a flattened cam lobe. Worth a quick check.

But yes, sounds like removing the cylinder head is next.

Adapt your compression tester to an air line.  Hook up compressor hose and see if the air comes out the intake, exhaust, or pcv.  

In reply to NickD :

did that.  Everything moves like it should.

I'd keep looking, right up to pulling the head before I started on the road to rebuilding the bottom end or buying another motor.  If you ended up rebuilding, the head has to come off, and if you're junking it, it doesn't really matter.

What Patrick is talking about is a leak-down test.  You put pressurized air in through the spark-plug hole and listen for where it comes out.  If you hear it coming out of the intake manifold it's a valve problem, but if you hear it coming out of the dipstick tube it's a piston/ring problem.   

The fancy leakdown testers have an orifice with gauges upstream and downstream.  The differential can be used to calculate percentage of leakdown.  You don't need anything fancy for what you're doing.  Take the hose for your compression tester and remove the valve stem so air can flow into the chamber.  Pull the fuse for the fuel pump, remove the dipstick and air cleaner and prop the throttle body open.  Set the offending cylinder to TDC, lock the motor down and connect the other end to an air supply.  You'll be able to hear the air escaping.

If you can get a bore scope take a look inside while turning the engine over by hand. That would be my next step.

Paul

Donebrokeit said:

If you can get a bore scope take a look inside while turning the engine over by hand. That would be my next step.

 

Paul

Also not a terrible idea. Harbor Freight actually makes a really good digital borescope that's a hundred bucks.

Either the valve is bent, stuck open, burned. or something in the valve train is holding one open.  5.0 still uses individual bolts on the rockers, correct? loosen the bolts on the two rockers for that cylinder a  turn or two and check compression again.

Then pull the head.

I have a leak down tester and a bore scope you’re welcome to borrow. 

Exhaust valve has a pie cut in it.

bent valve should be obvious...  valve will not be as high as others...  burnt not so

There is more to a leakdown test than shooting air into a cylinder. You have to position the engine so that that cylinder would have both valves closed before trying to pressurize it. 

As said, bent valve is often visible from the top because it won't come up as high as the others when it's 'closed'. The pie cut missing out of the valve scenario is due to valve overheating, likely from the rocker being too tight (valve sheds a lot of heat through the seat so if it doesn't seat right it doesn't cool right) or a very loose guide causing it to seat poorly and also not transfer as much heat away through the guide.

A cylinder with 0 compression isn't going to 'burn' anything as far as oil consumption. Try sticking a taser into some motor oil and see if you can burn it.  A lit match has more ability to burn things than a sparking spark plug in a cylinder with 0 compression.

NickD said:

Donebrokeit said:

If you can get a bore scope take a look inside while turning the engine over by hand. That would be my next step.

 

Paul

Also not a terrible idea. Harbor Freight actually makes a really good digital borescope that's a hundred bucks.

Not directly relevant to this thread, but I was disappointed to discover the HF borescope is too large for my 85 SBC’s plug holes. 

This evening I spent 2.5 hours taking things apart.  It's down to everything but the actual head itself (dinner interrupted my labor).  Tomorrow it'll come off and be taken to the machine shop for repair.  I sort of suspect that puting everything back will take a bit longer but hopefully not too much, it's getting familiar now.

A few years ago someone here (a fellow GRM'er) had some nice TITAN bore scopes thankfully i scooped up two the heads are tiny they even fit in to the small honda 10mm plug holes on bikes.  I've been able to read the numbers stamp on piston tops.   Now i see you can buy one for your phone that plug in to the usb port for under $20...

As this is push rod engine you can do leak down at BDC by popping out the push rods thus make it easyer to test as you don't have to hold the crank at TDC. worn ring will still show and while you might have a score in the wall at TDC that would make the leak more the ring would still be damaged.

Pete Gossett said:

NickD said:

Donebrokeit said:

If you can get a bore scope take a look inside while turning the engine over by hand. That would be my next step.

 

Paul

Also not a terrible idea. Harbor Freight actually makes a really good digital borescope that's a hundred bucks.

Not directly relevant to this thread, but I was disappointed to discover the HF borescope is too large for my 85 SBC’s plug holes. 

What vintage of Harbor Freight borescope? Because the ones from ~3 or 4 years ago have an end that is way too large for much use. Then a year or so ago they redesigned them and they have a much smaller tip

If Explorer heads were used, it's SOP to upgrade the valve springs before a cam swap.  Maybe somebody made a mistake reassembling one of the valves on that cylinder.  Regardless, I would want the head off the car trying to diagnose and fix a valve leak.

Tape a rubber glove over the tailpipe(s), the PCV port on the valve cover and the carb throat.

Now do a leak-down test.

The glove the gives you the finger is the culprit.

And this is exactly why I much prefer to buy motors that are untouched since they left the factory.  That is, I much prefer a JDM motor with 80K miles on it to a "rebuilt" motor with 5K miles. 

In hind site, a leakdown test (HF sells the tester pretty reasonable) while it was sitting on the stand should have been done.  You would have found that dead cylinder right off.  Tell us what you find. 

NickD said:

Pete Gossett said:

NickD said:

Donebrokeit said:

If you can get a bore scope take a look inside while turning the engine over by hand. That would be my next step.

 

Paul

Also not a terrible idea. Harbor Freight actually makes a really good digital borescope that's a hundred bucks.

Not directly relevant to this thread, but I was disappointed to discover the HF borescope is too large for my 85 SBC’s plug holes. 

What vintage of Harbor Freight borescope? Because the ones from ~3 or 4 years ago have an end that is way too large for much use. Then a year or so ago they redesigned them and they have a much smaller tip

I originally borrowed one, but when it didn’t fit I went to HF to buy a new one. IIRC the ends on both were either 7.5mm or 8.5mm, whichever it was didn’t fit my SBC. 

Well there is your problem..... used a flashlight to "highlight" the lack of valve closure.  To the machine shop tomorrow!

Patrick said:

Adapt your compression tester to an air line.  Hook up compressor hose and see if the air comes out the intake, exhaust, or pcv.  

This times 100.

Set the crank so that the offending cylinder is at TDC of its compression stroke (best way to guarantee that the valves are supposed to be closed).  Blow air into it and see where it comes out.  Intake or exhaust and you have a valve seal issue (burned, bent, dropped valve seat).  If it comes out the dipstick tube or PCV, you have a serious compression leak (rings, cylinder walls, hole in the piston)

Although I have to say... It doesn't matter what the problem is, the engine has to come apart.  I don't see anything wrong with pulling a head at this point.

Edit:  Oops.  didn't read the last post :)

New valve installed by local machine shop, lapped in and everything: $45.  Awesome!

That's cheap! I do like cheap repairs. So did the valve get stuck or bent?