Symptoms: '94 850 (not a turbo). Fuel pressure regulator failed while wife was driving, engine died, didn't restart. Replaced fuel pressure regulator, starter cranked engine for several minutes until engine caught and ran very rough with blue smoke for several minutes. Turned engine off, talked with Volvo service tech buddy. Changed oil (level was OK), pulled spark plugs (dirty but undamaged). Engine restarted, ran better but still somewhat rough, no blue smoke, but loud metallic tapping noise. Turned engine off, had Volvo tech listen. Doesn't like the metallic tapping noise but is not sure whether it is bent valve or hydrolicked cylinder.

I removed spark plugs for visual inspection of piston heads while turning harmonic balancer with wrench: bit of carbon but no visible damage. I don't have a bore scope. Following a hunch, I put shish kebab skewers into spark plug holes to show piston position and turned harmonic balancer with wrench. Rotating the engine with the wrench is pretty easy until cylinder 3 nears but has not yet reached top dead center, when it feels like something hits something else. The wrench gets very hard to move at this point. I think I could rotate past it, but I don't want to break my socket extension trying. The problem occurs both clockwise and counter clockwise.

Please help with diagnosis: I suspect a bent valve or other top end problem, not bent connecting rod or other bottom end problem. You agree?

Seems like the next step is to pull the head, inspect head and piston top for visible top end damage. (I think that's easier than pulling the oil pan to look for bottom end damage.)

Thanks for the help.

-_ Bruce

Using shish kebab skewers to indicate piston position is a great example of necessity being the mother of invention.
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-- Bruce / '82 244 (315k miles!), '86 745, '87 760Ti, '94 854

Compression test will find a bent valve - might of stuck down when the raw fuel washed all the oil from the valve stem - same idea as the "lawnmower syndrome" - search the term if unaware.

Condolences for the trouble - nobody wins when a car leaves you.

MAT

Being a Volvo tech myself who's seen both bad FPRs and hydrolocked engines (not on the same cars however), I'm trying to understand how this "bent valve" could happen from your description. A bad FPR either leaks gas through the diaphram into the vacuum line or causes excessive fuel pressure. In either case, the engine burns the mixture without hydrolocking. Hydrolocking can occur if an injector sticks opened causing a cylinder to fill with gas and I've seen that too BUT it doesn't bend a valve, if bad enough it'll bend a connecting rod.
A valve doesn't just bend by itself or by a hydrolocked engine.
I'd get another professional opinion before trying any engine work yourself as that's not an engine for an amateur (not simple like the good ol' 4 cyl red engines in comparison). BTW, it's alot easier to remove the oil pan than it is to pull the cylinder head BUT without engine experience, I highly doubt that you'll see physical damage by just removing the oil pan.
If you're determined to do your own work on that engine if need be, don't try it w/o getting the OEM engine overhaul manual (800-25-volvo) and be aware that costly special tools are also required to properly R&R the cylinder head. Having done that before several times myself, I wouldn't ever consider trying that job w/o the special tools.

Thanks for the reply, Dave. Here's some additional info. Taking your points slightly out of order:

The failed fuel pressure regulator was the first problem, and quite effectively disabled the car -- the engine just wouldn't start. Diagnosis was pretty easy: the fuel pressure had pushed the vacuum line off the regulator. Replacement required removing the fuel pressure rail including injectors. I'm pretty careful: nothing fell into the injector holes while the injectors were out. I vacuumed the injector holes and throttle body (while holding throttle open) to remove any dirt that might have fallen in, but I had brushed and vacuumed the injector seal areas before removal. After replacing all parts there were no pieces left over and no missing tools. I had not yet removed the plugs before restarting the car.

Restarting the engine was difficult and the metallic noise was present when it first caught. I doubt therefore that an errant piece or my actions caused the noise. I could, of course, be wrong -- and I am gonna find out.

After starting the engine and hearing the noise, I turned the engine off and called my Volvo tech buddy. He suggested checking the spark plugs, replacing the oil, and checking the timing belt. Plugs were dirty but undamaged, timing is dead on, and the oil smelled a bit but was the proper volume. I replaced the oil filter (always do when I change the oil) and added the correct quantity of oil (20-50 synthetic blend: summers are hot out here). Engine ran better but noise remains.

Volvo tech buddy says probable bent valve. Advises against compression test or leak-down test for same reason you give: every tap does more damage. Suggests removing head for a look-see. I started doing this today.

I do have the '94 850 Volvo service manual for the car (sections 2(20-22, 25-26), with service bulletins current to about 9 months ago. I will ask the service manager of my local dealer whether Volvo has released any service bulletins for that car that I don't already have. I have read through the cylinder head section (and most of the rest of the manual, too).

The only special tools required to remove/replace the head seem to be for replacing the front cam shaft seals. My Volvo tech buddy recommended that I not remove the cam gears until after the head is reassembled, when cam friction will help immobilize the cam. At that point, he advises marking the cam gear position very carefully, removing the gear, replacing the front cam seal, and then replacing the gear in the same rotational position. I have no opinion yet on how feasible this is. I'd value your opinion on this.

I hope to have the head off tomorrow. I'll post the results.

Thanks for the advice, Dave and James. I would never attempt this without the Brickboard providing access to knowledgeable people like you.

-- Bruce
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-- Bruce / '82 244 (315k miles!), '86 745, '87 760Ti, '94 854

If the timing belt didn't jump (which would have also been unlikely), I fail to understand how a valve could just bend. How does anyone explain that? Valves don't just bend by themselves, the bend from hitting something like a piston or foreign object (debris like a nut, etc if dropped in) and that can only happen if the t-belt jumped teeth (or broke) or something fell in the cylinder. Even if one fuel injector was stuck opened due to high fuel pressure or the was so much gas in the intake manifold that it sucked enough fuel in to fill a cylinder, the liquid couldn't bend a valve. A liquid filled cylinder CAN bend a connecting rod on compression stroke however.
It wasn't me who said not to do a compression test, I still say it should be done. I have alot of engine experience and if a valve were bent from hitting a piston (impossible by your story), once bent, it's done....out of the way. Valves are bent once and then completely out of the pistons way the next time up. Now, you could have a dropped valve seat or guide but I've never seen that with a Volvo white engine. If you're determined to pull the head anyway, a compression test won't matter and even if anything cyl head related is hitting, those pistons are tough and you won't do more than put small marks in them (worst case).

I'd suspect something else which is why I say to get another opinion BEFORE removing your cyl head.

Other special tools? You will need to remove sprockets to replace cam seals and to hold and position both cams for re-assembly. That's shown in the manual. There are tools that hold the cams in front and hold/position cams in back to the upper cam cover before you re-assemble it to the lower half. The cam positions are critical, you can't just be close. I wouldn't try doing that w/o those tools and be certain that the crank is at #1 TDC (crank mark lined up). Then you'll need the two large wing-nut type tools to carefully, slowly lower the upper cam cover downwords before re-installing all the bolts. Those tools are required to overcome the valve spring tension because it won't just bolt down easily otherwise.
The one time I had an 850 injector stick fully opened on me in the shop causing it to hydrolock while cranking, I too heard a metalic noise cranking only. I removed the cylinder and a guyser (sp?) of gas sprayed out cranking. No valves were damaged but a flywheel tooth broke off when it hydrolocked so I then had to replace the flywheel.
I'm telling you that so that you realize that there are other possibilties besides an unlike "bent valve".

Hi Dave --

Thanks for the info. I got your email after I pulled the head. Which was not easy, I must say.

Valves, tappets, cam shaft, head gasket all look fine. Bunch of carbon on piston tops. I'm not sure how to diagnose tappets, since the operative stuff is all inside, but read on because the problem remains even with the head removed.

Current set of anomalies:
1. Top dead center of cylinders 3 & 5 is noticeably lower than 1, 2 & 4 -- perhaps as much as 1.5 mm by eye. I'll check with my set of vernier calipers tomorrow. I'm bushed today.

2. With the head off, if I rotate the crankshaft with a wrench, an abrupt resistance remains shortly before cylinder 3 reaches TDC. It feels like something hits something else. The wrench gets very hard to move at this point. I think I could rotate past it, but I don't want to break my socket extension trying. If I rotate the cranckshaft counter clockwise (opposite of engine rotation) the problem remains at about the same place (now shortly after TDC).

Have you any suggestions for how to diagnose these symptoms?

So you were right: the problem is not in the head and I will have to see whether I can borrow the special tools you describe. It turns out that I needed to replace the camshaft seals, at least: the rear intake cam seal was leaking oil into the distributor.

Thanks for the help.
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-- Bruce / '82 244 (315k miles!), '86 745, '87 760Ti, '94 854

Don't plan on re-assembling the head until you know why there's resistance turning it as you're saying as (to me), that doesn't seem normal. I'm hesitant to tell you where to go at this point as I don't want to be the guy who might take you on a "wild goose chase" but IF the problem is a bent connecting rod (or two) and that'd be IF it hydrolocked (and still can't imagine how from your story here however), you'll need the head off as you'll then need to remove pistons (pan removed too).
As you can imagine, all this means alot more work, time, etc just to TRY diagnosing a noise with a non-running engine.

Who decided that the fuel presure regulator was the problem that disabled your car? How was that diagnosis arrived at? Perhaps the stall was related to the problem that you have now.

Any parts or tools missing from your last repair job? Has anyone worked on the car lately? One of my tenets on troubleshooting is - What happened just before the problem came to light? You had the plugs out.

If parts are clashing, you should not try a compression test, every tap is doing damage.

Some possibilities are - foreign material in that cylinder, timing belt has jumped, the engine stops because of a problem not in the cylinder, such as a broken cam.

I think that you have to take it apart or look in there with a scope - I'd find a scope. With luck, there might be something in there that you can remove.
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3 8s & 2 7s 725,000 miles total

It's done by using 1) a compression tester and then 2) a cylinder leak down tester to know exactly what's causing the compression leak.
Don't guess.