What could/would happen if your spark plugs weren't tightened and you drove the car, say, 200 miles?

1.You'd probably experience engine knock and misfiring if they were not down into the port deep enough.
2. They could possibly be shot out of the port from the force damaging the hood or other components in the engine bay.
3. Nothing at all. Maybe something tells you that you should just check the plugs and you discover they were loose and you tighten them.
4. The plug flies out and the empty port scorches everything from the heat and flames coming out.
5. Your car morphs into a marshmallow and you are forced to eat your way out

Most likely #1 and #3, although #5 would be awesome except for the fact that you would no longer have a car.

__________________

1990 190E 3.0L

I know i had one blow out on my V8 Rover .Noticed nothing , only a chug chug after it blew out of the head . Problem that i had was with the back plug on bank 1 lh side rear .It was in such a tight spot ,that you thought it was locked in but it was not . Just reset the plug and re fitted it .

There would be no scorching after the plug ejection since there is no spark to trigger combustion.

__________________
Prost!

I've seen thread damage a few times.......

What about the fuel filled air puffing out of the hole with a raw wire nearby with sparks coming off of it? Fire???

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Well, here's the deal. Just did a head gasket job on daughter's 190E 2.6. new guides, seals, and mill on the head. No time to give it a long test, she needed to drive it 3 hrs back to college as soon as I was done. I did get to drive it for 50 miles or so first.
Went smoothly, fired right up ran great, blew a pocket of some oil/foam back into the nice clean, new reservoir after I flushed it twice (with MB acid flush). Anyway, she calls me to tell me the car had no power climbing a fairly good mountain incline, had to floor it to keep it at 50. She makes it over the mount and about 15 -20 miles down the road, the car loses power and dies. No noises at all that she heard. cranks over but wont fire up. Tow driver described it as cranking as if it had no compression. About 200 mile now on it since the repair, running fine until the last 25 miles or so.
At first I was suspecting I hadn't tightened the plugs, therefore losing compression, then power, then finally stalling and no start as they backed further and further out.
I get it towed back, I haven't tried to crank it yet. I only removed the plugs to see what was up. They were tight. Cylinder 4 fought me all the way out. The end was totally smashed up. obliterated. all other plugs looked beautiful. That's all I had time to do. had to get to work. Disassembly starts tomorrow. Don't know what got in there, maybe the electrode came off the plug. I KNOW I didn't drop anything in there. Hope I find it salvageable. Let you know what I found in a day or two.

Loose plugs won't cause the problem you describe. Besides, you state that you didn't find any loose plugs so why pose the question to cloud diagnosis?

As for the mashed plug, something is bouncing around in the cylinder.

Possibly the intake manifold guide sleeve fell off into the cylinder , you may have not seen it fall into the manifold

Did you reset tensioner piston ?, if not, cam broke causing bent valves

When I posed the question, I did not even have the car back yet. I was tring to put together reasons why the car would have acted as it did (I've done a few M-103 head gaskets without any problems) so I'd have a place to start when I got it back. I remebered thinking wether I was going to torque the plugs or just tighten them by hand, but I couldnt recall actually doing either, so I thought it might be a possibility that I snugged them and never finished.
It ran beautifully before the job even with lots of oil in the coolant. I didn't change any other ignition or fuel related parts except the cap, rotor and the air flow boot, which was cracked underneath. And it started right up and ran beautifully afterward....for a couple hundred miles.

97 SL320, you don't think the cylinders would lose compression if the plugs were loose?

nulu, the manifold dowel might be possibility, but I would think I would have heard a little racket when it first started. Tensioner was properly reset.

The guide is easy to check unscrew 2nd intake manifold bolt rearwards, you will be able to see guide and check rear 2nd from rear same thing, only check if what you say , you have damage to plug and no compression

I think on twice the volume of air in the cylinder , as the piston went down due to plug being loose.Then as it came up no place for it to get out .Only a guess . Or you fitted a longer length plug. But i think --- Other thing is that coil spring type thread inserts ,may have been fitted in the past gone down as plug was removed or -- refitted. Good luck,and i hope its an easy fix .

Optimusprime, the cylinder can only hold as much volume as its size permits unless it forced in under pressure. Any air that can enter through the plug threads can escape the same way, even during the compression stroke.

I showed the mangled plug to the machine shop guy who did the guides, valve job and milling. He said it might be very possible to salvage it without delving into the bottom end if the piston isn't cracked or damaged too badly and the connecting rod isn't bent. He said heads can usually be repaired and he's seen some very beat up pistons that are still in running cars. I'll see when I get the head off again, working on it now, but not much time to do it.
What I don't get is why it would stall and not even sputter trying to restart. The other 5 cylinders seemed fine. Wouldn't the bad cylinder act as if the plug were just fouled and causing a miss? Shouldn't it still run or at least try to run? obviously something else is wrong.