Mercedes Diesel Misfire After HG Replacement

[JHZR2]

After that, I pulled the prechamber to look into the cylinder.

It comes out with a slide hammer. It usually takes a decent amount. This one came right out. Like first slide. Almost comically so.

The prechambers have angled injection, so what the photo shows is normal. It was wet with HPL EC as I’d expect as I only ran the engine a minute or so to maneuver the car.

I was really surprised how easy it came out considering how much I’ve had to hammer some others in the past. They have been known to be a cause of compression leakage, but I didn’t see or hear that during test or operations.

So then I put a scope down there. It isn’t the good one that @Trav recommended, just a cheap little one I had that gets some initial insights.

There didn’t seem to be any glaring damage. But there do seem to be some scratches across the crosshatch. I don’t know if those are horizontal or vertical.

 

[JHZR2]

I also pulled about five gallons of diesel from the tank. Not a speck of rust in what was pulled out. It actually looked good.

 

[JHZR2]

After mulling over this all night, I’m kind of confused. Is the bore scratched? The crosshatch looks great all around.

Similarly, why is it that I have relatively low leak down? 64psi / 72 psi is 88%, so 12% leakage. Ive heard discussions about 30% or more with working engines. Granted, I do understand that the delta between cylinders makes my 12% potentially worse since the other cylinders are far better.

I don’t really know if what I see is scratching, scuffing, or something else. The photos aren’t great, but if there is a scratch, it doesn’t seem particularly huge.

So what’s the next best bet? Piston soak? B-12, top end cleaner, or the like?

Is this looking more and more like a stuck ring, or something else?

 

[mattd]

After mulling over this all night, I’m kind of confused. Is the bore scratched? The crosshatch looks great all around.

Similarly, why is it that I have relatively low leak down? 64psi / 72 psi is 88%, so 12% leakage. Ive heard discussions about 30% or more with working engines. Granted, I do understand that the delta between cylinders makes my 12% potentially worse since the other cylinders are far better.

I don’t really know if what I see is scratching, scuffing, or something else. The photos aren’t great, but if there is a scratch, it doesn’t seem particularly huge.

So what’s the next best bet? Piston soak? B-12, top end cleaner, or the like?

Is this looking more and more like a stuck ring, or something else?

if it’s as loud coming out of the breather tube then something is wrong, especially if oil comes out the dipstick tube when you plug the breather.

Other possibilities include rings that are stuck in the piston grooves (grease may have hardened up not allowing piston rings to move freely, or from wear and/or carbon buildup )

If you truly only have 12% leak down that is not that bad. If the pre chamber is not leaking compression, pull the valve cover and make sure you have no worn out cam lobes, which can give you a dynamic compression issue but may be sufficient for a static compression test (although not really in your case). This would result with good leakdown numbers.

 

[JHZR2]

if it’s as loud coming out of the breather tube then something is wrong, especially if oil comes out the dipstick tube when you plug the breather.

Other possibilities include rings that are stuck in the piston grooves (grease may have hardened up not allowing piston rings to move freely, or from wear and/or carbon buildup )

If you truly only have 12% leak down that is not that bad. If the pre chamber is not leaking compression, pull the valve cover and make sure you have no worn out cam lobes, which can give you a dynamic compression issue but may be sufficient for a static compression test (although not really in your case). This would result with good leakdown numbers.

Under the valve cover versus the headspace above the oil in the pan is the same continuous volume. If the dipstick tube is below the oil level, of course oil will come out. It can only go one of three places, out the breather (the correct lowest impedance path for blowby and crankcase pressure), the sealed and gasketed oil fill hole, or the dipstick hole. I had the dipstick out.

The thing with cam lobes is that this isn’t an unknown engine. It’s a low mileage engine that ran beautifully before I had the HG done. Cam lobe wear is a long term process (or due to running without oil I guess, which would cause similar wear to the rest of the cam and engine).

I don’t know how I can’t truly have 12%. I did the test twice, and the results were consistent.

I think the real question is if there’s another path for air/compression to escape.

 

[mattd]

Under the valve cover versus the headspace above the oil in the pan is the same continuous volume. If the dipstick tube is below the oil level, of course oil will come out. It can only go one of three places, out the breather (the correct lowest impedance path for blowby and crankcase pressure), the sealed and gasketed oil fill hole, or the dipstick hole. I had the dipstick out.

The thing with cam lobes is that this isn’t an unknown engine. It’s a low mileage engine that ran beautifully before I had the HG done. Cam lobe wear is a long term process (or due to running without oil I guess, which would cause similar wear to the rest of the cam and engine).

I don’t know how I can’t truly have 12%. I did the test twice, and the results were consistent.

I think the real question is if there’s another path for air/compression to escape.

I understand all of that. I am simply trying to give you possible explanations to your problem.

 

[JHZR2]

I understand all of that. I am simply trying to give you possible explanations to your problem.

No I get and appreciate that.

What I’m trying to reason through is if there’s another leakage path. I think not. Its rings, back up the oil drain, and if not out an opening to air, it pressurizes the crankcase which pushes oil out the dipstick since the end is below the oil level.

But my point is really, it wasn’t like this before I gave it in. So it can’t really be wear or cams, as they were fine before.

That seems to reason it to the rings, as you mention.

So now it’s what co concoction to use. B-12, mmo, toluene, thinner, kerosive, diesel, gasoline, etc???

 

[JHZR2]

I had a little while with the car tonight. I used my borescope again to look at the scratching, and then applied some solvent for a piston soak. First, the photos.

I found this one pit. Only one from what I could tell. Two photos of the same thing.

I did notice this bright scratch in the oily black combustion zone. Two photos of the same thing.

And I tried to get better images of the lines, which I don’t think are below/deeper than the crosshatch, at least not the whole way, but might be. Hard to tell.

@Trav @Astro14 and @clinebarger Im curious of your opinion given these photos.

I then turned the engine over, set it to BDC, and put in some B-12 chemtool. I tried to deduce the level, but all I had was a wood qtip type stick that would not wet properly. I even added some MMO to try to get some coloration on the stick but no dice. I have an idea that I have a couple inches of fluid in there. Tomorrow night I’ll look again.

Question about piston soaks: it seems like it would make sense to turn the engine by hand down and up to force the rings to move a bit. But with all the solvent in the sump, and the fact that the oil pump will pick some of it up, I’m concerned with the prudence of turning the engine. Maybe just a few mm? But then it doesn’t get up and down. Thoughts??

Thanks again!

 

[mattd]

If it was mine the head would be already off it to inspect the cylinder closely and see what I can feel with my finger/nail.

 

[clinebarger]

Those vertical scores are pretty close to each other, Wonder if you have a busted compression ring?

A large volume of air out of the crankcase during a leak down isn't good.....Even though it should show up as high leak percentage??

Compare #1 & #6 to each other far as air volume out of the vent.

I've never worked on one of these engines.....But with diesel engine in general, If you loosen the oil cap while running & the cap wants to "dance"....You have a ring/piston issue.

 

[JHZR2]

Those vertical scores are pretty close to each other, Wonder if you have a busted compression ring?

A large volume of air out of the crankcase during a leak down isn't good.....Even though it should show up as high leak percentage??

Compare #1 & #6 to each other far as air volume out of the vent.

I've never worked on one of these engines.....But with diesel engine in general, If you loosen the oil cap while running & the cap wants to "dance"....You have a ring/piston issue.

I’ll have to dry the thing out and get the prechamber back in to do the comparison. I could readily hear the airflow when I did the leak down test on #6. I couldn’t really hear it that I can recall for #1.

Anything is possible, not sure how the ring would break, but I guess it “could be”? I don’t know that the air out was large. It just was enough that I could hear it. I couldn’t really inflate the glove I put over the end, at least not like a balloon. It obviously popped open with it hand held on the end.

Maybe it’s the best bet if it is a bad ring…??

Lift engine a bit, pull pan, unbolt piston, drive out from top with a dowel, super light hone in place, replace ring, install piston.

Straightforward, no??? Seems like there is less of an issue than removing the head again. Assuming we end up sure that it is a piston/ring issue…

Thanks!

 

[clinebarger]

Again....Not familiar with this engine, But I've never pulled a piston/rod from the bottom....
*The crankshaft is in the way
*How would you compress the rings for reassembly?

 

[JHZR2]

Again....Not familiar with this engine, But I've never pulled a piston/rod from the bottom....
*The crankshaft is in the way
*How would you compress the rings for reassembly?

It’s late. That makes much more sense, lol.

I thought I had read that someone had. I thought maybe the crank could be turned to maneuver the disconnected rod. I don’t know.

But it really does seem like removing the head and possibly the piston is the logical next step. Will need a cherry picker regardless, to help lift the head (I assume it is much safer for a newbie to do this than try to just lift it out by hand), and to help lift the engine in order to remove the oil pan…

Knocking on wood the piston soak works. Otherwise it seems like this is the next step…

Thanks!

 

[JHZR2]

If it was mine the head would be already off it to inspect the cylinder closely and see what I can feel with my finger/nail.

IMO easier said than done. This is a mechanical engine. So injection timing and everything else is contingent upon the cylinder/piston locations.

If I was to pull the head, I need to set cyl 1-6 to TDC, to retain timing when I remove the chain/sprocket from the cam. Then I can’t see down the bore. So thus I need to pull the piston too, which means pull the oil pan, which means lift the engine, which means the head needs to be on.

Nothing is hard per se. It is just logistics. The right order to be well thought out and planned to do everything that is needed.

And for what? The potential of a stuck ring? So I’m thinking that through for sure. I don’t see any skill that I can’t do, or haven’t done, except that I need an engine hoist, and I’ve never removed a head. But I’ve done all the leading up steps.

Those vertical scores are pretty close to each other, Wonder if you have a busted compression ring?

I was looking through the FSM, it seems to imply that one will see lines:

This is curious to me. So stripes are ok, which is probably argue is what I mostly see, and I certainly see honing marks. But I have 150 compression. And it’s coming from the breather system… not valves.

Very confusing still…. Broken ring makes sense…

 

[JHZR2]

Stupid me, I didn’t have my scope set fully bright.

Got some new photos…

The B-12 from yesterday was completely gone.

I added more and I could hear it drip down into the crankcase. You can see it sitting on top of the piston, but not much.

I turned the engine up from BDC a bit more, and added some MMO. No more drip, no noise. Level of liquid seems to be staying.

 

[JHZR2]

Here’s a curious thing. Look at the junk in the valve. I know I didn’t have a leak down issue through the valve but I need to look at it some more.

 

[tiger862]

I still say you have a bent valve. I would have removed head already to check piston, seats and valves.

 

[JHZR2]

I still say you have a bent valve. I would have removed head already to check piston, seats and valves.

I’m not opposed to that.

Thing is, how was it that my leak down was relatively low, and I observed the leak down through the crankcase vent that goes out the VC gasket!

I’ve never removed a head before but I’m confident that I can do it.

Thing is, if I then need to remove a piston, I need to lift the engine, which I can’t without the head.

So in reality I need to lift engine, remove oil pan. Cover. Then remove head. Check everything out. Remove piston if necessary. Then all the opposite.

 

[supton]

I turned the engine up from BDC a bit more, and added some MMO. No more drip, no noise. Level of liquid seems to be staying.

Bad taper to the wall? cross hatch looks too good for that.

I don't think the scuffs are a problem, but not a diesel guy.

You can always put the head back on to lift the engine, it's not like it has to have a good HG to do that. Nuisance to remove only to reinstall and then remove again, but it's not quite that bad. Thing is, if everything is leaning towards a bad ring, it's leaning towards planning to pull the engine so as to pull that piston.

I'm guessing it's a lighting thing, but is that why, looking around the piston, that it's not quite the same looking all the way around? It looks closer to the wall in some spots. Can you spin the engine up slightly, then back down slightly, make sure the piston doesn't rock? I don't think that's it, but that first picture just looks like a loose piston (but then it'd have piston slap, which isn't the complaint, so I think it's just lighting).

 

[JHZR2]

Bad taper to the wall? cross hatch looks too good for that.

I don't think the scuffs are a problem, but not a diesel guy.

You can always put the head back on to lift the engine, it's not like it has to have a good HG to do that. Nuisance to remove only to reinstall and then remove again, but it's not quite that bad. Thing is, if everything is leaning towards a bad ring, it's leaning towards planning to pull the engine so as to pull that piston.

I'm guessing it's a lighting thing, but is that why, looking around the piston, that it's not quite the same looking all the way around? It looks closer to the wall in some spots. Can you spin the engine up slightly, then back down slightly, make sure the piston doesn't rock? I don't think that's it, but that first picture just looks like a loose piston (but then it'd have piston slap, which isn't the complaint, so I think it's just lighting).

Good point about bolting the head back in case I need to pull a piston. Thing is, I agree - things are pointing towards it being a ring. So it would seem to make sense to just be proactive.

Regarding scuffs, note the FSM comment:

I don’t know how to differentiate, and there is no objective metric given. it does tell us that if honing marks are still visible, the engine is ok, and they are, all around…

I don’t need to remove the engine to pull the piston. I just need to lift the engine enough to get the pan to clear.

Other folks have driven them out from underneath.

The irregular gap does bother me some. These 3.5L engines are known as “rod benders”, and will wear the bore oval. But generally they will run ok and just start to consume oil at a huge rate. This engine ran perfect, consumed little to no oil, and didn’t have a low compression misfire. I don’t see how it could wear to the condition you see just from putting it back together and running it 10 miles.

I’ve tried reading a bit and it seems that pistons not being perfectly concentric in the bore is normal, especially when at dc because of how the skirt sits. But that’s just my rudimentary reading about it…