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Got tired of trying to "steam clean" the pistons, and took it for a road test. Did full throttle accelerations up to about 50, back down.
The exhaust got progressively smokier. After a little over a mile, the stalling issue returned with a vengeance. Still the exact same signature; some DTCs showing coil current too low, others too high. Stalled on switching transmission to reverse when turning around.
I'm really stumped. If there is a preignition problem, perhaps on cylinder 2, it would make sense that letting it persist would make it worse. I could try switching off injector #2 (and the rest of the injectors in series) to see if one cylinder's preignition is causing the problem.
After getting frustrated that the problem recurred, I got aggressive on the water injection on #2. Stabbing the throttle, letting up, running at higher rpm (3-4k), cycling on and off. It is cured.
There must have been jagged carbon deposits that were causing preignition, which made the system retard the spark timing, leaving sooty plugs and probably making the carbon worse.
I also was running Gumout Regane Complete, one large bottle per five gallons of gas, which probably helped.
Here is a picture of the water injection in action:
The black tube has a loose connection to the clear tube, so air is sucked in along with the water. This helps break up the water as it goes in.
To have a serious effect on the carbon deposits, you have to be pretty aggressive about the water. I gave it enough to generate misfires at idle.
Alternating full throttle to closed throttle probably cycles the piston top temperature, as the vacuum suction mostly only draws water in at closed throttle.
Update: After water cleaning cylinder 2, I was troubled to find that the problem recurs occasionaly - this when trying to clean a different cylinder.
I switched my endoscope to side view. Look what I found on the bottom side of one intake valve:
This, then, is probably truly the root cause. Note the protruding carbon point, which is known to get hot and act as a glow plug. This explains why cleaning alleviated the issue - it may have reduced the size of this stalactite - and yet why the carbon buildup on the pistons just was not that bad, yet the symptoms were clearly pointing to uncontrolled preignition on one cylinder.
Since I am still waiting for the weather to warm, I will rig up a pressurized water injection system and try doing this on the road, under full throttle prolonged acceleration. Manifold vacuum will not suck in water when the throttle is opened; an effective system has to have forced injection. This also associates with why doing water cleaning not under load is less effective; the combustion chamber, and especially the intake valves, just do not get that hot.
This buildup is mostly if not all due to oil leaking down the intake valve seals. I noticed oil puddled on top of the valve when the intake manifold was out. I am loth to put in the labor to install new seals on a 450 motor that has 185K miles; I will try a combination of Liqui Moly hydraulic valve treatment, which seems to greatly reduce the oil consumption, and avoiding short trips first - and then shop for a 550 motor.
In hindsight I wish I had DAS and risked only about $300 on a used ECU instead; possibly my Autel does VIN coding anyway. Live and learn.
Last edited by eric_in_sd; Mar 16, 2022 at 07:20 PM.
Looks like I finally reached closure. With a pumping water injection with a shutoff valve to prevent water draw from the vacuum, I did a series of full-throttle cylinder cleanings. Here is the #2 valve after the last clean:
The valve is mostly clean, a few chunks remaining. I am inclined to let time and longer trips finish the job; I will check after about a thousand miles. The piston is nearly imacculate:
With that much oil leaking in, I'd say the vehicle is nearly unusable for short trips. Will try some oil treatments to swell the seals and reduce the leakage, but otherwise really ought to shop for a 550 motor.
Well if you can find a Mb system with developer mode near you, and bribe operator with some libations that’ll take care of that. Glad you’re 450 motor is running tolerably well now but yeah that kind of oil leakage is limiting to its use.
Pretty sure my only recourse is getting my own system. Since I am the only Mercedes owner in town, I am probably also the only potential DAS owner! I could ask around over the Boise area.
I am wondering whether it would be worth doing the valve stem seals. I guess I will wait and see whether the Liqui Moly lifter additive does the trick. Or resign myself to doing a cylinder cleaning every so often.
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