Wednesday, May 11, 2005

Detroit Diesel Episode IV -- A New Hope

When last we saw our intrepid galactic explorer 8V92TA and his robotic sidekick, DDEC-II, they had fallen into the clutches of the evil Lord Filter Evader...

Seriously, nothing much happened today, on account of the fact that the service manager was out, as he was yesterday, due to a death in the family. So we spent the day relaxing in the parking lot and catching up on a few things.

However, at some point this morning, Virgil stopped by to chat, and asked us to fire the big Detroit up so he could have a look at the smoke. Nothing unusual was noted, which is to say that it sputtered some white smoke on startup, which went away almost immediately (it's in the low 80s here).

Based on this observation, and years of experience, Virgil has ventured a guess that we may not need liners or rings after all. His theory is that a lot of the dirt we sucked in past the air cleaner is actually packed up in the aftercooler. He showed me an aftercooler, and, after seeing it, I can understand his point. Symptomatically, he believes that if the cylinders were dirted out, we would be seeing lots of smoke at idle, and it wouldn't clear up at all, or maybe only when the engine was hot. Further, the black smoke we are seeing under load is more commonly a symptom of air starvation than blow-by.

The aftercooler is a device found only on turbocharged models. It sits in the valley of the engine, below the blower, and consists of fins and tubes, just like a radiator, to a depth of about five inches. The air passages are very small. This device transfers some of the heat from the charge air, which has been superheated by the turbine compression process, to engine coolant flowing through the tubes. This provides a cooler charge to the cylinders. (Similar devices commonly found on turbocharged automobile engines are sometimes called intercoolers, and often are air-to-air, rather than air-to-coolant, units.)

Virgil's theory is that, often, there exists a thin film of motor oil on the aftercooler, due to normal seepage around the blower gaskets. The combination of the oil film and the small air passages in the aftercooler can trap dirt, and the aftercooler can become plugged up with the resulting gunk.

While starving the engine for charge air doesn't do it any good, this is certainly better than having the dirt proceed all the way to the cylinders, causing excessive ring and liner wear.

Now you may remember that, when we were at Williams, I had a look for myself into the cylinders, and saw what looked like excessive scoring of the liners. So I grilled Virgil on this. His opinion is that vertical score lines are absolutely commonplace on these Detroits. He told me that if I could still see the cross-hatch (I could) then the cylinders were probably normal. Furthermore, he contends that a dusted engine would have the liners polished smooth, with neither scoring nor cross-hatch visible. I confess that, while I've had a lot of experience with big Detroits (mostly 149s) and looked at a lot of liners, I saw them mostly new or at end-of-life, and I don't have a lot of experience looking at liners at this stage of wear, andI have to bow to Virgil's experience here.

So, while some dirt certainly must have actually gotten into the cylinders, and the cylinder wear is probably higher than it should be, they may not actually need to be replaced. We will need to look at the tattle-tale grooves on the rings to know for certain.

I did mention that turbo boost pressures look normal, and asked if they wouldn't be higher if the aftercooler was, indeed, plugged, but Virgil reminded me that the turbo is almost certainly shot (being the first thing the dirt hit after the intake hose), which would tend to lower boost pressures. Sort of two abnormal circumstances adding up to one normal gauge reading.

So the plan now is to pull the turbo and the blower, and have a look at the aftercooler. And we'll all cross our fingers until then. And, of course, we'll have a look at the rings, at least on the left side where they are easy to reach.

Tomorrow they will be backing us in to our assigned spot in the yard. Work will probably not start until Friday or maybe Monday. In the meantime, we have our work cut out for us, dissassembling the wardrobe and pulling up the carpet to remove the access panel over the turbo.

1 comment:

  1. Egads, this is beginning to resemble "The Perils of Pauline." A cliffhanger every day. What will our heroes find when they open the thing up? Stay tuned? You bet!


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