An RV filled to capacity with seven women (a grandma, two daughters and four grandaughters) and, I think, six small dogs, came into the shop last night. It was a 2008 Chevy C5500 with 41,000 miles on it. The thing still smelled new and there was no corrosion to be found anywhere except the exhaust pipes. The RV had been well cared for and has been parked indoors during winters. They called us saying that it was in a speed derate condition. I think they came out of North Dakota or Minnesota and it had derated soon after departure. A shop replaced the fuel filter and they went on their merry way. 300 miles later it derated again and that's when they showed up at our shop.
I grabbed the work order, saw it was a C5500 and thought I had the job in the bag. Would take a couple of hours, but I thought I would be able to get them fixed up. One of our regular customers had a small fleet of those trucks and they all suffered, regularly, from the same problem these women were having. After having them tell me what had happened (always talk to the drivers, shit gets messed up when the information passes from driver, to service writer, to mechanic) I scanned the thing and saw P0101 and P0102. "This is a slam-dunk fix." I thought. Then I saw the odometer.
The other customer (the fleet) didn't start having those DTCs and derate problems until the vehicles had well over 100,000 miles. Those fleet trucks also idled a hell of a lot. Someone not in the know, see the previous post, would toss a MAF sensor into the truck and call it good only to have the vehicle come back the next day with the same problem. More parts, same problem etc. After bashing our heads against the wall trying to figure out what was going on, we solved the problem. The problem wasn't expected and wouldn't have been looked for, but we figured it out. What we found with the first fleet truck was a carbon problem.
All of the trucks in this story have the Isuzu 6.6L "Duramax" diesel engine (the LMM variety I believe). Let me tell you, the Duramax didn't get the nickname "Dirtymax" for nothing. They're decent engines and are reliable, but they're a real pain in the ass to work on. Access to just about everything sucks.
Try and follow along. Air entry into the engine starts at the air cleaner, passes through the filter, past the MAF sensor (air intake temperature sensor and MAF sensor are one unit), through some ducting, into the turbo, out of the turbo, more ducting, through the CAC, more ducting, through the intake air valve, through the intake heater, through a small manifold where the EGR valve is also connected then finally into the engine. Towards the end of that path is where the problems were found.
The little manifold bolts on top of the EGR valve. The two ports which allow the exhaust gas into the fresh air intake are about 1" by 2" (25mm by 50mm) and the dirty-ass exhaust puts a LOT of carbon through those ports. With the first fleet truck those two ports were completely blocked and the intake down into the engine was about 1/3 blocked. I piled up all the carbon I dug out and it amounted to a ball (non-packed) of about 4" (100mm) in diameter. Once clean and reassembled, the problem was gone. Well, until the carbon built up again. The next time we ran into those DTCs, a quick shove of a bore scope down the intake, a cleaning, done. Every now and again the carbon build-up would cause the EGR valve to fail, but that was an easy test. Use the scan tool to command the EGR valve to different positions throughout its range, a look at the values (commanded/desired position should match actual position) and if the numbers matched, EGR valve was OK. If the numbers didn't match a new EGR valve was needed. So, after all of that experience I thought the RV would be no different.
The RV, as mentioned earlier, only had 41,000 miles on it. I did the EGR valve check and it was fine. I pulled the pipe from the CAC and stuck the bore scope down the intake tract. Sooty, slightly oily (normal) but no carbon build-up to speak of. Exhaust wasn't restricted (air flow through the engine is the name of the game here), air cleaner was OK, ducting OK, CAC OK... Well, I guess the diagnostic procedure had to be run through.
I started with P0101 and not far into it a list of DTCs was given and if any of those were present they had to be dealt with first. That's quite normal. When faced with a number of DTCs, you pick the active DTC with the most occurrences and start there. P0102 was one that had to be dealt with first and I then got that procedure. I suppose I should explain diagnostic procedures.
When I was in tech school, these procedures were called flow charts because that's what they were. Search Google for "flow chart" and you'll see what they are. Start at the top, answer the "yes" or "no" questions (lots of testing involve in this by the way. Voltage and resistance measurements etc.) and you'll end up with a solution. The people who create these flow charts are really good at it, most of the time they're spot-on, but they're not infallible. Most of the flow charts end up with one of two things. "Replace ECM" or "Contact manufacturer". It's very rare to get that far, especially contacting the manufacturer, but it does happen on occasion.
ECMs are expensive. We recently replaced an ECM in a 2006 KW that has a Caterpillar C13 and our cost for the part and it's programming (we don't have the capabilities to program Cat stuff) cost a little over $3,000. When you're the mechanic doing the troubleshooting, you don't want to be wrong. Replace an ECM only to have the same problem means you missed something early on. A disappointingly large amount of mechanics don't go through EVERY step of the flow charts. Generally speaking, if someone says an ECM is bad and they have less than an hour of diagnostic time into it, they didn't do what they should have.
Troubleshooting start with basic stuff. Visual inspection for corroded terminals, broken/burned wiring, missing components, damaged components etc. A little further in and the ECM and sensor connectors will be disconnected, resistance checks done. Those terminals and wires have to be identified and confirmed, the connectors have to be removed, wire colors or numbers identified... it takes a lot of time. You want to be sure that the pin you stuck a meter probe into is the correct pin. Sometimes you get to "replace ECM". I go through the whole flow chart a second time just to be sure I didn't fuck up somewhere. As I go through the flow chart I tick off every step I do and, if a measurement of some sort is called for, I write that down right on the chart. If I get to "replace ECM" again, well, I guess the customer is getting some bad news. If the second run through ends up at another solution I go through for a third time.
The flow chart for this RV was alarmingly short. Sometimes you run into a short one. Scan, "Are any of these codes active yes/no", inspect intake for loose clamps... inspect exhaust for... monitor MAF sensor PID is the value within so and so? Measure resistance at pin 2. Is the resistance greater than five ohms? If yes, repair circuit. If no, replace ECM. I was startled at how quick ECM replacement came up. "That can't be right at all. I missed something somewhere." I said out loud not realizing all these women were watching the whole time. So, I ran through it a second time. Replace ECM. A third time wound up at the same spot. "Ma'am, I have some news for you."
I hate telling nice people that they're going to be stuck in place for days and will be giving us a lot of their money. I explained the fleet trucks and that their problem should be a carbon build-up problem but I wound up at ECM replacement three times. She asked me some questions, I gave answers, she asked how much an ECM would cost and when we could get one, I paused awkwardly.
"Well, it would probably be a couple thousand but I don't know for sure. All the dealers are closed for the night." Her tanned face turned white. I then had to tell her that new ECMs are blank and required programming, programming involves programming fees, new ECM probably won't be in stock... She asked where the nearest dealer was and I said "Just across town. They'll probably open up at 08:00. You're welcome to camp out in the parking lot if you choose to stay here tonight. She went inside the RV and they all had a pow-wow. She comes back out and says they have to be at a dog show, two states away, by 08:00 the next day.
"What will happen if we keep going? Will it derate again? Will the engine be badly damaged?" she asks. I can only say "I don't know. The engine will most likely derate again, but I don't think the engine will be damaged. You will be going through some not-so-nice parts of a couple of big cities though. At night." She then asks me what I would do. I'm always a straight shooter with customers even if they don't want to hear what I have to say. "I would call the dog show a bust and be at the dealer first thing in the morning. The safety of my family would be my first concern. You may make it just fine, but it could also go very, very wrong. If my decision to try and 'make it through' resulted in harm to my family... I wouldn't want that on my conscience. A dog show comes around every year, family only once."
They had another pow-wow. The lady came back and said they were going to try heading down the road. "OK, I'm going to make it clear on the invoice that it was your decision to try and make it." I said. They paid the bill, went out back to have dinner and when I looked out again they were gone.
I hope they make it and that the dog show was worth it.
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