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Old 08-01-2022, 01:20 PM   #1
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CAT9 Fuel Knock

Hi. I have a 2008 Beaver Motor-home with a Cat9-400 engine. I bought it last year with about 48K miles. I had everything serviced prior to a trip last fall. The unit seemed to be running fine however it did not shift into overdrive until you reached 65 mph, and then would remain in overdrive if I dropped back to 60 mph. We hit some freezing temps in Tenn., then hot weather in Florida where my problems started. I starting losing power and stalled a few times. I drained the fuel separator in case there was condensation built up in the fuel. I made it to my destination, however the fuel separator was now allowing air into the system. I changed the fuel separator (without the drain, only one available). I filled the separator with diesel, but had a very difficult time starting (despite using the primer). I had a mechanic help me get it started, and we headed back home. We were losing power again and check engine light now started flashing on & off. I pulled into a large diesel mechanic that works on 18 wheelers. Based on the codes, they diagnose the issue as 1 bad injector. The garage was very busy with many trucks being worked on, and I was told it would be a couple months which I was okay to wait. They actually changed all the injectors now. The mechanic now says there is a 'Fuel Knocking’ when the engine is idling, but goes away as rpm's are increased. He has replaced and moved the injectors around. He says he has tried re-calibrating the injectors with no change. He has dropped the oil pan and says the engine looks like new. He says it drives fine but is nervous about releasing with the fuel knock, but is essentially at a loss what is causing the knock. I appreciate any direction you can provide. Thanks, Dennis.
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Old 08-01-2022, 01:24 PM   #2
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Sounds like two VERY different diagnoses:


Air in fuel.


Bad injector.


Not very likely that both would occur at the same time.


Clearly, eliminate the "air in fuel line" issue first.
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Old 08-01-2022, 03:15 PM   #3
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Thank you Brett.

My first action when I was losing power was to drain the fuel separator, thinking I had condensation or bad fuel. After still losing power, I noticed that the drain at the bottom of the fuel separator was allowing a small amount of air into the filter (I could see small air bubbles thru the viewing glass at the bottom). So I installed the new fuel separator. The new separator does not have the viewing cup at the bottom.

I ran for a couple hours and then starting losing power again. That's when I pulled into the diesel repair shop and was told of the bad injector.

The mechanic replaced the injectors, but say there is now a 'fuel knock' on that one cylinder when idling. It seems to run okay on the open road, but their concerns are with the fuel knock when at idle. Any ideas what can cause the fuel knock. They said the engine itself looks like new.

Thanks, Dennis.
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Old 08-05-2022, 07:11 PM   #4
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Check your fuel pressure. I have had to change the regulator twice now. Once for low pressure and once for high pressure. Should be about 80 lbs at fast idle.
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Old 08-05-2022, 07:27 PM   #5
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The fuel knock on the same cylinder everytime? Or did it move with the injector. To test for bubbles/air you can remove the return fuel line from the engine and place it into a bucket of fuel and watch for bubbles as the engine runs. If you do then you have a suction leak. You also said they recalibrated the injectors? Did you mean putting in the injector trim codes?.
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Old 08-19-2022, 11:34 AM   #6
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Thank you gpounder and Cat.

The motorhome is still at the shop. They said they got past the fuel knock, and that is was running great. The next day they went to move it, and it was running worse then when I brought it in.

My original problem ..... I ran fine for 3 days to Orlando Fla. We spent 4 days and had a great time at the campsite. The motorhome started fine but conked out 3 or 4 times. It seemed to then be running okay until Ft Meyers (my next stop). After refueling, it conked out again as I was leaving the station. With some help from the Loves mechanic, we got it started and off the road and back into the parking lot with the rest of the 18 Wheeler's where we spent the night. The next morning, it started like nothing ever happened and again seem to be running fine. We went from Ft Meyers to Key West. We parked in a campsite for 5 days. Family had a great time. I changed the fuel filter after noticing the air being sucked in from the drain plug. (The new filter does not have a drain.) After that, I could not get the engine to start after much priming and multiple tries. The mechanic came and was able to get it started. We left for home. I started having problems around Daytona where lights on the dashboard started flashing for the first time. That's when I found the diesel repair shop that I drove into for diagnostic. The computer said it was a bad injector.

So the injector was replaced, and fuel filters replaced. He said the fuel pressure was low and had replaced the pump as well. He is now saying he believes it to be the ECM. He has already replaced the harness. I see in other posts that the Ignition Solenoid could be bad.

Any thoughts are appreciated. Thank you, Dennis!
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Old 08-19-2022, 01:03 PM   #7
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Ok it is possible the ECM failed, but this is not how they normally manifest a problem. A normal failure is- no communication no start. Strange diagnostic code. Lose flt or fls settings. When ever you see this kind of jumping around troubleshooting they are guessing. And usually they are guessing with your money. Make sure you ask-- ok its the end but who pays if its not. Also find out right now-- what you are paying for the other repairs- and then you have ask yourself should you be paying for this. Its important you do this because it makes them a little less likely to throw things at the problem.

Questions-- when the mobile mechanic primed you engine. How did he do it? Did crank and bleed the system- did he pressurize the fuel tank? Or did he remove the fuel filter and fill it then reinstall?.
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Old 08-19-2022, 09:29 PM   #8
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The mechanic did not do much or take anything apart, it was pretty quick. I believe he may have used starting fluid. I was up front turning the key. The engine started right up, and things sounded normal. No roughness in the idle. He checked a few things and did not see any issues.
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Old 08-20-2022, 06:52 AM   #9
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Could the Ignition Solenoid could be bad, and now causing the engine to run rough? I've seen that problem in other posts.

I did have another issue where the chassis battery did not have enough charge to start the engine without help from the house batteries. Before I bought it last year, it was sitting for a few months not hooked up to line power. I had the batteries checked at the dealer when getting it fully serviced, and was told they were fine. But that problem happened a couple times. The mechanic in Key West also checked the batteries and said they were fine.

Is there a way to check the ECM before replacing it? I watched a few YouTubes but still not sure.

Thank you!
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Old 08-20-2022, 07:57 AM   #10
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My ECM went out but it was a total loss all at once while driving down the road.
After replacing it I took note to how close the exhaust pipe ran to the ECM, as well as the ECM was just above the muffler itself. When I got home I pulled the muffler and pipe off and wrapped the pipe with header wrap. Even though the muffler had a heat shield above, there was a tremendous amount of heat in that large restricted muffler. I also replaced the muffler with a less restrictive one and much smaller around, but still was quiet. The side of the engine that has the ECM bolted to it stays much cooler now as well as the floor over the engine is much cooler. Another advantage to the less restrictive muffler is the turbo spools up much quicker. I did much research on the ACERT C-9 and the muffler has no bearing on any ECM readings. The muffler I chose is a 5 inch in 5 inch out used by Cat on construction engines.
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Old 08-21-2022, 08:30 AM   #11
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Thank you Will. I will checkout insulating the muffler from the ECM once I'm back on the road.

I guess my question now is ..... Can the ECM fail intermittently? My 1st problem was leaving Orlando when after staying at the campground a few days, the engine stalled a number of times. I thought I might have picked up bad fuel and I drained the fuel separator. I believe this caused a 2nd problem as the drain plug was now allowing small air bubbles into the line. I installed a new fuel separator, but still had the issues of the previous posts.

The mechanic in Daytona hooked up the computer and noted one of the injectors was bad. Twice he said it was purring perfectly, but then it starting running worse then when I brought it in. He believes it's the ECM. I've also seen mention of the solenoid. Not sure if that could contribute to the issues.
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Old 08-22-2022, 02:30 PM   #12
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My best guess is YES there could be an intermittent problem. Maybe heat or cold situations could cause thermal expansion or contraction within the ECM or possibly within the pins and plug. Good luck with chasing it down.
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