Engine Room Discussions

[sote2006]

I have a CAT 3126B engine which most of the time works perfectly but occasionally and at seemingly random intervals, the revolutions drop back to idle speed for a couple of seconds before returning smoothly to the previously set cruising revolutions.

On average this occurs about once an hour but may happen twice in quick succession or not at all for several hours. According to the previous owner of the yacht this has happened in previous years (the engine was new in 2006) but it didn't do it at all last summer when I was also on board as skipper.

The engine has never stopped completely as a consequence so the problem is more disconcerting than alarming but in my experience these kind of problem do not tend to fix themselves so I am eager to get to the bottom of it.

Has anyone had a similar problem with this kind of engine or have any suggestions as to what might be causing this behaviour?

We have investigated the fuel supply which is the obvious first place to check but there are no problems there.

I look forward to any suggestions

[dave10010]

Fuel supply is not the first place to check. You are likely losing the throttle position signal, which would cause the engine speed to drop to idle. Check wiring from throttle position sensor to the engine ECM. TPS may be defective.

[sote2006]

I've had a good look at the wiring from the ECU and it all seems fine with no loose connections. Also tried unplugging the throttle sensor and cleaning the contacts but the problem remains the same

[GreenChief]

I had a similar problem in a 3512. Fuel filters showed normal with no trouble codes. Watching the computer when plugged in showed the momentary vacuum across my racor pot. Changed all fuel filters and the problem resolved. Of course a loose connection could cause the problem too.

[sote2006]

Fuel filters was our first suspicion. We checked the Racor  which certainly needed replacemert and thought this might resolve the issue. Sadly not.  We have since changed it again (although it appeared clean second time around) and the fuel filter on the engine itself at the same time but the problem remains the same.

I has been suggested to us by a CAT engineer in Turkey that the problem lies with the high pressure pump, and we will explore this avenue.

We're told that we simply need to replace the pump but as it is a high  cost item I wanted to get other people's experience  before spending the money, particularly as there are no errors logged in the ECU.

[leebenn]

Check the connection between your ECM and the control system you are using. If it is a three wire connection(one for signal and two for twelve volt power) see if the two power wires are connected to switched power or if they go directly to the 72? pin connector supplied with the Cat engine.  If not the latter, make it so. The Cat ECM and the sensor in the control system have to operate at the same voltage for the modulated pulse signals to be properly interpreted by the CAT ECM. Sometimes switched power has voltage ups and downs as various systems in the boat (like bilge pumps, refrigerator interfaces, etc.) or even your charging system fluctuate the line power at the ignition switch. It took me two seasons and the fine people at Glendenning Controls to get to the bottom of this one. It took ten minutes to fix it.  Oddly, it only occured on one of my two engines. Good luck! 

[Zifer]

I have a similar problem on 2002 3126B. A bad Throttle Position Signal causes the engines to go to Idle. My problem arises when the GenSet (started after major repair to windings) is running and AC connected to GENSET. Electronic noise is causing the throttle position signal to go bad. I have not solved problem yet so I don't have genset on when cruising. When genset is at low power output noise is constant and throttle does not respond, when load is increased problem becomes intermittent. In your case the noise could be coming from any electronic devise close to the throttle position sensor. Use ET to see diagnostic code, you must catch it when it happens as for an unknown reason it is not stored as an event.

[bigscoot11]

We are having the same issues with a C15 engine on our boat. New ECM did not fix nor did new harness and connectors. CAT cannot find any fault codes or diagnostics. We are stumped and not happy campers.