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Cat Man
Cat Man, Cat Engine Expert
Category: Boat
Satisfied Customers: 8679
Experience:  23 years Caterpillar Engine Experience
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I have a marine 2002 3126 TA 420 hp mechanical 1zJ02113 about

Customer Question

I have a marine 2002 3126 TA 420 hp mechanical 1zJ02113 about 1850 hrs. After coming off cruise the idle surges +/-25 rpm when in neutral. In gear surging stops. Raising rpm top 1000 it also stops. CAT Manual says use buffer screw to raise rpm 10 above the idle stop setting. Other diesel boards say raise it 25-50 rpm with the buffer screw. I've tried that last time 680 with idle stop final 720rpm with buffer screw. Got a little better but still have the problem. What's your advice?
Submitted: 1 year ago.
Category: Boat
Expert:  Cat Man replied 1 year ago.
Thanks for asking your question. I will try and help you out.
There are two things I have seen cause this.
The first and most common is the engine has a worn injector control rack. It has copper bushings in the control rack. Over time when they wear, it will cause the engine to surge at different times.
Second is a worn governor.
I would try having the control rack replaced first. If the engine continues to surge. I would then suspect the governor.
Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 1 year ago.
What is involved in replacing those parts? Can you give me a ball park labor time estimate and parts for each repair. Also does it harm anything to just live with this surging? It stops after about 2 minutes or if it's in gear. Also you did not comment on trying to adjust the idle screws. Can adjusting them differenty fix this or compensate for the other worn parts? This isn't a huge problem and the motor runs fine I'm trying to decide what if anything I want to do about it.
Customer: replied 1 year ago.
As I said it is only doing this at idle AND in neutral and dampens out completely on its own in about 2 minutes. Would a worn injector control rack have this symptom or wouldn't it surge all the time?
Expert:  Cat Man replied 1 year ago.
Sure. To replace the control rack, all of the rocker arms will need to be removed. The rack cost about 350 dollars. It takes about 8 hours to replace.
The governor is not cheap. They run around 1200 dollars plus install. It takes about 6 hours to install.
It does not hurt anything to have the engine do this. I have seen a control rack adjustment cause a surge but only after someone has been in there and misadjusted it. If it has not been touched recently. I normally find it to be worn.
Not common for an injector to cause this. They normally miss or knock when they are the problem.
Please reply back if you need more help or have more questions. I am happy to assist.
Customer: replied 1 year ago.
What about trying to adjust the idle stop and buffer screws to stop this? Can you comment on how that's been done in your experience as far as how many additional RPMs to add with the buffer screw. The manual says add 10 rpm with the buffer screw the diesil board says add 25-50 with it. Also what is your recommended idle rpm? I've seen 700-750 is this correct.
Expert:  Cat Man replied 1 year ago.
Sure. I have only had success adjusting the low idle screw.
It's acceptable to idle between 650-750 RPMs. Cat recommends setting it to the lowest possible with no surging to save fuel.
Let me know if you need more help. I'm happy to assist.
Customer: replied 1 year ago.
So in this situation how would you adjust the low idle screws to try and sto this surging?
Expert:  Cat Man replied 1 year ago.
I would try going down to 650 first. If the problem continues. I would adjust up 25 RPMs at a time until I get to 750. You could find a good spot in that range to stop the surge. If you do not. I must suspect the rack or governor.
Customer: replied 1 year ago.
Are you familiar with the 2 low idle adjustment screws on this engine? I only ask because there is specific guidance in the manual that states one screw brings idle up to a preliminary rpm the second screw, the "buffer" screw brings it to its final setting. I'm looking for advice on what combination will have the best chance at stopping the surge.
Customer: replied 1 year ago.
Are you saying I should back the buffer screw all the way out ( so it adds no rpm) and just use the low idle stop screw to bring up rpm?
Expert:  Cat Man replied 1 year ago.
I see. I have not adjusted many of the two screw governors like you have. I have only used the same manual that you have. Sorry. I say it's a trial an error adjustment. No adjustment will hurt the engine and each engine is different. What's good for one engine may not be for another engine.
Customer: replied 1 year ago.
Can you ask someone in your field who has worked on this specific engine with the 2 screws and give me follow up advice? With the two idle setting screws there are too many combinations for me to go through with trial and error. Others who have worked on this mechanical 3126 may have seen this in the field and know more about it. This 2 screw adjustment problem was my original question.
Expert:  Cat Man replied 1 year ago.
Sure no problem. I will opt out and see if another expert can help.
Customer: replied 1 year ago.
Ok I'll wait for someone else to respond.
Expert:  Cat Man replied 1 year ago.
10-4.

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