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Tan, MerCruiser Certified Technician
Category: Boat
Satisfied Customers: 11052
Experience:  40 yrs. experience with complete engine repairs both inboard/outboard and MerCruiser Certified Technician.
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Having problem with maintain rpms at wot. motor is an erude

Customer Question

Having problem with maintain rpms at wot. motor is an erude 1990 150hp with vro disconnected. fuel vacuum switch bypassed to determine cause of horn sounding an intermitten beep. new powerpack and water pump with no overheating issues. Faulty heat sensors , what ts next?
Submitted: 1 year ago.
Category: Boat
Expert:  Tan replied 1 year ago.

Hello. My name is Tan.

Could I get the engine model number please?

When the engine starts to surge have you tried squeezing the primer bulb?

Have you disconnected the oil tank alarm?

Customer: replied 1 year ago.
E150stlesb, primer bulb is new and I don't think it is fuel issue. vro completely disconnected. tank removed. Horn was. . sounding at 2500 rpms. Disconnected fuel switch , horn stopped. Went to 4000 rpms held briefly then dropped back to 3000 and seemed port bank cylinders cut out and was running on 3. Could starboard heat sensor be sending false signal to powerpack to shut port side down?
Expert:  Tan replied 1 year ago.

The vacuum fuel switch when it sounds it is letting you know you have a fuel restriction.

The temp switches will only put engine into SLOW mode reducing the rpms not kill cylinders. How are you determining that you are loosing spark on 3 cylinders?

This could be a timer base or power pack issue.

Customer: replied 1 year ago.
Also when I start motor on muffs it only fires on starboard cylinders until I rev a few times then all are firing at Idle.
Expert:  Tan replied 1 year ago.

When the engine isn't firing on those cylinders do you have no spark or no fuel?

How are you checking for not firing?

Customer: replied 1 year ago.
Using spark tester with visual confirmation. Carbs rebuilt twice in last four years, stator 2 yrs old. power pack 1 week old .New, fuel lines from gas tank straight to fuel pump, no separator in line for testing purposes. When in SLOW mode what do you think max rpms would be?
Expert:  Tan replied 1 year ago.

2500 rpms. I would disconnect the temp switches on both cylinder heads and go run the boat if you are sure it isn't over heating.

Customer: replied 1 year ago.
both sensors or just the starboard with the 2 wires.
Expert:  Tan replied 1 year ago.

The sensor with the 2 wires is for quick start only on initial start up. The other 2 sensors with the tan wire os for temp. So disconnect both of those. Be sure you engine isn't over heating.

Customer: replied 1 year ago.
Will do , thanks for the chat. If motor runs fine just replace sensors?
Expert:  Tan replied 1 year ago.


Post back if you need further assistance.

Thank you...Tan