How to repair a 1984 Mercury 150 Black Max that won’t idle, and oil injection removal

[DB150]

I’m a machinist with no prior experience trouble shooting my Mercury outboard because it worked great for decades. I couldn’t find anyone willing to work on it, with the help of S&F contributors Rock, Rgsauger, and Onpad, I fixed it myself. Many thanks to those guys. Now it runs better than ever. My diagnosis and repair process is described below.

1. Problem: The motor would start and fast idle, but when I slowed down to shift into gear, it died.

2. Ultimate solution: replaced the stator and switch boxes, details described below.

3. More detail: Mercury, 1984, 150 ELPTO model: 1150724, serial: 6455388

4. The motor had been working fine and then suddenly, like flipping a switch, would not idle. If I pushed the release to advance the throttle without being in gear, it would start and fast idle, but when I slowed down to shift into gear, the motor stopped.

5. Hauled it home and took the cover off. It looked complicated so I started with the easy stuff.

6. Bought a Seloc Tune-up and Repair Manual for Mercs 1965-1986 because that is all I could find for my 37-year-old motor. The manual covers all 6-in-line and V6 from 1965-1986, I started reading and stapled together all the sections that don’t apply to my motor. Read it about 3 times before it started to make sense.

7. I found rubber dirt on the screen in the fuel pump so I decided to service the fuel pump and carburetors. I installed a fuel pump kit and carburetor kit (new gaskets and o-rings all around). I found the Seloc manual useful and accurate for servicing the fuel pump and cleaning the carburetors. Some orifices in the carbs were plugged with rubber bits. I also replaced the in-line fuel squeeze bulb which was shedding rubber into my fuel system. I started the motor, it still would not idle.

8. At this point I turned to the internet and found Scream and Fly. I submitted a post, Rock recommended a compression test to begin diagnosis; rgsauger weighed in too and concurred with the compression test and correctly guessed that I would need a new stator and switch boxes.

9. Obtained a compression test kit (BlueDriver Compression Tester Kit). I started the motor and let it warm up, then moved into the garage and checked compression with all of the plugs removed. It’s a V-6 with cylinders and compression as shown below if you are standing behind the boat and face the motor:
Left Right
cylinder/psi cylinder/psi
2/109 1/112
4/117 3/117
6/117 5/117


10. Compression results suggested that my motor is old but not dead. Compression of all cylinders is supposed to be within 10% high to low. My range was 109 to 117 so the motor passed.

11. Since all the plugs were out I used a spark tester (Lisle Spark Tester) to check each plug. This was the diagnostic key to solving my problem: I found NO SPARK on cylinders 1, 3, and 5. Spark was present on cylinders 2, 4, and 6. No spark on one side of the motor (1, 3, 5) suggests a bad switch box.

12. Visual inspection identified very bad (crumbling) insulation on two yellow wires that go from the stator to a square (1-1/2" x 1-1/2" ) potted circuit board (rectifier) that is just above the switch boxes. I observed bare, corroded wires in close proximity to each other.

13. Went back to S&F with my results, Rock suggested that the yellow wires from the stator shorted and wrecked a switch box. He offered that if the wires aren’t too bad they can be re-insulated and used. Mine looked really bad so I opted to replace the stator because it was at least 20 years old and I didn’t understand how to test it thoroughly.

14. Ordered parts: there are two switch boxes and they are sold separately, BUT THEY NEED TO BE REPLACED IN PAIRS so that they match (I don’t know why). I bought two from Sierra and an OEM stator from Boats.net. The stator took months on back order, but finally came through.

15. While I was waiting, I removed the oil injection system. It is prone to failure on motors of my vintage because of an internal plastic gear. Here is the step-by-step:
a. Obtain a block off kit. It should have a plug that is about 6" long like the one pictured below. It requires an o-ring (.489 ID x .070 dia) and two bolts. Do not install just a cover plate because there is a brass bushing inside the motor that is supported by the end of the plug.
b. Remove the oil tank in the boat and the lines to the motor. Plug the port to the crankcase that is connected to the line. It requires a brass 1/4-18 NPT pipe plug. Or do what I did: remove the existing fitting, squeeze the 1/4" end shut with pliers and seal it with a drop of solder, then re-install it.
c. Drain oil from reservoir on motor.
d. Remove the bracket and studs in front of the reservoir and remove the reservoir.
e. Replace the bracket and studs.
f. Remove the reservoir cap and its wires.
g. Remove the Warning Module. It will have three wires remaining after the injection pump is removed (brown, purple, and green), they are not needed. The powerhead heat sensor will continue to work without them, though when you turn the ignition key the system will no longer beep to confirm it is functioning. To test the heat sensor see below.
h. Remove the 2"-long linkage connecting the oil injection pump to the carburetors.
i. Remove the two bolts on the injection pump and remove it.
j. Remove the slotted bearing to reveal the end of the gear shaft.
k. Remove the gear shaft. DO NOT turn the flywheel while the gear shaft is removed. Continue installing the block-off plug.
l. Install the plug: the plug should have an o-ring in its groove (.489 ID x .070 dia) . Install the plug so that the flat side of the shaft faces the crank (inside). It will go in easily until you reach the o-ring, then it will be snug, but will go in with a firm tap of the heel of your hand. Push it all the way in until it seats on the face and install the two mounting bolts. You can use the original bolts, but you will need to shorten the long one for clearance.
m. Install a new piece of fuel line to connect the fuel hook-up to the fuel pump (1/4" ID fuel line).
n. Mix the gas and oil in the fuel tank (50:1).
o. Done


16. Installation of new parts: Rock advised that switch boxes are color coded so as long as you are not color blind it is an easy project. Make sure the yellow-banded trigger, stator, and coil wires on same box. Before installation, I took pictures, then removed just one wire at a time and installed it on the new switch boxes. Easy.

17. During the course of installing the switch boxes, I had to replace the stator. The Seloc manual was very useful for flywheel and stator removal. The flywheel nut was tighter than Dick’s hatband, it took a half-inch breaker bar and a secured strap to break the nut loose. The manual says 100 ft/lbs torque, I guess it was all of that. Another S&F contributor, Onpad, helped me out and recommended a flywheel puller (Zinger Mercury Mariner Outboard Lift,Flywheel Puller Removal Tool 91-849154T1,Lift Ring 91-90455-1). The puller worked great, put a little dab of grease on the mating surfaces to reduce friction on the parts that rub. Once disassembled, I cleaned the surface that the trigger rests on (it needs to be clean and dry), and reinstalled the stator. Use a drop of blue Loctite on the stator bolts and make sure the trigger moves freely after assembly.

18. Note: the yellow wires from the stator proved to be rotten all the way. If you see bad insulation near the rectifier, you should assume that you need to remove the stator and inspect all the wiring.

19. When the wiring is compete, re-install the flywheel and clean the lettering on it to aid the ignition-timing step. Re-install the flywheel cover.

20. Confirm timing: the Seloc manual was useful and detailed. Onpad and rgsauger also helped. In a nutshell, remove the spark plugs, confirm that the timer pointer is set correctly by finding top dead center with a gauge or caliper, then turn the flywheel counter-clockwise past .462, and then back to .462. The .462 value on the flywheel should match the pointer. Once the location of the timer pointer is confirmed, proceed to checking timing. Confirm no fuel in the motor and unhook the fuel line. Remove all sparks except #1, connect its lead. Ground the leads of the other five spark plugs to the housing. Confirm the battery is fully charged. If there is a spark advance module or idle stabilizer, disconnect the white/black wire from the switch box and insulate it. Next, I used a plastic wire tie to hold the timing arm all the way back so that the maximum spark advance screw was against its stop. Spin the motor over while observing the flywheel with the timing light. Read the value. Adjust as necessary to achieve the correct maximum timing based on the specs for your motor (mine was correct at 18 degrees). Tighten everything up.

21. Secure loose wires with wire ties the same way it was when you started. Pressurize the fuel system and check for leaks. Put the muffs on and give it a try.

22. Done.

23. To test heat sensor: The heat sensor sits in a blind hole. When it gets hot enough, it makes an internal mechanical connection that completes the circuit between the engine block that it is mounted in, and the wire it is connected to. To test the sensor, turn the power on and use a piece of wire to connect the engine block to the screw to which the sensor is attached. The alarm should sound continuously. The oil warning module is not needed for it to work.

24. My motor had an idle stabilizer. It had bad insulation and I was uncertain if it was working. After the procedure described above, the motor worked fine without it, so I removed it permanently.

25. Last words: I am not a good enough outboard mechanic to confidently advise people on Scream & Fly about their motors, so I will only be on occasionally. If you have a questions about what I did, contact me by sending an email. I’ll get back to you ASAP.

Pictured below: spark tester, compression kit, bad insulation on stator, block off plug, heat sensor, fly wheel puller.

[rgsauger]

Dan,
Wow! You are meticulous if you are anything! sounds like you went from one end to the other and now have a great motor. This is exactly what this forum and community is all about. Nobody knows it all but we all know some and when you put it all together, it turns out pretty good usually. Congratulations again!

Rob

ps. Your compression looks perfect for a stock motor. If you have not already, look up how to do a decarbon treatment on the motor. Basically, A can or two of seafoam, pull the spark plugs, hose each cylinder thoroughly, tilt the motor up to your highest trim setting, put the plugs back in and let it sit for an hour. When you crank it up, it will swear it is blown. Your compression will probably come up a bit

[Darkcloud]

DB150,
I own a 1985 150. Love my girl. Just read your post entirely and learnt about testing overtemp alarm. Mine has had an agricultural attempt at removing oil injection which I’m rectifying and I don’t know if my alarm works as I never hear it. Good way to test.
thanks for the post.

[pcrussell50]

and I don’t know if my alarm works as I never hear it. Good way to test.
thanks for the post.

I need to get on this myself. My horn works. But I can’t get the alarm to sound when I heat the button. I bought a used one off ebay and same thing. Neither of them turn on the horn when heated (I use a candle flame gently).

-Peter

[Darkcloud]

Yeah, I never even thought to check. I’ll short out sender first to check if alarm works. If not I can start looking at that.
Pete

[paradak]

Once the alarm portion of the overtemp system has been verified per above with grounding the wire, you can test the sensor by removing it from the cylinder head and heating it with a heat gun to ensure proper sensor operation. You can estimate temperature trigger point with an IR thermometer if available. My boat (1985 2.4L) did not have the audible alarm installed nor did it have a temperature gauge when I bought it. It did have a water pressure gauge but the tubing was bad so I fixed that first. I used the above technique to ensure the system is working properly (once I installed an alarm under the dash). I also added the sensor to the port head to drive a temperature gauge so I have both temperature gauge and overheat alarm. Gives a lot of peace of mind monitoring the temp gauge and having the alarm verified.