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  1999 Mercury OptiMax 200 Has Poor Starting and Idle

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Author Topic:   1999 Mercury OptiMax 200 Has Poor Starting and Idle
hais7474 posted 08-11-2015 08:23 PM ET (US)   Profile for hais7474   Send Email to hais7474  
I am hoping I can reach out to some of you vintage OptiMax experts out there. I have recently been having problems with my [1999 Mercury] OptiMax 200-HP outboard engine on my 21 Conquest. [The engine] has been very difficult to start and keep running at idle. [The 1999 Mercury OptiMax 200-HP outboard engine] will catch for a few seconds then die out. Sometimes over 10 starts [are necessary] to keep [the 1999 Mercury OptiMax 200-HP outboard engine] running, and I have to quickly put [the shift into] gear for [the engine] to stay running. [The 1999 Mercury OptiMax 200-HP outboard engine] also surges a bit between around 700 to 1,100-RPM. [An engine speed greater than] 1,100 RPM is great, no [problems with stalling].

I have the shop manual and followed the following recommendations. None of these corrected the [stalling and idle problems].

--replaced tracker valve diaphragm

--replaced Fuel Regulator Diaphragm

--fuel pressure and air pressure within specification

--replaced primer bulb, and

--checked fuel pumps

I finally took [the 1999 Mercury OptiMax 200-HP outboard engine] to a certified Mercury shop. The [certified Mercury shop] noticed a lot of carbon build-up and the [peak cylinder pressure under test] was only in [about 85-PSI]. Then the [certified Mercury shop] [performed a procedure to remove carbon] with [a solvent called] Powertune. They were able to increase the [peak cylinder pressured under test to] around 90-PSI on all cylinders. The [certified Mercury shop] also replaced the starter because they said it was going bad and dropping the voltage down to 5-Volts during startup. And they replaced an ignition coil that was suspect.

When I got back [the 1999 Mercury OptiMax 200-HP outboard engine], it did seem to run better at first. But it quickly went back to the original [behaviors, that is, it was hard to start and stalled at idle speeds]. I started playing around a little bit more by checking the temperature sensors and seeing how the engine would react to disconnecting one at a time. Disconnecting any of the sensors really did not made a difference in how [the engine] ran, except for the starboard cylinder head temperature sensor. When I disconnected [the starboard cylinder head temperature sensor] the engine picked up about 100-RPM and ran beautifully. I turned it off and restarted it and it fired up perfectly! I took an ohm meter to the sensor leads and it appears the sensor is still working correctly based on the resistance graph in my engine manual.

So I am stumped! Based on the wiring diagram of the engine, the sensor is connected to the ECM and the Throttle Position sensor. Could there be an [malfunction] with my ECM or TPS that is interpreting the signal from the temp sensor incorrectly, and it's messing up the fuel or injector mapping?

I am loosing my mind over trying to fix this problem! I appreciate an feedback.

Mambo Minnow posted 08-13-2015 11:08 PM ET (US)     Profile for Mambo Minnow  Send Email to Mambo Minnow     
I have the same boat and motor. My OptiMax engine has run very strongly. My major failure to date is a tilt/trim that leaks down at rest. Also, a lot of ghost alarms for low oil probably due to a bad magnet in the under cowl tank.

I did have some surging last September and suspected E10 fuel to blame. I took it to the local Mercury mechanic and he replaced an air temp sensor that he diagnosed as the faulty part. I don't have part no. with me, but it was a rather simple black plastic disk with an electrical lead attached that came from the air intake system.

I added a RACOR-brand filter between the tank and motor as a future precaution. Whaler did not install one other than the one that is under the cowl on the motor.

The DDT runs show a lot of Throttle Position Sensor error codes. Fortunately boat does not go to guardian mode. However, I am told these early versions have more moving parts to the TPS than later models.

Wish I knew the system better, but from the sound of things you may want to focus some attention with that TPS sensor and check out that air temp sensor.

Hais747 posted 08-14-2015 02:21 PM ET (US)     Profile for Hais747  Send Email to Hais747     
Thanks mambo. I ordered new [throttle position] sensors and they are arriving next week. I hope [replacement of the throttle position sensors] will do the trick. I've measured all the temp sensors and they seem to be in spec per my engine manual. If the TPS doesn't fix it I'll look at changing the sensors next. I'll update next week.
Mambo Minnow posted 08-14-2015 08:00 PM ET (US)     Profile for Mambo Minnow  Send Email to Mambo Minnow     
Good luck, let us fellow owners know how it all works out. It would be good to have in the archives here for future reference.
RevengeFamily posted 08-19-2015 06:06 AM ET (US)     Profile for RevengeFamily  Send Email to RevengeFamily     
I have twin 1999 Mercury OptiMax 225-HP engines on my 1999 280 Conquest. These beasts are incredibly finicky regarding cold starts and idling. One thing I did realize early--I've had this boat for six years--is that out fishing, I can idle along all day long if I want; but some how, they know when we are back at the marina. They can and will stall at the most inopportune time. And I'm not the only one to notice this. If, when approaching my slip, there is no wind or tidal flow, odds are they will idle perfectly. But if there is wind or tidal flow and I need them to be at their idling best, one of them will stall making docking absolutely miserable. I'm not one-hundred-percent sure, but I suspect that the fuel in the fuel rails begins to vaporize. The vaporized fuel can not be pumped into the fuel injectors so the engine stalls. They will start up again, but it breaks my focus on getting into the slip.

I have been doing my own maintenance on these motors since I bought the boat. This includes three complete rebuilds on engine blocks. Regarding hard starts and idling, healthy spark plugs and 95-PSI cylinder pressure are a must; all temperature senders need to be in working order; fuel rail pressure needs to be up to snuff; and of course (as hais7474 mentioned) tracker and fuel regulator diaphragms need to be in good working order--Norm

hais7474 posted 08-21-2015 09:35 AM ET (US)     Profile for hais7474  Send Email to hais7474     
Great info Norm!

Update on my progress. I replaced the Throttle Position Sensors and no improvement was noticed. I decided to de-carb the engine again with powertune. I filled the water separater with powertune, ran it for a minute or two, removed the plugs and filled each cylinder with it, rotated the flywheel by hand a few times and let it sit overnight. Yesterday I started it and a ton of smoke came out. It seemed to run a little better at idle after. So I did the same procedure again last night and will test her on the water today. The shop I brought it to supposedly de-carb'd it but it does not appear they did a thorough enough job.
Norm - you may be spot on about having at least 95psi compression in each cylinder. I was just short of that which may have led to the poor idle performance. Hopefully this last de-carb will bring mine back to life.

hais7474 posted 08-24-2015 10:58 AM ET (US)     Profile for hais7474  Send Email to hais7474     
Well I gave the engine a second application of Mercury Powertune decarb per the method I described in my previous post and took her on the water over the weekend. She starts up and idles much better. It still takes a couple of times to start it up but I think that is just the norm for a 16 year old Optimax. I still have some surging between 700-1000 RPM's so not quite sure what is causing that, but at least the idling issue is taken care of. I also noticed I picked up a few hundred RPM's at WOT.

So as has been discussed many times on here before by other folks, fuel additives and decarbing seem to be very important in keeping these engines running smooth. I will be using Quickleen now every time I fuel up and plan to decarb once or twice a year depending on my usage.

Over the winter I will probably send my rails out to get the injectors cleaned as well since I don't think they have ever been serviced.

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