Engine misfires in mid range

[Flytrade]

My starboard 6.2L Mercruiser just started misfiring in mid range. 2500 rpm and below, and 4000 and above, it works fine. However, between those rpms it misfires. By that I mean I get a few "beeps" indicating a problem, and the rpm decreases, and the speedometer goes to "0". When I activate the parallel switch, it seems to cure the condition. But then, 30 seconds later it happens again. I then got a fault readout on my tach which read - fault, TPS failure. I then had the throttle position sensor replaced, but that did not fix the problem.
Here is something that may bear on the problem. When I started the engine I had the ignition key to the on position, but accidentally had the circuit breaker for that engine in the off position. I therefore had no power to the start switch on the dash, and thought I had a bad battery, or left a switch on and drained the battery. I then used the parallel switch and the engine started normally. However, when I let go of the parallel switch, the engine died. I tried this a couple of times with the same result. I then found the circuit breaker in the off position, turned it on, and the engine started and ran as normal - that is until I got on plane with the engine at about 3500 rpm in cruise. Then the above problem started.
So, since this problem started right after I made the mistake of trying to start the engine with the circuit breaker in the off position, could this be the cause of my problem?
Since changing the throttle position sensor did not fix the problem, do you have any idea what I should be looking at to fix the problem. My mechanic does not know.
Thanks in advance.

 

[scoflaw]

How's your battery and charging system?

 

[Flytrade]

How's your battery and charging system?

My batteries are less than a year old and the charging system for both engines is right at 14 volts.

 

[scoflaw]

Need a mechanic with the tool to read that ecm.

 

[Flytrade]

I am out on my boat now, using my iPad. I still have the same problem, but now I’m getting a readout that says “Lost Engine Com”

 

[Flytrade]

Here is an update.
My mobile mechanic replaced the Throttle Position Sensor with no change.
I then took the boat to my local MarineMax dealer, and so far he has been trying to find the problem for 2 days with no success. Yesterday the service writer said he has his best mechanic on the problem, and they think it's a loose wire. However, I don't think that can be the problem because the engine missing is rpm dependent. Unter 2,000 rpm the engine works fine.
I also called Mercrusier and talked to their tech rep yesterday, and he does not have a clue what the problem could be.
Any thoughts you may have will be helpful.

 

[scoflaw]

Quite possible it has a loose wire or corrosion in the wire. "lost engine communication" sounds like a bad connection to me . That would be the least expensive thing to look for.
You don't want it to be a bad ecm but possible. $$$$$

 

[Flytrade]

Quite possible it has a loose wire or corrosion in the wire. "lost engine communication" sounds like a bad connection to me . That would be the least expensive thing to look for.
You don't want it to be a bad ecm but possible. $$$$$

MarineMax is looking into the "loose wire" possibility, but my thought is this problem is rpm dependent. The engine works fine above 2,000 rpm. However, we'll see what they find. So far not a thing, and that is very disturbing since they are the experts for this boat and engine.

 

[Flytrade]

Marine Max just called and said they found the problem. It seems a communication box - cost $70 - needs to be replaced. The expensive part of this is that it took them 7 hours to find the problem. But, I will be glad to have the boat behind the house again.

 

[scoflaw]

Please post back with more detail on this box. Never heard of it. They might be blowing smoke

 

[Flytrade]

Please post back with more detail on this box. Never heard of it. They might be blowing smoke

I'll let you know the name and part number when I get the bill.

 

[Flytrade]

Here is an update to the above engine missing problem.
MarineMax St Pete has now had my boat for 3 weeks, and they are no closer to fixing the problem. Two weeks ago, after a tech spent 7 hours, they said they found the problem part. It was ordered, but because of the holiday it took over a week to arrive. They then found this part did not fix the problem.
I have written both the service manager and the general manager, and even asked the general manager to call me, but have not received a response.
As far as I can tell we are back to square one, where we were 3 weeks ago, and this problem is obviously above their ability to analyze.
Where do I go from here? Wait another 3 weeks? Call/write MarineMax, Mercruiser, Sea Ray or Brunswick?
Your thoughts would be appreciated.

 

[Bt Doctur]

I then used the parallel switch and the engine started normally. However, when I let go of the parallel switch, the engine died. I tried this a couple of times with

You do not need a parallel swith to start an engine , Having the engine die means your not charging or not connected to a battery or both.
I suspect both engines are connected together electrically.
With 1 battery pos post disconnected see what works and what doesnt, disconect that battery and reconnect the other battery and see what works and what dosent.
Report the results.
If they are connected electrically you MUST separate the systems
2 engines require 2 battery switches and at least 1 battery EACH . The parallel switch is just a solenoid to connect both batteries together to start atleast 1 engine

 

[Bt Doctur]

My batteries are less than a year old and the charging system for both engines is right at 14 volts.

Maybe ,if they are true separate electrical systems ,connected together your only seeing 1 alt reading and ECM`s and sensors are electtically connected together

 

[Flytrade]

I then used the parallel switch and the engine started normally. However, when I let go of the parallel switch, the engine died. I tried this a couple of times with

You do not need a parallel swith to start an engine , Having the engine die means your not charging or not connected to a battery or both.
I suspect both engines are connected together electrically.
With 1 battery pos post disconnected see what works and what doesnt, disconect that battery and reconnect the other battery and see what works and what dosent.
Report the results.
If they are connected electrically you MUST separate the systems
2 engines require 2 battery switches and at least 1 battery EACH . The parallel switch is just a solenoid to connect both batteries together to start atleast 1 engine

Thank you for your thoughts, but both engines start and perform normally as long as I keep the rpm below 2000. I have two batteries for each engine, and the charging system is working as it should.

 

[Flytrade]

My batteries are less than a year old and the charging system for both engines is right at 14 volts.

Maybe ,if they are true separate electrical systems ,connected together your only seeing 1 alt reading and ECM`s and sensors are electtically connected together

You may well be right, but should not MarineMax be able to check this?

 

[Bt Doctur]

stuff like this is not mentioned in their training , it`s taught by experience.So, how many batteries do you have and how many and what type of battery switches eg: on - of , 1- both(all)- 2
you called them with a trouble and thats what their looking at. What you described.
Thats the direction they took.
a quick test. turn on 1 ign switch and see what gauges come alive .Then turn that one of and turn the other one on. see what comes alive.
Also try the start position
If you see voltage on both gauges with only 1 ign switch on, the motors are electrically joined
left a nj cell

 

[Flytrade]

Here is an update now that we are 3 1/2 weeks into this problem. MarineMax is coordinating with Mercruiser Tech Support, and we are no closer to solving this than we were on day one. I wrote an email to MarineMax CEO, so we'll see what he has to say - if he even answers. I say that because I have written several emails to MarineMax St Pete General Manager, as well as leaving him a voicemail, and I have yet to hear from him.

 

[El Capitan]

So this only occurs during “load”.
Only way to isolate and find problem is to test/diagnose under said “load”.

I can’t tell you how many times I had something “fixed” on dry land that when put in the water was “unfixed”.

 

[Flytrade]

OK - I have some good news - finally. I got a call from Mercury Marine Tech Support and they said the problem is in the computer. For some reason the computer is shutting down periodically, causing the engine to quit for a second or so, and during that time the speedometer is also losing power causing it to go to zero as well. To test their theory they had MarineMax sway computers between the two engines, and the problem followed the computer to the port engine. Therefore, they are 99% certain a new computer ($2K) will fix the problem.
However, I told them of my experience with MarineMax changing out parts, only to find that is not the fix. Tech Support they are so confident this will fix the problem they want me/MarineMax to order a new computer, and if that does not fix the problem then the new computer is on Tech Support.
I don't think I can ask for better than that. So, hopefully I'll be back on the water in a few days.