1999 Mercruiser 350MAG Issues

Just to give you a bit more info, I did a TPS sync test this weekend. Also, voltages are equivalent between the two sensors. The one thing I didn't do is measure the deflection of the throttle valves to verify. I've included screen shots of what they looked like at a steady state cruise. From a layman's eyes, it just seems like the SB engine is lazy. When throttles are sync'd (versus adjusting throttles to sync RPMs), the numbers between the two engines seem pretty similar. The exception to that is the RPMs and temperature. Notice the almost 6 degrees C difference? Is that enough to have this affect? The thermostat was changed in the SB engine. Is it possible a 160 degree stat was installed instead of a 170 (assuming that is an option)? I wouldn't be concerned about a 200 RPM difference at WOT but it seems notable that there is a 200 RPM difference at cruise RPM.

I am hoping to find the time to at least pull the plugs and look at them. I would like to do a compression test too but I'm not sure I'll have a helper.

Thanks.

Jon
 
Last edited:
I just wanted to leave a follow-up. I was getting nowhere with this problem and decided to swap the coils between the two engines. In the process, I found a lot of goop on the SB engine's coil. It didn't appear to be contact greese...at least none that I've seen. I cleaned both coils along with the actual connectors and swapped them.

The SB engine is now behaving better. It's WOT is still 300-400 RPM different than the port and still requires more throttle to match RPMs with the port, but it does spool up and doesn't flutter. This is verified by the GPH comparison at cruise between the two engines. This is the behavior I saw when I first sea trialed the boat and the dealer convinced me it was related to the steaming (which made sense...water does't burn very well).

With the fact that I've had the props reconditioned, could something else in the drive train be causing drag such that it would more energy to turn the same RPMs? One thing I have noted in the past month is that at low RPMs, the two transmissions sound different. The port sounds smoother...less gear noise. Both have good clean fluid in them and are at the proper levels.

Any thoughts?

Thanks very much.

Jon
 
Some coils are oil filled. If you seem to have 'goop' around the coil, I would suspect from your description that the oil is leaking out. A new coil may be your ticket.

Doug
 
V-drives? You have one left and one right hand prop, correct? Then one tranny has an extra gear (the reverse gear) engaged. That could be your noise... but I wouldn't expect it to have a big enough of an efficiency loss to cause your power issue... but I'm not an expert, just an engineer.
 
That's a very good point. I never thought of that. The sound isn't terribly different, but noticeable. Being that the fluids are clean, smell fine, and are at the correct level (and have never needed additional fluid), I'll chaulk the difference up to that. So I'll keep looking elsewhere.

One person I spoke with suggested a possible intake leak. Could that cause the need to use more throttle to get the same RPM's? How would the MAP sensor react to an intake leak?

Thanks.

Jon
 
That's a very good point. I never thought of that. The sound isn't terribly different, but noticeable. Being that the fluids are clean, smell fine, and are at the correct level (and have never needed additional fluid), I'll chaulk the difference up to that. So I'll keep looking elsewhere.

One person I spoke with suggested a possible intake leak. Could that cause the need to use more throttle to get the same RPM's? How would the MAP sensor react to an intake leak?

Thanks.

Jon

Did you look into a possible bad coil?

Doug
 
Incredible! I feel for ya bro. The first issue I see is:hit or miss troubleshooting. Please,don't take offense to that statement. It is not intended to be insulting or condescending-just an observation from someone who has spent years chasing wierd problems. A methodical,complete engine performance analysis is the way to correct this issue-not the"well maybe its base timing,maybe its injectors,maybe its the coil" method that rules the diagnostic world these days. Might as well get out a dart board and start throwing.

This is how I would approach your issue:
A. Perform a CHARGING system test!
1.Alternator peak output
2.Ripple test
3.Voltage drop tests for output ckt. and ground ckt.
4.Battery test
B. Base engine mechanical test!
1.Cranking vacuum test(compare to other engine)
2.Vacuum waveform tests(at idle and cruise,compare to other engine-I rec. a Fluke 193B scopemeter and a PV400 vac/pres. probe for accurate results).
3.Oil pres. test.
4.Running compression test(use the 193B again)and compare to other engine.
5.Dampener alignment(bring#1 to TDC and inspect dampener/timing marks for proper alignment).
C: Basic ign tests!
1.Coil peak available Kv
2.Base igniton timing,Computer controlled timing,advance curve-compare to good engine.
3.Primary dwell,burn time,burn kv under various loads-compare to good engine.
D. Fuel system tests
1. Fuel pressure and volume tests.
2.Injector balance and relative flow tests.
E. Engine control tests!
1.Retrieve DTCs
2.Voltage drop test power supplies and grounds.
3.Sweep test TPS and MAF(if equipped)
4.Sweep test all other inputs(water temp etc.)
5.Verify closed loop control.
At this point,you will have built a solid diagnostic picture and can make some decisions about further testing.
Note: I don't expect you to do all of this. What you should do is print this,take it to your technician and ask him if he understands ALL of this and can perform these tests.If not-FIND ANOTHER TECH,PERIOD. Sounds like you have wasted an entire boating season with guesses and maybes. Its unfortunate but,there are many people who can guess and change parts,there are very few who can actually troubleshoot. Good luck and hope this helps.
 
No offense taken...because you are exactly right. I don't think anyone would debate that. The reality of this situation is a dealer that sold a boat with issues and took their "best effort" to fix them. After spending several thousand dollars using the shotgun approach, they eventually gave up hiding behind the "sold as is" in the contract even though the problem listed in the inspection sheet wasn't fixed. This left me holding the bag.

A multi-step methodical approach like you listed is clearly the correct path to take. A competent mechanic that would understand your steps will probably be in the top 20% with an hourly rate to match. Most will admit that no matter what the machine, there are always a good number of obvious issues that can cause a problem. I was hoping for added "eyes" would potentially point out an item that I was competent enough to address that may have been the cause.

Unfortunately, that wasn't the case. So now I'm left with an engine that cruises "fine" but unfortunately is burning 3 to 4 gallons per hour more than the other engine because of the required additional throttle needed to match RPMs. With fuel costs leading us to spend all but maybe two or three trips a season anchored in the river, spending what it would take in labor and potentially parts to fix it is questionable.

For a person like me, that's a terrible pill to swallow. But sometimes the wallet dictates the choice. The problem is a challenging one that isn't going to have a simple fix. Unfortunately, between labor, parts, and fuel to do all the testing, it's just not in the cards.

Thanks very much for taking the time to reply. I most definitely have saved your response to keep in my tool belt if I'm not given the option but to diagnose it in the future.

Jon
 
I'm still confused because you stated you found 'goop' on the coil. As I said before, the coils are usually oil filled. If in fact the coil is leaking, your problem may be a faulty coil. Can you try swapping with a friend's motor or just purchase a new one? Coils aren't usually expensive and ohm'ing them out doesn't always show a faulty one.

Doug
 
I swapped it with my port engine. I found the goop in the process of swapping them. Neither engine changed their behavior unfortunately. :-( Good thing I'm not into trap shooting.

Jon
 
I am not sure what the final answer was to this problem or if advancing the timing 5 degrees is the fix, but I suffer from the very same problems and have yet to solve it. I do think my mechanic advanced the time 8 degrees but it did not work. Any final thoughts appreciated as I am really stuck and have replaced many of the same parts mentioned here and cannot get passed 3200 RPM, Single engine 99 L29 7.4L MPI in 270 Sundancer.

Thanks in advance.
 
Actually, I just resolved the problem weekend before last. Out of a sheer last ditch effort, and even though the dealer had done it already, I replaced the plugs and wires. Amazingly, it now runs like a top. In fact, it runs better than the port. I plugged my computer into them and they are dead on (with the exception of the 5 degrees of retard due to the fact I still had it 5 degrees advanced). Even when the RPMs "magically" came up in the past, they were never even close to the same on the computer.

It's a cheap process worth doing if it hasn't already.

Good luck. Hopefully you won't lose a season to a $75 fix like I did last year.

Jon
 
Last edited:
With the difference in RPM, I think you will find the gear box ratios are different between forward and reverse. To compensate for this you should have one prop with about 1" more pitch than the other.
If you have the "v" drives contra rotating ie. one in forward at the gear box and the other in reverse, may be you should check this out.
Each engine will be carrying a different load if the prop pitches are the same.

BTW:- (I'm getting old and retired and starting to forget some of the tricks !)
 
Last edited:
1999 310 Sundancer, T-350Mags, Inboard

Items replaced to date by dealer:
- Exhaust manifolds
- 3 of the injectors (they broke them)
- fuel pressure regulator
- MAP sensor
- cap, rotor, wires, plugs
- computer (swapped with both the port engine and a "known good")
- fuel filter many times



So...to the issues/questions.

The engine runs fine at low to medium RPMs. Anything above ~2800 RPMs it has the following symptoms:
- Oscillating RPMs (~200RPMs)
- Inability to maintain high RPMs many times limited to 3200 RPMs regardless of throttle possition.
- When RPMs drop down, they will surge back up without throttle position change by over 500 RPMs.
- When RPMs won't spin up and throttles are mashed, it sounds like it's laboring and trying to spin up but just won't. At times, it almost seems like its starving for fuel.
..........

I had very similar situations.
Especially, replaceing
- 3 of the injectors (they broke them)
After going thru 6 months of diagnosis and most of what was said above, I had all of the injectors tested.
Gas flow tested and ohm tested.
Turns out that 2 of the brand new injectors were bad and not spraying properly.
1 of my original injectors had a lower ohm reading of 14 ohms vs. 16 ohms.
So, I repaced all 3 with new ones and had them tested before they were put back in.

Walla,
That was it. Runs better than ever now.
Gas usage is the same for both engines and RPM's, etc. good.
I'm now getting about 1 MPG which I feel is great....

Nobody looked at - or - wanted to look at the injectors because 2 of them were new. I said test them all and found the bad ones.

Moral of the story, here in my case is:
Just because it is new, does not mean it is not defective.
New (bad) injectors cost me 6 months and a lot of time, money and agrevation! :smt038:smt038:smt038

Lee
 
Lee that is unbelievable. Getting screwed over with failing new parts is too much. I have still not solved my problem but will report in when I do.
 

Forum statistics

Threads
112,944
Messages
1,422,730
Members
60,927
Latest member
Jaguar65
Back
Top