1999 Mercruiser 350MAG Issues

Agree with Grant. Check the AIC motor. Again, if you can borrow a tester there is a "Reset AIS motor" test that you can fire. It will cause the AIS motor to close (RPMs drop to about 450) then go back to normal. If you don't get the drop, you have a failed AIS.

On the other hand, if you don't tear the gasket, it's easy to swap AIS from side to side.
Best regards,
Frank
 
Irish Eyes: You bet. Revs up fine in the slip. But I also am not comfortable running it high idle in the slip for too long. I'm trying to be a good neighbor. But it definitely seems like it's under load. I wonder if I drop it quickly to idle when it's happening, go to neutral, and rev it up if it will behave the same. Ha...another test. :)

geriksen: Funny you mention that. I asked them about the whistle and they told me it must be the flam arrestor. Nice huh? Thanks very much for the suggestion. I will add it to this weekends tests. Is it Friday yet????

PlayDate: Unfortunately, I haven't had fuel pressure readings yet. But I hope to this weekend. I desperately need to rule out fuel and fuel delivery. The injector damage happended when they did the heads. I simply think it was poor parts management during the work.

I truely hope one of you very kind fellow boaters have hit a bulleseye. I hope to come back on Monday with valuable feedback or even better...identification of the problem.

I'll see if I can pick up a couple of IAC gaskets before the weekend. Any other expendables if I decide to do any part swapping?

Thanks again!!!
 
fc3...my primary goal this week is to find a tester and fuel pressure gauge. No luck so far but I haven't run the well dry yet. :)

Thanks.
 
Frustratingly....no. I did verify the timing is correct. But that was all I was able to test yesterday. I bought a fuel pressure guage, but the guage leaked. I didn't want to go through the trouble of hooking up the portable gas tank until I can test that. I also wasn't comfortable swapping the IAC or TPS because I didn't have time during the week to get to the store and buy gaskets for them (just in case they tear).

So I did verify the timing....that's at least one step. Unfortunately, I wasn't expecting it to be wrong. I have a sneeking feeling it is either the IAC or fuel pump. Hopefully next weekend I will be able to test them. Outside of foul weather...I will be testing them!!!

Thanks for all the help and interest. I will most definitely update everyone when/if there is any progress.

Jon
 
Well, not as successful a weekend as I hoped. I did get a new fuel pressure guage and it worked fine...but weather prevented me taking the boat for a ride. Since I was asking a friend to sit back in the cockpit and watch the guage, I felt bad asking him to stand in the rain while I was nice and dry.

I did, however, do a deep dive on the top end and found that the IAC at a minimum was already replaced. The TPC may also have (it seemed cleaner than the port engine's). Not that this rule them out, but it is food for thought.

So I'm hoping for a fuel pump problem at this point.

By the way, when running at idle in the slip, it was hanging at about 24psi. When I brought the RPMs up to 3000, it dropped to around 22psi. I didn't (for some unknown reason) plug it into the port engine to compare. Also, after being off for a while (20 mins+), the pressure went up to ~28/29 psi. Does all this sound normal?

Thanks all. Hopefully the planets will align this weekend. Forcast looks good so far...knock on wood.

Jon
 
I don't have my manuals at hand, but your pressures sound low, and there should not be a 2 psi drop at cruise. How are your filters?. If the filters are OK, then check the fuel pressure regulator, the fuel pump, the shut-off solenoid, and plumbing connections.

Best regards,
Frank
 
I'm kicking myself for not putting the guage onto the port engine too. I almost want to drive down there tonight and test it on the port engine. Maybe I'll be <cough> <cough> sick tomorrow or Wednesday. I would be very happy if you are right and the pressure is low. Because that is a quick fix...at least for me. :)

I'm going to test the other engine to compare them, but does anyone know what the pressure should be? That way I can also validate the guage.

Thanks very much.

Jon
 
Most normally aspirated gas engines have rail pressures of 40-45 PSI. I bet yours should be around there, too.
Best regards,
Frank
 
Well, I guess this can be considered progress...although the motor is not running any better.

Today, I did side by side comparisons of the fuel pressure at the rail. Both were identical, both in the slip and under load. The interesting thing is that the pressure was around 22 psi regardless of the RPM while in the slip. But under load, it was ~30+ psi. So fuel pressure isn't a problem.

I also attached a portable fuel tank with fresh medium grade fuel to the fuel filter inlet. There was no change in the engine's behavior.

I looked at the IAC and TPI. The IAC has most definitely been replaced. I'm not sure yet when or by whom, but the IAC on the port engine still has the original black paint on it while the starboard is nice and shinny. The TPI, although it doesn't look new, has definitely been removed because I can see the screws are slightly stripped. I ask myself if that rules them out or makes them more suspect.

So the behavior today, regardless of which tank I ran from, was the same. The engine seemed to run fine up to and including ~3500 RPMs. It would struggle at any RPM greater. If I mashed the throttle, it "sounded" like it was trying to accelerate (it even got a whole lot louder), but was being held back. Fuel pressures were up in all cases.

I just sent my first email of the season to my friends at MarineMax asking them for a list of all the work they have done to it as well as a list of all parts replaced. I'm sure they were happy to hear from me. :)

If anyone has ideas going forward, please let me know. In either case, I will reply back when/if MarineMax answers my questions.

Thanks very much!!!

Jon
 
Advance curve, TPS, knock sensor, bad injectors, restricted intake or restricted exhaust come to mind.

Best regards,
Frank
 
By advance curve...you mean timing right? But if it is a timing change (or not a timing change when one is needed), wouldn't that mean the computer isn't doing something it should be or is getting a wrong input?

TPS and knock sensor...those are both things many people keep mentioning. I may just bit the bullet and swap them between the two engines. I'm just concerned about those screws on the starboard engines TPS.

Question: Going down the route of it being a sensor problem, can the engine be run under load with it in base timing mode?

Jon
 
By advance curve...you mean timing right? But if it is a timing change (or not a timing change when one is needed), wouldn't that mean the computer isn't doing something it should be or is getting a wrong input?

TPS and knock sensor...those are both things many people keep mentioning. I may just bit the bullet and swap them between the two engines. I'm just concerned about those screws on the starboard engines TPS.

Question: Going down the route of it being a sensor problem, can the engine be run under load with it in base timing mode?

Jon


As RPM increases, so does the engine timing. I am pretty sure that the MPI engines do not have centrifugal advance in the distributor, but the symptom of stumbling above a certain RPM sounds like a bad curve. Yes, a bad input can cause that. The inputs to the processor for timing, this is off the top of my head and NOT from the manual, coolant temp, TPS, knock, and RPM.

The thing with TPS is that if the TPS signal is out of spec, the processor can do a couple of things, like retard the timing, disregard the TPS, hold the engine to a "safe" RPM, etc.

I'll probably be cutting the shrink wrap on Just Ducky soon. If you want, I can ship you the Rinda tester, which is on-board, for a couple of weeks. You'll know right away if any sensor is out of range with that tool.

Best regards,
Frank
 
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Just because that IAC had already been replaced doesn't mean it is good. I would still test or swap it.
We have had brand new ones fail.
 
Frank, thank you for offering your tester. That is very kind of you. I very much appreciate the offer. I think, however, that I am going to invest in http://tunertools.com/proddetail.asp?prod=MEFI-Scan. I think it will help me even with tuning/adjusting for best fuel economy. With fuel prices, it may pay for itself in the end.

BUT......I am officially losing my mind now. It was a beautiful day today so we went for a ride knowing it would be a slow/awkward one. Well guest what...the engine ran like a top. Outside of about a 300RPM WOT difference, it ran fine. Spooled up fine...ran perfectly smooth at high RPM, and never hickuped once. Because of this, I ran it longer, expecting it to eventually start having issues. It never did.

The previous test was running the engine off a portable tank. All I did was disconnect the fuel line into the filter inlet, capped it, and connected a fuel line from a portable tank. The engine still had it's issues immediately when the RPMs started to pass ~3200 RPMs. At that point, I simply reconnected the onboard tank, tested it for leaks, and went home. Nothing changed. No wires, no bolds, no plugs, nothing...I didn't even flush the fuel line from the tank. I just capped it. But somehow, the engine ran fine today.

The only thing I can think of is that I plugged a fuel pressure guage into the fuel rail at the same time as the tank test to make sure the fuel pressure wasn't dropping. Obviously that drains some fuel from the rail...but not much. Could that have jogged something lose and freed something up?

Any ideas? Was it possibly a fluke? I think she is mad at me for some reason. She knows I have a meeting Wednesday with the dealer to discuss the engine problems!!! :)

I'm convinced there is a logic problem....and that it will happen again. This software will tell me because I can plug it into both engines, log the data, and compare. I'm considering just telling the dealer to save their money bringing back in their "expert" and just give me $500 and I'll fix it myself. $200 for the software/cable and $300 for gas and parts. I don't think it's anything in the rotating assembly. That wouldn't be intermittent right? So it's either a part or a wire (which I can fix).

What do you think?

Jon
 
I don't know about the "log and compare" methodology. But you should be able to study the results of one engine and compare them to the other engine if it starts to run poorly again. I'm going to agree it's probably not an internal engine problem. Usually see these as electronic components and wiring harness failures.

That software is a pretty good deal. My Rinda tester was about $300 when I bought it in 1999. No idea how much it would be today. Still, much cheaper then the $2,500 Ford tester, the $4,000 one for Jaguar, or the $7,000 diagnostic system for Porsche! (I think those are the right prices.)

Best regards,
Frank

Best regards,
Frank
 
Yeah, I think getting the software is critical to finding this problem. What do you think about the $500 offer to dealer? The remaining $300 won't cover a wiring harness but if I narrow down the sensor that is giving bad data, I could repair that part of the harness or just replace the wires. No sensor is over $50 so even if I replace all of them, it won't be $500. We validated the fule pump and they just replaced the fuel pressure regulator. All other sensors are very accessible.

Sound reasonable?

Thanks very much!

Jon
 
Maybe. It's very rare, but the EFI module itself could be bad. Price it before you decide what might be a good idea. I don't know if letting the dealer off the hook easily is ever a good idea.

Best regards,
Frank
 
Good (?) news is that the problem didn't magically fix itself. Right out of the gate it had the same problem this weekend. According to the dealer, they either replaced or swapped all the sensors. The only piece they didn't replace was the distributor pickup...so I replaced it @ $52 of my coin. It was the old style (not sealed). Replacing it didn't fix the problem. I did find some oil under the sensor and rotor assembly. Is that normal? It wasn't too much. I could clean it up with a couple of swipes of a rag.

I did get my MEFI Scan in on Friday and made some ScannerPro recordings. I'll post them up here tonight after I get them off my laptop. Maybe somebody can take a looksee.

I was able to record a WOT test with max RPM around ~3600. I'm still stuck on the fact that even when the engine does spool up, it does so much much slower than the port. It takes a good 5 to 7 seconds longer for the starbord motor to get even to the high 3000's. Props were both reconditioned and drives seem fine.

What a nightmare...

Thanks all.

Jon
 

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