350 Mag mpi leaky exhaust gasket

[silver_power]

I changed exhaust manifolds and risers to my engine 350 mag mpi (2010 sn:1A348183). After about 40 hours a hissing noise from the one side appeard. Finally yesterday during idle I put my hand near the manifold close to spark plugs and I felt obvious exhaust gas to my hand. So I am sure that the hissing sound is due to axhaust gasket leak. Beside the noise, one thing I notice is that the engine has excessive fuel consumption at idle and poor acceleration at low rpms. To be more specific , the engine normally at idle (no gear) has 550rps and i see 2,5lph at smartcraft gauge. Now after the hissing sound appeard the idle rpm are 510-530 and the consumption is 3,0lph.The wot performance is very good (5100rpm 58mph@gps). Is it possible that the exhaust leak is responsible for the idle symptoms?

 

[Bt Doctur]

exaust gasket or head gasket

 

[silver_power]

The exhaust gasket is leaking for sure...I felt it clearly to my hand. No problem with head gasket.

 

[silver_power]

I thought that the exhaust gasket leak symptom is only the hissing noise of the engine. That is why I am asking if this could also related with the idle high low consumption.

 

[alldodge]

If your motor has catalyst then the exhaust leak can cause issues

 

[silver_power]

It has not catalyst....
Anyway first thing is to change the exhaust gasket.
Thank you ...

 

[silver_power]

Concerning the low idle I mentioned before, how can I check the iac valve with diacom scan tool? I run the test , but as I change gradually the % dc of the iac nothing happens, it does not change the rpm. Only if I change the duty cycle fast then it will change the idle rpm and at about a sec it will idle again at 550rpm. Is this correct or during test if I push it to idle at ex. 600 it will stay there until I stop the test with the scan tool?

 

[alldodge]

You change things and the ECM reacts to those changes to compensate. Look at the ECT and MAP readings. If the ECM things the motor is cold it will supply more fuel, same goes if there is a vacuum leak or if it thinks there is one

 

[silver_power]

Well in about 2 weeks I will take off the boat for winter period. Bt Doctur is absolutely right telling me that the issue could be blown head gasket as well .First thing I will check is the compression to all cylinders. If the results are ok, I will change only the exhaust gasket and hook up a vacuum tester and the scan tool. But IF the compression results are not good, then I will pull intake manifold to change the head gasket of the one side of the motor. Is this method correct?

 

[alldodge]

Sounds good

 

[silver_power]

Thank you very much.....
The reason i am telling you all my concerns is just because I do not trust in Greek mechanics even though they are mercruiser authorised.
I have all the tools and diagnostics and I could do it exactly as sevice manual describes. Thank you very much again.

 

[silver_power]

BTW what MAP (steady)reading should I have at 550rpm (idle) and what at 800rpm with fully warmed engine and what iac dc (%) . Thank you very much.

 

[achris]

Concerning the low idle I mentioned before, how can I check the iac valve with diacom scan tool? I run the test , but as I change gradually the % dc of the iac nothing happens, it does not change the rpm. Only if I change the duty cycle fast then it will change the idle rpm and at about a sec it will idle again at 550rpm. Is this correct or during test if I push it to idle at ex. 600 it will stay there until I stop the test with the scan tool?

Watch the spark timing (in the software). If the IAC is working properly, then the spark timing at idle should be steady. If the IAC is faulty, then the ECU will use spark timing to control idle speed. And the ECU will set an IAC fault code.

Chris....

 

[silver_power]

The spark timing @idle fluctuates from -2 to 8 degrees (watching diacom scan tool) really fast..but ecu does not throw any fault...and iac is about 23-25% dc whatever hot or cold....

 

[achris]

Unplug the IAC and measure the resistance with a DVM. Should be around 10 ohms. If it measures out ok, then pull it off and see if the spindle is stuck. If it is, use some carb/throttle body cleaner and clean it out. If you'd like to replace the IAC anyway, try Summit, Autozone or O'Reilly and get an IAC for a Ford Windstar. (SMP AC423/Duralast TV278/Hitachi ABV0006/Delphi CV10128)... $45. (less than a third of what Merc want for it. :facepalm: )

Chris......

 

[silver_power]

Perfect...thank you very much.
And could you please tell me what MAP reading should I have at 550rpm (idle) and at about 800rpm. I will have hooked up a vacuum gauge connected as well the same time with scan tool ...
I am asking for map reading at 550rpm (idle) because everybody sais that a healthy engine should have around 16-18mmHg of vacuum at idle...but mercruiser has low idle about 550rpm and so maybe a reading of 14-15mmHg would be fine....
So what is normal MAP reading??
Thank you again.

 

[alldodge]

Depends on the motor, age, health but what your looking for is if its low as in under 14 and if it is not steady

 

[silver_power]

40hrs ago the vacuum gauge showed 15mmHg steady at 550rpm and 17mmHg at 800rpm. So it was good....Anyway My priority now is to check head gasket , exhaust gasket, iac .I will post again as soon as I pull the heads on about two weeks. Thank all of you.

 

[silver_power]

Today I jumped in the boat to make a compression test. Spark plugs 1,3,5,7,2,8 perfect. but 4,6 not good. I post some pics of 2,4,6,8 spark plugs .4,6 are at the middle. I then made a compression test expecting that at 4,6 cylinders I would find the leak and that I would have zero compression at that cylinders.....results: 165,165,170,170 , 165,165,170,170. Perfect results. ......so maybe the leak is just a leaky exhaust gasket and not from a blown head gasket. But why the 4,6 cylinders spark plugs are so bad???.....maybe shorted spark plugs? Any ideas?
Spark plug wires, dizzy cap, rotor totally new .....
Maybe a cylinder isolation test with diacom will help...but I think that diacom isolates two cylinder at a time. How can I know what cylinders it isolates at a time?

 

[alldodge]

If you have the wiring diagram below, there can be only two choices, and they fire four injectors at a time. To verify look at the wire colors on the injectors. If you only see Lt Green/White and Lt Green/Purple along with Pink/White, then injectors are fired 4 at a time.



If you have more colors then they are fired according to how many colors. My manual is dated Dec 2006, and yours being a 2010 then it could be different

As for the color, 4 and 6 look to be rich and would either have the injectors flow tested, or swap those injectors in another location to see if it moves. The best is flow testing, and again the MAPT controls how much fuel.

During a run what temp does the diacom show?