2006 Mercruiser 496 HO starts then stalls
 

Hello,

My apology if this questions has been asked many times. I did a search but could not locate an answer.

As the title indicates, my 2006 496 HO start then stalls. I called for service. I was told they had to replace both fuel pumps (Low & high pressure). They contacted me said the boat is running. I went to start the engine -- same problem. The engine cranks then stalls within 5 seconds. The engine never runs normal then stalls, more link a rough idle. I replaced the IAC, no change. I try to advance the throttle while I'm trying to get the engine to run. It dies.

I called the tech to come back to the boat. He connected his computer for diagnoses. It indicated a both cam and crank sensors were bad. He replaced them. Now the boat will not start -- almost like its not getting a spark (where is that old distributor cap. I will see if its providing a spark!) I asked him if either one of the sensors he replaced could be causing the engine not to start? He said NO. He came back the next day. Spent 8 hours trying to figure out the problem. He disconnected all the electrical connections from the boat network to try to isolate the engine. He connected a started button to crank the engine. The engine never started.

He said the boat needs to come out of the water (Formula 370 Super Sport). The engine needs a new wiring assembly. You have got to be kidding! The engine started before you messed with it. Now it will not crank over. So he said, I have not other options.

Today, I decided to reconnect everything he disconnected. I removed the cam & crank sensors he installed. I looked at all the connections looking for any wires that might have been frayed/damaged/etc... I checked all the fuses, the lanyards, anything I found on the internet that provided some clues.

The engine now starts (again) but stalls. I am back to where I started after spending $1300 for travel, labor, and parts to put me back at square 1 again. I am not a mechanic but I can read and follow directions. Can someone help me figure out what is wrong with my engine? Initially. everyone I spoke to said it was -- 100% the IAC. From your description, its is the IAC. Now that I replaced it. What other options do I have?

Thank you in advance for your recommendations.
Jeff

One of the two sensors he replaced is bad IMHO. If you make things worse back track

Thanks Wobble. I did back track. I removed and replaced the two sensors he replaced. I put back the old ones. That is how I got the engine to start. It seems to me that the ECM determined that one or both sensors was wrong. Maybe that is why the engine did not get a spark to start the engine. Who knows....both sensors could have been the wrong? I have no idea. They only thing I was able to figure out was how to get the engine to start. That is more than the Merc Tech could figure out. So the engine starts, rough idle, then dies within a few seconds.

Here are the items that were replaced:

Low Pressure Fuel Pump
High Pressure Fuel Pump
IAC
With these 3 items replaced, the engine starts, rough idle, stalls within 5 seconds
replaced these two items:
Crank sensor
Cam sensor
Now the engine will not turn over. It will crank but not start
I removed the crank and cam sensor, the engine starts, rough idle, then stalls.

Any more ideas?

Does this engine have a Fuel water separator? If both fuel pumps were replaced, do I have to replace fuel filters? Are they separate from the fuel pumps?

How old is the gas ?

When he changed the pumps did they also change the screen under the pumps ? They also get clogged

The fuel is only 3 months old. I was out on the boat one day. It was running great. Then when I was ready to leave, I could not get the engine to stay running. It would start then stall. I'm not sure if he changed the filters. Where are they located? Do they look like inline filters?

Jeff

Was fuel pressure verified before and after the pumps were replaced? Does the engine start right back up?
Please to help us help you edit your earlier posts, an engine that cranks is turning over but does not start. When you write that it "won't crank over" I started to assume the starter wasn't engaging.
Some basic diagnostics not parts replacement can lead you down a more informed and cheaper path.
Is there an oil pressure safety for the fuel pumps on a 496 or does the PCM turn them on based on rpm ?

When your boat starts for the 5 seconds does your tach work ? NO start and no tach signal 99 % of the time is a bad crank sensor. I have seen one time were the reluctor on the crank that the crank sensor reads had a few scars on it and caused very hard start ups and the engine ran like it was 180 out of timed with total max rpms of 3 k at full throttle. The engine always had a cam sensor code.

Is this a DTS / 496 ?

Was the kill lanyard bypassed to eliminated it out of the mix.

key on engine off what is fuel pressure ? And when the engine starts what is fuel pressure running, plus will fuel pressure stay steady on the gauge?

I think bad fuel, water in the fuel is one possibility.

Hopefully you do have a jack of all trades craigslist tech working on your boat. Any good tech that has been in the business long enough should know to run / start a problem child boat with his own fuel supply with fresh gas (3 or 6 gallon remote fuel tank & fuel line ) along with checking / changing all fuel filters and checking the inside condition of your cool fuel system especially if it the GEN III cool fuel system. Also a clear jar with your fuel taken from your fuel tank to check the condition of your gas.

If this tech has not done any of this nor checked fuel pressure, I would look for a current up to date Mercruiser certified tech period to start with the current basics and move forward along with scanning the motor.

Mercruiser has had a big problem with the GEN III cool fuel system of corrosion and paint peeling inside causing huge fuel restrictions in return with running problems or no start conditions .

Also a TPS way out of range or faulty will cause a motor to start then stall immediately.