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Merc 1997 350 MPI EFI Surges

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Hi, I have a 1997 Merc 350 MPI EFI with 125 hrs on it and it was running fine until a few days ago. It starts up fine then the rpms drop to 400 from 600 and almost stalls before it increases in idle speed up to about 1000 rpm an the engine seems to be making a sucking more air sound. Once I give it some throttle in neutral it seems to go back down to 600 rpms and stays there. Out on plane it will rev to 5000 rpm and runs perfect for as long as I hold the speed I am at but if I slow down and then accelerate quickley, it will bog down and I have to pull back the throttle a bit before I advance the throttle slowly to get it up to full rpms. If I am at idle speed in gear and gun the throttle a bit it did stall and then the engine would turn over for a while before it would start which is not normal for an EFI engine which always starts on the first shot. I changed the fuel/water filter and put in injector cleaner but no change occured. The filter did have some crude in it though. I always hear the fuel pump making 2 sounds when I turn the key to on. Any opinions.
Thanks
 
Re: Merc 350 MPI EFI Surges

Check for any air or vacuum hoses that are damaged or missing. Then check Idle air controller, Throttle position sensor, fuel pressure regulator. Best to put it on the computer and have it checked.
 
Re: Merc 350 MPI EFI Surges

Hi, thanks, that was quick...can the IAC affect the engine at higher rpms or just at idle...also where is the throttle position sensor and how do you check the fuel pressure regulator. It could not be the fuel pump ...
thanks for the advice.
 
Re: Merc 350 MPI EFI Surges

If it is stuck, the IAC will allow bypass air into the plenum, effectively a vacuum leak that gets past the sensors. Reseat the connector. Throttle position sensor is located on the throttle body. Where the throttle plate pivot shaft goes thru the body the sensor is on the side opposite where the cable attaches. Fuel pressure regulator looks like a little can with a air hose attached to it. If it is not working the fuel pressure may be low.
 
Re: Merc 350 MPI EFI Surges

every time I heard that dreaded sucking sound on an MPI and those symptoms it was the IAC valve.
 
Re: Merc 350 MPI EFI Surges

it seems to be the air pressure switch.... took it off, cleaned it and after reinstalling motor runs great.
 
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