502 MPI Mercrusier Ideling problem
 

I have twin 502's in my 35' Fountain after they idel for about 5-10 minutes they start hissing threw the intake and stall out.
The motors both do it but not at the same time. If I throtel them up they will back fire sometimes they will stay running other times they will still stall.

The motors are Multi port Injected and the computer showes no codes. The motors run fine at crusing speeds

Sounds like they are running out of fuel. Check the fuel pressure.

Dave

How many hours on the motors? Carb's or EFI?

i won't preach the scanner deal anymore.... but if the injectors are filthy they will go lean at small pulse width (idle)... but there are a dozen things it could be.... all of which you would know for certain as opposed to guessing if you had the scanner for your motor. the FIRST time you find out the REAL answer as opposed to throwing 500 worth of parts at it that AREN't the answer, you will understand...

You guys aren't reading the original post. MPI's don't have carbs and he obviously has a scanner because he said "No codes".

the hissing sound most likely is your IAC motor opening to raise you idle speed for some reason, use your scan tool and check your IAC counts, also look at your tps voltage, should be .5 volts at idle . it sounds like it runs ok cold but after 10 minutes are so is when it starts acting up[warm motor] . you might clean the butterflies and take out the IAC and clean with some carb cleaner, if your IAC counts are high and your tps voltage is low[.4 or less] then you could open your throttle screw a bit, i am worried a bit about the backfire, that sounds like it is running lean, if the engines are VST [early model] then there is a small tank at the back with an electric pump in there and a filter screen that cloggs up, you can remove the lid, be careful with the oring, and pull out the round filter screen and clean it, there is also a setting for fuel level in this tank that supplies the elect. pump

Multi port injection, 164 hrs

When you hear the "hissing" become louder it's not because there is anything wrong with the IAC, in fact the opposite, it's doing it's job. It's trying to keep the engine running but it's either leaning out or loading up or something so badly that the IAC can't keep it running even when it's all the way open. Did you check the fuel pressure??? How do the plugs look??

You said that it's a 1994, so I assume you have Vst tanks and they are probably the problem. There is a diapraghm that gets pinholes in it from the gas. The raw fuel then dumps into the #8 cylinder causing the motor to run rich and your Iac is opening up to keep the motor running which is why you hear the hissing. It's also the reason that you do not get any codes.

I had 1994 mpi's and went through this, I think I also have a bunch of parts laying around

check your ect[engine coolant temp] as you warm up, see if the coolant temp sensor goes out of spec as it warms up, this controls the fuel during warm up [along with the map sensor].If it goes super hot due to a false reading from the sensor it would lean it out,and as said before check fuel pressure,all though if it runs fine on the top side,I wouldn't think the pressure would be the problem,but still check it to confirm[at idle and wot].my two cents worth. turbomopar34