A mercruiser 4.3l MPI with a low weak idle hard to start

[samseoane]

Hello Everyone,

I have A mercruiser 4.3l MPI with a low idle between 450 and 500. Here’s what I done so far.

2008 engine no alarms no beeps

1New Fuel Filter
2 New IAC and cleaning of the throttle area
3 fuel regulator on top with fuel rail checked for blockage no issues
4 new spark plug wires and spark plugs.
Took off all the spark plugs and checked
Compression all cylinders 120
5 checked for spark on all 6 holes of rotor cap with timing light no issues
6 fuel rail psi 40.
7 Diagnostic computer connected no Faults.
8 Duty Cycle on IAC as seen by computer stays on at 99% all the time (NEW IAC NO HELP)

 

[Scott Danforth]

Your compression is way too low and your fuel pressure is low

Your compression should be between 140 to 150 or more. Check your gauges

Your fuel pressure should be 43psi minimum.

 

[alldodge]

With the IAC at 99% the ECM is trying to increase the rpm.
Your compression is low if you gauge is accurate

What does the MAP and Temp show?

Does it get better when the motor warms up or stay the same?

 

[vroom ZOOM]

Just a thought... do these motors have an IAC bypass adjustment that could be clogged?

 

[achris]

Just a thought... do these motors have an IAC bypass adjustment that could be clogged?

No. But they do have an IAC muffler that needs replacing annually.

 

[QBhoy]

Assuming your gauge is off. You’re missing the main cause of this on the mpi. The famous diz cap !

 

[samseoane]

Hello Everyone was off the boat for a while. The compression and fuel pressure are good. im showing 140 and 43 for fuel

 

[samseoane]

i accelerated engine here a little bit to try to match other one at 600 RPM. Here it shows the IAC at 99% and Water pressure sensor 0 PSI. Both replaced REFERENCE ATTACHED

 

[samseoane]

With the IAC at 99% the ECM is trying to increase the rpm.
Your compression is low if you gauge is accurate

What does the MAP and Temp show?

Does it get better when the motor warms up or stay the same?

Hey motor stays the same always surging low idle . I Attached a pic at the end of the thread showing the motor

 

[alldodge]

140 is better but not great

What is the deviation of the compression numbers?

When idle is below 400 rpm the ALT voltage is low, temp is cold and MAP is where it should be "IF" readings are correct

When MAP shows 9 psi the voltage should read about 2.8V
At 8 psi reading should be about 2.3V

There might be a vacuum leak

 

[samseoane]

All the cylinders are between 140
And 143

 

[samseoane]

Where would the suspect vacuum leak be and what are the bet methods to check thanks !

 

[achris]

Somewhere along the head/manifold gasket. Use a bit of carb cleaner and spray in the suspect area. When the engine revs change, you found the spot.

Chris.......

 

[alldodge]

Can spray wd40 around the intake and throttle body to see if ram changes.

 

[samseoane]

thanks for all the help guys a little dark here in south Florida but will definitely hit it tomorrow

 

[samseoane]

im going to start another thread on my dads boat hopefully you can pitch in!

 

[samseoane]

Fixed the sea water pump pressure sensor. Sprayed starting fluid around intake manifold and no luck on any RPM changes. Idle stays very low

 

[alldodge]

What fixed the seawater sensor?
What is the motor serial number?

Might be the Throttle body factory adjustment has moved. Look close at the stop screw and see if it has been moved

 

[samseoane]

I pulled off the connector and noticed it has the green salt buildup did a contact cleaner with a toothbrush. The SN is ia309885

 

[alldodge]

Think that is another puzzle is with the water pressure showing zero the available power should be less then 100%. Also I would expect to see guardian mode active