Idle Air Control Valve problem

[1994almeida]

I have the boat, now on trailer, with 1999 Volvo Penta 5,0 Gi (TBI), with SX drive.
When I reduce the power quickly (from 1500 to 650 rpm) the Idle Air Control Valve is not quick enough to maintain the rpm, they drop under 650 rpm, and the engine dies.
If I drop the power slowly, the engine runs normally, and the rpm maintain normal at 650 rpm.
I clean the IAC and perform the test. Disconnect the IAC harness, Restart the engine and the rpm are higher by more 200 rpm.
Reinstall the IAC harness connector and the idle speed gradually return to original rpm within 30 seconds. Using a test lamp connected to ground, I probe each of the four IAC harness terminals and are OK
Any ideas to identify the cause of this problem, should I have a IAC or EEC problem, or other ?
Thank you

 

[alldodge]

I have the boat, now on trailer, with 1999 Volvo Penta 5,0 Gi (TBI), with SX drive.
When I reduce the power quickly (from 1500 to 650 rpm) the Idle Air Control Valve is not quick enough to maintain the rpm, they drop under 650 rpm, and the engine dies.
If I drop the power slowly, the engine runs normally, and the rpm maintain normal at 650 rpm.
I clean the IAC and perform the test. Disconnect the IAC harness, Restart the engine and the rpm are higher by more 200 rpm.
Reinstall the IAC harness connector and the idle speed gradually return to original rpm within 30 seconds. Using a test lamp connected to ground, I probe each of the four IAC harness terminals and are OK
Any ideas to identify the cause of this problem, should I have a IAC or EEC problem, or other ?
Thank you

You could really use a scanner to figure this out. Make sure the channel feeding air from the IAC through the TBI is clear. If it is then need to find out if your fuel pressure is within spec and then check the MAP

 

[1994almeida]

I haven?t a scanner.
I clean the IAC and the air channel through the TBI
I am not able to find out if the fuel pressure is OK, and what kind of tools are necessary.

Thank you

 

[alldodge]

Lets start with checking grounds, follow harness and make sure grounds are clean and tight. Check vacuum hoses, not sure if your MAP is bolted to the intake or has a hose on it. Take the Throttle position connector off and make sure contacts are clean. Don't know if you have a MEFI 2 or 3 but here is the way pin outs are (this is Merc but VP are the same pins on MAP and IAC)


If you have a vacuum gauge see what the reading is at idle.

Tools needed to fuel pressure can be found at any auto parts store. Need to tee into the fuel line going to the TBI

 

[1994almeida]

The MAP sensor is connected to TBI with a hose. The hose seems ok.
Removed the Throttle position connector, IAC connector and ECU (MEFI-3 from DELPHI 1623009). Contacts and grounds are clean.
I have to buy a vacuum gauge.
Next days I will run up the engine and I will inform.
Thank you.

 

[Fun Times]

Hi there, Check to see if the inside of the throttle body where the throttle plate butterfly would almost rest when closed is clean and not deeply grooved from wear. Clean both sides of the butterfly plate and the sounding areas of the throttle body. You may have a carbon type buildup affecting your sudden idle.

Also the linkage on/to/at the throttle body could have excessive force trying too close the plate too much due to maybe the throttle cable could be adjusted too tightly which could cause wear to the whole moving mechanical system of the throttle body.

Also if you find your throttle body dirty, check a few of your spark plugs for excessive carbon buildup just so you know their condition.

 

[1994almeida]

Hi
I think is the linkage on/to/at the throttle body that have excessive force trying too close the plate, due to the throttle cable is adjusted too tightly.
Some days ago, when I reduce the engine power, the engine not return to the recommended RPM +/- 600. Moving the Throttle lever to full close position the throttle doesn?t close totally, so the RPM doesn?t drop to 600 RPM.
So I adjust the pivot eye of the cable (2 turns off). Perhaps to much force too close the plate. I have to correct this.
The throttle plate butterfly rest when closed and is clean and not deeply grooved from wear
Thank you