Idle too high- shop cant diagnose-- over rev IAC, ECM

[GHBAY]

I love my boat, but like many of you, the amount of money it has been costing me is starting to **** me off...

near the end of last season, after the engine was put back in after the last of the hull work, she wasn't idling properly.

VP 2001 4.3 Gi efi

idle was racing in neutral- warm or cold engine. I posted here back then asking if it could be the IAC- consensus was that could cause the issue.

jump forward to now- boat is at the local VP specialist. They are a bit baffled- no codes are getting kicked out identifying the problem. So far they have looked at:

IAC- they don't think is the prob
Thermostat- was stuck, they replaced it- think it may have been contributing but are about to retest now and aren't confident that is the problem
all throttle connections- investigated, not the problem

The most recent discussion suggested I may have a bad ECM. I disnt quite follow the explanation, but was told ""your IAC is getting "70" from the ecm- another similar engine I tested on is a fraction of that""

I am hoping it isn't the ECM as they are freakin pricey from what google tells me. Any other thoughts from you folks what could be causing this? Any advice?

thanks

 

[Bt Doctur]

from my limited knowledge the idle speed is controlled by the TPS(throttle position sensor) a variable resistor that sends input to the ECM module .
The TPS is controlled by the throttle cable. So with the cable disconnected and the TPS held closed, what is the rpm reading?If still abnormal, substitute a different TPS sensor and compare readings before spending money for a replacement ECM.
Also be aware of a air leak in the intake manifold , or the throttle body unit that bolts to the intake manifold will increase idle speed also.,

 

[Thalasso]

They can be repaired All Marine ECM Repair - volvo ECM Free Diag With Repair‎

http://www.ecmfram.com/ [h=2]1.855.788.1688[/h]

 

[Glastron_V210]

Classic stuck IAC valve or vacuum leak symptoms. The IAC gets it's air via the throttle body, usually a little hole about half the diameter of a dime.

Start the engine, then block the IAC intake with your finger. The engine should stall, or at least chug hard. If it only diminishes slightly, you have a vacuum leak. Try a smoke test to reveal the leak, or use the ether trick, propane trick etc (at your own risk of course).

My bet is on a vacuum leak.

It could be the driver of the IAC valve in the ecm. 99% of issues are not the ecm though, they are the iac or vac leak.

Sometimes the IAC, and/or the passage in the throttle body gets gunked up...check for cleanliness and clean as necessary. It can stick the IAC open causing high idle rpms.

The idle speed is controlled by the ecm reading rpm (usually for a distributor signal), and then adjusting the IAC as necessary for the target rpms.

When cold, the target RPMs are elevated, and reduce as the temp gets lower. Like 1200rpm down to 700rpm range as it warms.

It could also be that the throttle plates are not fully closing. Check throttle linkages for correct operation (Seems like that has been done above, but make sure!)

If it's higher than that, there is something wrong!

Chay

 

[Tail_Gunner]

Its not your ecm or extremely unlikey. Can you get your hands on a vacum gauge and a volt meter????