Vacuum Leaks

[JONSNELL]

What would be best way to check for Vacuum leaks ?

Spraying with something and listening for a change in engine pitch but what is the best thing to spray ?

Thanks JON

[rodbolt17]

easy way? watch the ISC valve opening.
vacum leaks on an EFI engine lead to higher than normal idle.
the ISC will close to an abnormal low percentage to try to make the idle speed fit the ECU map.

not many places to leak on the F115.

[JONSNELL]

I have nothing to compare mine to but the IAC starts at 100% on start and then reduces to 30% .

What I'm confused with is when I run the active test on the IAC valve there is very little change in RPM ( like 100 max ) and I cant hear it when I run the static test.

Current idle rpm is around 900

[rodbolt17]

there are several ways to increase the idle speed.
add air,add fuel but not much or add ign timing.
30% is slightly low should be in the 40% range.
are you positive the valve is actually moving?
the ECU simply COMMANDS a move.
it has NO way to know if it acctually move.
the % you see is a REQUEST not an actual mechanical position.

base ign timing on that engine should be about -4* ATDC.
if its at 10BTDC you will see an increase in RPM at idle.

10*BTDC is a default setting,its where the pulser coils are physically mounted.

not much to leak on that engine.
could be between the throttle body and intake or head to intake,I personnaly have never seen it.

I have seen misadjusted throttle shutter screws cause a backyardigan wanted to play with the idle speed.

have you done a link and sync on the shutters as per the service manual?

[JONSNELL]

I ran the diagnostic on a friends F60 today so now I have a comparison. On that outboard I could hear the IAC valve on the static test and on the active test it raised the RPM to 1900 when it opened it to 100%.

So now the mystery continues. On my outboard when I run the static I do not hear anything and on the active I get little to no RPM change. With the old IAC and a restriction in the hose we got an idle of 850RPM with a replacement ( secondhand part ) without restriction I'm getting about the same.

So maybe the signal form the ECU is not reaching the IAC .. bad connection other thing is my replacement IAC is not operational either just that it is at a position that allows a 900RPM idle as opposed the the one I removed which was open making the idle 1500RPM

Have looked in the workshop manual and cant locate any test for the IAC.

I think I will try a volt meter on the plug for the IAC when I run a static test to see if current is reaching the IAC.

Thoughts anyone ?

[rodbolt17]

its a stepper motor.
two leads are constant 12V and the other two are ECU ground paths.
its a fairly simple setup.
toggle either ground and the motor turns.
one way to open the air passage the other closes it.
the ECU cannot and does not monitor did the valve actually move.
could be your wireing or ECU is bad,could be your ISC is bad.
the ISC is easily tested on a bench.

[JONSNELL]

Based what others have said re sticking IAC valves and it burning that part of the ECU out if that is the case with mine then would it still show a % when I run the active diagnostic test or show a % on the engine monitor ( it currently does ). At no time have I heard the valve move.

[99yam40]

You would think that if nothing changed in the parameters that it was trying to change then it would keep trying to adjust more and more to get them where it wanted them, But I have no real idea

wait on Rodbolt to post again to make sure what it would do..


Would not hurt to do a bench test to see if it moves and make sure you have the 12 V to the 2 wires

[JONSNELL]

Just checked the plug with my multimeter and have 12.3v at the plug and when I run the static test the voltage changes.

Going to bench test my replacement IAC and see what I come up with.

Good news ECU still working - bad news my replacement IAC my be non operational.

Appreciate everyone's help

[JONSNELL]

Yep my new/secondhand IAC doesn't work. So I will order a new one and put an end to this saga.

[rodbolt17]

thats kinda what I was leaning you towards.
the % reading you see on the laptop is simply what the ECU requested.
the ECU is rather stupid and assumes the request was carried out.
thats why your reading was so low.
all the ECU sees is the idle RPM is high based on TPS and MAP inputs.
so it tries to lower engine speed by closing the ISC.
but the ECU cannot and does not monitor the actual mechanical position of the ISC.

yes a stuck ISC can and will smoke an ECU.
typically you can temp gun the ISC and see its about 210-230*F.

thats a sure indication that its stuck.

the 12V is constant and supplied by the harness.
the ECU will toggle the ground path to move the valve open or closed and depending on the amount of the request it selects which ground path.

if the valve fails to move,mechanically stuck, the ECU keeps toggling the ground to move the engine RPM.

thats when the ECU overcurrents,gets hot and dies.

[JONSNELL]

In the interim while waiting for the new IAC to arrive would it be OK to leave the electrical plug to the IAC disconnected and operate the motor ? Reasoning if you don't have a circuit then the ECU cant overload it and blow it.

I could put the old one back in and put the hose clamp back or leave the replacement in - either way that will achieve an acceptable idle RPM.

Based on me getting 12v to the IAC my ECU is still good so don't want to risk it.

When I measure the resistance on the IAC's with the old one I get 30 odd ohms on two of the posts and the replacement has nothing.

Thanks Jon