blown formula

If your engine surges, does not immediately follow the throttle movement, dies when put in gear or runs up when taken out of gear- then you probably have either a TPS or IAC sensor to be tested & or adjusted.
First- the equipment needed:
1/2" wrench or socket to remove the throttle body
7/16 wrench to disconnect the throttle cable
5/16 wrench to loosen & tighten throttle cable adjuster
1/8" ball end allen wrench
Digital dc voltmeter or Mercruiser scan tool (best & easiest)
Timing light.

1st-Warm the engine to operating temp, then set the base timing as required for your engine configuration
(Procharged set at 10* base in base timing mode (scan tool)
Do not take a timing reading off the scan tool at idle, it
varies radically. Use only the timing light.

2. Disconnect the throttle cable from the throttle boddy.
3. Remove the throttle body from the intake...... carefully unplug both cable plugs from the unit. Now you have good control over it, turn it over and find the two set screws that adjust the Primary (left) & the Secondary (right) throttle blades. Of course the Primary opens first but with EFI, the idle is NOT adjusted on the Primary side. Remove part of the loc-tite holding the screws in place.
To explain: The IAC sensor measures the amount of air supplied thru the IAC Tube between the throttle blades. This is a fixed size and can only flow so much air. The IAC is adjusted with the screw under the secondary blade of the throttle body. Therefore, it is a "relationship" adjustment!! The more air that flows through the Secondary throttle plates is aire that does NOT have to be supplied by the IAC (Idle Air Control motor) The IAC actually pushes a valve in and out in reaction to the air tube flow.

TPS- identify the TPS mounted under the throttle body but over the Primary throttle blade shaft. There is an adjustment screw ajacent to the sensor.
4. Plug the cable leads into the TPS sensor. Plug in the scan tool to the ECM. Turn the ignition ON, but do not try to start the engine. Set the scan tool to TPS voltage (or connect DVM) then set the adjustment screw for the lowest value & still touching the throttle arm stop. This may be .045-.065 volts.

5. Reinstall the throttle body, but do not hook up the throttle cable as yet. Don't forget to plug the control leads into it.

6. Restart the engine. Rewarm the motor.
7. On the scan tool select -IAC voltage
Now before we adjust the IAC we are going to test the IAC control. Slowly put your finger over the IAC tube and partially block it. The engine should start to stumble, lose rpm or die. This is good & means the IAC control is good. Now to adjust it.
NOTE: I used a flexible 1/4 socket drive shaft to snake under
the intake with a torx wrench in it to reach the allen
screw adjustment as it is not reachable behind my
intercooler with a plain allen wrench.
Insert your allen wrench or ajustment tool into the 1/8" screw. Turning the screw in (clockwise)will reduce the IAC voltage reading (it opens the secondary throttle blade). Turning it counter-clockwise closes the blade causing MORE air to come thru the IAC tube & increasing the voltage. DID YOU NOTICE THE IDLE SPEED NEVER CHANGED??? The total air flow at idle remains constant, it is the SOURCE of the air flow that is important to the ECM & sensors. I recommend you verify the required setting thru a Mercruiser Manual or you engine builder, or ECM Programmer. Arizona Speed has recommended to me to set to .035v. It works. Not to disparage knowledgable Merc mechanics, but most "professed" Merc mechanics in smaller shops have no idea what the values should be. Ask to see the book.

8. Final & last step- recheck your base timing. You might actually be surprized... it can change on you.
Another NOTE: The spark timing shown on the scan tool at 2000 rpm can vary by a wide margin because of the Merc timing tables by 400 rpm either side of the 2000 figure. This reading is not absolute- do not trust it. Use only the timing light to calibrate timing!!

This was a hard fought learning process for me. I hope it helps you all. I am beginning to figure out that EFI is about relationships, not necessarily direct readings or adjustments. Adjust one thing to make something else change......
it can make you...

........

mats

Blown Formula,
very interesting. Being a boat owner sure is a learning experience........

Mats

Vinny P

Great info. Thanks

blown formula

Your welcome.... I guess I did ok, no one has contradicted me yet..... but if I am wrong... tell me... I am always willing to learn more or a better way!!

US1 Fountain

iTrader: (1)

TTT

Would this also be an cause of a 502efi that dies when the throttle is given a quick rap?

Turbojack

yep. The ecm opens the IAC as the throttle is opened. When you shut down throttle quickly the IAC lags behind to let the RPM's come down slowly.

Dave_N

Great post! You have obviously given this a lot of time and thought. There are a few points that I would disagree with.

The IAC is not a sensor at all. It provides no feedback to the ECM. It is a stepper motor controlled by the ECM.

The IAC moves a pintal toward a seat to control air flow, not in reaction to it.

You would normally go for 0% position, not a specific voltage

This would not be a good test of IAC function. You would want to watch IAC postion during idle and rapid throttle changes and compare desired IAC position with actual IAC position on your scan tool to determine proper operation.

Base timing can not change based on anything you do with the TPS or IAC
If I am incorrect, please feel free to let me know.
Dave

tomcat

Thanks to all... for starting this thread, bringing it to the top and clarifying some points. I have a question about engine stalling when put into gear.

Just so I understand, you adjust throttle blades so that warm engine idling is done on the throttle blades, with IAC in 0% position. This is done to give the IAC the most room to increase idle air when the engine is cold, or when it is loaded by putting it into gear.

You adjust the TPS so that it is reading correctly with the new throttle blade position. This is done so that the TPS reading isn't too high, which might cause the ECU to turn off idle spark control.

Now when you put the engine into gear both the IAC and the idle spark control should be able to catch the stall and recover to the desired idle speed, right?

After doing the above the engine still stalls. What gives? The other engine in the boat has identical ECU programming and it has no problems going into gear.

Dave_N

The IAC position is displayed in a range of "counts" (from 0 to 150), not percentage, or voltage. You dont want a 0 count at idle. You want a position of about 40 counts. That allows the IAC to move either way as necessary. If you adjusted the throttle plates for a IAC position of 40 counts, then you could adjust the TPS for 0%.

If you are stalling when put in gear, these adjustments will not help. Don't mess with them. You probably have a bad IAC motor, or bad connections at the IAC. Check the IAC activity with a scan tool or swap them side to side if you don't have a scan tool.

Dave

Dave_N

Also, be aware that a high or low IAC position during idle is normally a symptom of another problem, not an adjustment problem.

Dave