quickdecision

Hello, I was wondering if someone can please help me with a problem I am having with my 1999 Merc 502 MPI motors. Both motors when fired up on the hose start up and run fine. When motors are up to proper temperature, a quick flick of the throttles will kill the rpm's and both motors stall out. Moving in another direction, when the boat is in the water and cold, they both start up and shift fine. The problem occurs whent the engines are warmed up and I am trying to dock or pull into marina, both motors will die when shifting into forward or reverse. Boat is at mechanics and it does not show any faults on the computers with regards to IAC sensors. Big question can the IAC sensor or motor be bad even if the mercruiser diagnostic equipment shows it is fine? I am trying to be proactive and understand this efi stuff. Yes both motors are doing the same thing. All shifter cables and neutral safety micro switches at helm have been replaced in last year. Help Please!!!! Thanks

Ted G

Yes, it is possible for the port for the IAC to be clogged, or there can be build up on the plunger shaft which will cause stalling. If your tech has the right scan tool he can look at IAC counts, if they are at like 99 and the think is idling low then there is a mechanical failure keeping it from increasing idle. I have even seen burned up IACs on 496 motors not set a code, but it will cause 2 beeps every minute or so.

Griff

If they are 1999 engines then they should be 502 MPI's............Correct??????

Do both engines use a common fuel tank??? Both engines having the same issue would indicate that something common between them is causing the problem.
Its not likely that the same problem happened at the same time to both engines. How long has it been happening???

quickdecision

Griff, yes both motors are MPI's. We just had motors rebuilt when I bought the boat last year. Previous owner did not indicate this as a problem. Both motors are drawing off of the same tank but like I stated earlier, they both run fine when cold and at throttle. Boat goes out for test on water today with computer, so hopefully an answer will be had, I want to go boating. lol

bobl

Has it been doing this always since the engine rebuilds? One possibility is the base timing is not set properly. Base timing being off much either way can really screw up the idle.

quickdecision

once again the boat is idling fine it is only when it is shifted into forward or reverse is the RPM's diving low enough to where it is cutting out the motors.

Vinny P

What rpm do the engines idle at in neutral?
Once again, to repeat what BobL said... check the base timing. If it is not set correctly, the engines can idle fine, but once the load of putting them into forward is applied, they can stall out.

quickdecision

Vinny both engines idle around 900-950, Base timing is at 8 degrees.

Vinny P

I found this while running a search here on OSO.. Maybe this will help you.....

How to adjust IAC & TPS" On Merc EFI

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 body.
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!!

Griff

900-950 rpms at idle is way high for 502mpi's. They should idle around 800rpms.

A vacuum leak will cause a high idle and also cause it to stall when put into gear.