View Full Version : timing how to

01-02-2011, 07:50 PM
I have an 350 mpi (mefi3) and want to check the timing. i read that I have to put the ecu in base timing mode. how do you do this? just want to use a timing light and learn something new.

01-02-2011, 09:06 PM
That may be somethin u have to do with a scan tool unfortunatly
I just purchaced the mercruiser scan tool. Only because it's needed
with computerized fuel injection... Old school tune-ups are long gone
good luck with that. Does the engine run well. R u just checkin
the timming???

01-02-2011, 11:50 PM
If its running fine leave it alone..If it was off the engine would surge with efi..

01-03-2011, 07:37 AM
The least expensive way would be to get a Codemate code reader from Rinda. There is a switch on it to put the engine in base timing mode. They go about $40 and are compatible with MEFI3. Cheap way to read DT codes also. Other than that, you can get a scan tool. They are a lot more $$$.


01-03-2011, 12:08 PM
Service bulletin No. 97-15 TO: SERVICE MANAGER TECHNICIANS PARTS MANAGER Timing 1998 350 Mag MPI and 7.4L MPI Models Models MCM 350 Mag MPI Alpha and Bravo Sterndrive Engines: Serial Number 0L000001 and above. MIE 350 Mag MPI Ski and 350 Mag MPI Inboard Engines: Serial Number 0L000001 and above. MCM 7.4L MPI Sterndrive Engines: Serial Number 0L000001 and above. MIE 7.4L MPI Inboard Engines: Serial Number 0L000001 and above. NOTE: This does not apply to any other 1998 model EFI or MPI engines. Timing Procedure Change The Delco ECM used on these 6 engines is different from the other engines for 1998 and for 1997 and older models. Because of this, there is a slight change to the ignition timing procedure. 1. With all other ECMs, the engine’s speed has to be manually advanced to 1800 RPM before setting the ignition timing. 2. The ECMs used on the 1998 engines listed above have the RPM advance programmed in them. The engine speed will automatically increase to a predetermined RPM when the timing tool is connected to the DLC harness connector or the scan tool is put in the ’service/timing mode’. 3. The rest of the timing procedure remains the same as before.

01-03-2011, 05:35 PM
ok am I to assume that turning the distrutor wont do anything becuase its all done electronically? cant I at least check it at 1300rpms with a timing light?.

01-03-2011, 05:41 PM
Turning the distributor will still adjust the base timing, the MEFI controls what the timing is for a given load/RPM. If you set the distributor to an improper timing setting everything will be off! Buy the timing tool then you can do it right, it cannot be done without the tool!

01-03-2011, 07:17 PM
ok so what does this tool really do. does it place the engine at the proper rpm (in my case 1200rpm) and keeps it there as well as locking out any additional advance due to increase rpm and load?

Dr. David Fleming
01-03-2011, 09:18 PM
The timing tool on a 502MAG MPI is a plastic plug with a jumper wire that jumps across two terminals in the diagnostic access plug on the top of the engine. This is the same terminal used to access the Rinda diagnostic tool or a lap top programed to the engine. (Imagine R2D2 accessing the Death Star computor system)

Basically it sets ECU into base timing mode so that a induction timing light (Snap On or equal) may be used to program the ignition timing by strobe flashes on the vibration damper which is the small flywheel on the front of the crankshaft.

The engine running the strobe light is pointed to he spinning damper and it will flash evey time #1 cylinder fires. This will momentarily show the spinning index mark as stationary against the pointer attached to the engine (sort of holographic vision if you are not framiliar). By loosening the clamp bolt that attaches the distributor to the engine the distributor can be rotated counterclockwise or clockwise to advance or retard the base timing of the engine. I believe 7* is the base timing specified in the Mercruiser Service Manual.

This whole procedure is spelled out more briefly in the Service Manual and is a "no brainer procedure" to a mechanic. To the unaquainted it has a bit of a mystic ritual vision to the paying customer - that something beyond his capacity is being adjusted - spinning engine and all - don't get your tie, neck gold, shirt sleves or timing light wire caught into the serpentine belt while the engine is running. This will pull you and or your body parts into harms way.

Base ignition timing is one of the most important adjustments on any engine.

01-04-2011, 06:54 AM
is it true that you can put the ecu into base timing mode (or service mode as some might call it) simply by jumpering pins A and B on the plug?

01-04-2011, 07:36 AM
is it true that you can put the ecu into base timing mode (or service mode as some might call it) simply by jumpering pins A and B on the plug?

That will do it also. Might as well stick in the LED and read codes also. Or, go back and read post 4 again and make it real easy. Codemate is a good tool to have handy for the MPI systems.
Don"t take this the wrong way, but the jumper procedure wasn't mentioned because nobody wants to see a good ECM screwed up by someone not knowing what they are doing. The code reader and scan tools are pretty much foolproof.

Here is one from ebay. Less than $40 delivered.


01-04-2011, 04:18 PM
Make sure you have the right one for your particular EFI.

joseph m. hahnl
01-04-2011, 05:35 PM
This gives the basic idea of the jumper lead method

01-05-2011, 08:07 AM
Make sure you have the right one for your particular EFI.

There is only one.:wink: