mjb

I'm having a timing issue on one of my 502 MEFI 1's. Maybe some efi guru's can help with this question.
At cruising speed(3500 RPM)I understand that the ECM selects the baseline timing advance from a look up table based on RPM and MAP reading. The ecm will reduce that baseline advance if the knock sensor is activated. Then the ecm sends this final advance reading thru the IC wire to the distributor.

QUESTION: when you hook up a scan tool(RINDA/MERCRUISER) and read the timing advance(at 3500 rpm's cruising- not at idle), is the advance shown on the screen the baseline timing from the lookup table before any knock reduction or is the final adjusted reading that is being sent to the distributor??

The RINDA tech and several mechanics don't know and I can't pinpoint my problem until this question is answered. Hopefully someone on this board is sharp enough to know.
THANKS-

Dave_N

Base Timing (ECM assumes it is set to factory spec) + ECM supplied advance - Knock Retard = displayed Spark Advance.

In other words, the Spark Advance shown on the Diacom display is the actual timing advance from TDC, and includes the Knock Retard if any. You should read the same advance on the Diacom screen that is shown on your timing light.

Dave

mjb

Dave- you were just one of the ones I was hoping to hear from....My timing light(new MSD non advance) at 3500 RPM is showing about 4-6 degrees less advance than my scan tool on my starboard engine only. The engine is clearly less powerful than the port engine. My knock retard is showing 0* at 3500. All sensors seem good- no fault codes.
ASM remapped by boxes and Jim told me to bump my initial timing to 10*. He says he remapped the ecm to account for the extra 2* advance. I switched ecm's between engines and the problem stayed with the starboard engine so I don't think the ecm's are the problem.
I wanted to check to see if maybe I was actually running on the distributor "limp home" curve instead of the ecm curve so I cut the white ic wire with the engine running. The engine died...telling me that I was running on the ecm curve, not the dist. curve.
So now it seems as though the distributor(new MSD) is doing something different than the ecm is telling it to do. I'm thinking that it may be a defective distributor or dist. module??? any more thoughts from anyone on what would cause this.
THANKS-

Dave_N

I hope my posting is not keeping anyone else from jumping in. Please do.

I may have suggested some of these things before, but I would;

1) swap distributors - to identify the distributor as the problem or not

2) check timing advance with the MSD box disconnected and running stock type inductive ignition - to confirm advance is the same with or without MSD

3) Check proper function of Knock Retard system. It may be coincidental, but the amount of retard you are showing (about 5 degrees) is the exact amount of maximum Knock Retard, like when a code 44 is set. I think I would force a code 44 by disconnecting the Knock Sensor and then checking the advance to see if it has retarded an additional 5 degrees, or if it stays the same.

Dave

mjb

Thanks Dave- I'll try your suggestions this weekend and repost my results.

mjb

Dave- Could you please clarify your comment #3 pertaining to the knock sensor.
In a properly running engine, is a code 44 set anytime the knock sensor is working and pulls out timing... or is it set only if your knock sensor has malfuntioned?

When you disconnect the wire to the knock sensor in a properly running engine will that prevent the sensor from working(ie...prevent it from pulling out timing) or will it do just the opposite and make the ecm think that there is a major knock thereby pulling out the maximum(5*??) timing?

Thanks-

rv

Isn't this just a case of the timing of the distributor position is out of sync with the ECM. In other words, rotate the distributor when at 3500 until the timing light matches the scan tool. Now the engines actual timing is matching what the ECM is asking for.

Yes, this will change the base timing but that isn't as important as accuracy of timing at 3000 and up.

Rick

txriverrat2001

This may not be related - but I ran into a 502MPI that would not adjust timing - the module was defective. It would still run - but not advance.

Just my 2 cents!

mats

If I understand you correctly the timing light shows an advance that is 4-6 degrees lower than the scan tool at 3500 rpm. Have you confirmed that you have 10 degrees initial advance (in service mode) on the starboard engine?

Dave_N

Sorry if I was unclear. I also forgot your setup is MEFI1. Which means you would not get a code 44, you would actually get a code 43 if the ECM detected a problem in the knock control system.

I know MEFI2-up systems go to full Knock Retard (4.94 degrees) when a Code 44 is set, but i'm not 100% sure if the MEFI1's do the same thing when a Code 43 is set.

The MEFI 1's use a seperate Knock Module that supplies an 8 to 10 volt signal to the ECM when NO knock is detected, and remove that voltage when there is knock, and the ECM retards the timing. Might be a good thing to check to make sure you have that 8-10 volt signal at the ECM, and check to see if it goes to full retard (like it should) when you disconnect the Knock Module. Then check to see if the voltage is removed (and the ECM retards the timing) when tapping on the side of the engine block.

Go over the entire knock system, including the ECM's ability to detect a problem in the system and set a code.

Dave