Absolutely frustrated with this Mercruiser engine

Have John hand-deliver it to him. The only reason I can't feel any more sorry for @Hoplite808 is that he's in HAWAII!

 

Yup, this has been my point for a while, the ECU very likely keeping the max RPM to this... coming from one that wrote SW like this for 20 yrs, on very similar packages. There are many ways to prove this. Put tabs up or down (or I/O trim, can't remember package), bring 10 friends on the boat, etc. If it always goes to 2k rpm, then it's the ECM calling for it. And since there is SW written to make the max RPM 2k under certain conditions, it's likely that the ECM is the cause.

 

Yup, 1st mentioned wonky ECM in post 11 and got poo poo'd.

 

Usually they work or don't work at all. Lightning strikes, putting 12 volts onto the wrong pin...will turn it into a very expensive brick.

Since this ECM is antique.....the only way to tell it is working properly is to probe the outputs of the ECM which Mercruiser specifically states not to do. They do that for a reason because 20+ years ago inductive diagnostic tech was really expensive and Mercruiser wanted to keep maintenance within their dealer network. Probing the injector pins improperly can/will create new problems.

In the real world.....if it turns out that the ECM is locked into a RPM reduction mode....that will answer the question he has been chasing but won't solve his problem.

Hopefully, Service mode may override the issue. There is scattered information that indicates it will......but it won't fix an internal RAM, PROM or driver problem.

 

And those are very good points. As electronics age they degrade in performance. Consider that even though the data shows the injectors are being cycled at a rate that doesn't mean they are actually being cycled as commanded. The ECM drivers could be operating at a fraction of the electrical current needed to open and close the injectors. The counter to that argument is that RPM wall at 2000 would be a bit more variable I would think.

 

Exactly !!!

Several pages ago i suggested to carefully tie up the boat in a secure place and rev it in gear while the boat is moored . If it also then revs to 2000 exactly then you know its all ECM introduced . There was some chat talk about this here but this very important point was forgotten again .

seriously - If it was my boat i i would with logic nail down what the cause is , then conclude what the root of this cause it , then fix exactly the responsible part and go boating .

Hoplite808: if you want to move forward follow mine or ericinmichs hint . Its crucial to know if the ECM locks at 2000 intentionally or not !

 

So to help with the determining if the ECM is limiting rpm by turning off half the injectors at 2,000 rpm you have multiple choices depending how much money you want to spend.

Power Probe 4 -does it all and is super easy to use. Just put it in correct pin and it will monitor the injector pulse and a whole lot more. $200.
https://www.amazon.com/Power-Probe-Case-Acc-Diagnostic/dp/B00NPY02EW


Noid or test lights - simple way to determine if the circuit is firing. Hook up one end to the battery and the other end to the injector signal/ground port. If you hook up two of them, to the correct ECM ports you can monitor both bank injector pulses through out the rpm range. $10-12 each.



Scope -$400-700


Regardless of which path you pick.....this should answer if the ECM is at fault without putting your life in danger.

The test light approach basically answers if the ECM is firing the two banks continuously. It won't answer if there is sufficient voltage or if the harness has an issue. Hope the videos help.

 

Just keep in mind that your ECM only has one trigger wire for each injector bank. When the ECM fires an injector it is actually firing four at the same time on one bank. Modern EFI systems have a wire dedicated to each injector from the ECM which makes troubleshooting a single injector problem easier.

As @ttmott and @Ericinmich both indicated there can be other issues.....but finding out if the ECM is actually firing both sets of injectors through the rpm range would really would help solve this mystery.

 

I dropped a bank of 4 injectors on my 99 gen 6 502 and it would not go above idle.
I don't see it holding at 2k on 4 injectors. Quite obvious looking at the plugs which ones were working.

 

Thought I would share a different story. Last week one of our Maytag washers (two years old) stopped working and I went down to DC to look at it. I pulled the codes which indicated that the Shift Actuator needed to be replaced. $25 part on Amazon delivered the next day.

I installed it and it still didn't function properly. This week I meet Maytag to repair it. He pulls the codes and announces the Shift Actuator needs to be replaced (great news that the machine didn't tell him a different story). I fess up and told him I had already replaced it. He looked at me for a moment and then asked me to help him check the harness between the control board and the Shift Actuator.

He was under the Washer and I was up by the control board doing a continuity test on a six wire plug. Out of six wires.....four worked and two were broken. He said: "Common problem which fools our techs 9 out of 10 times." He indicated that the harness is wrapped so tight at the factory that any major vibration (this is a washing machine that tenants use) will cause the wires in the harness to break over time.

Sometimes it is something very simple but finding it takes someone who knows where to look.

 

I really don't know if this is the issue but the way this antique works is that both banks are firing up to 2000 rpm and then it cuts off one bank. So for your 502 you would have to get up to 2000 rpm then cut a bank to see how it acted. The reason they designed it this way is so you would still have control around the dock. It allegedly restores power to both banks around 1200 rpm.

Hopefully....he can at least get the testing done on these two wires and we can move on to either a resolution or something else.

 

Probably easier to send the ECM to OBD in Ca. for testing. He's been doing this for a long time and can repair most issues and he does have ones for sale .

 

I mentioned it many posts ago and haven’t seen if you did this….

Add fuel when it hits 2000 rpm.
Figure out a way to spray carb cleaner into a vacuum hose or the intake, will it exceed 2,000 rpm with the extra fuel added??

 

Nope.....I suggested that you monitor the two injector signal wires at the ECM which is accessible.

 

I think we're all starting to get a complex from you continuing to refer to our 90's boats as antiques...

 

Shoutout to 370Dancer for sending OP an ECM. That should narrow things down.

 

At the correct voltage and current.