74 L mercruiser EFI going into guardian mode

"After my engine is warm, it i

"After my engine is warm, it is going into guardian mode intermittently without an alarm. Put a tester on it and all the engine parameters are normal. Put a fuel tester on and pressure is normal. No codes showing. Alarm tests fine. Replaced temp and oil sensor, no help. Any ideas?"

How do you know its in Gardian

How do you know its in Gardian mode?

"It runs fine all day long bel

"It runs fine all day long below 2500 RPM. When running above 2500, it will start running rough until I pull the throttle back to a setting that is below 2500. I can't think of a situation other than RPM reduction (guardian mode) that would cause this. It seems to me that if it were an ignition problem, it would occur at all RPM's. It has been suggested to me that it might be the ECM computer. I should also tell you that I am not a mechanic, just the owner. My description of this problem to a boat mechanic is where I got my idea that it is going into this mode. He said it limits the motor to 2500."

"Try new fuel filters; may be

"Try new fuel filters; may be starvation. "

"OK, but it doesn't do it

"OK, but it doesn't do it unless the engine is run for about 45 minutes....still think this may be it?"

What year do you have? Are yo

What year do you have? Are you the original owner? What fuel system service/repairs have ever been performed?

The TPS (Throttle Position Sensor) could be on the fritz. The fuel pump and/or relay could be on their last legs. The wiring to the fuel pump on some models uses an in-line "quick connector" that can corrode; one fella had it happen to him recently. There may be some corrosion on the inside of the engine barrell plug. Same goes to all sensors and the computer conectors.

"It is a 1999. I have been go

"It is a 1999. I have been good about having annual services done, which includes fuel filters. Although I missed the last annual service and it has now been two years since this has been done. No other fuel related work has been done."

"First, go here, [url=""]http:

"First, go here, http://forums.iboats.com/showthread.php?t=251571
and down load the Mercruiser service manual, Service manual 23. As you have checked the computer with the scan tool (above) you will have to do the manual checks as outlined in the service manual. As you will see, if all those checks are completed OK, you will have to replace the ECU. Study the manual and you will see that there is only 3 faults that will put the ECU into the "limp home mode", high water temperature, low oil pressure, and low oil level in the out drive oil bottle. The ECU on your motor will be a MEFI 2 unit, but check the serial numbers on the engine as a few were fitted with the MEFI 1 and the MEFI 3.

I have been down this track before and made absolutely sure it was the ECU, as here in Australia they are almost impossible to obtain and are US$1,600 ! (ouch)

Chris, Hervey Bay, Australia."

"Also, you can download Servic

"Also, you can download Service Manual 24, as there is a very good description of how the ECU works. That is in Section 5D of the manual. This manual is for the 305cid and 350cid motors but the principals are the same and these engines use the same ECU as the 7.4l (454cid).

Chris"

"Thanks for all the help guys.

"Thanks for all the help guys. I will not be able to pursue any of this until next month, as I am out of town. I was hoping that I might be able to get my hands on an ECU to plug in and see if the problem goes away, but that is probably a pipe dream."

"I can not recall anything abo

"I can not recall anything about guardian mode limiting the engine to 2500 rpm. Early engines with MEFI1 computer did have a power reduction mode set at 2800 rpm. The prm was activated when either drive lube was too low, if engine oil pressure was too low, or if coolant temp was too high.
Please note that the computer uses different senders for temp and ol pressure than the gauges.
However, if your engine is verified to be 1999 it probably has a MEFI3 unit which does not have the prm activated. Best thing to do from here is to hook up a fuel pressure gauge and a scan tool. Check fuel pressure under load and sensor input to the computer on the scan tool. Also be aware the problem may be heat related like a faulty ignition coil or trigger coil in the distributor. A skilled tech with proper tools and equipment should be able to solve this within a day or so."

"Just fixed another 7.4 MPI. W

"Just fixed another 7.4 MPI. Wouldn't go over 2800rpm. No codes, fuel pressure OK, everything on engine OK, number 6 spark plug always saturated with fuel, changed the plug, changed the plug lead, changed distributer cap (new). made no difference. Pulled injectors, number 6 OK as were all the others.
Found a service bulletin advising 2 batches of bad distributer caps being delivered from China. Replaced the cap with one with BRASS posts rather than the aluminium ones.
Perfect, fixed the problem."

"Thank you all for the help.

"Thank you all for the help. It turns out that I was mistaken, as it was not going into guardian mode. I was tricked into thinking that it was because the engine would run rough and the RPM would be limited to 2500. Under 2500, it would run fine all day. As it turned out, the ignition pick up was bad. Changing this solved the problem."