I think I may have found out why my 5.0 EFI won't rev past 3,000RPM?

[Kuneha]

Hopefully some of you remember me.

I have a Regal 1999 with the 5.0 Mercruiser EFI (TBI).

I have done so many things to the boat in pursuit of fixing the lack of WOT and power. Such as...

Batteries
MAP Sensor
Distributor Cap and Rotor
Fuel Pump
Oil Pressure Sensor
Heat Sensor
Oil Change
Spark Plugs

And much more.

I got sick of the mechanics not finding the issue & charing me money for it, so I sought out your wisdom but a lot of the things I was asked to do went over my head as I do not have most of the equipment to do even the simplest of things.

After everything that was changed I thought perhaps it is an ignition issue as the timing seemed a bit unstable. Fuel pressure is 30PSI throughout the rev range and under load so again, it must be an ignition issue.

Where the rotor and cap was changed but there appeared to be ignition timing issues, I thought to check the ignition sensor that is inside the distributor and this is what I found:

This looked worse before I took it out, but it is rusty and pretty disgusting looking.

Anyways, I read through the Mercruiser manual which states how to test this and it says that it should have 100 ohmes from the wires. I don't even get a reading. So I tested from the contact points on the PCB board inside and still no reading. A continuity test shows nothing either so I can only assume that this is the reason why my boat doesn't like to rev over 3000 RPM and why the ignition timing seems a bit jumpy and off.

I haven't changed it for a new one to test this out as I have to wait a few days before it arrives but from your experience, does it appear that I have FINALLY found the problem?

 

[Scott Danforth]

I would change it, however doubt its your problem simply because if that sensor doesnt work, your motor doesnt run.....period

no where in your list of things you changed does it state that you checked the contents of the fuel filter or checked the anti-siphon valve. what was your fuel pressure?

 

[Kuneha]

This confuses me as I found a thread whereby someone suffered the same issues as mine boat and he changed this sensor and it was all good. It seems to me that the sensor works in some way where the engine will still run but the ignition timing is all messed up. I have also done further reading and have found that Mercruiser has updated this sensor's design to a more reliable one.

There is absolutely no continuity or ohms reading so surely this is the issue?

Fuel pressure is all good, 30 PSI under WOT on and off load. There is no fuel issues.

Let us assume this isn't the cause of my lower power at WOT.

What issues would this cause then as it has failed this following test straight from the Mercruiser manual:

1. Disconnect the two sensor wires from the wiring harness.
2. Using an Ohmmeter, check for an ohm reading between the two sensor wires. If reading
is less than 100 ohms, replace sensor.

It doesn't read any ohms, I get no reading at all so it is 100% busted. However you gentlemen say if it is indeed broken, the engine will refuse to run. I'm beyond confused because the engine runs. It just sort of bogs around 2900-3000RPM.

I did a continuity test to make sure all the wires are working and that the contact points I was using on the little PCB were clean and had continuity just to make sure I had a clean connection for the ohms test. Nothing shows. This is a new multimeter too. There is no connection from one wire to the other, there is a break somewhere in the circuit.

 

[Scott Danforth]

30 psi is 15 psi too low. Should be 43-45 psi (3 bar)
Your running out of fuel. Find out why, and you will get your performance back

 

[achris]

30 psi is 15 psi too low. Should be 43-45 psi (3 bar)
Your running out of fuel. Find out why, and you will get your performance back

Nope. Spec for those engine is 30psi (by the Merc service manual, #24, page 5C-2)..
(It's EFI, not MPI)

Chris........

 

[Scott Danforth]

My bad.

 

[Kuneha]

Just to update this thread as I know people often try to find these types of thread for answers but the OP never usually returns to tell people if they ever found the problem.

The boat has been taken out of the water and I've managed to take off the other riser to inspect the flaps.. both were missing.

I removed the drive and found them in this exact position:

Flapper has blocked the exhaust entirely.

I've not tested the boat yet but this seems the likely cause.

Regards.

 

[Big Gee]

Thanks for posting your findings. eace:

 

[Mischief Managed]

Did I help you with that diagnosis on Regal Owner's Forum? Seems really familiar...

 

[Bondo]

Ayuh,.... That'll do it all right,...

 

[nola mike]

Had a car with a plugged cat once, similar sx. Took forever to figure it out.