I have some strange problem with my Mercruiser 496 Mag HO.

It all started last year - the engine going in to guardian mode sporadically, for example sometimes when i fire the engine up or just going at idle, or above. But most of the time when I going over 3700-4000 rpm.

The rev limit kicks in about ~ 4200 rpm and a ”warning check engine” message appears on my Livorsi gauge.

I have the mobile VesselView and it doesnt tell me anything about the problem.

I cant see engine data like rpm and so on on my mobile VesselView but it finds the engine and connects to it.
I have no problem to see the engine data on my gauges, there is all fine.
I also tried with a friends VesselView, and its exactly the same with no engine data connection.
Also connected it directly on the engine harness at the back of the engine - no engine data.

We have tried to connect the engine with a computer from a shop, and it tells me I have problems with almost all of the sensors. If i remember right we could see engine data on the computer. Like rpm, oilpressure and so on.

Last month the oilpressure sensor went bad. Zero oil pressure. Swaped it with water pressure sensor and it showed oil pressure again. Changed the water pressure sensor (original the bad oilpressure sensor) to a new one.

Before this season i changed the impeller so that one is brand new.
I flushed the water system, oilcooler and heat exchanger from seaweed and **** so its clean. Also changed the o-rings and gasket to heat exchanger.

The engine is a MAG 375hp original, but has been upgraded to a HO by a authorized company here in Sweden. HO cam, the small throttle hole is drilled and ecu upgrade, also whipple has tuned the ECU to a stage 1 whipple….

I starting to think about a ecu problem, but I dont know.

Any ideas?

Thanks

Why are you thinking ECU when the diagnostics are pointing to a sensor or sensors? Which ones?

if i remember correctly, it told like all the sensors.
crank, cam, tps, ice and so on.

I will ask my friend if he took a copy of all the fault codes.

If things are that waked out Id be looking at all the ground wire connections

You also stated How long have you been running it since then with it going into guardian mode? Problems usually get worse when you ignore the initial warnings.

There is no quick fix on these engines if you don't have the diagnostic tools. Otherwise you are throwing parts at it. When it may be something simple like F-2 said like a ground.

This was actually one thing I saw.
One ground cable at the back/port of the engine was located on the port manifold. I changed its place so now it has ground directly on engine.

Havent tested it in water after this

Just a couple of hours to see if the problem was there after I changed impeller and flushed the water side.

Last year it didnt run much either with the problem.
Basicly took it up just right after it started.

I also did a compression test recently.

1 - 155 psi
8 - 163 psi
7 - 164 psi
2 - 162 psi
6 - 168 psi
5 - 169 psi
4 - 165 psi
3 - 170 psi

Need to erase your pcm fault history and rescan for active faults with engine running. Engine gaurdian is doing its job limiting rpm so you don't hurt the engine.

If you provide us active fault codes we can help. Anything else is just a shot in the dark.

I understand. Thats much easier, going to try to borrow a scan tool again.