Guardian or ignition failure

My 2005 4.3L mpi s.n.0w064797 really has me stumped. So as I said in my first post I’m having an issue where I’m unable to go over 2800 rpm AFTER approximately 10 minutes. New cap,rotor,wires,plugs (again) oil and filter, fuel filter. Water pressure sensor,temperature sensor, have also been replaced. Engine temp runs 170 and never higher. fuel pressure is 42 key on,38 at idle, did wot test and pressure was 41 psi. I believe this is acceptable. IAC replaced and that’s what stopped the beeping. But.... still can’t get over 2800 rpm.
Ordered a 160 thermostat, due in tomorrow. Spoke to the local merc tech still looking at mid August before I can get in for a scan. So any and all ideas are welcome.

Here’s what Happens: run motor to 170 degrees at dock, take out on lake and can run wot (4800) or even say 3100 no problems for around 10 minutes or as I did today just started it up on went straight out 10 minutes later won’t go over 2800. If I keep it just under 2800 and I check all the trim so we’ll just go over 2800 it’ll drop to 2600 and bounce back back-and-forth 2600 to 2800 however if I try to give it more throttle it’ll bounce down to 2200. No alarms so my question is is this going into Guardian mode or am I having an ignition breakdown say a coil or a distributor sensor.

Does the warning horn sound off when you turn the key to RUN?

Yes 1 beep.

I would say that the ECU is NOT invoking Guardian mode...

It would probably be wise to repeat the fuel pressure test when the issue develops...if it stays solid, then that suggests an ignition issue...would also be prudent to scan the ECU to see if it has found any anomalies...

Thank you for the quick response. Does 40 to 41 psi sound good for fuel pressure , I can run the test again tomorrow. unfortunately I cannot get it scanned until the middle of August that’s what a couple of marinas have told me. do you think it could be the ignition module/coil or should I look at the distributor.

what brand of cap & rotor did you use?

I wold ensure the fuel pressure is consistent before and after the malfunction...that would rule out the fuel system and point to ignition.

It is aftermarket pwc I think. I thought What I ordered was quicksilver. Think amazon pulled a switch a roo on me.

All aftermarket caps and rotors are junk.... OEM only on these parts. What shape is the ignition coil in? Rusted and swollen?

I have seen many a post stating the aftermarket caps/rotors are junk ,unfortunately I paid $89.00/20.00 respectfully thinking I was getting Quicksilver components. As for the coil (and the boat as a whole) it looks great no corrosion,I see nothing leaking from the coil either. This boat was well maintained and only used on a lake.

The reason I’ve been reluctant to purchase OEM cap/rotor now is if distributor needs to be replaced it comes complete from Mercury.

I’m assuming it’s not Guardian mode so my question is would the scan show if there’s an ignition component breakdown?

Failure of a complete disti is rare... and besides, caps and rotors are routine replacement items... Think of it as a spare ( a good idea anyway) I usually only get 2 trouble free seasons of use out of mine, regardless of how many hours I run, however, I'm in a salt water environment.

I don't want to seem harsh or critical, but if the cost of an extra cap/rotor is of concern.... sell the boat.

Re: buying engine parts on Amazon.... best stick to known reputable sources, for example the folks who host this forum. While MARINE ENGINE.COM offers aftermarket alternatives for many items, they also sell OEM, and the difference is clearly marked. It's a foolish economy to shop for engine parts solely on price.

I totally agree with buying OEM parts, the reason I bought from amazon was because problem started just before we had
family up for the 4th and local mercruiser dealer didn’t have cap/rotor in stock and wouldn’t have been in before the 4th. As for the amount of money it was only to say at those prices I thought they wouldn’t be aftermarket. I looked up #’s and sent to wife to order because she has prime (2 day shipping). Believe me I was pissed they weren’t Quicksilver-which is what I believe I ordered.

I don’t hear it as a miss fire it seems more like it’s hitting an rpm limiter. I’m no novice,I have machined ,assembled,and raced many different engines from stock 4.3l marine long blocks,to all out drag race sbc,bbc,fords, Etc.... I’m not trying to toot my horn just letting you know I’ve been around engines in some form for the last 45 years. That said I have little to no experience with computer controlled engines.

I’m going to order the Mercruiser cap/rotor when I go pick up my thermostat later today, in the meantime is there something else I need to check. All advice is greatly appreciated.

So I just looked at amazon to see what I ordered-sure enough the Mercruiser items where substituted for aftermarket brand and both myself and wife missed it. Ok lesson learned. Mercruiser parts ordered.

Now the plot thickens-I installed the new 160 degree thermostat today and run the boat for 18+ minutes with no issues. The lake was very rough so wasn’t going to have wife drive the boat today so I could check the fuel pressure. After 18+ minutes I couldn’t go over 3600 rpm! Now 3600 is way better than 2800 but I’m now thinking water temperature sensor ( I installed a new 1 Tuesday) so after 48ish minutes of running up to 3600 rpm I decided to install the old temperature sensor bingo engine won’t rev over 2800 rpm. Quick change back to new 1- 3600 rpm. Also engine now seems like it is getting to temp on gauge 173 degrees.

I will now order a temp sender, maybe a 140 thermostat? As well.

sounds like the temp sensor has lost its accuracy...if so, it will drive the operating point of the air-fuel curve too rich if the sensor is 'low'...

that's a case where having a scanner would help a bunch....

being on the lake, I'd suggest the engine will be happier with the 160 deg F t-stat...the 143 deg F t-stats are needed for operating in the brine, to minimize the minerals sticking to the cooling system passages...

So you agree 173 degrees is not to hot. I will stay with the 160. As far as the scanner-I’m almost ready to buy 1 myself, still 3 weeks before they can get me in. Dumb question does the battery need to be disconnected in order to reset the ECM When replacing sensors ?

if the 173 is accurate, under load, and the T-stat is 160, i dont see an issue there.

Time is money and if you make the 'investment' the dividend is you now have some independence from a shop....you can also help you marina neighbors in your current situation...

No need to reset the ECM when you change sensor units...they are read in real-time...you can clear any codes but since it hasn't been scanned there may/may not be needed. Any code history may be of interest - your choice on that...

New sender comes in next week so I’ll find out if 173 is correct. As far as the investment into the scanner yes I was thinking the same thing only thing is we’re new to the state itself and only know if few people. I still think I may order a scanner on the fence right now. I am going to order another temperature sensor. I have seen a post stating that Guardian will prevent rpm when cold-I assume you wouldn’t get an alarm because the ECM only sees the engine as to cold. Maybe the new sender will shed some light on the issue.

I truly appreciate ALL the responses ,they have saved me $$$ as I was ready to buy a distributor and coil.

Engines with 160 Tstat ( vs 140) and 170ish operating temp have a number of advantages...
1) oil stays cleaner
2) more economical fuel burn
3) likely better engine life ( as it runs closer to the original designer's operating temp assumption).