Mercruiser 4.3l mpi low rpm limit problems

A while back I had a local mercruiser garage to take a diagnostics on my engine.

It's a 4.3L MPI 220 HK 2003 mod in a Bayliner capri.

I had a problem with the engine being difficult to start, the crank was fine, but it would often stopp abruptly when it got ignition.
I had checked that gasoline pressure, nozzles and ignition were working.

I told you about checks I had done earlier, like disconnecting different sensors and that it didn't seem to matter on engine performance.
I was just told that I should leave it as it was originally and not think about it any more.

When I returned after making the diagnosis, the answer was that I needed a MAP sensor and a cooling water pressure sensor.
But from what I have read, an error on the coolant pressure sensor typically puts the engine in "Limp mode" and does not make it difficult to start.

When I returned to the boat to check the engine, several of the pistons were full of salt water.
So this was the condition the engine was left in after 2.5 hours of diagnostics.
The conclusion from the diagnosis was that the engine starting problem would be solved by buying these two sensors.

I installed the two new sensors, but still the same problem.

I decided to dig in to the problem, and long story short, it was a bad grounding to the MAP sensor that led to the ignition timing to be very "advanced". Hence during the cranking this caused the engine to start the opposite way and suck in water from exhaust.

So the 1200 dollars I paid for diagnostics and sensors resolved very little of the engine problem.
Instead, I risked being left with a broken engine if I hadn't emptied the cylinders shortly after.


Now I'm back to a problem that had for the last 2 years before this event. The problem is that the engine stagnates at about 4000 RPM under WOT when driving (5000 RPM at WOT in free) with the MAP sensor plugged in, while the engine is running at 4500 RPM at WOT when MAP is disconnected.
The acceleration is bad without MAP but come still it reaches closer to the max speed it had previously.

By the sound, it sounds like it is the same rev limiting mechanism that holds it within 5000 rpm when free, as the one that holds it within 4000 rpm when in drive. I have tested different trim tilts and propellers, to the point where the propeller is almost free spinning. Still the rpm limit in drive is quite constant.

I would have liked someone to look at it, but have to say I'm a little lost to see who I can trust to solve this problem.

I hope this is something someone here may have the opportunity to find an answer to?

Current status is that I have now changed all the sensors, except the crank shaft sensor. I have checked this sensor with oscilloscope and it looks like the readings are fine.

plugs, cables, distributor cap are also changed.

I don't have anything to offer except to say that the MAP sensor input is only part of the picture. On the positive side, the engine runs! So there is something not set properly or some parameter is out of spec. I wouldn't put all the blame on your MAP sensor.
What you say about the MAP causing hyperadvanced ignition timing is just not possible. MAP effects fuel only, not timing.
If it were me, I would get out the book, and go step by step through the tune up procedure. Check timing, plugs, wires, etc. Also, if you don't have the diagnostic tool to check the performance of the entire system (sensor ranges), you are pretty much flying blind.
The water in the cylinders is a symptom...don't ignore it. Go find a better mechanic. Sounds like they don't know anything about fuel injection which is lame in this day and age.

PS I dunno what you are referring to earlier posts. According to your post count, this is your first one.

Post the engine serial number.

Docksidemarineservices said:

Post the engine serial number.

Click to expand...

Serial number is:
M626080

Hi thanks for your replie.

I will try not to state facts beyond my observations.
But I did use a timing light gun, and it showed about 20 deg advanced timing during cranking.
Thinking a failing ground to the map sensor was like a perfect storm for this sensor. As it would not have a reference but a 5v input that may ripple out at various random voltages from the sensor output.

But yes the map sensor is new and wiring should be fine. Hence I don't think the map sensor is the source of this rpm problem.

But still wether it is hooked in or not seem to play a role in my rpm issue.

Take a look at the ignition coil PN 863704T

Is it rusted, is the metal coil that wraps around the plastic housing growing from rust and separating the top towers where the High Voltage lead connection and weather pack plugs are?

If you had 5v reference issues you would have a few more running problems.

Hi Thanks for replie


I did change to a new coil on a previous occasion. But no change.

Actually I wired up my old and new coil together. Tested the spark and saw the spark got about twice as strong.

Running the boat with this dual coil set up actually made the rpm stagnate at about 200 rpm less. I.e. about 3800 rpm when running.

Two things jump out at me having re-read the post. The first is to be sure that you have a fully charged battery and that all the connections between the engine and the battery are clean and tight. Low voltage will play many dirty tricks on you, and have you replacing all kinds of parts that don't fix your problems. So just double check that.
Second thing that will give you fits is the ignition amplifier module. In your case it sits next to the coil. I dunno if this is a thing that you can replace separate from the coil itself but if you can, I would try another one. They boost the (very weak) spark signal coming from the distributor into a higher voltage pulse that gets kicked up via the coil to the center pin on the diz cap. If it's not working you will have hard starting, weak spark, rough running, poor gas consumption, all kinds of bad stuff.

Oki cool

I like the point about the connections leading to the battery.
Because this problem got worse after a heavy rainfall.
I have kept it dry most of the time since then, but with little effect on the problem.
I'll check and refresh the connectors tomorrow

EHS said:

Oki cool

I like the point about the connections leading to the battery.
Because this problem got worse after a heavy rainfall.
I have kept it dry most of the time since then, but with little effect on the problem.
I'll check and refresh the connectors tomorrow

Click to expand...

Cleaned up and checked all connectors leading from battery and ecm grounding. But still same problem.

I would start with a full fuel system cleaning, Pull the injectors and have them serviced by a pro with a Machine to actually check their flow rates.

Disconnecting the MAP and getting more RPM is because it defaults to a increased pulse width which fattens up the fuel mixture. That is what leads me to suggest servicing the injectors, I would also service the fuel cooler.

Fuel pressure is important, too high or too low will cause different issues. Run 87 octane fuel for best performance.

Thanks

I have already checked the fuel injectors.
Took them out and wired them up and pressing through carb cleaner
The boat accelerates good no apparent issues there.

I have checked the fuel pressure and it stays at 43 psi with some increase towards wot.

Docksidemarineservices said:

I would start with a full fuel system cleaning, Pull the injectors and have them serviced by a pro with a Machine to actually check their flow rates.

Disconnecting the MAP and getting more RPM is because it defaults to a increased pulse width which fattens up the fuel mixture. That is what leads me to suggest servicing the injectors, I would also service the fuel cooler.

Fuel pressure is important, too high or too low will cause different issues. Run 87 octane fuel for best performance.

Click to expand...

Pulsing them with carb cleaner is not how you service injectors.

Thanks for the reply

But before I put more money to this.
Would not faulty injectors cause a more gradual and general symptom.
Not just only show symptoms at 3200 rpm.
Note: I checked the pulses from the ecu to the ignition coil. And in free I see pulses start to drop out at 5000 rpm whilst the pulses start to drop out in the same way at 3200 rpm when in gear. So for me it very much looks like the ecu is trying to limit rpm at 3200 rpm when in gear, But I just can't understand why the ecu decides to do so.