2002 Mercruiser 4.3MPI surging at top RPMs after running a while

[Maclin]

While testing the new prop another problem showed up about 30 minutes in. At first all was great, throttle response was good, no surging, could cruise at 4600 - 4800rpm for minutes at a time. Then it began to cut out and surge at anything above 4600, then 4400 then 4000. It seemed to "allow" me the 4000 rpm for the last 10 minutes or so. After that I putted around the Marina for about 10 minutes, idling was ok, never faltered there. I ran it about an hour 3 weeks ago or so and none of these symptoms showed up. Temp was the same throughout both runs. just below 160. Always starts ok, just seems to need to crank about a second or two longer than I would expect for an MPI, but this is my first Mercruiser MPI so not sure if that is any kind of issue.

Supposedly had recent service And the test drive went fine, and the first hour-long run I did solo went fine, and I had put 10 gallons in which about topped it off. I am able to get ethanol free, but have no idea what was in there before. Boat had been used all last season before they traded it in this year.

Between both runs getting close to a half a tank used so far since I got it.

I am going to put new fuel separator/filter on and inspect the contents of the old one as soon as I can. Will add new fuel and try again as soon as I can. Just wanted to see if any of the experts here have had this type of thing come up and what end result for a fix might have been.

Engine Model:
MCM 4.3L MPI

Engine S/N:
0M325971

 

[Bondo]

I am going to put new fuel separator/filter on and inspect the contents of the old one as soon as I can.

Ayuh,.... I suspect the answer will be in the fuel filter,.....

 

[alldodge]

With it taking a while to start, I'll lean toward weak fuel pump. If you can turn key to ON, wait till pump stops. Turn OFF, and repeat twice more. If it fires right up after doing it three times, the pump is either weak or there is a bad restriction or weak pump

 

[achris]

Mercruiser MPIs do take a bit more cranking to start. The fuel injection system is 'open loop', ie, no oxygen sensor. All the fuel/air/revs mapping is done straight out of the computer memory, with no control feedback (exhaust oxygen sensor). When the engine is cranking the computer tends to 'over-rich' the mixture (you can smell it as the engine finally catches) and with no sensor, it doesn't know any different....

Your problem could, as suggested, be a fuel pump. You'd need an EFI fuel pressure gauge to check it though.... But changing the filters is a good start....

Chris......

 

[Maclin]

Good stuff guys, thanks! I can try the startup test on the muffs at home this weekend. Was wondering primarily if the crank time was normal or not, seems consistent. However, after thinking about it some, I have only started it once on each outing and once at home before to check water flow. I have never had to restart once I had it going, so I do not have a feel for how it would start after just a short down time.

 

[achris]

A restart within about 10 minutes of shutdown is usually instant.

 

[mconner52]

if u use non ethanol fuel and there was a tank of ethanol fuel, since ethanol is a cleaning agent in fuel the ethanol could clean the tank and send it through the fuel filters, it sounds like you have a fuel restriction I would check all the filters and the anti siphon valve on the fuel tank and run it on a portable 5 gallon fuel tank, with it being harder to start I would check the distributor cap since u had it in storage the contacts in the cap can get corroded making it harder to catch spark in the cap

 

[Maclin]

Reporting back:

Changed out the water separating fuel filter. Poured contents from old one into a mason jar, totally clear. No separation of any kind after more than 30 minutes.

Was preparing the mating surface for the new filter and felt a lot of roughness on parts of the surface that the gasket seals against. I scratched it with my fingernail and was getting a cloud of green dust from some kind of corrosion I guess. I took some 220 sandpaper and went around it several times until the paper started to not pick up anything. I did not go grab a mirror but it felt very smooth with my fingers, about 1000% smoother than when I started. I then finished installing the new filer. I have no idea what any ramifications are from it being like that or for it being cleaned up.

Hooked up the water and turned the key on and waited until the fuel pump stopped cycling. I did this several times, then started it. It waited about the same 3 seconds or so then fizzled out. Figured it was from the filter change out leaving some fuels gaps. Did the fuel pump cycle 3 or 4 more times then tried again. Got about the same wait but it started and stayed going like normal. I tried it a couple more times and it started almost immediately.

I then tried about 30 minutes later and it started almost immediately.

So all in all some ambivalent results regarding any standout diagnosis. I probably won't be able to run it on the water again for a while due to work schedule.

So these things will probably throw a code or two that can be checked? What kind of a scanner could be used?

Thanks everybody!

 

[alldodge]

You probably had some air in the lines even with turning the key ON/OFF. My initial thought was to do it before changing the filter. In any case I don't see any code being thrown to find. Your motor should have a MEFI-3 and it doesn't have a fuel pressure sensor that I can remember right now.

Your motor is starting like mine currently. If I let it sit a week or more it will crank over a few seconds before starting. After it starts I can sit most all day, and when I turn the key to head home, it doesn't even turn half way before it fires up.

If this continues, I would look at the fuel pressure regulator screen next

 

[km1125]

The fuel injection system is 'open loop', ie, no oxygen sensor. All the fuel/air/revs mapping is done straight out of the computer memory, with no control feedback (exhaust oxygen sensor). When the engine is cranking the computer tends to 'over-rich' the mixture (you can smell it as the engine finally catches) and with no sensor, it doesn't know any different....

It is an open loop system, but so is every other MPI system when you are starting. Even if you restart on a hot motor on a closed loop system, it still takes a few seconds before the system goes closed loop.

The system will send more fuel during cranking on a cold start than a hot start.


OP: Check to see if there is a 2nd filter somewhere in the line. I'm not sure on that specific motor but a lot of MPIs have a second smaller filter. The surging problem could have been caused by the gasket surface you cleaned (it might have been sucking in some air), or you still might have a restriction in a filter.

You said this happened while testing a new prop? The surging didn't happen on the old prop? What is the difference in the props? If the new prop is loading the engine more, the engine needs more gas so if there is a restriction it will surge.

 

[Maclin]

Thanks AllDodge. BTW, me too (all Dodge).

 

[Maclin]

km1125 I have only had this boat for a month or so. I had only been on it for the test drive, then once it was mine I had it out for an hour the first time for a shakedown cruise, adding the 10 gallons in just before. This was when I realized that for me it was underpropped, but I did not experience the surging. I may have been a little gunshy though, but I remember it being 4000-4500 most of the time. I obtained another prop and then took it out for about an hour and this showed up. It was fine for the first 20-30 minutes before it happened.

 

[Maclin]

The sucking air was what I was/still hoping was going on when I found that crusty stuff. The filter was on tight enough but when I looked it over real good after that I did see some white powdery looking stuff on about half of that gasket. Having a smooth machined surface has to be better than just putting it on over the rough.

 

[Maclin]

Went out for about an hour yesterday. Water was very rough so had little chance to run it at top RPM's for more than 30 seconds at a time. I had changed the filter and decided to do the dizzy cap and rotor for other reasons. I put in 20 more gallons of non-ethanol fuel to top off the 38 gallon tank. Should be all non-Ethanol by percentage now.

After 30 minutes I ran it WOT, tach showed 4900 as I trimmed up. After 10-15 seconds it started the cutting out, but this time it seemed very deliberate. No RPM alarm, but just acted more like an on purpose event. When I backed down to 4500 it went away, water was bouncing me around quite a bit so hard to see exact. After that I ran it at 4500 for same 30 seconds or so and no cut-out, seemed ok. When running in the times previous, the RPM it would falter at got lower and lower, and the cutout seemed more like a fuel starve. So, the results were somewhat encouraging but jury still out until I can get more than 30 seconds at a time at the target RPM's.

It does seem my prop choice with the 21p Michigan Wheel is good to keep me at top R's without hitting rev beeper.

 

[alldodge]

You sure your tach is accurate?

 

[Maclin]

I think so. The tach is steady, it was me bouncing around some and couldn't watch it real close at times. Numbers look about right to me: 4500rpm, 1.62 ratio and 21p prop, boat speedo at 45mph, propcalc shows 18% slip which could be improved probably. Seems to be able to cruise at the 4500rpm without the cut-out.

The cut-out shows up at about 30seconds after a steady 4900 tach reading with throttle all the way forward, adjusting trim.

 

[Maclin]

I meant to add that the issue seems to have improved somewhat from the initial occurrence.

 

[alldodge]

With 18% slip on an 18 footer make me think something is wrong. Start getting above 10 is bad enough, but 18 your into cruiser slip. I think your hitting the rev limiter. Are you using gps for speed, or the speed-o?

 

[Maclin]

Speedo. My handheld GPS walked off I know there is room for error with that 18% somewhere so not too worried about it yet.

When I first got it there was a HighFive 19p on it. It hit the rev limiter too easy with that one, I got the audible alarm but may not remember exactly what went along with it, this is my first electronic boat engine. With the 19p, two of the times I got the alarm was when I was trimming to hit the sweet spot and ventilated the prop. The last time I got the alarm it was not ventilating but hit 5k rpm as I was seeing just what the top obtainable RPM was. No cut-out before that though if I remember right, but it did get to the 5k pretty quickly and I may not have noticed. I changed the prop to a 3 blade 21p right after that, only ran the HighFive once.

Do you know if there is a cut-out before the audible alarm? I know when I ventilated the 19p and got the alarm there was not as much load on the engine and the RPM's tried to run away. This time the RPM's were not going above the 4900 into the 5k range.

The next time with the 21p is when I noticed the surging that seemed to start happening at lower and lower RPMs. After the work I did to try to fix that, this time it was more like a cut-out and the RPM that it showed up was the same. Maybe it is better after the fuel filter "fix" and changeout, and the cut-out was always there at 4900 as a safeguard. Or maybe I have found one of the reasons the previous owners traded it in for their new bigger Cobalt

 

[alldodge]

I don't have the answers to the current issues, just trying to figure out what numbers are accurate and which ones are guesses.

As an example: Use your 21 pitch, 1:64 ratio, 45 MPH at 4500 RPM and 18% slip

Reduce slip to 8%, (nominal) and your speed goes to 50 MPH
Bring RPM to 4800, keep slip a 8% and speed goes to 53 MPH

All this would be great for your 4.3
My 23 foot Rinker, 7.4, 1:50 ratio, 21 pitch and at maybe a hair above 4800 RPM I'm hitting right at 60 MPH with close to 6 % slip

If you could reduce slip to 6 % you would hit 54. The only 4.3 I know of which is designed to go above 4800 in your model years is the SKI models which can reach 5K. So if your getting 4900 rpm you could be hitting the rev limiter and also with reduced slip reaching 56 MPH