4.3 Bravo III suddenly wants to die under 1200RPM.

[Jimmy Buoy]

Hi All, Quite a day today. Finally got the boat splashed today for the first time this year after doing some heavy repair work on the drives and transom assemblies. Things were going rather well for the first hour or so with the boat running well. Then after a quicker run, I decided to take a look at the ER again and found that the STB Water Distribution Housing had come apart at the seam and spewed water all over the ER and other engine. I had just replaced this part with a new one since the old one had cracked where the blue plug screwed into the housing. Great...

Fortunately the engines are fed water from through hull fittings, so the fitting for Stb was shut off to give the bilge pump a break. We then started limping back using the Port Engine which was set at 1150 RPM making about 5.5mph and after about an hour and getting near our destination, the engine began to stumble and began to lower its RPM. I decided to bump up the RPM to 1500 quickly before it died and the engine responded by gaining new legs and ran fine at 1500RPM until back to the marina.

Problem was as soon as the engine speed was reduced the engine started slowing down again and headed for a stall. That was going to make docking fun. I decided to open the Stb water valve, start up the engine for docking and let'r wash the bilge some more.

Any ideas? What RPM does the IAC stop being a factor?? Could a plugged up fuel/water separator cause this (thinking not since it ran great at 4500RPM an hour before).

I hooked up my Rinda Analyzer which showed "no codes" but the fuel pressure was listed as 2.1 psi - seems very low. The engine was running very cool as well with the temps showing the high 90's throughout the afternoon which seems odd, but it should still run ok even cold.

 

[Jimmy Buoy]

One other symptom as I think back. While trying to dock, and trying to manipulate the throttle up and down to keep it going I did hear a couple of "POPS and WHOOFS" (maybe some backfiring) while trying to give it some throttle to keep it going.

After getting situated at the dock, I did start it again. Started right up and idled fine but when I put some load on the motor by placing the drive in Forward, it started to slow down and stall. During this time I had the Rinda hooked up and the fuel pressure stayed at 2.1 psi.

 

[Jimmy Buoy]

Thinking back, the engine went from running fine for an hour at 1150 RPM to changing its sound a couple of times (like a short drop in revs then recovered) then within a few minutes wanted to slow down and stall unless the revs were increased to 1500. That sounds electrical to me, being so sudden.

The ER did get soaked with the split water distribution housing happening at high revs. When I raised the ER hatch, water was dripping off. the underside of the hatch and both the engines looked like they had been drenched with a fire hose. Is it possible that the water while running the one engine for an hour at slow speeds had some water work its way into its ignition components? Would that make more sense in this case?

 

[OllieC]

Hi All, Quite a day today. Finally got the boat splashed today for the first time this year after doing some heavy repair work on the drives and transom assemblies. Things were going rather well for the first hour or so with the boat running well. Then after a quicker run, I decided to take a look at the ER again and found that the STB Water Distribution Housing had come apart at the seam and spewed water all over the ER and other engine. I had just replaced this part with a new one since the old one had cracked where the blue plug screwed into the housing. Great...
View attachment 129626

Fortunately the engines are fed water from through hull fittings, so the fitting for Stb was shut off to give the bilge pump a break. We then started limping back using the Port Engine which was set at 1150 RPM making about 5.5mph and after about an hour and getting near our destination, the engine began to stumble and began to lower its RPM. I decided to bump up the RPM to 1500 quickly before it died and the engine responded by gaining new legs and ran fine at 1500RPM until back to the marina.

Problem was as soon as the engine speed was reduced the engine started slowing down again and headed for a stall. That was going to make docking fun. I decided to open the Stb water valve, start up the engine for docking and let'r wash the bilge some more.

Any ideas? What RPM does the IAC stop being a factor?? Could a plugged up fuel/water separator cause this (thinking not since it ran great at 4500RPM an hour before).

I hooked up my Rinda Analyzer which showed "no codes" but the fuel pressure was listed as 2.1 psi - seems very low. The engine was running very cool as well with the temps showing the high 90's throughout the afternoon which seems odd, but it should still run ok even cold.

That’s not a good sign. I just bought one of those too and installed it last weekend because of the crack where the plug is.

 

[Fill Ess]

Yikes -- your BRAND NEW water dist housing broke?

 

[OllieC]

I seem to diagnose this with every stall thread because of the situation I had last year - probably because it cost me $2k when I didn’t have to spend that kind of boat bucks.

Mine also stalled under load and not in neutral at the dock. I did have a water pump leak as well. Turned out it was my crankshaft sensor at the bottom of the motor. My thoughts were water sped up the corrosion on it. It’s only a magnet with one wire which syncs the motor crankshaft with the ECM. I didn’t get any codes, nothing. Just stalling. Mine was stalling to the point that it would catch itself and go into restart mode by beeping and turning on the fuel pump.

Under the advice of the shop, who couldn’t figure it out either on their laptop, thought it was the ECM. So I spent $2k on a non-returnable ECM when all it was was that stupid sensor. $70 at Napa. Stupid that when that sensor goes bad it doesn’t throw codes.

Again, not sure if that’s the case with yours, but I think the 6’s that you have, have the sensor as well. Someday my replies will help someone

 

[Jimmy Buoy]

That’s not a good sign. I just bought one of those too and installed it last weekend because of the crack where the plug is.

That is exactly what happened to my original one. Put the blue plug in this spring, leaked, hand tightened it, still leaked, so used pliers to tighten more and the hole broke right out. Contacted the supplier - they're going to send a replacement.

 

[Jimmy Buoy]

Yikes -- your BRAND NEW water dist housing broke?

You bet. Never would have predicted that but with anything plastic that has a seam, it is possible to have a failure. Really makes me wonder how some of those captains out there just turn the key, go and never take a look at the ER for issues. The pic was taken while at idle. I can only imagine what it looked like in there at over 4000 RPM's moments earlier!! It was sure wet in there.

 

[Jimmy Buoy]

I seem to diagnose this with every stall thread because of the situation I had last year - probably because it cost me $2k when I didn’t have to spend that kind of boat bucks.

Mine also stalled under load and not in neutral at the dock. I did have a water pump leak as well. Turned out it was my crankshaft sensor at the bottom of the motor. My thoughts were water sped up the corrosion on it. It’s only a magnet with one wire which syncs the motor crankshaft with the ECM. I didn’t get any codes, nothing. Just stalling. Mine was stalling to the point that it would catch itself and go into restart mode by beeping and turning on the fuel pump.

Under the advice of the shop, who couldn’t figure it out either on their laptop, thought it was the ECM. So I spent $2k on a non-returnable ECM when all it was was that stupid sensor. $70 at Napa. Stupid that when that sensor goes bad it doesn’t throw codes.

Again, not sure if that’s the case with yours, but I think the 6’s that you have, have the sensor as well. Someday my replies will help someone

Thanks for the tip! My 4.3 engines are very similar to your 5.0 (actually the same as a 5.7 subtract 2 cylinders) and they do have the crankshaft sensors at the bottom of the plastic timing chain covers. I'll check them out for sure. Any ideas on how to test them once removed?

 

[BillK2632]

If that much water was sprayed all over the engines, I would bet that is the cause of the idle issue. My first thing would be to get everything dry and check all the connections I could - go from there.

 

[Jimmy Buoy]

If that much water was sprayed all over the engines, I would bet that is the cause of the idle issue. My first thing would be to get everything dry and check all the connections I could - go from there.

Thanks for the comment. I haven't touched it today, but tomorrow I should be able to get down to the boat and start checking various connections to see if there is any standing water or if the warm weather has helped to dry things out. I'm hoping that patience here pays off and I'll find that the fire hose spray of water is the culprit. I'll update as things progress!

 

[OllieC]

Thanks for the tip! My 4.3 engines are very similar to your 5.0 (actually the same as a 5.7 subtract 2 cylinders) and they do have the crankshaft sensors at the bottom of the plastic timing chain covers. I'll check them out for sure. Any ideas on how to test them once removed?

I saw something on YouTube that had you put it on an ohm meter and wave a piece of metal by it. it’s just a magnet. I figured it was cheap enough and just bought one and installed it. After all, I was already throwing good parts after bad starting with the most expensive.

 

[Jimmy Buoy]

Windy day at the marina today, but I installed the original water distribution housing on the stb engine (after doing a make-shift repair until a new part arrives) and fired her up with the Rinda attached. The water housing held fine, engine ran fine (like before) so I checked out the fuel pressure to compare it to the 2.1 lbs/inch from the stalling port engine. Same reading on the stb as the port - so I assume fuel pressure is not the issue.

I then started the port engine which had been stalling the other day but now it runs just fine (at the dock in Neutral and Forward - idle speeds only). Either it was water intrusion in the ignition system causing the issue or it won't reveal itself until running the engine for over an hour. The jury is out on this until I get to water test more.

Also, that is the engine that didn't even get to 100 degrees while running. I've got my laser temp gun on board now, so I can check on that when water testing. If its not getting to operating temps but running well, would my first order be to pull the thermostat and check to see if it's stuck open? I've heard some thermostats are designed to fail "open" to avoid engine damage.

 

[Jimmy Buoy]

Took the boat out today and was again disappointed. Didn't even get on plane but after travelling at 1550 rpm for 10-15 minutes both engine temps were not even to 100 degrees f. That's one issue but should be easily solved by examining the thermostats (if there are any thermostats in place).

Decided to try and lower the rpms to 1000 or so and the port engine began to lower its rev's to 700 then back up to 1000. Crap. Same issue as last time, so water wasn't likely the issue (would be dry by now). Turned around and went back to the marina at 1500 but once the engines were brought down to idle (710 or so) both engines began to drop down as low as 540, shake, then recover, then drop and finally stall out. Both did this at different times. They'd start right up, recover and repeat. So once again docking was fun since port engine wanted to stall unless I played with the throttle while the stb engine was ok until putting it in neutral before backing into my slip - then it stalled. Nice.

I used my Rinda analyzer on both engines. No faults. Then I proceeded to do the IAC test with each engine while idling at the dock. Both had the same result. Rinda starts of with the IAC at "0" in test mode and shows the rpm at 690 or so. You can then use the arrow UP to increase the IAC from "0 up to 100 for a max". No change in RPMs when I increased the parameter. Then after decreasing it back to "0", I pressed the arrow DOWN and found that once at "15" down the rpms started lowering. By the time I'd gotten to "35" down the engine got to 550rpm and stalled. Same thing for both motors.

Should I not expect increasing the parameter from zero to 100 to cause the idle RPM's to increase? Lowering the value into the negatives below zero lowers the RPMs. If that's the case, I think I have a new spare IAC that I can install and test the same way.

Anyone have experience with testing the IAC's with a RINDA??

 

[Jimmy Buoy]

Installed the new IAC on the port engine and NO CHANGE. Still runs well upon startup but after running for 10-15 min, in and out of Forward, and running it to 3000rpm in neutral the motor starts to falter as it is brought back to idle. Even with bringing the idle back slowly you can tell around 1000rpm that it wants to drop on its own. Giving it some throttle will bring it back to life but lowering it back to idle will kill the engine. It will start right back up, but after 20-30 seconds the rpms will drop from 720 slowly until it is below 500 then stall. Not looking like the IAC.

Same thing with the Stb engine but it seems to take a bit longer before it starts to falter more like 20 minutes. I do have a pair of spare ECM's so I swapped the Stb ECM from the current new one to the original one and hooked it up to the Rinda before starting the engine. All looked nominal. Started the engine and it ran normally (had not been started in days) for 20 minutes at idle, then started to falter - same as before. Not an issue with the ECMs is my conclusion.

I've ordered a fuel pressure tester and will be testing this weekend. I'm starting to believe that the engine is starving for fuel when it falters based on the slow lowering of rpm's. The engine does bring its idle back up several times only to falter again and again and finally stalling under 500rpm. If it's fuel starvation (only at 1100rpm or less) I'm thinking either fuel pump or fuel pressure regulator (smaller one on the cool fuel unit). Leaning towards the pressure regulator since fuel pumps either run or not receiving a constant 12vt power source (not variable). It provides enough fuel to run over 4000rpm without any hiccups. Coming back to lower rpms is the issue.

I pulled a spark plug and it looks fine, other than some black carbon on the metal base. Gap is correct. Insulator is a good colour. Also used my borescope to look into the cylinder. One valve was open and looked new and silver, the cylinder still has cross hatching on the walls (motors have 50hrs) but the piston head has some black carbon spots on it. Probably a bit of carbon due to the engine not achieving operating temps (80 degrees max - another issue) but looks quite normal.

Anyone have any other suggestions?

 

[Jimmy Buoy]

Received the Fuel Pressure Tester and went to the boat today. I was expecting this to be a fuel issue, either with the fuel pump or with the pressure regulator.

Attaching the tester to the port engine first and turning on the ignition, showed 43psi while the pump cycled & 38psi after the pump stopped. I waited two minutes during which time the pressure only dropped to 36psi. The engine was started - 38psi and held until going to 2800rpm quickly which dropped the gauge to 36-37psi. Came back to idle and like normal the engine stalled still at 38psi. Started it again right away without issue and it operated nominally at 38psi. Shut the engine down then, changed the fuel filters and ran fine as before with the pressure holding at 38psi.

The Stb engine was next. Same results regarding fuel pressures but the engine refuses to act up and stall coming down to idle from 2800rpm. The engine ran for over 1/2 hr at the dock before it finally showed the stalling issue. During that time the pressure kept up to 38psi even going to 3000rpm and back to idle. When it went to stall coming back to idle, the fuel pressure climbed to 43 just before it stalled (probably since the engine lost vacuum at such a low rpm - so considered it normal). Changed the fuel filter and it ran as before @ 38psi.

I'm stumped. Doesn't seem to be a fuel pressure issue. Both engines were still running under 85 degrees F. but I don't think that should affect a MPI engine.

The PO says he replaced the distributor cap, wires, and plugs so I decided to pop a cap off and take a look.

Looks rather new (like a 50hr engine would look)

And the rotor looks ok

Pulled a plug and it looks new-ish but has some carbon around the outside of the ring (running @81 degrees is probably causing this with a rich fuel/air ratio)

and the correct gap is .060 while the plug is .058 - close enough

Like I said - stumped! Unless anyone has other suggestions, I guess I'll have to call in the professionals...

 

[Jimmy Buoy]

I've been dealing with an "underheat" issue with both motors not wanting to get above 85 degrees F and have ordered new t-stat housings (already installed new t-stats with no change). A long shot, but if the new housings allow the engine to get to 160 degrees or more it might just cure my stalling issue. I read something on the net that stated that Merc MPI engines don't start utilizing the variable IAC valve until reaching operating temps. While cold, the ECM just dumps more fuel into the cylinders and leaves the IAC in an open position?

Probably just wishful thinking on my part, but I'll wait to see what happens if I can get these engines to reach their normal operating temps!

Other suggestions are more than welcome.

 

[Jimmy Buoy]

Problem Cured! Looks like operating at normal operating temps DOES make a difference. No stalling now that temps are nominal. Whew!