4.3 MPI Cold Engine Start Issue

I've been living with this issue since owning the boat for the last year now and would like to nail down the cause. Starting the engines cold (not run for 24hrs or more), both have the same symptom. Turning the ignition key on, the system has one long beep while the fuel pump runs and pressurizes up the fuel rail then stops - NORMAL. Turning the key further engages the starter motor and the engine starts up like any normal engine and idles at about 700rpm but after 5 seconds or so begins to slow down and stall out at about 450rpm. Turning the key to engage the starter 5-10 seconds later and the engine once again starts normally but this time remains running without further issues. The weird thing is that BOTH engines do the same thing! Double start feature??

These engines are new long blocks with 70hrs on both and run well other than the cold start issue. Ignition Cap/Rotor have recently been replaced without any change. I've put a fuel pressure gauge on the Schrader valve and both engines have 44lbs of pressure while the pump cycles and once the pump stops sit at 38lbs. Once the engines are started pressure remains at 38lbs and can fluctuate between 36-39lbs with throttle input changes (nominal). Even with the cold engine stalling pressure at the Schrader valve held at 38lbs.

I do have a Rinda Code Reader and with the ignition on and engine off, the following info displays for the Stb Engine...
Coolant Temp 62F, Manifold Pres 14.2psi, Barometric Press 14.3psi, Manifold Air Temp 66F, TPS V#1 Volts 0.55 vdc, TPS#1 Position 0.1 %, Battery Volts 14.3 vdc, Sensor Power 5.03 vdc

After starting the Stb engine twice (as usual) the readings change as follows...
Engine Speed 725 rpm, Coolant temp 84F, Manifold Pres 7.0psi, Barometric Pres 14.9psi, Manifold Air Temp 66F, Throttle Pct 0.0%, Block Pressure 4.9psi, Battery Volts 14.4vdc

Did the same test with the Port Engine with similar results.

The Port Engine's IAC had been replaced during the summer (had a spare on board) resulting in no change with this strange cold start behaviour.

Any ideas or help would be appreciated. All comments are welcome even if you don't consider yourself an expert!

Oil pressure switches?

No clue, but what happens if you goose the throttle after the first start? Can you keep it running then?

Wanderer 290 said:

Oil pressure switches?

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I believe older models using a carb had oil switches to cut off the 12vt power going to the electric fuel pump if oil pressure were to get below 15psi, but the MPI's have an oil pressure sender communicating with the ECU which operates on 5vts (3 wires 1-5vt+ 1-gnd and 1 signal wire with variable voltage from 1-5vts depending on pressure 0-100psi). There is an additional oil sender on the same "t" connection to the block with a traditional analogue oil pressure sender that uses one brown wire on the end of the sender which provides info for the analog dash oil pressure gauge. It relies on being grounded to the block. No oil pressure switch to cut off power to the fuel pump on the MPI's that I know of.

Golfman25 said:

No clue, but what happens if you goose the throttle after the first start? Can you keep it running then?

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Haven't tried that since most fuel injected engines shouldn't need that. It'd be worth a try tomorrow and I'll get back to you! Something is definitely wrong and it is repeatable with each engine with EVERY cold start. Should make it easier to diagnose.

I've got a 1988 pontiac firebird that's been doing that for years. Runs perfect after the second start. I don't regard it as a big deal.

scoflaw said:

I've got a 1988 pontiac firebird that's been doing that for years. Runs perfect after the second start. I don't regard it as a big deal.

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Agreed it's not a big deal but I just find it odd and would like to see if I can diagnose and fix. I must be bored...

Google searched the symptoms and found one owner with the same issue and says the problem stemmed from the oil filter used. It was an automotive one that did NOT have a check valve, allowing the oil to drain out over time. Next start the oil took 5 seconds or so to fill the filter allowing the engine's sensors to detect low oil pressure and shut down the engine. I am suspicious of that fix. Are not the oil senders first in line after the oil pump to receive oil which then travels next to the oil filter and in the case of most boats the remote oil filter? If so, would not the senders report good oil pressure upon startup anyway??

My boat's prev owner put NAPA Gold 7099 which is listed as an alternative oil filter on Merc's site. According to NAPA the Gold series has Silicone Backflow Valves as well. I'll be changing the oil end of season anyway so maybe I'll just get either the right Merc or Quicksilver filters and see if that makes a difference.

When I anticipate the stall I give it a shot of gas and alls good. When it got progressivly worst I would give it a good drink of Seafoam directly down the IAC passageway.

I have the 350 mag mpi in a 05 320. According to the Mercruiser owners manual for my engines the cold start procedure is to fully advance the throttles one time, then pull back to idle then slightly advance them, then start engines. So that's what I do and works well for me everytime, whether it's been a day or a week or more. I then let them run around between 700-800 rpm until they get close to operating temp and reduce throttles to idle speed. Warm start procedure is slightly advanc throttles and start. This entire process is counter intuitive to fuel injection, but that's what manual outlines so that's what I do. Maybe not apples to apples, and long post to get to my point. What does the owners manual for that engine specify as the start up procedure?

That crazy, that's what the IAC is supposed to do.

MiLoII said:

I have the 350 mag mpi in a 05 320. According to the Mercruiser owners manual for my engines the cold start procedure is to fully advance the throttles one time, then pull back to idle then slightly advance them, then start engines. So that's what I do and works well for me everytime, whether it's been a day or a week or more. I then let them run around between 700-800 rpm until they get close to operating temp and reduce throttles to idle speed. Warm start procedure is slightly advanc throttles and start. This entire process is counter intuitive to fuel injection, but that's what manual outlines so that's what I do. Maybe not apples to apples, and long post to get to my point. What does the owners manual for that engine specify as the start up procedure?

Click to expand...

Don't want to hijack this thread, but I have similar slow starting issues on my 05,
320 MAG MIE Horizon -- Just checked my Mercruiser manual and I don't see
the above starting procedure. Please help as to where this is located

Thanks
Rich

Don't overlook the basics, complete tune-up , cap, rotor, wires, distributor module, pickup coil, compression and fuel pressure test along with fuel filters. go along way with solving issues like this. This will give you a base line before any other advanced testing

scoflaw said:

When I anticipate the stall I give it a shot of gas and alls good. When it got progressivly worst I would give it a good drink of Seafoam directly down the IAC passageway.

Click to expand...

Just went down to the boat and tried your suggestion. Had the throttles set and ready to give it a shot of gas and both engines started up like normal, then started to slow rpm's to stall so I gave a little throttle and the engines responded by increasing rpm's. The stalling was averted and I was immediately able to drop the throttles to idle without issue and both engines ran normally afterwards. Never thought I'd have to do that routine with mpi engines but for now it's a work around to avoid the double start routine!

d

MiLoII said:

I have the 350 mag mpi in a 05 320. According to the Mercruiser owners manual for my engines the cold start procedure is to fully advance the throttles one time, then pull back to idle then slightly advance them, then start engines. So that's what I do and works well for me everytime, whether it's been a day or a week or more. I then let them run around between 700-800 rpm until they get close to operating temp and reduce throttles to idle speed. Warm start procedure is slightly advanc throttles and start. This entire process is counter intuitive to fuel injection, but that's what manual outlines so that's what I do. Maybe not apples to apples, and long post to get to my point. What does the owners manual for that engine specify as the start up procedure?

Click to expand...

Interesting - and thanks for the reply! I have not even bothered looking at the manual since I've had 8.1's mpi in my last boat and just like a car, it needed no throttle action to start and idle no matter what the temperatures. I'll have to research that in the manual for sure, but as said in my previous post giving the throttle a little nudge once started avoids the stalling even if only bumped for a couple of seconds before dropping to idle.

Jimmy Buoy said:

Just went down to the boat and tried your suggestion. Had the throttles set and ready to give it a shot of gas and both engines started up like normal, then started to slow rpm's to stall so I gave a little throttle and the engines responded by increasing rpm's. The stalling was averted and I was immediately able to drop the throttles to idle without issue and both engines ran normally afterwards. Never thought I'd have to do that routine with mpi engines but for now it's a work around to avoid the double start routine!

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How old are your IAC mufflers? If they are Old pull one out and try starting and running without it. Will not hurt anything just makes loud hissing. Also, you mentioned you had spare IAC. Was it one you bought or did it come with boat?

scoflaw said:

That crazy, that's what the IAC is supposed to do.

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I read somewhere that the IAC doesn't even start fine tuning the intake stream until the engine reaches operating temps. Before that I believe it just opens to it's widest setting and the ECU applies more fuel via the injectors until the engine warms up.
Maybe I'll pull the filter media out of the intake and see if that makes any difference. It could be dirt restricting the intake enough to cause a choking effect upon startup.

Check and make sure the throttle body's themselves and throttle plate's are clean on both sides of the plate's. Side note: IAC's do the most work when the engine is cold.

Jimmy Buoy said:

I read somewhere that the IAC doesn't even start fine tuning the intake stream until the engine reaches operating temps. Before that I believe it just opens to it's widest setting and the ECU applies more fuel via the injectors until the engine warms up.
Maybe I'll pull the filter media out of the intake and see if that makes any difference. It could be dirt restricting the intake enough to cause a choking effect upon startup.

Click to expand...

Yes pull the filter and pour some Seafoam in that passageway while it's running

scoflaw said:

Don't overlook the basics, complete tune-up , cap, rotor, wires, distributor module, pickup coil, compression and fuel pressure test along with fuel filters. go along way with solving issues like this. This will give you a base line before any other advanced testing

Click to expand...

Agreed. Keep in mind that the engines only have 70hrs. I've checked the plugs and they look good, the wires appear quite new. Caps/rotors replaced since there was some corrosion (not much but...). The distributor does not have a module for the mpi engines, pickup coils have NOT been replaced. No compression test but can't imagine an issue at 70hrs. I've looked into a few cylinders with a borescope and the cylinder walls still have cross hatching and the piston heads seem relatively clean with just a few spots of carbon. Fuel filters replaced. Fuel pressure testing showed proper psi during cycling with ignition on, held pressure for 2 minutes before cranking, and held the proper pressure while running.

I don't know much about the pickup coil, but it was taken off the original engines which had 300hrs. Is there a way to test the coils?

I was thinking the MAP sensors may not be working properly, but after scanning the engines both at rest and running the data seems to indicate they are both providing good pressure info and air temp info so probably not the issue.

Jimmy Buoy said:

d

Interesting - and thanks for the reply! I have not even bothered looking at the manual since I've had 8.1's mpi in my last boat and just like a car, it needed no throttle action to start and idle no matter what the temperatures. I'll have to research that in the manual for sure, but as said in my previous post giving the throttle a little nudge once started avoids the stalling even if only bumped for a couple of seconds before dropping to idle.

Click to expand...

MiLoII said:

I have the 350 mag mpi in a 05 320. According to the Mercruiser owners manual for my engines the cold start procedure is to fully advance the throttles one time, then pull back to idle then slightly advance them, then start engines. So that's what I do and works well for me everytime, whether it's been a day or a week or more. I then let them run around between 700-800 rpm until they get close to operating temp and reduce throttles to idle speed. Warm start procedure is slightly advanc throttles and start. This entire process is counter intuitive to fuel injection, but that's what manual outlines so that's what I do. Maybe not apples to apples, and long post to get to my point. What does the owners manual for that engine specify as the start up procedure?

Click to expand...

Found a manual with the startup procedure for 4.3 mpi engines ...

https://www.manualslib.com/manual/1139541/Mercruiser-Sterndrive-4-3-Mpi.html?page=56

The surprising thing is step #5, "If the engine doesn't start after 3 attempts"? It advises to advance the throttle only position to 1/4 throttle.

Even more surprising is step #6, "If the engine doesn't start after step #5"! Wow. It then advises to advance the throttle to full then back to 1/4 and try starting again.

Kind of unacceptable to have to expect to have to try more than 4 or 5 times to start your boat engine. I'd think that after two unsuccessful attempts, the manual should advise to have the engine checked for issues.