MPI Hard Starting

"Hi There wondering if any one

"Hi There wondering if any one could shed any light on a problem i'm having with my 2002 350 magnum. When i come to start the boat it is an absoloute nightmare to get going, coughs back and almost sounds like it's trying to run backwards?? Once i do get it going the engine runs fine and will start up every time for the rest of the day.

Anybody come accross this before?"

"Ayuh,......

How long has i

"Ayuh,......

How long has it been since the motor has had a Complete Tune-Up,+ Service,..??..??....."

Just recently had a full servi

Just recently had a full service and a new battery.Once it starts it's fine and will start every time. Leave it for a few days and get the same problem all over again. Turn the ignition to crank it over and over then when i release the key it sometimes limps into life coughing and spluttering.Give it a little throttle and then it's fine.

"Going to parade my ignorance

"Going to parade my ignorance here about the specifics of the mercruiser MPI system, but shed some light on general areas in which to look.

I suspect you are very lean on cold start.

All gas engines must have compartively far richer fuel mixtures under a cold start. Carbs acheive this with the choke.

That said, fuel injected engines use three general EFI strategies 1)SPEED DENSITY: use manifold pressure, temperature, throttle position and engine speed to calculate the air volumes and therefore fuel requirements, 2)MASS FLOW: uses an air mass meter to physically measure the incoming volume of air, or 3) a combination of the two strategies.

The physical enrichment is managed by lengthening the pulse width of fuel injector pulses.

Since i suspect you have a lean condition on cold start, and I think Mercruiser uses the first strategy - i would suspect an engine air or coolant temp sensor is non functional.

Anyone know if you can scan this thing for stored trouble codes?

Also - make sure you've got a strong spark. The richer mixtures provided on cold start are harder to ignite with a weak spark.

Low fuel pressure and or an air leak could also cause the issue but you would have other symptoms as well.

just thoughts."

"MistaHays:

"just thoug

"MistaHays:

"just thoughts" were good ones. I am looking at the block diagram for troubleshooting EFI. The fuel "prime pulse" needed to start the engine comes from the following: a yellow wire is connected at the slave solenoid where the yellow w/red stripe starter solenoid wire connects. This wire runs to a rectifier (line filter for AC volts?); the outputs are a green wire and white wire that join together as one; the white wire continues to the fuel pump to supply voltage to it at engine start.

Once oil pressure is achieved the power for the fuel pump is supplied from the purple wire that comes from the ignition coil thru the oil pressure switch where the wire color changes to yellow and then goes to a different rectifier. Its outputs, green and white wires, are tied together and are connected in paralell with the first rectifier's output to the fuel pump.

My theory for the hard start issue is that the first rectifier above is open (or the wire is defective or connection at the solenoid is corroded) and does not supply voltage to the fuel pump to provide a priming pulse when the key is turned to start. Once oil pressure is achieved after the engine cranks a little, the oil press. switch closes and it sends voltage to the fuel pump and it begins to provide fuel as told by the ECU (based upon the conditions it recieves from its sensors) to the engine; thus the stumbling start. The rectifiers are not shown as part of the EFI unit in this diagram.

To prove my theory just run a 16 ga. jumper wire from the slave solenoid connection where the yellow w/red stripe wire connects to the white wire on the fuel pump and turn the key to start. If the fuel pump kicks in immediately; problem solved.
Hope this helps solve the start issue."

Just a quick update.Borrowed a

Just a quick update.Borrowed a ddt to check for fault codes - nothing all good. Tried the jumper wire with no improvement also
tried a new IAC and to no avail. It's a bit of a difficult one to diagnose as once it fires up it's fine then i have to wait 2 or three days for the troubles to begin again.

It's had a new distributor cap and rotor which did seem to improve it slightly but not to how it should be.

hate it when an engine gets the better of me.

Is there a fuel pump relay in

Is there a fuel pump relay in the circuit? Yes? Contact to prime circuit could be burned and failing intermittently.

"You should check your fuel pr

"You should check your fuel pressure at the test point on the fuel rail. It should be about 40 psi, before you start and while running. If not, you likely have a fuel pump problem, or relay as Guy says.
You can borrow a proper fuel pressure gauge or buy one quite inexpensively at AutoZone or NAPA.
My 6.2 MPI has 2 pumps; one is a booster that sucks fuel from the tank and feeds it to the cannister filter, then a second that is housed in the Coolfuel module down low on the port side. This second pump provides the 40 psi needed to the fuel rails. If one or the other is not doing its job, you will not be getting sufficient pressure.
Rod"

303hound:

It sounds like

303hound:

It sounds like Rod has figured out where your problem is located since the jumper did not work. Are you checking for voltages along the way espc. for drops in voltage? A failing pump can be located by the amperage that it draws. If you know the baseline draw you can see if there is an increase in amperage at a pump when running--that pump is getting ready to fail.

"Another thought:
You could t

"Another thought:
You could try a shot of standard fuel injector cleaner thru the fuel. This would work best if the fuel level in your tank is somewhat low, like less than half full.

Sometimes when injectors get fouled they cause hard starting when cold, but work just fine thereafter.

Your problem may be that simple. It's likely the first thing (and the cheapest) to try.

Rod"

"I don't think it has anyt

"I don't think it has anything to do the fuel system. The symptoms you describe are consistant with low voltage during cranking.

When your cranking the engine, the voltage is so low that the coil can't produce a good spark and/or the ECM is not functioning. The moment you release the key the voltage jumps up, and if the engine is still rotating, it may start, or it may act like it's trying to run backward.

Check your battery, connections, ect., or you could have a bad starter that draws too many amps when cranking.

Dave"

"To test dave's idea, watc

"To test dave's idea, watch your voltmeter while cranking. It should not drop below about 9 volts. If it does, your battery is weak, or you have bad connections, or something similar.

A quick easy test would be to jump it with another boat (running), or known good battery. If she jumps into life, there's your problem.

Rod"