PDA

View Full Version : VERY HARD TO START MERCRUISER 74 MPI 1999



mstanley
09-11-2007, 02:48 PM
"I HAVE BEEN WORKING ON A 1999 MERCRUISER 7.4 MPI WHICH IS REFUSING TO START. WHILE CRANKING IT WILL SEEM TO PICK UP SPEED BUT JUST WON'T START.
I HAVE REPLACED THE FUEL PUMP AND THE FUEL PRESSURE REGULATOR, THE IGNITION RELAY, PLUGS, CAP, ROTOR, PLUG WIRES, PICKUP COIL, COIL, AND THE CONTROL MODULE ON THE DISTRIBUTOR I HAVE ALSO PUT IN NEW FUEL AND A NEW FUEL WATER SEPARATOR. I MUST ADD THAT IT HAS STARTED TWICE IN THE LAST TWO WEEKS AND ONCE STARTED SEEMED TO RUN OK BUT AS SOON AS YOU TURNED IT OFF IT WOULD NOT RESTART. TIMING SEEMS TO BE RIGHT BUT IS IT POSSIBLE FOR THE IGNITION TIMING TO BE OUT OF TIME WITH THE INJECTION TIMING. THANKS FOR ANY HELP YOU MIGHT BE ABLE TO GIVE"

ratmower
09-11-2007, 10:30 PM
"Mark
Sounds like the only thing you havent replaced is the engine itself.
I would be checking the whole wiring loom for bad connectors or loose wires, check all your grounding points.
You have replaced the fuel pump but are you sure it is working correctly ?
Get your local marine guy to do a code check on your computer and see if there are any fault codes stored.
Cheers
Peter C"

bill999
09-12-2007, 12:13 AM
"Try starting fluid to get it started. If it starts, continue with short squirts of starting fluid just to see if it will continue running. If it dies on its own than you have a fuel issue of yet undertermed cause.
Make sure you have good ventilation and fire extinguisher handy. Engine could spit/backfire back thru air intake. Do not over spray start fluid, keep it in the intake."

mstanley
09-12-2007, 06:02 AM
"Bill,
I have tried starting fluid several times but it really did not seem to help."

mstanley
09-12-2007, 06:06 AM
"Peter,
I will check all the connectons next. I have not taken this boat to a marine service man because the closest is about an hour away."

ratmower
09-12-2007, 06:39 AM
"Mark
let us know how you go, just make sure eveything is tight and clean, 0.2 of a volt difference in specs in some circutes is enough to give problems in a fuel injection system.
Try wobbling all your connectors to make sure they are firm and making good contact.

Cheers
Peter C"

mstanley
09-13-2007, 09:50 AM
"Peter,
I have started to look over all the wiring and I have found the grounding studs to be very rusty. Would this explain why although I am getting fire my friend says the fire does not look right in that it is sometimes yellow and jumps around on the plug instead of being blue and going from electrode to electrode. Does the spark need to be a certain heat to ignite. If so this might be my problem.
Thanks for everyones help."

ratmower
09-13-2007, 06:55 PM
"Mark

You are looking to get a nice blue looking spark definitly not an orange/yellow looking one.

Clean up those grounding points but also check what voltage you are getting at the positive side of the coil with the ignition in the on position and let us know what it is.
Make sure you pay particular attention to all the connections around your ignition pack and coil.
Be care full when you are winding over your engine looking for sparks aside from the fact we do not want you blowing yourself up, you need to make sure that you have the body of the plug you are looking at, well grounded out to the engine block properly.
If I am looking to visualy confirm that there is good output out of the coil I will rather use a cheap spark teater that is grounded out to the engine or open up an old plug to about a 3mm gap and ground it out to the block using a jumper lead.
When ever you are checking for a spark make sure you have your bilge well ventilated.

Cheers
Peter C"

mstanley
09-14-2007, 11:57 AM
"Peter,
I cleaned the grounding points but still seem to have a weak spark. The input to the coil reads a little more than 12 volts. When I try to start the motor it seems like the spark is not hot enough to ignite the fuel and then after a bit becomes flooded.
Thanks for your help."

guyjg
09-14-2007, 01:26 PM
"Mark:

Make sure your battery is fully charged and terminals are clean. Old cables? May be corroded internally which drops the voltage and current.

Try a different coil output wire and make sure the rotor tang that connects to the center electrode in the cap is actually making contact.
You can substitute a plug wire for the coil wire to test it.

Does a computer contol your timing and injectors?
If not, since you changed the pickup in the dist., reset the timing."

mstanley
09-14-2007, 02:18 PM
"Guy,
My batteries have full charge and I have already replaced both battery cables. I will check the coil wire and the rotor tang. My understanding of this motor is that the computer controls the timing and the injectors but I will explore that a little more.
Thanks for your help"

bill999
09-14-2007, 03:12 PM
"Yes to a previous post, a weak spark will be killed by air/fuel mixture in comb chamber.
Check your coil, use ohm meter, Primary should be 1 ohm and secondary 12-15k. This is qeneric values, actual would be listed in manual for your engine.
Check you plug wires with ohmmeter. If you use resistor wire, resistence should be 250 ohm per inch of wire and max of 1000 per inch.
If they are not resistor wire, just check for continuity."

gkm24
09-14-2007, 03:30 PM
What voltage is at the coil when it is cranking?

guyjg
09-14-2007, 05:47 PM
Mark:

I know you replaced everything except the engine. Did you run the ignition test that I gave you. There may be a defective part in the new ones.

ratmower
09-14-2007, 06:35 PM
"Mark
Are you 100% sure you got the right coil when you replaced it originaly ?

As Grahame mentioned above, what is the voltage at the + side of the coil when you are cranking your engine over.

You have replaced a lot of things, did you start to replace everything to fix the problem you have now, or is this a different problem from what originally prompted you to start replacing everything.

Do you have a manual ? Computer controled ignition systems have what is known as a base timing point, you need to make sure that your base ignition timing is set as per procedure and specs for your engine.

If your fuel/air mixture is to rich as Bill said your engine will flood just like if you had a carburetta and your choke was jammed on, some computer systems are designed so that when you hold the throttle flat out while cranking they will shut down the fuel pump so as not to over fuel the engine.
make sure your plugs are dry, hold it flat out and see if it starts.

Try those few things and get back to us.

Keep Looking.

Cheers
Peter C"

mstanley
09-14-2007, 07:50 PM
"Guy,
I did run the ignition test and rotor and cap turned out ok. I also went directly to the coil with a plug wire and checked the spark. It was yellow-orange and looked weak. So it seems it is the coil(this is the second coil I have tried) or something befor the coil.

Thanks for all your help."

mstanley
09-14-2007, 07:54 PM
"Bill,
I will run the test you suggest tomorrow and get back to you.
thanks for your help"

mstanley
09-14-2007, 07:56 PM
"Graham,
Do you mean to test the high voltage side of the coil or the voltage coming into the coil when cranking.
thanks"

mstanley
09-14-2007, 08:06 PM
"Peter,
I replaced the coil with a match from the coil that was on the boat when I bought it. The motor was doing the same thing with the first coil and I just assumed it was weak so I replaced it. Maybe the first coil was wrong it looks the same as the manual but Ill check the part numbers. Base timing for this motor I think is 8 before tdc I have place the motor at tdc and then lined up the rotor and cap. It has started twice over the last few weeks using this timing method but other than those two times refuses to start. The manual does say that wot will stop fuel flow to allow recovery from a flooded condition and I have tried that over and over. I will try a dry start next time to see what happens.

Thanks for your help"

mstanley
09-15-2007, 12:41 PM
"Graham,
I checked the voltage at the coil and it was 12 volts plus with the key on and not cranking. When you move the key to the cranking mode the voltage drops about three volts. Is this normal?
Thanks for any help you can give."

guyjg
09-15-2007, 05:41 PM
"SELOC manual states the following:

Special timing words for engines equipped with EFI. Identify the Red/Yellow lead at the ECU. Normally this lead is NOT connected to anything. However, when timing an EFI engine, this Red/Yellow lead MUST be grounded to the engine block. Failure to ground this lead will produce a retarded timing adjustment. When done disconnect this lead and protect it from accidental grounding. Start the engine and adjust idle speed to specs."