Mercruiser 5.0L '98 w/ carb - cranks but won't start

[chip1]

I have a '98 mecruiser 5.0 carb I/O... Fuel pump isn't coming on. Checked fuel pump by applying 12v directly to it and it seems fine. I can actually run the engine if I do this... So then I check the voltage on the other end of the pigtail (purple w/ yellow wire plus ground) and while cranking I get anywhere from 9.5-11.5v fluctuating, which doesn't seem to want to run the fuel pump!!! I have removed the purple w/ yellow wire from the starter and cleaned it, I have also jumped the oil pressure switch (purple to purple w/ yellow) and neither of these have helped. Already replace fuel pump and backflow valve in tank.

Also, had the alternator/generator replaced at the start of the season....maybe I should check the purple wire from alternator to ground as this would be the primary supply voltage to the fuel pump during cranking...yes?

One more question...is there any possibility that the alarm warning circuit could impact this problem? (It may be coincidental but once it was all previously running it seems that things eventually got screwed up
once the oil warning wire (tan) was reconnected)?

Next steps in the diagnosis?

 

[hivoltg]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

The fuelpump should draw it's power from the ignition circuit (Purple wire) in the engine's harness, it will route thru the oil pressure switch for saftey, that is the fuel pumps primary running circuit. From the small excited terminal on the starter's solenoid, there's another wire (Purple/Yellow) that runs to the fuelpump to run it while the starter is cranking the engine, but before there's oil pressure to turn it from the other circuit. Some systems also use a relay to power the fuelpump for a few seconds when the key is turned from off to on.



Im not sure on your boat if it has another sensor not letting the fuel pump run like a low outdrive oil reservoir sensor or something. If you post a serial number im sure some of the experts can confirm that.

 

[alldodge]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

Your ECM controls the initial turn on of your fuel pump. When you turn your key it sends a signal to the ECM and it turns it on for 3 seconds which you stated is not happening. This is why when you bypass the oil switch it still will not come on, and why it comes on when it is hot wired. The below link is a good resource to troubleshoot your system. Have a run through it and let us know how it goes or if additional questions come up.

http://www.google.com/url?sa=t&rct=...HwILDTweG4kXdvqfw&sig2=L_766O4UamqS7bH6bJOv2g

 

[chip1]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

Your ECM controls the initial turn on of your fuel pump. When you turn your key it sends a signal to the ECM and it turns it on for 3 seconds which you stated is not happening. This is why when you bypass the oil switch it still will not come on, and why it comes on when it is hot wired. The below link is a good resource to troubleshoot your system. Have a run through it and let us know how it goes or if additional questions come up.

http://www.google.com/url?sa=t&rct=...HwILDTweG4kXdvqfw&sig2=L_766O4UamqS7bH6bJOv2g

I didn't know there was an ECM in a carbureted system? I thought that was only for FI....

 

[alldodge]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

I didn't know there was an ECM in a carbureted system? I thought that was only for FI....

Your right my mistake, should pay more attention, sorry.
What is your serial number

 

[chip1]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

Your right my mistake, should pay more attention, sorry.
What is your serial number

serial # 0L072065

 

[alldodge]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

I have a '98 mecruiser 5.0 carb I/O... Fuel pump isn't coming on. Checked fuel pump by applying 12v directly to it and it seems fine. I can actually run the engine if I do this... So then I check the voltage on the other end of the pigtail (purple w/ yellow wire plus ground) and while cranking I get anywhere from 9.5-11.5v fluctuating, which doesn't seem to want to run the fuel pump!!! I have removed the purple w/ yellow wire from the starter and cleaned it, I have also jumped the oil pressure switch (purple to purple w/ yellow) and neither of these have helped. Already replace fuel pump and backflow valve in tank.

Also, had the alternator/generator replaced at the start of the season....maybe I should check the purple wire from alternator to ground as this would be the primary supply voltage to the fuel pump during cranking...yes?

One more question...is there any possibility that the alarm warning circuit could impact this problem? (It may be coincidental but once it was all previously running it seems that things eventually got screwed up
once the oil warning wire (tan) was reconnected)?

Next steps in the diagnosis?

Lets try again, Red from the battery goes through your circuit breaker to red/pur which is routed to your ignition switch. When your ignition switch is turned to run it supplies 12V to the pur wire. The Ignition switch pur wire supplies power to th alternator and most everything else.

You stated you can connect a jumper from the bat to the pump and the engine will run. Power comes from the ignition switch not the alternator connection. Check your connections at the main pig tail connector from engine to dash wiring harness. Jumper across the run circuit in the key switch PUR to RED/PUR. If this jumper works then it's either the key switch of the terminal connectors at the switch. If this doesn't work you have a bad connection or the PUR wire in the harness

 

[chip1]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

All Dodge, thanks for the response.

As a point of clarity, if the run circuit (ignition switch) was bad would the engine still crank? It cranks just fine... I think initial power to the fuel pump comes from the Starter solenoid while cranking. It currently appears that there is not enough voltage at the fuel pump during cranking to get gas into the carb. When I hotwired the fuel pump directly and then turned the key it started and ran fine...

Are you saying that the ignition switch could still be a possible culprit in the voltage loss I am seeing? I will check the run circuit tonight after work....

I am also thinking that any of the wires to the alternater should be checked. In the spring when I first started it, it alternator was rusted frozen and the torque bent the alternator backets to 90 degrees from normal (the belt never even snapped). So residual damage to the connections could be a great place to look.....

 

[alldodge]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

All Dodge, thanks for the response.

As a point of clarity, if the run circuit (ignition switch) was bad would the engine still crank? It cranks just fine... I think initial power to the fuel pump comes from the Starter solenoid while cranking. It currently appears that there is not enough voltage at the fuel pump during cranking to get gas into the carb. When I hotwired the fuel pump directly and then turned the key it started and ran fine...

Are you saying that the ignition switch could still be a possible culprit in the voltage loss I am seeing? I will check the run circuit tonight after work....

I am also thinking that any of the wires to the alternater should be checked. In the spring when I first started it, it alternator was rusted frozen and the torque bent the alternator backets to 90 degrees from normal (the belt never even snapped). So residual damage to the connections could be a great place to look.....

With this being a carb engine their should be enough fuel in the carb even without the pump running to start the engine; That is unless the carb has a leak and it dries out. When you crank the engine 12V comes from the starter selonid to the pump bypassing the oil pressure switch. Under normal conditions there is enough fuel in the carb to start the engine without the pump. The connection is there to supply fuel if the carb is fairly empty. Once the engine starts, the key is released and 12V comes from the run side of the key switch through the PUR wire. Oil pressure increases, oil pressure switch closes and passes the 12V to the fuel pump an it is turned on again.

You can apply 12V direct to the pump and it will run, but it will not run if the jumper is removed. So there are two issues, you cannot start the engine when cranking meaning the PUR/YEL wire is not supply the correct voltage. The engine will not run with the oil pressure switch being bypassed (PUR wire).

By the way, in your first post you mentioned the Tan wire going to the oil pressure switch, the "Tan" wire should go to the water temp sendor. The "Light Blue" wire goes to the oil pressure sender.

Check your 90AMP fuse on the starter post, make sure it has not been blown. Don't think it has because you can crank your engine, but something has either been left off or a bad connection.

The possiable causes are key switch, bad connection at starter, corrision or mis-wiring

 

[chip1]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

So the mechanic at my marina looked at it...ran through several of the same tests I did and he says he noticed the choke needed adjustment. After he adjusted it it started right up?!?!? He says he waited overnight and went back again today and it started right up....He even to it for a "water test"...and no problems....

I 'm not buying it...I think it is intermittent either a short to gnd or corrosion on a connection. I should be able to start a cold engine on an 80 degree day with the choke wide open!!!

Need more investigation as well as a new mechanic!!!


I guess I'll spend time on the boat tomrrow and hope I don't require a 4th tow back....

 

[alldodge]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

Something doesn't add up and I look forward to the further investagation

 

[Afiorenza]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

Something doesn't add up and I look forward to the further investagation

I'm having the same issue with my 2005 mercruiser 5.0L carburated engine. Where is the oil pressure switch located?
My engine runs fine if I jump the electric fuel pump directly to the 12v battery. But the pump dies when I connect the purple/black connector to it. I have checked the voltage and cleaned the connector, but no success.

 

[Bt Doctur]

Re: Mercruiser 5.0L '98 w/ carb - cranks but won't start

usually just above the oil filter has 2 wires going to it

 

[chip1]

So I know this is an old thread and I have now been having this problem for over a year.... Last fall I replaced the Oil Pressure switch since that is the ONLY thing I could think of that would cause the fuel pump to stop being energized!

I ran it this year so far 4 times out and no problems until yesterday. Same old gremlin was back. Cruising at 4000RPM and the thing just dies...It won't restart because it has used all the available fuel in the Carb float bowl. I created a set of jumper wires with alligator clips and if I jump 12V to the fuel pump, I am good to go! Obviously the fuel pump power supply is cutting out somewhere (and completely random). The schematic only shows the Purple wire going to the Oil pressure switch and then to the alternator.... My next examination will be that connection back to the wiring harness (or if it directly connected to the alternator) to see if there is a bad connection. Not ready to go buy a new wiring harness yet. Without jumpering the fuel pump, it will not restart until the next day....really weird.

Any other thoughts?

Also, when this happened last year, I spliced a fuel bulb into my gas line between the tank and the fuel pump and I was able to limp home as long as I was pumping the bulb often enough.

 

[Bt Doctur]

power to run the pump comes from the purple wire.it powers the choke, alt lead, ign coil, etc. the purple wire goes to the pressure switch, the other side of the switch goes to the pump.Remember the main harness plug powers everything.If your sure the fuel pump is losing power start tracing the wires back.
If you do find the pressure switch bad again, wire in a cube relay to handel the amps of the pump and not the thin contacts of the pressure switch.

 

[chip1]

Bt Doctur thanks for the input. I will try to follow it back - unfortunately I will have to wait until it happens again! (I can probably force it by inviting lots of guests on the boat!!!) :lol:

I didn't check yesterdays failure but previously when it happened, I tried jumping across the oil switch - effectively bypassing it - and it still supplied no power to the fuel pump...so I surmised that wasn't the problem...

I also missed a previous suggestion from AllDodge "Check your connections at the main pig tail connector from engine to dash wiring harness. Jumper across the run circuit in the key switch PUR to RED/PUR. If this jumper works then it's either the key switch of the terminal connectors at the switch. If this doesn't work you have a bad connection or the PUR wire in the harness"

which I will try as well...but actually....if the key allows me to still turn over the engine would this test still help me determine a faulty key switch?

I am to the point where I am convinced the purple wire has a fault somewhere

 

[Bondo]

.if the key allows me to still turn over the engine would this test still help me determine a faulty key switch?

Ayuh,.... Welcome Aboard,...... That only means the key switch has power, 'n can send it down the Start circuit,.....

The Ignition circuit could still be dead, from the key switch,....

2 separate circuits, 2 separate wires,...

 

[chip1]

Ayuh,.... Welcome Aboard,...... That only means the key switch has power, 'n can send it down the Start circuit,.....

The Ignition circuit could still be dead, from the key switch,....

2 separate circuits, 2 separate wires,...

So it has not happened again yet but I looked behind the dash and the PUR wire goes from the key switch in series to just about every gauge on the dash and then I assume back to the engine. If ignition circuit was bad, then I would think that all gauges would go dead when the motor dies correct?

I pulled the connector apart where the pigtail of the wiring harness meets the dash wiring but saw nothing visibly wrong. Still need to have it fail again before I can use the multi meter to better identify issue.... Don't really want to unwind all that electrical tape that wraps the wiring harness together but that may be the next step... It looks like the wiring harness pigtail is actually screw mounted to the engine back behind the distributor while the other male connector that goes to the dash is just plugged into it (and somebody hose clamped it to the female end so is doesn't fall apart ?!?)

 

[chip1]

Too funny! Just found this son Sterndrives.com...

The Instrument Harness connects the engine to the dash's keyswitch and gauges.

The Motor Harness and the Instrument Harness connect together through a large "cannon" plug. This plug is a likely source of problems as it can work loose or the connector pins can corrode and make for poor contact. The plug is designed to be held in place with a standard hose clamp. If the hose clamp is overtightened it can ruin both harnesses.

Mine seemed to come apart VERY easy when I pulled it off the other day...and I didn't even loosen the hose clamp !!!

 

[chip1]

So after 2 seasons I found my problem. It is an intermittent connector at the Fuel pump snap connector. I was able to jiggle it around and have the pump go on and off randomly (when I was connecting directly with 12V from the battery to the wire coming off the oil pressure switch). This explains why neither the starter circuit nor the Oil pressure circuit would let it run when it randomly occured...and also why if I hot wired the fuel pump to 12V directly it seemed to run fine. Don't worry, I only hot wired it directly when I was stranded out on the water

Now I just need to figure out what kind of replacement plug connector I can find to change the existing one out....