1996 Mercury 200 EFI Fuel Pressure Loss

[mllr90]

Having issues troubleshooting fuel pressure loss. About every 6 or so attempts to start, the motor will crank, but not start. It sputters a little a first, and then won't do anything but crank. After the boat has sat overnight, it will start right up like nothing happened. It also starts on the muffs every time (of course) with 35 psi. at the high pressure shrader valve.


The mechanic replaced the high pressure fuel pump and cleaned out the VST. The next time the motor had failed to start, I put the fuel pressure gauge on after the high pressure pump and the readings jumped between 0 and 20 psi. The 20 psi spikes would rapidly go back to 0 psi. I put a timing light on each cylinder, and they all had a spark with no abnormalities I could see in the timing light. The high pressure pump is running, and has between 2 and 12 volts on the negative terminal. I jumped the negative terminal on the high pressure pump to ground and it did not change the 0 to 20 psi fuel pressure.

Once I got back to my shop, I found the VST was full of oil. I assumed the low pressure pump was failing and the oil meter filled the VST with oil.

I then replaced the fuel line and primer bulb. I also rebuilt the low pressure pump. The boat ran well on plane, but had a rough idle for a few days. And then it was the same fail to start. I was not there to troubleshoot. Now I've drained the VST and it looks like like the 50:1 ratio you would expect.

Now I'm heading to the lake to repeat the failure this time. Anyone have ideas on what to look for? Is there a way to test the EFI ECU? If the new high pressure pump is running I'm assuming it's working, is this a fair assumption? Is there a way to test the fuel pressure regulator?

Thanks in advance for your help! Any advice is appreciated.

 

[mllr90]

UPDATE:

Went to the lake. Started troubleshooting with the merc service manual. The fuel pressure would surge between 30 and 39 psi. I pulled the fuel regulator off. When the pressure was 30 psi, there was no flow. When the pressure was 39 psi, there was a solid stream of fuel. So that checked out OK. Then I measured the volts across the positive and negative terminal of the fuel pump. They were all within spec with the key off, on but not running, running below 2000 rpm, and running above 2000 rpm. So then I took the negative terminal (black with purple stripe) off the pump. Then I connected the the negative terminal of the fuel pump to ground. The fuel pressure then went to 39 psi and stayed there until I motored back to the ramp. The boat would not get on plane at this point even with 39 psi fuel pressure, so there may be other issues.

I got back to the shop, and the oil tank (in the boat) was pressurized. So I ordered a new check valve for that. I also did a compression check Starting at cyl #1 and going up numerically, 122, 112, 125, 118, 118, and 100 psi. Is the 100 psi compression on cyl #6 an issue?

Right now my plan of action is to send the injectors off for cleaning, have the EFI computer tested, and replace the oil pressure crankcase check valve. Am I on the right track? Thanks in advance for any advice!

 

[Dukedog]

disconnect both wires at fuel pump coming from ecu harness and tape 'em up... find tha purple on tha starboard side of tha motor...thats gonna be your "new +" for tha pump.. its a key on switched hot... neg side of pump goes ta any motor ground.. (can leave it like this forever if ya want to).. this will take tha pump driver circuit of tha ecu outta tha equation.. pump will run 35 ta 39 lbs anytime key is on and eliminate tha pump as a problem if tha pressure stays up... when testin' and it acts up but tha pump is still runnin' (vst probably not keepin' up) pump tha bulb ata rapid rate fillin' up tha vst.. if motor comes back ta life tha mechanical pump and/or fuel supply lines are bad....

if tha thing wouldn't plane but seemed ta be runnin' smooth, just not wind up like it should ya might look at tha stator.. bad high side.

tha oil tank is supposed ta be at pressure.. thats what pushes oil up to tha motor tank.... its a closed system.

 

[mllr90]

Thanks for the reply. I did run the negative terminal straight to ground. The fuel pressure went straight to 39 psi. The boat still ran like ****. Just sent the EFI/ECU and the injectors out for service. Will keep you posted.

 

[mllr90]

UPDATE: Sent the injectors off for service. Two of them were running a little dirty. Also sent the ECU off to Brucado. The fuel delivery circuit was running several volts low. Tony at Brucado sold me the Brucado ECU unit and it went on with no issues. Started the motor up. It started much better. It started consistently. Fuel pressure was a consistent 39 psi. I think the new oil check valve solved the hard start problem as it would fill the VST with oil and no gas.

Anyway. The boat still had trouble getting on plane. It was definitely not making the power it should. About half of the time the engine prop would blow out (cavitate). So back to the shop. I noticed the stator had a coil that showed discoloration from what I'm assuming was heat. I hooked the yellow stator leads to an oscilloscope. One lead was making 15 volts, but the phasing looked dirty. The other lead was only at 3 volts peak-to-peak. So I took the stator off. One coil was burned up. I could get an ohm reading across the stator wires themselves of about 100,000 ohms. So that was bad. I also found one of the regulator leads had direct short to ground. So I have ordered a new stator and two new regulators.

Will keep updating.

 

[Dukedog]

100 lbs is low.. 118 is approachin' borderline 115 and lower (for me anyway) need ta check on occasion after ya get it straightened out...

tha "blow out, cavitation" (a little is good) is a set up and/or trim, prop problem ya may need ta work on too!.. again, after ya get tha motor healthy....