ECM 15 amp fuse keeps blowing- 2000 5.7GSi

The 15 amp fuse powering the ECM keeps blowing and I can't pin down a short. All the harnesses are intact and the connectors seem to be corrosion free. The SELOC wiring diagram for the 2000-01 5.7 GSi engine doesn't show my engine's fuse block but using the 4.3 diagram that appears to match, it looks like this 15 amp circuit feeds the ignition/fuel relay. Could a defective relay be causing the issue? SELOC doesn't have a troubleshooting procedure for this very frustrating issue.

If I replace the 15 amp fuse with a 20 amp it isn't blowing. My ammeter is showing an inconsistent draw, sometimes well under 15 amps (11-12) and sometimes spiking to just under 20 amps. There does not appear to be any other issues. The starter cranks normally. 1387 hours on engine; original starter and alternator. Alternator is putting out 13.2 volts at 2500 rpm with normal load (GPS, VHF). Could it be a faulty alternator diode feeding back to the relay?

Any assistance would be greatly appreciated.

Is this a new problem? IFSO when did it start?

Captain Bob,
New problem May 12 at 1387.1 hours. Engine wouldn't start. Original relays, original starter, replacement fuel pumps 4/2014 at 1116 hours. Original fuel pumps were okay I was just being proactive. Ignition key tests okay. I pulled the fuel pump and fuel/ignition relays yesterday. Both show very slight corrosion on one terminal. Replaced both relays. Keeping the 20 amp fuse in the circuit the boat is starting and running normally. I'm just perplexed why the 15 amp is blowing as soon as KO. Rechecked all the harnesses and they seem to be perfect, no fraying, burned insulation, etc. What is really interesting is the SELOC wiring diagrams prior to '99/'00 and after '00 all show this as a 20 amp fused circuit.

My GUESS... The replacement fuel pump draws more current that your original?? Could be a change by the OEM in the pump design, i.e. more pump power/more current draw or if you used an aftermarket, it was an "equivalent" and not an exact replacement. The purpose of fuses is to protect from catastrophic failure that would lead to a fire. Going from 15A fuse to a 20A should be OK. If you start to blow the 20, then you have a real problem. The slight corrosion on those points should not cause fuse to blow. Did not see where you replaced fuel filters???? Did you ever determine why the engine would not start in the first place? BTW... fuses ratings and current capacity... fuses are generally designed to carry the rated current, generally indefinitely and blow at a certain defined time at 2x the rating (@ 68 degrees & faster if hotter )...

I was thinking it was possibly fuel pump related but I replaced it with VP OEM pump. But you could be correct that they now sometimes draw more than 15 amps. Filters are replaced every season, 4/1/17 at 1381.5 hours so they have just under 6 hours. I have the stock OEM filter and added a Racor filter about 12 years ago.

As for the original no start last Friday May 19, I was out drift fishing and occasionally starting the engine to move. Maybe 4-5 times with no issue. Went to start it, engine was still probably warm, 130-140, it kicked once and then stalled. Went to start it again and no beeps, engine cranked, no start. Water temp was 66, air 83. Full tank of gas. I thought it might be the ICM and since I didn't have a spare with me and it was getting dark I called for the tow and started troubleshooting the next day.

The ICM checked out (was replaced when I rebuilt the distributor 6/14 @ 1123 hours). Coil was also replaced at the same time. Checked the fuses at the engine fuse block and the 15 amp ECM fuse was blown. Replaced it, started right up and started two more times. The next day it was blown again, blew as soon as KO no crank. Replaced again with 15 amp, same story. Replaced with 20 amp and checked current draw. Seems perfectly fine with the 20 amp. So I guess I'll just keep my eyes on it.

BTW, big thanks for jumping in.

Was the original coil bad?.... IF not (and you still have it) put it back in. Again.... OEM or replacement coil. Alternatively.... check to see if coil is getting hot when it runs.

No the original coil was good just decided to replace proactively, I still have it. And I replaced it with a OEM coil. Let me see how warm it is while running. Good idea!

7/3/17 Update
Capt Bob,
So it had been running normal with the 20 amp fuse until the other night, maybe an additional 10 hours since the first incident. A few days ago I was drifting and starting the engine multiple times. On about the 5th or 6th restart the fuse blew again. I replaced with another 20 amp and it immediately blew. I replaced with a 25 amp and it restarted and I made it back to port running normally. I tested the coil and both the primary and secondary circuits were outside of the normal range so I replaced it with a new coil on Saturday. I replaced the 25 amp fuse with a 15 amp and the engine started with no issues. I ran it for a few minutes and restarted it a few times with no problems.

Yesterday I started it up cold in almost 90 degree ambient temperature. It ran for about 2-3 minutes at the dock and then quit, first time it has quit while running. I checked the fuse and it was blown. Replaced the 15 amp with a 20 amp and it immediately blew. Replaced 20 amp with a 25 amp, same story. I have checked grounds for corrosion and have found no apparent issues. Today I intend to replace the fuel pump with the original pump that I saved.

Do you have any other ideas what could be driving this issue?

Do you have access to a digital multimeter?

Check your fuel pump and ecm relays for dirty connections as well as all the engine connectors. A poor connection can create resistance, high resistance equals more amp draw. Pull all connectors apart and clean with electrical cleaner and put back together with dielectric grease. Sand all grounds metal to metal including components that use mounting bolts for a ground. When you put the connectors together use a jewelers screwdriver to pus all the pins in on both sides of the connectors.

7/3/17 Update
Capt Bob,
So it had been running normal with the 20 amp fuse until the other night, maybe an additional 10 hours since the first incident. A few days ago I was drifting and starting the engine multiple times. On about the 5th or 6th restart the fuse blew again. I replaced with another 20 amp and it immediately blew. I replaced with a 25 amp and it restarted and I made it back to port running normally. I tested the coil and both the primary and secondary circuits were outside of the normal range so I replaced it with a new coil on Saturday. I replaced the 25 amp fuse with a 15 amp and the engine started with no issues. I ran it for a few minutes and restarted it a few times with no problems.

Yesterday I started it up cold in almost 90 degree ambient temperature. It ran for about 2-3 minutes at the dock and then quit, first time it has quit while running. I checked the fuse and it was blown. Replaced the 15 amp with a 20 amp and it immediately blew. Replaced 20 amp with a 25 amp, same story. I have checked grounds for corrosion and have found no apparent issues. Today I intend to replace the fuel pump with the original pump that I saved.

Do you have any other ideas what could be driving this issue?

Yes I do have a digital MM. What should I be checking?

I have replaced the fuel pump and ECM relays and checked connections for corrosion and I always use dielectric grease on every connection I pull apart. There were 7 ground wires on the coil mounting bolt. They were clean but i cleaned each and re-bolted with grease. After I replace the pump I intend to pull the harness off the ECM and recheck for corrosion, I have previously reinstalled those harnesses with grease. The main harness on the back of the engine is the only harness I haven't pulled apart so that is also on my list.

With engine off
1)Set MM to ohms.
2)Find two points, one either side of the fuse that keeps blowing and measure the resistance.
3) record this value as "R"
4) Set MM to volts
5) Start engine and immediately measure the volts "V" across the same points that you used to measure ohms.

The current thru that point is equal to V/R, i.e. volts divided by ohms. Pay attention to the decimal points. Make sure you are using Volts as volts and not mV ( as some MM will do for you)

Fuses are designed to carry ( under normal temperature conditions) the rated value indefinitely and blow after some delay at 2 times the rated value ( unless its a fast blow or a slo blo).

If you are seeing less than the fuse's rated current ( as calculated), watch it for awhile and see if the volts you read spikes.

YES I know that MM often have DC amp scales, but rarely at the current level needed to diagnose this problem... and measuring DC current for prolonged period will result in the internal shunt in the meter heating up... unless you have a really good industrial MM.

Update- problem resolved
The underlying issue was the throttle body injector harness was shorting out between the spark arrestor and the throttle body. After almost 18 seasons and 1400 hours the insulation on the wire was starting to break down.

Glad you got it sorted out and thanks for posting the resolution, others may benefit from this thread. Good job and safe boating.

monterey262 said:

Update- problem resolved
The underlying issue was the throttle body injector harness was shorting out between the spark arrestor and the throttle body. After almost 18 seasons and 1400 hours the insulation on the wire was starting to break down.

Click to expand...

Any way you can post a picture where? I’m chasing the exact same problem