Undiagnosed issue with 1991 175 XRI

[jpd_hack]

Hey all, I have a 1991 Mercury 175 XRI 2.5L with serial number ODO50927. It's ran well for the most part since I bought my boat in 2020. I had fuel lines replaced when I first bought it, have done routine maintenance (fuel filter, water pump, etc) and also had the injectors cleaned by Don Weed over the winter. It was running better than ever at the beginning of the season.

In May, it started behaving oddly when running at full throttle. It would run the usual ~5800 RPM at 57-59 mph and then would suddenly stutter and start running low 50s, then sometimes pick back up. This worsened and when running full throttle it would start shuddering badly and barely stay on plane. I'd throttle down and then could get the boat back on plane and run fine as long as I stayed outside of WOT (e.g. 4500-5000 RPMs no problem). Some days I'd take it out with no issue and could run full throttle, other days it would behave this way. About a month ago, I fished a 10 mph speed limit chain of lakes, and when returning to a typical lake afterwards the boat would barely get on plane (i.e. it would take a good 20 seconds and a couple football fields to get out of the hole) and would shudder at all RPMs. I took the boat out again to a different lake and magically the boat got on plane fine and ran great up to about 4500 RPMs but would go no further.

I took it to the local Mercury mechanic and he's said it has good compression and spark. They noticed a pinched fuel line (I believe between the fuel filter and pump assembly) and replaced it and lake tested it and said they were unable to get it to plane out. They've been unable to diagnose the issue after a couple weeks at this point. Based on some research here, it sounds like the ECU may be the likely culprit. Is there anything else I should look into? My understanding is that shipping the ECU out to Simon Performance Technology for diagnosis and repair would be my primary option as I've previously seen Don warn that used ECUs might have similar problems.

Any help is greatly appreciated.

[turfy49431]

Sounds like the fuel driver in the ECU. I had the exact issue. Sent it to Simon Performance and it runs perfect now

[JOE54]

ECU problems usually don't run fine up to wide open , DOES NOT sound ECUish to me , need todo a fuel pai test thru all rpm ranges as well as spark test / peak voltage tests at coils

[jpd_hack]

The mechanic said that it’s not running on cylinders 2 or 5. I’m planning to send the ECU in to be tested.

[EuropeanAM]

2 and 5 are different drivers, if it were drivers it would be 1-2 and 5-6 that were down.

Almost certainly ignition related, possibly trigger.

[jpd_hack]

Thanks Don, very helpful. The mechanic said they weren’t seeing issues with ignition components but I’ll ask about that in particular.

[jpd_hack]

I’ll also add that spark is good and consistent across all cylinders and the mechanic said the coils are good. If it was an issue with the trigger, would it still be getting spark but no fuel to the cylinder? Apologies as my understanding of these components is quite limited.

[JOE54]

can still spark and trigger be bad

[jpd_hack]

Mechanic says he’s “positive it’s not the trigger”. This Mercury dealer ran out of ideas so I have the boat back at home. I shipped the ECU out for testing and there are no issues with it. I guess I take it to another mechanic and start looking at upgrading my boat earlier than expected in the meantime

[leonreno]

My problem last year was the trigger, it tested out with bad resistance, which is an easy test. Luckily triggers are also not crazy expensive.

[EuropeanAM]

My problem last year was the trigger, it tested out with bad resistance, which is an easy test. Luckily triggers are also not crazy expensive.

One other often-missed problem is that the trigger's HARNESS may be deteriorating inside the wire sheath. Always flex the harness and leads while performing a resistance test on the trigger. If readings don't remain virtually constant (very little or no change), it's likely the trigger will need replaced.

That harness DOES flex as you move the throttle.

[jpd_hack]

Thanks Don, will try to test this myself and report back.

[JOE54]

advance throttle to wot and test trigger ,hopefully "positive " did that when tested

[jpd_hack]

I resistance tested the trigger and stator today. I was by myself and have a hot foot so had to do some jimmy rigging to get the throttle moved and still be able to test resistance back at the engine.

I’m a complete newb at this but by testing brown to purple wire, white to brown, and purple to white resistance readings were all in the 800-900 range.

I tried the stator as well while I was at it and was getting approximately 1400 between blue wires and 400 between red wires.

It sounds to me like this is barely within spec for the trigger but haven’t found concrete numbers for the stator yet.

Thanks again all for the help.

[jpd_hack]

Looks like my stator testing was incorrect based on https://www.go2marine.com/Product-Gu...ercury-DVA.pdf. I tested blue to ground, blue/white to ground, red to ground, red/white to ground and saw numbers around 3850 for the first two sets and 110 for the latter two sets which appears to be within spec to me.

[JOE54]

stator test > blw to redw , b to red then red to grnd then red white to grnd , no test for blue or blue white to grnd readings should be close to same on first set then second set

[jpd_hack]

The ECU is bad after all. Brucato told me it passed initial testing but it did fail load testing. I’ll hopefully report back with good news once I have a new ECU on the engine. Thanks all for the help.

[JOE54]

glad you found problem