150 optimax rough/unstable idle, stalls when shifting into gear. Runs ok above idle

[Michsnet]

Mercury Optimax 150. I purchased it new with the boat in 2008
SN: 1B532030
185 hours on the clock
No diagnostic codes
New battery
120 PSI average compression x6 NMT 10 psi variation.

This problem started somewhat gradually with the rough idle. The stall shifting into gear started about 2 months ago. It restarts fine & runs great above 2500 RPM. It runs rough from 1000 to about 2500 rpm on gradual acceleration. Above 3000 RPM runs like a sewing machine. If you hammer the throttle its smooth as can be, hole shot is good as it's always been.

I started simple, fuel filter & spark plugs, the high dollar NGK plugs. no change. I had coils go bad under warranty years ago, low power rough idle etc. so I replaced them too - no change whatsoever with the idle problem. Dealer replaced the fuel lines with the coils back then too. 2011? I think.

I'm a retired auto repair technician. I bought a factory service manual for this motor. I bought a basic scan tool for this gem. How hard can it be to fix? I initially checked fuel & air pressure with a non-liquid filled gauge; the readings bounced a lot. Bought liquid gauges, re-checked & the bounce was almost unnoticeable.

Fuel: 90psi
Air: 80psi

I looked here & elsewhere for pattern failure issues. I went with the known issues, in spite of low hours it is a 2008 engine, age matters. I removed the fuel rails, bought the special tool. Replaced the tracker diaphragm AND the fuel pressure diaphragm. OE merc/quicksilver parts. I pulled the air regulator & cleaned the gunk off the screen. I found a service bulletin about the direct injectors. I pulled out the direct injectors (mine has the blue ones) no debris & they flow okay by looks anyway. They pass the resistance tests; however, you can't declare something good with an ohm meter. I do not have the actual tester/tools to correctly check them. They hold 40 pounds of pressure, that's as high as my tester I rigged up goes.

The neutral/in gear switch is working correctly. It will die at idle in neutral too within a minute or so of startup. If you either raise the r's in neutral or throw it into gear it will stay running but not below 1000 rpm & preferably 2500 rpm.

If this were a car with an EGR valve I'd bet money on it being stuck open part way; exhaust gas leaking into the intake, this is exactly the symptoms that would be: runs fine top end. Runs like crap at low rpm, stall at idle. A leaking direct injector allowing compression/exhaust gas into the air side of the fuel rail might do something similar? I pulled the reed block. I had a couple OE stainless reeds I could see light through & it had clearly leaked & allowed gunk into what I call the air plenum where the MAP sensor mounts. I replaced the reeds with Chris Carson reeds. I installed the stuffers & new gaskets. It's never ran that good on the top end, it went from runs well to haul ass on the top end, but it did nothing to change the idle problem.

I pulled the VST. There was some gunk in the bottom. I cleaned the bleed hole & replaced the needle valve. The high-pressure pump seal was corroded in the housing. I broke the high-pressure pump taking it apart. Replaced the high-pressure pump & seal with mercury OE parts. Didn't change anything about the idle problems. I replaced the diaphragms in the low-pressure mechanical pump too.

I found a set of direct injectors on e-bay. I swapped them with mine, no change at all with the way it ran. No Guarente they aren't bad too. I'm sending a set of fuel & direct injectors off the be cleaned & tested. If the fuel rails come off again, the injectors are getting replaced with known good ones. The fuel injectors aren't leaking but.....

The map sensor seal was leaking a little & the barometric pressure was off a little. I replaced it. It's a GM car map sensor; part numbers & function match 100% - no change
Since its a known issue I bought an OE throttle position sensor - replaced it, no change
The intake air temp sensor reads pretty much spot on matching ambient air temp. If I understand correctly the ECM uses a different strategy for spark, air & fuel below 3000 RPM vs above, correct?

Everything I've done has helped something & was probably due or past due to be done; sooner or later but it still won't idle. The ECM is about the only culprit that could do this IF the direct injectors aren't an issue, correct? If the inputs are good & it's mechanically sound what else could it be? Since this has no O2 sensor or mass air flow sensor it has to guesstimate idle fuel & air based off the sensors it does have. I'm at a loss here. Any advice would be a greatly appreciated. The ECM is a lot of $$$ but far short of the $20K to replace it. Just wanting to be sure I haven't missed something.


Update: I had the e-bay direct & fuel injectors inspected & cleaned by Chris Carson's marine service. It may have helped a little, but it still runs rough & stalls at idle. It was suggested to drop the lower unit & look at the exhaust tube/tuner to see if it was loose. I pulled the powerhead instead & looked, it was tight, however I finally found something conclusively wrong, something I could point to & say it was actually bad: the gasket under the driveshaft housing plate aka the part the exhaust extension tube is bolted to.

It looks like one side of the engine is running rich OR the blown gasket contributed to it? see original post for all the normal things I've already replaced that can cause this. I did not disconnect anything except the shift linkage when I pulled the power head up to inspect this area. It did run better but still doesn't want to idle. I'm going to look at the throttle linkage very close & watch the TPS volts to see if it changes from the Key on engine off voltage to in gear with no throttle change. It's odd that at times it runs really great top end & other times okay but 5-6 MPH several hundred RPM short on the top end. The TPS is a new mercury part. I'm thinking this is about the only variable I might be missing IE a worn or out of adjustment part of the linkage is altering the "learned" idle TPS volts throwing the fuel curve off in left field at times.