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1998 Optimax 150 DFI cold start stall issues

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westwater

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1998 Optimax 150 DFI cold start stall issues.
Starts right up then dies. After numerous tries (10++) willrun and starts perfectly warm.
Fuel pressure 90psi and spools right up and holds
Air pressure 80 psi and spools right up and holds
Spark and both injectors working on all 6 cylinders
No fault codes
TPS on spec (both)
Air compressor temp on spec
Engine block temp on spec
Air intake temp on spec
MAP on spec
Crankcase position sensor – don’t know but my understandingis that if not working/correct engine dies.
All of the above are via computer diagnostics on ECM. Thisthing is driving me nuts and with the price of these parts I don’t want tothrow money at it..
Oddly if I pinch the return line to the vapour canister AND put a screwdriver into the throttle plate to open it the thing starts fine. I think I’m just cheating a sensor though and not really that practical moving forward.
Please does anyone have the missing piece to this puzzle. Other than the least expensive ones which sensors should I start to replace first?



 
Right at cold startup I thought maybe the compressor was worn and needed time to generate pressure but that doesn't seem to be the case. Alo pulled apart the vapour canister to make sure no blockage of high pressure fuel screen but all clean. Throttle plate and linkage also seem OK.
 
Thanks guyjg for the TSB link. It's a little different on the DFI but I have pulled the check valve and made sure it's clean and operable. The motor has 465hrs and has been regularly serviced if that has any bearing.
 
So today I pulled the temp sensor on the block. With it out but still grounded I heated it up with a heat gun and it all started and ran perfect. Applied ice and it reverted to previous state of stalling right out. There isn't a temp sensor in town so I have to wait to replace it. Am I correct is assuming the colder the temp the more fuel the ECM delivers? I will report back once I re and re the temp sensor. In the meantime if my one can chime in on my theory feel free.
 
Yes, sounds absolutely correct.

And I am aware that the temp sensors on the very early ETEC's caused a similar issue when running in cold water. They would shut the engine down to prevent issues from running "too cold".

In that case BRP reflashed the ECU's because it wasn't really an issue, just a perceived one, but in your case if the temp sensor is on it's way out it sounds reasonable that the motor won't start.

Temp seems to be a "bug" issue with the early model DFI's in general (both hot and cold). Merc did some changes to the cooling system design on the Opti's a few years after your model was built to correct issues on the "hot" side..
 
You say the Direct and fuel injectors are working but are they clear? Once you get the engine started, let it warm up. Get a can of power tune and spray it right in the air compressor while the engine is running and in the water(not on muffs). Spray it in until the engine takes off, do not let it over rev. Do this until the can is gone. Retest your cold start.

Is the prop hub clean or covered with black goo?
 
Thanks for the input. Bought a sensor and installed it. Same issue so not the sensor. Put a whole can of lovin' through the air compressor. Same problem. At lease with two sensors I don't need to wait for it to cool down as I can leave one in the block and the other at ambiant temp and just swap harnesses. I guess next to play with is the air intake temp sensor. What else could cause too rich on startup?
 
I've never pushed it that hard in neutral to my recollection and I don't remember it hitting the limiter. While this has been going on on occasion I have advanced the throttle to try and cold start it. That was before I was aware the it was a too rich issue. The tps would alarm in those occasions and it would reset once the throttle was backed off (or reset with the key off. All that being said I can't say with 100% certainty that it has not happened. I am assuming there is some significance to your question. Also rechecked the tps and air intake temp and on spec.
 
Same problem here. Same symptoms, same engine+year, similar #hours, identical fuel/air pressures. I can also read ECM data real time and can confirm temp, tps, etc. working properly. Starts and runs great when warm.


I can add one more data point. On my "home" lake (3,500 ft. Elevation), the cold start problems are severe. On a lake at 1,000 ft. Elev., the cold start problem is much less severe. This seems consistent with too-rich when cold. Lower elevation means denser air, which requires more fuel.


I have seen other posts refer to ECM reflash to change timing and A/F ratio to resolve. However the shop I was directed to, Simon Motorsports, said "your engine is too old for us" even though they list 1998 DFI as an available custom tune on their website. Any other shops out there able to do a custom tune on this engine?


Would love to get this fixed. Will work with anyone to get to the bottom of it. Glad to see I'm not alone!
 
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I had put that motor and all the work I put into troubleshooting it out of my mind this winter. With spring at hand last week I got back at it. My mechanic had recently been at Merc school and ran into an instructor with some insight into that beast. Apparently when these motors came out and were introduced to the race side they had temp issues. The solution was to bypass the block temp sensor and use a race specific part. That's as far as he got. I want back to my theory that I could trick the ecu into thinking it was warm by installing a resistor that emulated the sensor "warm" reading. I had been down that path last fall but it turns out the resistor I used was faulty. I got a potentiometer from a local electronics store and with the resistance graph in the manual played with the resistance to find the sweet spot, hoping all my troubles would be solved. No such luck as there was no such sweet spot. I think I ended up settling on 180 ohms and put together a harness and resistor that bypasses the block sensor. The motor now starts cold, but only after 5-7 tries, which is better than it was but not optimal as I'm sure the starter will crap out one day or something else, who knows. I'll run it this way until it blows up and buy a Yamaha. Down side is I'm sure it will blow up at the most inconvenient moment, it is a Merc after all. The only thing I haven't ruled out and is mentioned in the service manual is the wiring harness. If there is corrosion there our symptoms could be the result. Seems a long shot and as it's a center console setup the harness dips and doodles through the hull. PTA to confirm as an issue or not. If you have better access to your harness let me know if there's faint hope on that front.
 
A buddy of mine had a 225 from the early vintage like this. It would constantly start then stall right out when cold. What he did (and what I learned to do) is to keep cranking the starter for a extra second after the engine fires. For whatever reason that kept the engine spinning and firing long enough to overcome the instant stall.
 
leaky said:
A buddy of mine had a 225 from the early vintage like this. It would constantly start then stall right out when cold. What he did (and what I learned to do) is to keep cranking the starter for a extra second after the engine fires. For whatever reason that kept the engine spinning and firing long enough to overcome the instant stall.
Click to expand...

Wow-after all this time, effort and frustration your suggestion seems to work like a charm. Starts up 2nd or so time cold and such simple fix. Can't figure out why as the bendix has disengaged, maybe there's a prime cercuit in the ecu based on cranking. I don't care anyway-it works.
Many thanks to you and your buddy. I love these forums.
 
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