Help diagnosing MS500I

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ZeroJunk

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The saw will start and idle for a couple of seconds and cut off. If you are quick getting to the throttle it will rev up and sounds normal, like you could cut with it normal, but if you drop back to idle it will run maybe 5 seconds and cut off. I thought it might be the fuel pump, so I replaced that as well as the hoses. Same thing.
I have not run a diagnostic on it, have the software, but I know on the other models they don't tell you much if they aren't running.
Compression is good.
Any ideas ?
 

CobraChevelle

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The saw will start and idle for a couple of seconds and cut off. If you are quick getting to the throttle it will rev up and sounds normal, like you could cut with it normal, but if you drop back to idle it will run maybe 5 seconds and cut off. I thought it might be the fuel pump, so I replaced that as well as the hoses. Same thing.
I have not run a diagnostic on it, have the software, but I know on the other models they don't tell you much if they aren't running.
Compression is good.
Any ideas ?

how did you get the software?
 

cookies

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Think your loosing the coil pickup below 2k rpm? its fuel injected so many the map/baro sensor is faulty or has oil in it? It should use the flywheel magnets to coil pickup to fire spark and time fuel injection so possibly the magnets are weak and unable to drive the system?, the pickup is failing? can you record the sensor outputs during running until it shuts off at idle to see whats falling out or throwing out of spec signals?
 

weimedog

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The saw will start and idle for a couple of seconds and cut off. If you are quick getting to the throttle it will rev up and sounds normal, like you could cut with it normal, but if you drop back to idle it will run maybe 5 seconds and cut off. I thought it might be the fuel pump, so I replaced that as well as the hoses. Same thing.
I have not run a diagnostic on it, have the software, but I know on the other models they don't tell you much if they aren't running.
Compression is good.
Any ideas ?
If that was an autotune , prolly running fat and a field reset would do wonders.
 

ZeroJunk

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If that was an autotune , prolly running fat and a field reset would do wonders.
I really don't know what if anything they have in common with an autotune and solenoid carb.
The other mechanic is working tomorrow. I may go in and see if we can keep it running between us long enough to see if we can get the program to tell us something.
 

alexcagle

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Did they do a reset on the MDG-1 ?
There is a tiny screen under where the supply fuel barb goes into the injector. They can get plugged up, but rare.
It can be pulled with pliers easily by wiggling back and forth. Getting the screen out is a job for a paperclip.
I personally don't see the need for three screens. (Fuel pickup, screen in the fuel pump, and one screen in the injector.
 
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Did they do a reset on the MDG-1 ?
There is a tiny screen under where the supply fuel barb goes into the injector. They can get plugged up, but rare.
It can be pulled with pliers easily by wiggling back and forth. Getting the screen out is a job for a paperclip.
I personally don't see the need for three screens. (Fuel pickup, screen in the fuel pump, and one screen in the injector.
I think were going to see many instances of wood pulp from ingress into the fuel tank clogging filters and the injector proper too.

Injectors have some pretty fine tolerances, I do see the need for the 3 filters, and yet I think over time were going to see more injector issues as wood pulp is different to the dust from the concrete saws that have run this system for much longer.
 

ZeroJunk

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The reason I don't think it is a filter is because it will rev up just fine if you are quick enough to get your finger on the throttle. It's too big a saw for me to drop start it with my finger on the throttle, and I don't even know if they will start that way.
I will take that filter out and clean or replace just the same.
I'll have to find out about the special cable as well.

I'll reply if or when I get it going and let everybody know.
 

alexcagle

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I have seen more fuel related issues this summer here in Oklahoma, than any other year I can remember.
I have encountered both tru-fuel and motomix fuel that was several years expired.
Some of my customers running non-ethanol have solved their hot restart issues by switching to premium fuel from a busy gas station that turns their fuel over quickly, resulting in fresher fuel.
We are living through a time where gas quality isn't consistent. This may or may not be true for the rest of the US.
 

SimonHS

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I have seen more fuel related issues this summer here in Oklahoma, than any other year I can remember.
I have encountered both tru-fuel and motomix fuel that was several years expired.
Some of my customers running non-ethanol have solved their hot restart issues by switching to premium fuel from a busy gas station that turns their fuel over quickly, resulting in fresher fuel.
We are living through a time where gas quality isn't consistent. This may or may not be true for the rest of the US.

I swear by Fuel Fit. I put the dose in the fuel can when I fill up. I've never had a fuel problem since I started using it. Mowers, saws and strimmers start easy every time, even after sitting over winter.

https://www.briggsandstratton.com/eu/en_gb/products/care-products/fuel-fit.html

Screenshot_20220727-181623_kindlephoto-46173898.png
 
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I have seen more fuel related issues this summer here in Oklahoma, than any other year I can remember.
I have encountered both tru-fuel and motomix fuel that was several years expired.
Some of my customers running non-ethanol have solved their hot restart issues by switching to premium fuel from a busy gas station that turns their fuel over quickly, resulting in fresher fuel.
We are living through a time where gas quality isn't consistent. This may or may not be true for the rest of the US.

FWIW, I had some motomix that was several years old. I finally used it with no problems. Mixed the quart with 1 gallon of mix I did with premium gas.
 

ZeroJunk

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Go to a properly trained STIHL technician and let him do it. Without the proper knowledge things can be messed up fast and easy.


You are correct as far as doing the computer diagnostic. But, guess who does all the rebuilds for the properly trained Stihl technician, including MS500I's .
 

alexcagle

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Check the injector O ring where it enters the crankcase, these can leak over time 👍
Have you had one leak under vac/pressure test?
Those o-rings on the injector are near impossible to install incorrectly.
If it's not lined up right, it won't go in, and the plastic injector housing will break.
688959-8b5285f19bed61749bc8309b905909f0.gif
 

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