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Discussion Starter #1
Hi All,

I am wondering if you can help, please. I have a 2004 JD GX355 which has been a very strong work-horse. A little loud and rattly at idle, but OKAY at WOT.
First off, video of the problem -
No Load
It doesn't matter if the machine is at idle or WOT, the machines revs will drop to an almost stall level.

With Load
At WOT and starting the deck, the machines revs will drop right down to almost nothing and it'll throw black smoke.

I have tried changing the solenoid but to no avail.

Anyone have any thoughts what could be wrong?

Thank you,

Arestes
 

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Wow. It looks like the engine isn't bolted down tight. I had a Briggs V-Twin shake like that, it was running on one cylinder because it had a broken rod and thus was unbalanced.


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Listening to the video, it sounds like it's starving for fuel. This could be a debris in the tank, clogged filter (in tank or in-line), or a collapsed fuel line. These items would give you the sound in the video...BUT! You said "throw black smoke", so that tells me you ARE getting gas, it's just not burning. Do a compression test and a leak down test...I don't have specs on motor, so can't say what "numbers" you're looking for. Keep us posted, Bob
 

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Discussion Starter #5
Hi gents,

Thank you for your response - I have two GX355s and the vibration of the engine seems to be normal. I would do a comparison video, but the second one is also poorly after winter.

Have ordered in a compression test kit in, will try and do this between the UK being on lockdown..

If anyone has any other ideas, I’m all ears!

Arestes
 

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thats a diesel lol, how many hours and when was the last time you changed the fuel filter? i would change the fuel filter and would run some fuel treatment.
 

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Discussion Starter #8
Hi chaps - yes they’re diesels. The GX355s were a strange machine in the range. They’re a really strong engine but are notorous for overheating and cracking cylinder heads. Just be mindful of the radiator and the radiator cover. If they’re not seated properly, the radiator protector wears a hole in the radiator and... overheat.

That aside, the machine has only had diesel in it. In the Uk fuel is expensive, but there is something called Red Diesel, which is normal fuel dyed red. This is fuel for commercial use, which ride on mowers qualify for. It could be fuel quality (from age) which is the issue?
But the fuel filter has just been changed, along with transmission filets & oils, etcetc.
If I remove the outlet of the fuel filter and turn the ignition, fuel pumps out of the filter nicely.

Does anyone know how the fuel inlet is regulated ?
Could well be a blocked injector or line. Will give those a clean out next.

Arestes
 

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for a diesel - for this to happen, it will be fuel flow problem 90% of the time. Either filter, for fuel pickup, or fuel pump failing (not 100% failed yet), etc.....

if it throws black smoke (ie running too rich, or incomplete combustion) - it's either air delivery, or injectors... could also be fuel pressure problem at injectors - the pressure needs to be there for the injectors to atomize the fuel... if pressure it's low, it will just dribble the fuel into the chamber resulting in incomplete combustion.....
 

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Discussion Starter #10
Good points you’ve raised there, Wally.

Winding things right back to basics..
For combustion in a diesel, we need Fuel (atomised), Heat (spark plug?), Air & compression.

When the revs drop we can hear the engine struggling not to stall, but at high revs & load we get reallly low revs & black smoke.
My question is, what device on this mower would be dictating the fuel & air mix? Is this digital or mechanical?
The pump seems fine if the black smoke is anything to go by.
Could it be air-intake or a faulty butterfly valve which is misbehaving or lose? This could cause improper airflow.

Hunting for a list of things to look into - the service manual arrived today, so will give that a good read aswell.
 

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Pretty sure all diesels have no air control (butterfly valve). The amount of fuel is controlled by the timing of the injectors. The amount of air is constant. The pump controls the pressure (actually it provides the flow, the resistance to the flow creates the pressure). The injectors spray pattern could cause issues.
 

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Pretty sure all diesels have no air control (butterfly valve). The amount of fuel is controlled by the timing of the injectors. The amount of air is constant. The pump controls the pressure (actually it provides the flow, the resistance to the flow creates the pressure). The injectors spray pattern could cause issues.
Spot on.
RPM is controlled by qty of fuel added. The governor will regulate fuel as load is applied, to increase fuel to increase power to keep the RPM steady. Air is 100% open all the time.
In cases of extreme air filter plug or other restriction, the amount of air flow may be reduced, especially at high RPM (higher volume being pumped), but if air flow drops low enough, you will have insufficient pressure on compression, to detonate the fuel. Remember, it's pressure that causes the detonation - not compression ratio. In that case the engine will start to stumble and misfire..... slow down... and then start firing again since at lower RPM lower airflow is required, and the plugged filter will be able to keep up.....
In a border line condition, you may get black smoke under high load with high-rpm, when the fuel being metered into the chamber is at say 100%, while air is at 90% due to filter being plugged. But at low RPM, or low-power conditions you will never see that because the fuel flow will never be at 100% in those conditions. In that instance, black smoke can only be caused by incomplete combustion - and that would be caused by poor fuel pressure (poor atomization), or plugged injectors (poor atomization).

As for speed ramping up initially, and then lowering and dropping - that to me sounds like fuel flow.... the pump can't keep flow-rate high enough to keep up with demand... and to repeat, fuel flow is what regulates RPM in a diesel.
There normally is a low pressure pump in the tank, that feeds the fuel-rail HP pump (injector pump) which produces fuel pressure - sometimes up to 3000psi, depending on the engine. If the LP pump cant supply sufficient flow to the HP pump, the HP pump will not be able to build sufficient pressure to atomize the fuel properly, which may result in black smoke despite low rpm and low power level..... in that instance, a plugged intake, failing LP pump, or plugged fuel filter may be the culprit.
Alternatively, if the HP pump is failing, it may initially build pressure but after a few seconds begin to drop that pressure and cause the issue... i cant think of a failure mode that would result in that behavior - but it's possible I guess.....
Lastly, a plugged injector would behave badly regardless of RPM or power level.....so you would not see RPM going up and then dropping... it would be bad the whole time.....
 

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Discussion Starter #13
Quick update - today I went to the mower and it’s brother, which hasn’t worked for around 18months due to lack of time to work on it.

had a look at the fuel lines, visual inspection OK.
Had a look at the air-filter & connections. All OK.

Started the mower without the hood and it ran without fault for over 30minutes. Idle, WOT, cutting deck Engaged. Cutting deck engaged with power flow. Cutting thick grass & collecting.
No problem what so ever.
Drove uphill, downhill. On a slant. Nothing made the issue reappear.

this tells me the problem likely still exists but is intermittent.
Electrical connection ?
Air flow sensor, if there is one?
Partially blocked fuel-lines or injectors?
Weather related ?

I am very confused.
 

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crap in the fuel tank plugging the intake screen.... it can drift away then get sucked back in at the right moment....

or - loose connection to the low-pressure pump... pump works well, tractor works well... pump stops runinng, HP pump starts starving of fuel....
 
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