Inside look at the Honda igx ECU, STR - self tuning regulator

[SlovenianUsername]

First some history. I bought a used spider lift that was fitted with honda igx440. The thing ran good only in summer, but even then, one had to always use starter spray to for cold starting.

The igx series has no manual throttle, instead it has an ECU which drives the so called self tuning regulator, STR for short. When ECU senses that the engine is loaded, it picks up the engine rpms. When not under load, the rpms go down, engine goes to idle speed.

Problem with my engine is, it is surging really badly while idling. When I for example want to extend the boom, the STR picks usually up the rpms, and it has a nice sound. But also what happens very often is, the rpm doesn't go up, and the engine just shuts off in a few strokes. Like a half dead horse when put through burden.

The problems usually start when it gets a bit colder, 40 degrees fahrenheit, or 5 celsius, I am certain to have problems.

Today I made a bit of a progress, I decided to open the ecu. That's the thing with the red LED on, the low oil shut off.




The protective rubber is at some spots melted away. Altough the pcb looks ok, no visible damage.

Then you remove the PCB, you just lift it off, and underneath you get another clue. Two small stepper motors.https://www.nipponpulse.com/products/Motors/motors-rotary-tin-can-steppers/PF35T-48R4
its operating temperature is -10 to +100 degrees celsius. They must have melted the rubber, but ok, let's move on.


I unscrewed the 4 philips screws, and the next thing unveils. Finally I come to the part that now opens the carburetor butterfly valves. One for each butterfly valve.


So two gears, for two butterflies, and underneath them, something black with a spring- and this is the only thing you can actually grab and turn, the thing has a lever on the outside of carburetor. When the engine is working, the lever doesn't make any difference, whichever way you turn it. No change at all.
Anyone know what that is?
But I don't think that's the culprit, as the engine was working really good in summer. I don't know much about carburetors, but just thiking out loud: could one just hard wire the thing to stay always in full throttle mode? So let's say could I force the butterfly valve to stay always open?

Oh, and the last thing. What if the damn thing, this ECU is working good. Could it be that the load sensing part is not working? How does a machine even send a signal that the hydraulics is under load?

Feel free to discuss, any input is appreciated.

Edit: just a link to a picture with feature of ECU http://visman-co.com/other/ENGLISH/Engine/gx_series_commercial_grade_igx45.htm

 

[lwn9186]

I'm no expert but it sounds like the idle jet needs cleaned. Take the plastic screw out keeping track of how many turns it takes. Carefully pry the plastic jet out and give it a good cleaning. I would also take the main jet and emulsion tube out and clean them. The hole on the main jet is very small so sometimes it takes a strand if wire to clean it. Spray carb cleaner in the idle jet hole.

Same as this..............https://www.youtube.com/watch?v=uksV4JFsFc0

 

[SlovenianUsername]

Main jet and emulsion tube (the one with many small holes, am I correct?) are clean. The plastic jet I never opened, I will give it a try.

I think I will anyway need to hardwire an open throttle, because I accidentaly turned the stepper motor gear, and I don't know if it the stepper motor will know its open and closed position.

Here is a video I made at the choke side. I try to show how the lever doesn't do anything. So there was never any choke at all.

 

[SlovenianUsername]

And here is the throttle valve

 

[legdelimber]

Sorry that I know nothing to help you.
But dang that looks like an awfully complicated way for Honda to avoid putting a couple of 12volt solenoids onto a regular, mechanical governed engine.

Is there any chance that the fuel module unit from the igx390 can be used?
I have a bad feeling you've already looked into that idea and had no luck.

Does any company make an engine with simple remote controls that can be simply operated by electrical switches.
If so, how expensive to change engines? and what other parts have to change with it?

 

[davidwyby]

@Hondadude

 

[SlovenianUsername]

Sorry that I know nothing to help you.
But dang that looks like an awfully complicated way for Honda to avoid putting a couple of 12volt solenoids onto a regular, mechanical governed engine.

Is there any chance that the fuel module unit from the igx390 can be used?
I have a bad feeling you've already looked into that idea and had no luck.

Does any company make an engine with simple remote controls that can be simply operated by electrical switches.
If so, how expensive to change engines? and what other parts have to change with it?

No worries on the helping part , just talking also helps

The ECU or ECM of igx390 looks very similiar, at a first glance: https://support.surefireag.com/medi...nd_Throttle_Motor_Parts_Diagram_5.18.2022.png

Currently I'm thinking more in a line of how to retrofit my carb to have manual throttle. What that means for me in real life situation is, that when I will lift off the ground with my spider lift, I will be up there with throttle wide open. No means to turn it down a bit. But, I don't really care, I have a ignition switch in my basket. So I come up there, I turn the engine completly off, I then proceed to cut the tree or whatever I am doing. And when I need to move two meters to the left, I fire the engine from the basket. And again I'm at full throttle, and again i shut it down when im in position to work.

The idle feature sure is nice, but not really necessary. And also considering that I have the option of 220V integrated motor, which I use whenever I'm near an outlet. The only thing is, the petrol engine is necessary for tracks to operate. For example, to load it on or off the trailer, you can't do it electrically.

One could also scrap the IGX440 engine for a new GX390, manual throttle. But again there is an unknown: will the 390 be able to run the hydraulics pump, or will it be too underpowered? There is a 49 cc of a difference between them. 438cc vs 389cc.

 

[SlovenianUsername]

I try to pry out the idle jet here

does this go out just with prying?

 

[lwn9186]

The idle jet can cause the exact symptoms you are having as it controls mixture at idle but also off idle also. I wouldn't by- pass anything until the idle jet is cleaned and the unit is tested.. The motors more than likely will find there way once the unit is fired up. The computer should take over and control them.

 

[lwn9186]

I try to pry out the idle jet here

does this go out just with prying?

Pry gently on both sides at the same time and it should pop up. has very small o-rings you don't want to damage.

 

[SlovenianUsername]

It poped off, it looked really clean. Altough different to what an idle jet looks like on a gx series, look here, timestamped

mine has no brass insert. And a really small hole, picture attached. I think I'll buy some spare idle jets and have the hole measured and if it's the same size, I will enlarge it a bit. I think it's worth a try.

Edit: I ordered carb cleaner, should come by post any day now. I will clean everything with it when I get it, maybe there is something in a hole that i can't see.

it's the correct jet, at least to this picture, size 45

https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/honda-engines/engine/gx/gx440/gx440iu-qta2-engine-jpn-vin-gcawk-1000001/carburetor-2

What does the plastic screw sitting just above the idle jet even do? The hole doesn't seem to be leading anywhere. Attached is the picture of which screw I'm talking about.

 

[lwn9186]

The plastic screw retains the jet and also serves to set the idle on some models. In your case the servo motor set the idle so more than likely only used to retain the jet.

 

[lwn9186]

Just noticed to the left of the idle jet is a idle mixture screw. Appears to be set on the lean stop. If you turn counter clockwise you should be able to richen the idle. There are stops to limit how much it can be turned.

 

[SlovenianUsername]

I see what you mean, it really is on the lean side, the butterfly can close completely. I have a good feeling that the thing will work with a bit of tunning. Ofcours if the servos find their way to reset to a known position zero

I also found out why the choke doesnt work, the lever that does the closing and opening is broken at the end. Video attached. Not an interesting one, I put it on here so it's all nice and documented.

 

[SlovenianUsername]

What I also found out today, I was wondering how does the engine ECU load sense. How does it know when to pickup the rpms, let's say that I pressed on the valve, or that I'm on the move with machine, surely there must be some sensor on the hydraulics.

It turns out, no. The load sensing is a part of the engine itself. Somebody please take a look at this video, time stamped:

The sensing part is actually the rpm of the (power) coil. The pulse of the coil is wired back to ECU. Something like that, I can't completly get my head around it. But yeah, I think it helps my case, there is no sofisticated sensor on the machine that is broken.

 

[legdelimber]

I'm reading just to try and learn a little here.

Any of you fellows know if this unit is counting frequency of the field poles (for rpm number) and comparing it to throttle position to adjust the throttle position?

Choke shaft is another issue. Can he still obtain one? How expensive is it?

 

[SlovenianUsername]

I tried to get the choke shaft out. The idea was that the choke shaft was inserted/pressed through plastic float holder. I tried to pry it off, and the thing broke :=) . Picture attached. I'll epoxy nicely epoxy everything back together. But the question still remains, how did the guys in factory insert the choke shaft?

 

[Hondadude]

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[Chrisbowen]

Hi chaps, I got a IGX390 on the weekend on a pressure washer and had problems of good load for about 30 seconds then the engine going into idle. Ram great but I could never bring the revs up again. Turning it off and on again gave me 10-20 seconds of power.

Ok you can easily make run at full speed. Behind the key start there is a bullet connector connecting a green & yellow wire. If you disconnect this this bullet connector the butterflies fully open and you get full power. I did it last night. Very easy. You could also add a simple toggle switch to connect the wires to go back to idle.

I noticed after I did this and then tried intelligent mode again ( green yellow wires connected ) it worked a bit more than before but not perfectly. My theory being that that assembly is gummed up or the stepper motors are a bit buggered.

Either way I can get full power now easily. It would be very cool to get selectable RPM/throttle but for now I’m happpy enough. If anyone can figure out a way to do it please let me know.

 

[SlovenianUsername]

I actually ordered a new carburetor. The engine now runs better under idle, but still not perfect. Still surging, although a lot less than before.
Under load, it runs normal, like before.

Could it be that we are having the same problem @Chrisbowen.
The problem might not be the carburetor at all (again, mine definetly had broken choke butterfly). Right under the gas tank, there is a thing with a funny name, petcock. It's a thing they install instead of a manual fuel valve. Inside the petcock is a membrane which opens the fuel line when the engine produces enough vacuum, otherwise it stayes closed.

Look it up, I for one, never heard of it before. Unscrew the fuel tank, you will see three lines, fuel line, return line (dunno how you call it) and a little short stubby line that attaches directly to a engine block. That's the one.
So... what if the membrane inside the petcock is worn out, or stuck or something, and the engine under idle speed doesn't produce enough vacuum for it to open?
My hypothesis can be easily tested. I will install a make up gravity fuel tank.
Maybe you have the same problem as me.

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