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Discussion Starter · #1 ·
2010 TRX420 FM. Bought used in 2015 and, aside from the occasional dead battery, it's been doing great until recently.

I used it a few time in the weeks prior to the problem starting with no signs of trouble. Battery if fresh, replaced a few months ago, and had full charge. Fuel is fresh and full tank.

Around the beginning of November I started having a hard start problem. It didn't want to start and stay running. Going off memory here. It would crank over no problem, I could play with the throttle and it would fire off and die. I could hold it off idle and keep it running. After holding it off idle for a bit it eventually stayed running on its own. That was around 6am and temps were in the 50s. I rode it on into the stand.

When I came out of the stand around 10am the temps had warmed up to around 70. The thing fired right up and ran no problem. I want to say that I didn't have any problems that afternoon either, but temps were still in the 70s-80s. Didn't use it the next day and dad brought it back home later in the week.

Since having it back home I have replaced both the petcock fuel filter and the fuel pump filter, they had never been replaced and I figured it wouldn't be a bad idea to do so. Installed a recoil start kit I've had sitting on the shelf for years. I verified that the fuel pump is working. I verified that the IACV is working. Installed a new spark plug.

Yesterday afternoon, with the temperature around 60, I got it put back together enough to test fire. It cranked over well, but it wouldn't start. Didn't attempt to fire off.

Verified that I was getting fuel to the injector. Verified that I was getting spark. Swapped plug back out for old plug and it would at least attempt to light off if we were playing with the throttle while attempting to start it, but it wouldn't run.

Today with temperature right around 70 I went out to investigate again. Before doing anything I decided to test fire again. It started right up. Runs like crap, but it started and idled. Try to give it throttle and it dies. Dad says that it smells like its running rich. I can't verify, my smeller isn't working great right now.

Since then I removed all of the Recoil starter components to rule out anything there, it didn't make a difference. I removed the IACV again, cleaned the valve and bore, and relubed the valve with a new o-ring. Nothing that I have done seems to have made any difference.

I'm not really sure what to do or try next. I'm almost at the point of throwing parts at it and seeing what sticks, but that gets expensive quick.
 

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What you have done so far really for the most part wouldn't affect how the engine would run. Probably a sensor has gone bad and causing the EFI to go faulty. You can install you recoil, it wouldn't affect running whatsoever. First I would take off the injector and clean it. It is probably a O2 sensors or somthing of the sorts. Honda has readings in there manual to test a sensor with a multimeter to test wether its bad or not. I would pull up a diagram and find the location of every sensors and check them after cleaning the injector. You should find your culprit in there. There usually is a airtemp sensor in the airbox, since your engine changes with air temp my guess is that sensor has gone bad.
 

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Discussion Starter · #3 ·
I said I removed all the recoil starter components, but what I really did was just sweap back in the original harness bypassing it. That was just to rule out the components or hgarness as cause of the problem. I think I actually did that before it would even try to start at all, but when it didn't make a difference I changed things back to the new harness again. It has run, though poorly, since the recoil starter stuff has been installed.

The problem is what sensor? There aren't many on this thing. There is no O2 sensor. From what I can find there is a CKP sensor, combination TP/MAP sensor, and ECT sensor, and an IAT sensor.

It definitely seems to be temperature related, but even when it starts it doesn't run well. Of course that could still be temperature related if it is causing the fuel mixture to be incorrect. It seems to be dependent on ambient air temperatures rather than coolant temperatures, so would think IAT instead of ECT.

The manual for this year really sucks for troubleshooting individual sensors. The only ones I see values for are the IAT and ECT, but to test those you would have to have a way of holding them at a controlled temperature specified in the repair manual. I don;t have a way that I can figure to hold those compoinents at that controlled temperature to perform the test. Maybe there are other instructions/procedures for tesating that I am overlooking somewhere.

In interest of time I ordered a bunch of parts for it yesterday, I want this thing running right by next weekend if at all possible. Bought through Amazon so I've got a nice return window incase they aren't needed. Got a throttle body assembly, fuel pump assembly, and a IAT sensor on order, all OEM. There are three controls built into the throttle body assembly (TP/MAF, Injector, IAVC) and it was cheaper buy the assembly than potentially buy all three individually. Fuel pump incase I figure out how to hook up a fuel pressure tester and find out it's not meeting spec, this is the part I most expect to be returning. IAT sensor for the reason mentioned above. The ECT is the only one I didn't think about, but as said above I think it's more likely to be IAT.
 

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What are the temperatures they want you to test at? I’ve been able to test sensors by sticking them in a mini convection oven I use for powdercoating/cerakoting with the wires sticking out the door. Do remember that every part you use may not be able to be refunded. Once you use that fuel pump it is deemed hazardous and Amazon will not take it back(in my experience). Have you checked for consistent spark? I’ve seen people get wrapped up over fuel issues not realizing that their spark was intermintant due to a fried coil. Let’s ignore all that though. Your engine if the compression is good shouldn’t be an issue especially if the valves were checked. This leaves your throttle body’s sensors or injector. I am certain it is your IAT sensor. All the other components really wouldn’t affect an idle (in this case your machine barely idles if I’m correct) if the IAT sensor is bad it will affect every other system of the EFI. If it was any of the other sensors your machine would start and idle like a dime every time but the second you give it any sort of throttle all hell breaks loose. In your case hard starting indicates a bad IAT sensor that is telling the ecu to give it way too much fuel. What I like to do to eliminate confusion is disconnect all fuel lines and electrical components and make sure that it still sparks well when this happens. Then shoot a little gas down the throat and bottle feed it. This lets me know it’s the fuel system. You could try unplugging the IAT sensor and seeing if it changes. Really anything that make the machine respond will point you in the right direction. Good luck.
 

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Discussion Starter · #5 ·
What are the temperatures they want you to test at? I’ve been able to test sensors by sticking them in a mini convection oven I use for powdercoating/cerakoting with the wires sticking out the door.
Not high. Around 70 degrees for each of them I think, I don't have the PDF open and I'm not going to go pull it up right now. If it were higher temperature I could used an oven (IAT) or heated water (ECT) to create a stable temperature for testing. Ambient temperatures are not currently adequate for testing at 70 degrees and I don't have a good way to create a steady 70 degree environment for testing. If they gave a range (IE: 0.2 @ 40 and 0.8 @ 70) I could extrapolate from there, but they only give a single value.


Have you checked for consistent spark?
After installing the new plug it wouldn't fire. Pulled plug wire, spark tester wouldn't fit. Inserted old plug, held to frame, cranked engine over. Good strong spark, not weak or intermitten. Assumed new plug was problem, easy enough to swap out, so reinstalled old plug. Problem didn't go away, so I kept new plug as a spare (haven't tested it). Nothing wrong with old plug, pretty sure I changed it in the time I've had the ATV.

Do remember that every part you use may not be able to be refunded. Once you use that fuel pump it is deemed hazardous and Amazon will not take it back(in my experience).
I am aware, at this point the only part I expect to be returning is the fuel pump, but if it turns out to be a problem I didn't want to be waiting for it to show up. I probably won't be able to work on the ATV again until Monday or Tuesday and if I waited until I verified fuel pressure, I figured it would be another week before I could get the part in. Turns out it actually shipped from a Honda dealer about 20 miles away, so didn;t really matter. I'll be trying to connect fuel pressure gauge before installing it, because I honestly don't think that it is the problem.

Whether it solves the problem or not, if a part gets installed I won't be trying to return it. If the new part didn't solve the problem I'd keep the old one on the shelf as a spare, if it did solve the problem the old one would get binned.

I’ve seen people get wrapped up over fuel issues...
I was hoping that they might be the cause of the problem, but I was of the mind set that it was good maintenance to replace the fuel filters anyway. It's 12 years old. I bought it at 5 years old and I don't know what the previous owners did. I know that in the last 7 years it's primarily been fed pump gas from a gas can. Those gas cans are typically exposed to the elements, so not exactly a clean fuel source. No water and used enough used that it's always fresh fuel in the can, but not necessarily clean.

Turns out there was plenty of trash in the bottom of the fuel pump, and the filter in it was nasty. Not clogged and not causing the problem, but installing a new one wasn;t a bad thing. Also got it converted over to the new style vertical filter setup. I don't understand it, but apparently there was a known issue with the original horizontal filter orentation. The petcock strainer was also disentegrating in the tank. When I pulled the valve out there was nothing left of the screen part, just the solid bits of the stem. Once again not the cause of the problem, but needed to be replaced.

I am certain it is your IAT sensor. All the other components really wouldn’t affect an idle (in this case your machine barely idles if I’m correct) if the IAT sensor is bad it will affect every other system of the EFI. If it was any of the other sensors your machine would start and idle like a dime every time but the second you give it any sort of throttle all hell breaks loose. In your case hard starting indicates a bad IAT sensor that is telling the ecu to give it way too much fuel.
The IAT sensor will be getting used, it's the first part I'm going to try because at this point I'm of the same mind about it being the most likely issue. It's also (relatively) cheap, quick, and easy. If that fixes it, everything else will be returned. Next, if I can get a gauge connected, I'll be testing the fuel pressure. I'm about 85% sure is within spec though. If neither of those fixes the problem I'll be throwing the throttle body assembly at and crossing my fingers.

At this point I'm hoping (and thinking) that the IAT sensor is going to fix it and I can return the rest, but I didn't want to wait on parts to arrive as I said previously. It not running is costing me opportunities to put meat in the freezer, I can't drive the truck into the places I hunt and its too far to walk from where I can park it.



You could try unplugging the IAT sensor and seeing if it changes. Really anything that make the machine respond will point you in the right direction. Good luck.
Would unplugging the IAT sensor put it into a limp mode or something? I'm not dismissing the idea of unplugging it and trying, I'm just wondering what the response whould be assuming that the IAT were working properly.
 

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ECUs respond differently. There’s no harm in trying since if the ecu responds by leaning out the mixture then you’ll confirm that the issue is that sensor. Usually the machine will try to work without the sensor but it’s not possible for it to run well. If it were working then it would probably die on the spot. In cases like these the atvs ecu isn’t very advanced you can unplug things one at a time and if you get lucky and there is a positive response then that sensor is probably the culprit. Kind of similar to how people test coils on a car. I’ve had some luck with it but again it’s not full proof. I’m more then certain it’s not the fuel pump though. The only issue you can get with them is weak pressure, since your machine changes how it acts with temperature it kind of eliminates that factor. Gasoline dosen’t get harder or easier to pump with temperature. If you want to eliminate the pump factor just make sure it pumps into a cup and if all else fails replace it if you couldn’t have gotten a tester on it but it’s a rare day to see a pump go on atv. I’d recommend testing a handful of sensors your can get to easily. First take them and read them even though the temp isn’t optimal, if there is one that is very off you found it. You can also just bring them into your house and let it sit for a day and test them. When I used to test electrical components temperature really didn’t matter unless it was too a extreme (low 10 or high 100) even then it would barely affect the reading. Again what you could try is just blocking off the fuel rail and for safety unplugging the injector, making sure it still sparks and then getting it to run off starting fluid or gas in a squirt bottle. If it fires off and you can get it to rev then it means your issue is a sensor in the throttle body. Remeber that without the fuel to the injector, the efi is basically deemed useless and by shooting fuel down the throat your doing what a carbeurator would and your now the only source for fuel. The engine could care less aslong as it gets gas and the ecu dosent cut the spark if one of the connectors isn’t plugged in. I use this method a lot in diagnosis so you could give it a try if you want, can’t really harm anything. The more you find out about the issue, the faster you can fix it when parts roll in.
Edit: If you want to try what I said you probably only need to unplug the injector rather than blocking off the fuel. Pretty easy trial.
 

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Discussion Starter · #7 ·
First thing first, it's fixed.

Unplugging the IAT sensor did nothing, no change. New sensor arrived Monday. Installed it and no change. Not IAT sensor, but it's a fairly cheap part so don't mind.

I was never able to test fuel pressure. The fancy fuel pressure test set I thought I had is actually a compression test set, my fuel pressure tester is just a basic unit like you'd find at the local parts store and I didn't have the stuff to adapt it. Never actually though it was the fuel pump, but like I said previously I didn't want to wait and have to order if I found out it was. Fortunately it wasn't the issue either and I never installed it so it'll be going back to Amazon in the next few days.

I'm still not certain of exactly what the bad part was, but my brand new OEM throttle body assembly arrived yesterday. Installed it and the thing fired up and ran perfectly. Old injector was obviously working because it would run, albeit poorly. The old IACV was working when removed from the throttle body, but I can't say for certain that it was when installed. I never really tested the TP/MAP sensor, but after removing and inspecting the potting material was cracked. New throttle body assembly was less than buying each of those parts individually.

It's running good, I'm happy. I'll install new air filter and finish putting it back together tonight.
 
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