2010 RTV 500 repairs

[kmelander]

I hope you have replaced the spark plugs, which solves 85% of the poor running issues with the Subaru engine used in the RTV500.
We sell and lease a lot of RTV500s, they are very popular with farmers/ranchers, horse folks, landscapers, and housing communities. Several of the local colleges and hospitals have a couple or more.

It is pretty simple to do all of the components ohm tests and the harness ohms tests covering the harness and the ECU. The components are for the most part easily accessible.

There is a fuel filter on the inlet side of the fuel pump in the fuel tank. Remove the pump/sender assembly and wash the sock filter.

I did change the plugs with no luck. Then I changed the injector, also no improvement. Throwing parts at this thing is expensive. I asked the mechanics at my local dealer if the coils were known to partially fail and create this issue but they were less than encouraging. They did however mention the fuel pump specifically so, that was encouraging. I'm not a FI expert. Can a bad fuel pump, presumably causing low fuel pressure to the injector, cause a rich condition? Even if it were, it seems improbable that it would persistently misfire. Also, I don't think it's misfiring on either cylinder with prejudice, it's seems to be both cylinders misfiring. At least that's what the spark plugs are telling me. It really feels like an ignition issue to me. I'm really hoping its the coil, since it's a simple fix. An interesting conclusion - the test for the coil 2ndary is measuring resistance from plug cap to plug cap, which means this coil is not cylinder discriminate. It must fire both plugs every 360 deg. This means one cylinder is always firing at the top of the exhaust stroke? That seems...odd. I don't know, maybe it's common in the world of these 360 deg. parallel twins. As I've said previously, I've never worked on one of these engines, or any other engine configured like this.

I will say this, again, the guys at the dealership were great. They were sympathetic and encouraging. Very busy and stretched thin on time, but still talked with me. Ultimately offered to handle it for me if I get stumped, which was encouraging but made my wallet twitch nervously.

 

[lugbolt]

yep low fuel pressure can absolutely cause misfires. I'm dealing with a Polaris 850 at the moment (at home) which has a misfire, to "me" it's an obvious ignition misfire-but it has 42 psi fuel pressure. Should be 60. New fuel pump comes in today; and I'm thinking it'll solve the issue.

UPDATE: Yep-60 psi fuel pressure with the new pump, runs great.

 

[kmelander]

29.95k ohm on the 2ndary of the coil, 3.5 ohm on the primary. 12vdc to the pin. It's just barely out of spec on the high side of the secondary. I guess that means replace it?

 

[kmelander]

yep low fuel pressure can absolutely cause misfires. I'm dealing with a Polaris 850 at the moment (at home) which has a misfire, to "me" it's an obvious ignition misfire-but it has 42 psi fuel pressure. Should be 60. New fuel pump comes in today; and I'm thinking it'll solve the issue.

UPDATE: Yep-60 psi fuel pressure with the new pump, runs great.

That's great! I'm waiting for my new fuel pressure test kit. I did find my old one but it doesn't have any fittings to go to a barbed connection. I re-set the governor control linkage (per shop manual) and tested some sensors last night. The IA and ECT (temp) sensors checked out ok. I need to repeat the "Air pressure sensor" test. I measured resistance between the 3 pins on it but the manual does not give these values. Instead they are giving voltage specs. I sure hope the fuel pressure is the ah-ha moment for me too. It's a bit humiliating to be getting my butt kicked by this little wheezer engine. Something else that crossed my mind is electrical grounds. I imagine the ECU and associated electronics are sensitive to this and ground issues have a way of elusively creating problems. If the fuel pressure checks out ok, I'll add that to the list of seemingly random items to check.

One thing that's bugging me...my compression reading. The shop manual states 165#-185# as acceptable. I'm getting 200# on each cylinder and that feels odd to me but I can't create a situation in my head where an improperly timed/adjusted valvetrain would cause this, or could it? Another interesting point; the ecu isn't recognizing any errors (I assume) because it's not throwing any idiot lights on the dash. This could also be an indication that the ecu is bad I suppose. I've never wished for a bad fuel pump before, but I sure am now.

 

[kmelander]

I tested the fuel pressure and it was fine. It started off at about 34# with engine off and climbed to 38# with the engine running. Then I decided to test the crank sensor, which tested ok thankfully. Then I decided to test the intake air pressure sensor.

This was a bit confusing by the instructions. Hopefully I performed the test correctly. Specifically, I think step #10 is out of order. At least I can't figure out how to measure the resistance of the wire between the ECU connector and the sensor pin with the ECU plugged in and powered up. I do think I found a problem...with the ECU! Step #11 calls for a resistance reading between the center pin of the harness connector and body ground. I performed this test with the ECU plugged in and powered up. When the fuel pump was running, I was getting around 33 ohms. Once the fuel pump stopped, I was getting a constant bounce reading between 1.x and 3.x ohms. According to step #13 and the table at the bottom of the page, the ECU is bad? With the key off, I do get a reading of 0 ohms between body ground and the center pin of the sensor. All of the other tests passed.

 

[kmelander]

Update:
Based on the MAP sensor testing, I ordered and received a replacement ECU. I purchased it used from an Ebay seller. I installed the new ECU and performed the same MAP sensor test to see if anything was different, which to my disappointment, it was not. One demerit for the WSM on this. The center pin to ground resistance behaved the same way. I started and ran the engine which ran exactly the same as it did with the original ECU. The only bright spot was that the ECU cooling fan did run for a period after starting the engine, which did not happen with the old ECU. It may just be a software version difference. The model year this part was removed from was a 2009 and mine is a 2010.

The Ebay seller had some additional take-off parts available. Specifically, the MAP sensor and the ignition coil, which I purchased. I'm still of the opinion that the engine is running too rich. I'm not able to bench test the MAP sensor as I don't have a 5 vdc power supply or a handheld vacuum pump. The cost was reasonable enough to just get it and try it. I'm hopeful this could be the issue as I believe it's the sensor directly responsible for trimming the base fuel map based on engine load. In other words, it could be responsible for an over rich A/F mixture if it's out of range. The coil is another concern. If it's weak, I think it's possible it could be causing the same symptoms.

The only other piece of the puzzle I can think to mention is the perforated metal element that was stuffed into the tailpipe, before the bolt on spark arrestor. It was quite badly plugged up with carbon and I opted to remove it completely, which was admittedly a chore. You can-not just pull it out. There is a pin welded to one side of the pipe, through the "media". The only way to get it out was to drill the weld and separate the pin from the pipe. I just used a sheet metal screw to then plug the hole. I have to imagine this "element" was creating, or increasing backpressure on the engine. Maybe that's why it was there, I don't know. This could be the reason, or a contributing factor at least, for why I'm seeing compression readings of 200# instead of the 185# maximum specified in the WSM. That seems "thin" but it's the best explanation I have at this point. I suppose it's possible that the cylinder head had been resurfaced in the past, but I didn't notice any evidence of that. Another concern on this point, if the compression is higher, the MAP may be lower, which would undoubtedly cause the ECU to enrich the A/F mixture. I do have a vac gauge and I will take a some readings for reference. I'm not sure how to overcome that issue, if it turns out to be the problem. Either a suitable, rescaled MAP sensor, or a carburetor retrofit as previously suggested by someone. That will be the extreme and I can only theorize about how to accomplish that. I suppose I would simply remove the injector and figure out the fuel delivery and governor /linkage. I wonder if there are any aftermarket ECU's that could be adapted to allow access to the tune parameters? That would be ideal, but ultra complex I think.

So to summarize, I'm waiting on the replacement MAP sensor and ignition coil and I'll perform a MAP vacuum test to get some baseline figures. I don't recall seeing that value specified in the WSM but I'll take another look for it.

 

[kmelander]

The struggle continues. I replaced the MAP sensor 1st. No difference other than a harder start than usual on the 1st start. Normal after that. Then the coil, which did improve the running characteristics, but not eliminate the misfiring. Under load (driving) full throttle it ran good. Part throttle is still unacceptable. So, I still have not found the gremlin in this engine. I'm wondering about the crank sensor, which is a 5-star PITA to change. I'll need to be certain that's the problem before I undertake that project.

To do that, I'm thinking I'll need to tap into that circuit and log it somehow. Feels like an oscilloscope meter will be required?