RX4 Hard Starting Issue
 

My 2020 RX4 has been experiencing hard starting issues all year that I haven't been able to solve. The bike has roughly 26k miles on the clock. It will usually take 3 attempts on the starter before the bike will start from cold. If you restart it immediately after shutdown it will start first try. There are no DTCs present, and the bike runs like a scolded dog once started. Maintenance wise, the bike has a fresh Oil change, Valve adjustment, spark plug, and air filter. I have changed the ECU and fuel injector (I had this issue before hand and no change). I cleaned the MAP sensor though I still will still likely order a replacement. I haven't found an easy way to access the throttle body to give it a good cleaning without removing the airbox. Any ideas?

Injector lines losing prime/pressure. Make sure new Injector is not dribbling. Fuel line check valve? Check spark when cold. My 4 cyl van was not pumping fuel on a cold start and took a few tries. It was the head temp sensor acting up. Just replaced that and it also ran better. No codes till it totally went out.

Bumping the starter a few times to cause multiple primes of the fuel system make no difference. The fuel filter has been changed as well.

If you haven't already check fuel pressure when cold. Remove injector (still connected) to see what it's doing when cold and cranking. Spray engine starter or gas in to see if it fires when hard starting. Has to be fuel delivery or spark.

I have checked the fuel pressure in the past. It's a shame that they didn't provide a fuel pressure test port. Item seems like the auto enrich feature is not working. Either the MAP sensor is feeding the ECU bad tem and airflow readings. Or, the O2 sensor has leaned out the fuel trim so much the the close loop starting fuel trim has been modified. I don't suspect a throttle position sensor issue.

I've hurd 2 issues for this on the RX4 one is the ECU they seem to have a problem and will be problematic in making spark on starting it will also miss periotically if your getting a miss every once in a while I would just buy a new ECU and swap them and see if that cures it.
fuel pumps not developing proper pressure and thuss the injectors don't work right
if you can put a pressure gauge on the injector line to see what the pressure is
not sure what is within specs but CSC would be able to tell you
..
Bob....

Bob, I have a new ECU (GoBoom tune) installed. Issue pre-dated the ecu replacement. I just changed the MAP sensor and checked the spark which seems strong. I'm still leaning towards a fueling issue. The bike runs and pulls without a hiccup once started. I've still been commuting to work on the bike. It takes 3 trs on the starter to get running. Priming the ignition multiple times makes no difference. I have no ability to check compression with the bike having a camshaft driven compression release and obscure 8mm spark plug thread.

Sounds like your still having the issue... My RX4 started first try every try till the day I sold it ( too heavy for me) I suspect as you do a fuel issue as well, it just sounds like a fuel problem to me ! the only other thing that it might be is SPARK PLUG doubtful, but I have seen sparkplugs give a starting problem before only on starting but not in running but it is a 1 in a million chance if you know what I mean....
Like the other guy said it sounds like the fuel is leaving the line to the throttle body when sitting.... so it takes a second or two to get fuel to the engine when starting
pull the tank off and look for damp areas on the fuel lines, if you find one that looks suspisious just replace the line, there is no sense in messing with it ! sense fuel injected bikes use a really high fuel pressure to make them operate (something like 30 p.s.i.)
any leaks at all are suspect.... although I doubt you'll find any sense the bike runs great once fired up it could be a check valve in line if there is such a thing on the RX4
if so I would just replace it.
I do know there is a fuel filter on the RX4 that is in the gas tank I think hooked to the fuel pump..... but you said you replaced it so that shouldn't be it.
it's also possible that your new ECU is bad.... if you can put the old one back in for a test just to see if the problem goes away.
I don't suspect the sensors MAF or Throttle position, but I do suspect that the wiring to them could be getting bad, look for rubbed wires and bare spots while your in there.
if they were bad you would get a fault though I think.
......
I hope you find it without having to dismantle the entire bike !
....
Bob.

a trick you might try is just turn on the key and wait for a minute or two and then hit the starter if it fires up right away then you know that the gas is leaving the line going to the injector.....
Bob.....

Sounds like a slightly weak battery to me

Before you buy anything, try jumping it with a nice charged car battery to see if it starts immediately when cold. If so, it is indeed a cold cranking amps issue. New battery would probably fix it.