Dynamite_Goat

Hi all - need some help with an '05 RX8. We replaced the worn-out motor with a low-mileage JDM unit a couple of weeks ago. Started and ran well for approximately 40 miles, then stalled coming to a stop and would not restart. Parked it for the night and it started right up the next morning, then stalled after a short amount of time. Had it towed to a friend's shop and we've been trouble shooting it over the past two weeks.

The car starts just fine when cold, sounds great, idles well - then promptly shuts off after about two minutes. It will not restart after that - seems like its flooded but really can't be sure. The next morning, it starts and runs perfectly again - then promptly shuts off after about two minutes. This cycle repeats itself indefinitely - we get about two minutes of run time when cold, then it shuts itself off for no apparent reason. The result is the same whether at idle or holding the RPMs up. Seems like an electrical problem but we're stumped. Also seems strange that it ran just fine for 40 miles or so.

Here's what we've done so far:

Replaced coils, plugs and wires
Checked compression with a rotary tester - all good
Checked and cleaned SSV, VDI and VFAD valves (SSV was stuck)
Checked all three vacuum actuators
Replaced fuel pump and confirmed pressure
Cleaned MAF and confirmed proper signal
Swapped throttle body and intake from original motor
Reset the PCM

From my research, there is no real difference in the JDM motor, harness or sensors that would keep it from running with the PCM that came with the car. I have not been able to determine what may be kicking in around the two minute run-time mark that would cause it to shut down. I have a scanner and can log any useful information. Any ideas?

Dynamite_Goat

166 views and no one has any ideas?

9krpmrx8

iTrader: (46)

Buying a used Renesis is up there with one of the worst decisions ever.

You need to compression test the engine.

200.mph

iTrader: (9)

thats like raw ******* a hooker

Dynamite_Goat

Like I said in my original post - it has good compression.

9krpmrx8

iTrader: (46)

What were the numbers? With engine warm at idle what is the STFT?

Dynamite_Goat

All six were between 110 and 120 psi with a tester from rotarycompressiontester.com. Not new but should be good enough to run, right? The original motor is getting a full rebuild in a few months.

It starts, idles and revs just fine - then promptly cuts off after two or three minutes so it never gets fully warmed up. I haven't checked the fuel trims since I don't believe its sufficient time to kick into closed loop. Can check though. Anything else I should log in the two minutes it will stay running?

9krpmrx8

iTrader: (46)

110-120psi is a pretty big difference, sounds like you are just throwing out numbers.

The car starts in closed loop.

Dynamite_Goat

Sorry - not just throwing out numbers - don't have them in front of me but that was about the range. 10 psi isn't a big deal based on this:

Rotary Tech Tips: How to compression check a rotary engine

Will look at fuel trims and report back. Thanks for the help.

TeamRX8

iTrader: (25)

I'm not sure if the car starts in low speed pump mode or if it switches after the initial warm up, but you might check the low speed fuel pump resistor to make sure it didn't die. Just guessing really, but something must be cutting either spark or fuel after 2 minutes. Or so it seems.

Dynamite_Goat

This is useful. I'll check this as well. Thanks

9krpmrx8

iTrader: (46)

Depending on the cranking RPM, 10psi could be the difference between good and bad (well borderline anyway). My starter spins around 300-310RPM.

The starter replay can be problematic so you should test it.

Dynamite_Goat

Good information. The new rotarycompressiontester.com tester adjusts the results based on cranking rpm and altitude. Nice feature and removes some guess work.

The car starts right up when cold. The no-start is only after it shuts itself off after a few minutes. It's not really a hot-start issue since it never really gets hot. One of the local auto-electric guys is thinking its the ECU after seeing it first-hand. I'm not so sure but we'll see.