HELP! Computer? Knock sensor?
 

I have a 2005 rx8 that blew the motor. Replaced with a used motor with good compression (120 psi on all faces when cold). Brand new coils. The new motor starts and runs great when cold but when it reaches operating temp it turns off. It doesn't bog down or run awkwardly it just turns off the same as if I was to turn the key. Do I need to have the computer re-flashed? Is there a computer re-learn process when replacing the engine? :dunno: Thanks for any help!

If you haven't reset the ESS, you need to do that. However, an abrupt shutoff when running is frequently a fuel pump failure. Be wary, because a fuel pump failure can indeed blow the engine. One member I knew personally lost their first engine to a fuel pump failure, and then the pump failed again with less than a mile on the replacement engine, while it was still in the dealer's hands, and blew that engine too.

Who tested your engine a compression test should be done hot not cold. A 120psi test at cold could become a 90 or 70psi hot and last I read on racing beat that was a fail.

I cleaned and reset the ESS. Nothing changed. I would lean towards the low compression when it gets warm but I would imagine I would be getting a misfire code then. No misfire codes.... I'm confused. Also, once it is warm it will not start until it has cooled down (This makes me question the cold compression vs. warm compression) I think I'm dealing with bad car voodoo.

Perhaps a rebuild is in order.

If baffled as to how the fuel pump failing would ruin the motor. I know my fuel pump is good though.

No you don't have to have a miss fire to have bad compression. Yes cold VS hot there is a big difference the speed at witch the engine was cranked can make a difference between pass and fail too.

The shut down when hot from low compression is typically a very different shutdown from a failing fuel pump (or other fuel interruption). A fuel pump failure shuts off the engine abruptly, a low compression stall is more of a spluttering drop in revs that just can't come back up.

Neither will refire for some time, although one is because the engine has to cool down, the other is because the fuel pump has to cool down.

Misfires are POSSIBLE with low compression, but will not always appear.

Fuel pump leens you out and cooks your apex and side seals. But I'm sticking with you got the shaft with your replacment engine compression test cold.

Thanks for all the insight, guys! I'm going to have another compression test done when the car is warm just to verify it's all good. If that is the problem I guess I'll be ripping this motor out and rebuilding it. Atkins Rotary is the only place I have found for parts....do you gentlemen know of any other places or should I stick with Atkins?

A fuel pump shutdown will cause a lean spike momentarily, which is what is dangerous to your engine. The RX-8's fuel pump typically doesn't slowly die and lose pressure. It's possible, but >90% of the time, our fuel pump is pushing 100% pressure/flow right up to the moment that it shuts off. That lean spike can be ignored by the engine if it's low load and/or low RPM. High load and high RPM is where that lean spike breaks one or more apex seals, which then the bits of those destroy the rest of them.

"Leaning out and cooking the seals" isn't usually a fuel pump related problem for us. It can be if there is a fuel pressure leak, but that is usually more obvious with a heavy fuel oder inside the car.

Lots of good venders here on the site just check the vender list and web sites.

RIWWP is the man he teaches and corrects me all time. He and a few others are the go to guys.

Awesome! Thanks guys. I'll update you with my progress :) Right now I have to put a transmission in an Chevy Aveo for a friend. I also am just about finished with my 2011 Infiniti G37. I'll be either selling the RX8 as soon as I figure out what the issue is or modding the hell out of it. I haven't decided which yet