Hey all,
I just put a rebuilt motor in my 2004 rx8. It's been developing worse and worse hot start issues (won't start now without sitting for 20-30m). I'm at a loss for what to do next. Here are the steps I've taken so far:

• Compression test by Mazda (9.1,8.8,8.8 @ 310 rpm for rear rotor - 9.0,8.8,8.2 @ 310 rpm for front rotor. They said it was done after it was warmed up, I have no way of verifying.) They also gave me a video: https://app.truvideo.com/v/A47imU?s=...5m9QKrPyEFjJL1
• New plugs // plug wires (new set when the engine was installed AND new plugs after 500 miles + new plug wires to verify they're good yesterday)
• New fuel pump
• New shitty injectors (i'm sending the OE ones to get flow tested//cleaned//flow matched this week)
• New E-shaft sensor
• Confirmed temp sensor is reading correctly via MazdaEdit
• Chased down vacuum leaks (still possible I missed some I guess)
• 20 brake press reset
• Throttle body reset
• Accelerator pedal reset
• Swapped coils front to rear (car has a p0302 code, this does not change the code)
• Put an inductive timing light on each plug wire (I'll be honest, not exactly sure what I was looking for here, but they all looked the same)
• Cleaned MAF and installed a new MAF after cleaning did not change anything
• I've verified and reinstalled the injectors like 4 times now, I think I have the order right based on the wire colors.
• Verified the coil pack // plug order a few times now, again based on the wire colors going to the coils.
• Cleaned a bunch of grounds && installed a new ground strap from intake to chassis and chassis to battery.
• Fresh o2 sensors when the engine was installed
• Fuel pump resistor removed (direct wired for now)
• New fuel pump relay on the way (not here yet)

Car mods:

• Bennitbuilt d585 coils (does it both with dwell adjusted per the excel sheet and stock tune)
• sohn adapter
• AEM intake (does it with stock too, tested this already)
• BHR midpipe
• premix (1oz/gal during break in)
• air pump delete
• "street ported" by engine builder
• Weapon-R catch can

Other symptoms: p0302 (though I struggle to tell when the car is running, just notice the blinking CEL), bad idle (unconfirmed, but the throttle body reset // accelerator reset may have fixed this).

What am I missing? Could the dealer have messed up the compression test and done it cold? Would those numbers when cold come out to really bad compression when hot? Anything else to try?

Appreciate all the information on this forum, it's been immensely helpful so far.

 

1) Return back to stock coils, stock tune, and reconnect the fuel pump resistor.
1-b) Why would you even disconnect the fuel pump resistor in the first place?
2) Who is the engine rebuilder?

 

1) I'll buy some stock coils. I don't think I have the stock ones. Are the ones on rockauto acceptable or should I be sourcing exact OE ones?
1-b) Was based off some googling that it could be overheating and causing fuel pressure to drop out, the theory being providing full speed to the fuel pump all the time would rule this out. I've got a fuel pressure gauge and in-line adapter coming to verify pressure is right, but this was the quick and dirty way to rule it out.
2) bennett built

 

"Street ported"?
Have you gotten OBD data while it's running, fuel trims, airflow, afr vs. rpm, etc?

 

"street ported" - whatever that means in bennit built land. I honestly don't know.

I've got a crap ton of logs. I'll post some when that laptop isn't in use (the SO uses it to WFH) tonight.

 

So a probably relevant symptom: If I idle the car from cold to warmed up, it idles fine. It reaches temperature, continues idling fine for a few more minutes then suddenly dies. At that point, the car refuses to start, barely even trying (like there's no fuel).

I've got an adapter coming in the mail so I can put a gauge on the fuel line, which should prove if this is happening.

 

If you have the old fuel pump it may be worth it to try swapping the old one back in. It is unlikely, but I suppose it is possible the fuel pump is heating up and could be having issues. When the car dies on its own and you cycle the key, what does the fuel pump sound like? I suppose if you wanted to see if you are getting fuel, you could cut ignition and do a sort of deflooding procedure when hot with the plugs out. Maybe Double/triple check the injector wiring.

Not for nothing, but for me I wouldn't trust dealers to do anything properly. But if they are showing you 130psi or probably around 120psi@250rpm that should be great compression. Assuming that it is when the engine is hot. Under 80psi@250 rpm Trailing slot is when you have hotstart issues. If the engine is really tight tolerances you could get amazing compression when cold, but then terrible compression when hot due to binding seals. If you plan on staying with the rx8 for the long run it may be worth it to buy your own compression tester for around $350.

When I was doing my research for this I think the general troubleshooting others said for this was 70% of the time it is compression. 20% ignition 10% fuel systems with the computer tying everything together obviously. It would have been better to hold off until after the rebuild runs to do all these new parts around the car. Downgrading back to your old stock parts/tune might be the cheap way to troubleshoot, assuming the car was running before the rebuild.

 

I think before doing anything else fuel wise, I'll wait for the pressure gauge to come and i'll verify that way. Likewise, injectors when I get them back from flow testing, I'll do another test of the order.

I considered buying a compression tester, but the car was already at the dealer for a handful of recalls. This is still probably the right idea.
I bought basically a rolling chassis. The old engine's second rotor had two cracks in the rear iron and a ton of heat scoring and was not running. As a result, I assumed the ignition system was trash, along with a number of other components that were either clearly bad or suspect. It's worth noting, the car hadn't run in probably 7+ years based on the tires.

Some of the parts (AEM for example) were to replace stock parts that had become cracked and damaged. Yeah, I know it's not the most liked intake compared to stock, but the additional space in the engine compartment is the most important factor. The car will be a track car, once it's running reliably and the plan is to add hood venting to improve cooling.

 

some logs attached.

 

start with the fuel pump resistor. That resistor is critical to maintain pump life. You are literally burning your pump up overworking it for nothing and may have already irreversibly damaged it at this point. Im boosted with over 300whp as are a few other members, fuel pump resistors intact.

 

It's already back in. After it didn't change behavior, I put it back. Just wanted to verify it hadn't gone bad, the plan wasn't to "delete" it. Car didn't even see more than a warm up + 1-2 miles without it.

 

Confirmed it's compression. I don't know how Mazda got the numbers they did - probably tested the engine ice cold.

That engine didn't last long.

 

Yeah just cuz mazda gave numbers doesn't mean they are correct. Already had mazda compression test one of my cars before and they said the compression was 120-130psi all around......got my own tester and I was getting 65-70psi. On a separate occasion I had similar numbers given to someone who was trying to sell me a car....few months later saw the car for sale saying it wouldn't hot start.

 

Yeah, clearly. Figured it'd be worth having them do it while recalls we're done. Instead it wasted a month of my time troubleshooting other things because I thought compression was ruled out.

 

My cars compression was slightly less than that and wouldn't even start unless you bump started it and then it wouldn't idle. You're lucky yours will start. Question...how many miles have you put on the rebuild? Did you reuse housings? What condition were they? I have read that it can take quite a while for a rebuild to bed the new apex seals to reused housings.