Hard cold start issue - EJ22 swapped RX7

I'm having trouble tracking down an issue with my EJ22-swapped RX7 and hopefully the GRM hive mind can point out where I'm going wrong.  The car is a first-gen, 1980 RX7 with nearly all the running gear transplanted from a 1995 Subaru Legacy.  This includes the engine (n/a EJ22), stock ECU, fuel pump, coil pack, injectors, etc.  It runs fine (a little laggy throttle response, but I don't know if this is normal or related), but has issues starting when cold.  When starting after it's been running for a while, it starts right up.

Things I've looked at:

• it starts right up with starter fluid when cold, so I'm fairly confident it's a fueling issue
• fuel pressure (when priming, goes up to ~60 psi, but drops relatively quickly to 20 psi after about 5 seconds, then to 0 psi after maybe a minute)
• fuel pressure was tested with the fuel line connected directly to and terminating at a gauge, so any leak-down observed was unrelated to fuel injectors potentially stuck open
• turning the key to the "run" position multiple times before starting to build up fuel pressure has no effect (probably due to leak-down issue mentioned above)
• new, cheap fuel pump- I'm not 100% confident that the hoses to the pump are fully tight
• fuel pressure regulator- no leaks to the vacuum side, but don't know about functionality otherwise, although this seems to be a pretty robust component
• Worth noting is that the fuel pump assembly was transplanted onto the stock RX7 fuel tank and has an imperfect seal around the tank gasket.  I assume this tank should not hold pressure, but I want to cover all of my bases.

Any ideas?

How clean are the injectors? Cold starts would be the most sensitive to a crappy spray pattern.

Race fuel pressure regulators bleed down that quickly, but most stock ones don't. They usually hold pressure for hours. I would replace the fuel pressure regulator.

GameboyRMH said:

How clean are the injectors? Cold starts would be the most sensitive to a crappy spray pattern.

I'll have to check on those.  I took a look this morning but they were stuck in place, so I didn't try forcing them.

MadScientistMatt said:

Race fuel pressure regulators bleed down that quickly, but most stock ones don't. They usually hold pressure for hours. I would replace the fuel pressure regulator.

On this engine, this should be test-able by pinching shut the fuel return line, right?  I'll try this first.  It looks like decent replacements are $60-70, so I don't want to test this particular part via parts cannon.

Thanks for the tips, gents.  I'll post back about results on these two things.

update:

Fuel injectors looked clean and clamping the fuel return line had no effect.  But I realized I had already tested this when I connected the fuel feed line to a gauge directly (no tee, but terminating at the gauge) and saw a pressure drop.

So if fuel pressure is a potential cause, I could add an inline check valve to the fuel feed line.  That should work, in theory, right?  I think the "right" solution would be replacing the fuel pump again and checking the lines for potential leaks, but the fuel pump setup is a huge PITA to deal with and inline check valves look like a cheap part to test out.

I added an inline check valve to my volvo V70, then later figured out that due to the E36 M3ty Volvo ignition it needed specific plugs gapped on the small side.  Cheap copper plugs also worked.   

I added an inline check valve to the fuel feed line and no change.  I also checked the spark plugs and they look decent and were gapped within spec.

I also finally fixed the OBD2 port wiring and was able to read some codes.  There was one for a failed coolant temp sensor and one for a failed heater circuit in an O2 sensor.  I'm trying a coolant temp sensor first and then will look at the O2 sensor.

I don't think either of those are likely to be your problem, but MAYBE the coolant temp sensor.

Does your fuel pump run while cranking? I'm not sure if it's supposed to on Subarus (my EJ25 swapped VW was swapped by someone else, so I don't know for sure) but if it's not running that might give you these symptoms. The cars I'm used to will run the pump to prime and then turn off until the starter spins.

I do agree that the fuel pressure is bleeding down too fast.

Some cars have two fuel pump relays, one for ignition on to prime then a different one to run.  Fun to diagnose.  On the Volvo one was a standard relay and the other was, well different.

Can you hear the pump running?  Do you have voltage when you expect it at the pump?

The rx7 should have a return line back to the tank.  Is the subaru return less?

Is the pump submerged in the tank?

Loose fittings post pump will result in fuel being sprayed out.  If this is happening, it for sure would cause your symptoms.  However, you would see it.  Unless the connections inside the tank are suspect.  In that case, it would just sort of recirculate inside the fuel tank.  

A couple months later and I'm back.  I thought I'd get notifications for responses but I guess not.

tl;dr: Connecting the stock temp gauge to the ECT wire between the temp sensor and the ECU causes this issue.

It turns out this was caused by a wiring error on my part.  I had connected the temp gauge in the cluster to the ECT sensor line going to the ECU, since the previous owner had connected an aftermarket temp gauge the same way.  Cutting this wire fixed both the P0115 error code and the hard cold start issue.

I tried fixing a potential fuel leakdown/bleed issue with a new pump and adding a check valve to the fuel supply line.  While doing so, I double-checked the connections from the tank to the engine, tightening any suspect hose clamps.

Hopefully this helps at least one person down the line.

So wait, are you saying you have an AWD rx7?

Robbie (Forum Supporter) said:

So wait, are you saying you have an AWD rx7?

That would be cool! But no, it uses a BRZ trans going to the stock diff.