RB25DET start fuel cycle question, starts & immediately dies

[DigitalBHP]

Our project is a RB25DET into a 1990 240SX. After attending to the myriad details involved including the misadventure detailed in my post to this forum “RB25DET high oil psi causing much white smoke?” the happy day of the (second attempt at a) first start came and all went well. As did test drives and a few days of using the car to drive on a few short in town trips. The fun came to an end last Sunday when after one of these short trips the restart failed.

We are currently in a state where when attempting to start the engine it fires for a couple of seconds then dies. The ECU is an Apexi FC, its monitor shows sensors are active and its trying to open injectors and fire plugs. The start then quickly dies behavior reminds me of a clogged fuel filter. This one was new during the engine swap. I checked it and fuel flows easily through the filter from inlet to outlet. While reinstalling it I connected the inlet and with an extra hose observed the fuel flow into a gas can as the ignition was switched on. Repeated attempts showed consistently as the ignition switch was cycled from off to on an initial little spurt of fuel followed by nothing. Bad fuel pump relay? Swapping it had no effect. Traced the electrical circuit back to the in tank fuel pump and a volt meter (and the whir of the pump) shows initially power is sent to the pump and then is shut off. Just as the fuel spurts from the hose, and just as the car starts and dies when the fuel line is reattached.

So guys, educate me. Is this normal behavior? Is the ECU shutting off the pump in response to some other failure elsewhere in the system, or even part of a normal start cycle? Should a high pressure fuel pump used in a injected engine flow continuously into my test can when it is disconnected from the rest of the injection system?

[Carl H]

ecu will cycle the pump for approx 3 sec to prime the system and will shut off if it does not see CAS rotation.
might have a bad cas connection or a bad cas.

if you pull out the cas (and do unplug the coils and injectors) then you should be able to spin it and hear the pump kick back on.

[DigitalBHP]

Thanks a 1,000,000 to Carl H your hint to look into the CAS was spot on. The connector was not quite seated, re-plugging it and the fuel pump stays on during start cycle. Not quite out of the woods yet, engine will only run as long as throttle is held down to keep revs around 1000 RPM. Engine dies rather than settle into idle. In what I am expecting to ultimately prove to be a related issue we have something flaky going on in the wiring harness. When the car is switched OFF via the ignition key switch parts of the system stay on. For example the dash lights stay lit until the battery is disconnected. Next step would seem to be finding the broken switch or relay or short circuit or missing ground that is causing that behavior. Once that is sorted see if she wants to idle.

Hints and suggestions welcomed!

[Carl H]

going out on a limb you have an s13?
check your lower harness wiring...more specifically your charging circuit.

wouldnt hurt to pull codes

[DigitalBHP]

Wow, Carl H you are good. I feel like an audience member who volunteered for the magician’s mind reading act and find myself surprised answering “why yes, my dog’s name is Penny and yes we are working on a Nissan S13 project car!”

The update on the weekend’s progress is two steps forward, one step back. Following Carl’s hint to check the charging circuit I reexamined the Nissan 1990 240SX wiring diagram vs. how our car is wired. The diagram shows four wires, color coded WHT, BLK, WHT, & WHT-RED. The first white wire was already connected, it is the heavy gauge wire terminated in a stud lug that carries charging current onto the main battery bus. I had stopped there telling myself the RB25 alternator must be a single wire system. Revisiting it and with the help of afternoon sun sneaking in under the radiator hose I was able to find the recessed into the alternator plug into which the housing holding the WHT & WHT-RED wire pair could be plugged. Looking and feeling all over the alternator I was unable to find another plug or stud to which a black ground wire could be connected.

Testing late Saturday with WHT, WHT, & WHT-RED wires now connected the car’s ignition switch was working correctly switching all circuits off when in the OFF position after cycling on. Car would start and run but die when trying to settle down to idle. Looked again for bad relays or poor sensor connections without any luck. The Apexi FC ECU display showed normal looking RPM, injector firing, and ignition advance as engine was rev’ed. Finally resorted to adjusting the throttle cable to increase the amount the throttle plate stays open when pedal is at rest and got it to idle at ~700 RPM. At this point I thought we were back on the road.

Sunday began with reinstalling the rear remote battery box (removed for access to fuel pump in gas tank) so we could do a test drive. As a precaution we pulled all six plugs and they looked OK, maybe a little rich with some flat black soot but no sign of something like a blown head gasket. Car started but once again would not idle and would only run if RPMs held above 2000 RPM indicated on dash tachometer. Warming oil did not help. Checking the Apexi FC display showed RPM, injector firing, and ignition advance monitor readings all frozen! The only readout that would change was RPM that was mostly stuck at 2850 with occasional jump to 2875, neither of which correlated with the dash tachometer which our ears told us is correct. Definitely a shut off the car and think about this moment.

So, how does an ECU suddenly loose contact with all its sensors? At this point I am wondering if the still missing black ground wire connection to the alternator leaves it to form a poor ground thru the engine so we were ok on Saturday and not so ok on Sunday as happens with crappy grounds. I am wondering if when the alternator ground is poor if it is floating up the ECU ground until the engine sensors go out of range? We got this car with the RB25 already installed so I am not certain if the alternator is off the RB25 long block or original to the 4 cylinder 240SX engine. Thus I welcome comments from any of you who have wired either style.

[Carl H]

i normally just rewire the ka lower and plug everything in on rb swaps, cleaner and more effective than splicing all the wires on the lower harness.
check the ecu plug, make sure it is fully bolted in an not cockeyed or slightly loose.