Sat for two days, now won't start/run.

[NocturnalEmber]

78 280Z 2+2

 

I finally got around to doing some more work on the Z, and for every step I take its almost as if its two steps back.

Two days ago I changed out the engine/ecu harness because mine had virtually every clip broken.

After I connected everything back up I started the car to verify it was running okay. Engine seemed to idle fine, rev fine. I did rev it once and it died when the RPMs settled, but it started right back up and ran fine from what I could gather. I shut the car down. I have not touched it since.

Two days have passed.

I go out to road test the car, and it wont start at all, just cranks. I will occasionally hear it try to start here and there, but its endless cranking otherwise *unless* I give it throttle during this period, then it will run, albeit horribly, as long as I keep the throttle applied. Full throttle will get it to maybe 1,200 rpms, all the while misfiring and smoking horribly (it doesn't smoke when it normally runs.)

 

Honestly at my wits end here, I've changed out and replaced a bunch of components along the way for the sake of troubleshooting and just due to age, and now this. Absolutely nothing happened to the car that I know of between two days ago, turning it off after replacing the harness, it running fine then, and trying to start it today and this happening. I did notice that after the fuel tank replacement, the car will leakdown to zero psi overnight, seems to hold pressure fine though during cranking/running (when it ran two days ago.)

If anyone has any advice I'd appreciate it.

 

Google doc below of what I've done so far:

 

https://docs.google.com/spreadsheets/d/1eRC31JowXWN3sx6-Y-zguYGn5HORvq0jeS9s4Pt3rxM/edit?gid=1386834576#gid=1386834576

Edited June 14, 2024 by NocturnalEmber

[Yarb]

Dropping down to zero psi IMO is not critical. After bringing my car back to life running very well, when I returned in the morning the pressure gauge would be at zero. When you turn to start position what does the fuel pressure read. Monitoring that while the engine is running will give you an idea where to look next . Please post back your findings .

[NocturnalEmber]

1 minute ago, Yarb said:

Dropping down to zero psi IMO is not critical. After bringing my car back to life running very well, when I returned in the morning the pressure gauge would be at zero. When you turn to start position what does the fuel pressure read. Monitoring that while the engine is running will give you an idea where to look next . Please post back your findings .

Pressure at cranking and running is 32~ psi.

[Yarb]

Doesn’t sound like a fuel delivery problem. I would go back and double check every single harness connection and the contacts one by one. Take your time. Also the resistor packs next to the wiper reservoir. Those are a main connection to the voltage going to the injectors. Every connection needs to be very clean.

[NocturnalEmber]

4 minutes ago, Yarb said:

Doesn’t sound like a fuel delivery problem. I would go back and double check every single harness connection and the contacts one by one. Take your time. Also the resistor packs next to the wiper reservoir. Those are a main connection to the voltage going to the injectors. Every connection needs to be very clean.

 

I checked them all a second time after they were connected two days ago and again tonight. As far as I can tell, absolutely nothing was changed between when it was running and idling fine on its own with the new harness and earlier when it wouldn't start. 

All the connections were/are secure and clean. The car was turned off while running fine, sat for two days, and now this.

[Zed Head]

1 hour ago, NocturnalEmber said:

Full throttle will get it to maybe 1,200 rpms, all the while misfiring and smoking horribly (it doesn't smoke when it normally runs.)

Check the fuel pressure regulator vacuum hose for raw gas.  Maybe the diaphragm ruptured and the engine is sucking in excess fuel.  Running super rich.

[NocturnalEmber]

1 minute ago, Zed Head said:

Check the fuel pressure regulator vacuum hose for raw gas.  Maybe the diaphragm ruptured and the engine is sucking in excess fuel.  Running super rich.

 

I actually thought to check that, though not for the same reason - I wanted to make sure it wasn't a vacc leak. No gas in the vacuum line coming off the FPR.

 

What gets me is what can happen to the car when it is off, with the battery disconnected, over the course of two days? It's in a sealed garage, no one messed with it, and it was actually running when i turned it off. Just doesn't' make any sense.

[Zed Head]

Your symptoms match how the ECU's fail.  You might try banging on it.  Many people have used that as a diagnostic method, there are stories on this forum about it.  Apparently the solder joints crack at the main connection and banging on it brings them back to life.  It it works it's a clue.  Fist or foot, it's worth a shot.

[NocturnalEmber]

1 minute ago, Zed Head said:

Your symptoms match how the ECU's fail.  You might try banging on it.  Many people have used that as a diagnostic method, there are stories on this forum about it.  Apparently the solder joints crack at the main connection and banging on it brings them back to life.  It it works it's a clue.  Fist or foot, it's worth a shot.

 

I mean at this point what's some free maintenance going to cost me? I'll go give it a kick or two and report back.

[NocturnalEmber]

@Zed Head I went back down to the car, verified it still won't start. 

At that point I took your advice and Slapped the ecu around a bit, and  it got to where it would start and idle. It did sound like it was running a bit rough, but it would start, idle ( 800~ rpm) and rev. Do i turn my attention at this point to the ECU? Didn't do anything aside from smacking the ecu and shaking it around a bit.

[SteveJ]

15 hours ago, NocturnalEmber said:

@Zed Head I went back down to the car, verified it still won't start. 

At that point I took your advice and Slapped the ecu around a bit, and  it got to where it would start and idle. It did sound like it was running a bit rough, but it would start, idle ( 800~ rpm) and rev. Do i turn my attention at this point to the ECU? Didn't do anything aside from smacking the ecu and shaking it around a bit.

Yep, it's the ECU. When you have to resort to percussive maintenance, it's pretty much ready for a restoration.

[cgsheen1]

17 hours ago, NocturnalEmber said:

@Zed Head I went back down to the car, verified it still won't start. 

At that point I took your advice and Slapped the ecu around a bit, and  it got to where it would start and idle. It did sound like it was running a bit rough, but it would start, idle ( 800~ rpm) and rev. Do i turn my attention at this point to the ECU? Didn't do anything aside from smacking the ecu and shaking it around a bit.

When you pulled the connector off the ECU did you check the contacts on both the plug and the ECU?

We've seen water damage (corrosion, oxidation) on the ECU connector from cracked and/or leaking windshield gaskets.