purchased this car after sitting ten years. Diagnosed bad fuel pump replaced cleaned fuel lines and replaced filter, drained tank. cleaned plugs and regapped Starts great, as idle comes down from high idle car starts to stumble and will pop and snort when trying to bring rpm's up.

 

the car cant tell what temperature its motor is. Could be a lot of things. Start testing. Look for vacuum leaks.

 

Welcome to ZDriver and to owning a 280zx!

Ignore that statement... there is a CHTS, coolant sensor, and air intake temp sensor... all of which help the ecu (and you) know what temp the motor is at.

Always start w/ the basics...

Check ALL sensors and connections (they have a tendency to corrode).
Check timing.
Check / read spark plug tips (this will tell you what's going on in each cylinder).
Check for vacuum leaks (possibility since you say it comes down from a high idle).

 

Old gas or rusty gas tank, maybe!!

 

more than likely a vacuum leak or even an exhaust leak.

 

I have an 83 280zx n/a. When I first start it, it runs pretty good for a few mins then its like flipping a switch and it starts running like dog crap, missing, pops out intake and exhaust randomly. I'm at a loss with this right now. I've replaced the coil, ign module, dist, cap, rotor, fuel psi reg, fuel filter, spark plugs, charcoal canister, cleaned the egr, has correct timing, fuel psi. Chts seems to be working correct and according to the ecm the O2 is working correctly. My manual is not doing me any good now. The longer it runs the worse it gets. Eventually it won't go over 1,500rpm and that's even tough. It will idle ok with a Lil miss.

 

Did you ever find a solution to this problem. My Z is having exactly the same problem you describe.

Cheers, Seady88

 

hoco... have u considered the afm

 

Have you gone through the testing from the FSM and metered out the harness? I hate to harp on the same things but connectors, grounds, battery cables, sensors especially the AFM and CHTS are major sources of problems. Personally I replace all the vacuum lines, battery cables, wiring harness connectors (which is why I made a kit up to replace these), run an extra ground from the battery to the ECU then the ECU to the intake plenum near all the sensors and rebuild the AFM. Then I start the car and start trouble shooting

 

You need to check the Vac. Advance on Dizzy. Also you TPS Throttle Poss.Sensor. The plastic clip inside the TPS breaks. Is fixable by drilling rivets and opening box and clean and inspect the plastic Half circle shaped clip that attatches to the Trottle Valve. Also your Vaccume TTV is probably leaking at the front and make sure the Vaccume lines are not leaking. cmakes sure you dont have to much oil in motor. Let me know how it works.

 

I realize this is an old thread, and too that the OP did drain his fuel tank and replaced his fuel filter, but after trying all suggestions, and then some, I finally found the reason why my 79 280zx which had been sitting for several years would run sort of OK until it got warm, then cough, spit, and backfire thru the AFM. And I couldn't decide for sure since I only refueled it once, but it seemed to run quite a bit better right after I pumped 7 or 8 gallons into it. Ran sort of good for a while, then back to the spitting, coughing, backfiring.

The problem: My fuel pickup filter inside the tank was partially clogged with rust; particularly at the lower section of it. That thing is about 2 inches in diameter, and about 2 inches tall. When there was enough fuel in the tank to cover it completely, there was enough unclogged area to supply the engine. As for it running almost ok when first started, I assume the cold engine injector picked up the slack till the ECM shut it off. Other than cleaning that filter and cleaning the tank using the electrolysis method found on this site, I did nothing else - haven't even changed the regular fuel filter yet, and the car runs like new.
It's easy to check - just 2 screws holding the access hatch under the rear cargo area, then 8 or so screws holding the sender unit into the tank, and the three hoses need removing. Takes a bit of contorting all that stuff (pipes, float wires, floats) sticking into the tank, but not a difficult task.

I might mention also that the fuel pressure was generally within specs(or pretty close) at idle, and usually pressure did jump up to around 37 PSI momentarily at WOT as the manual states it should.