Hey everyone-

Newbie here with a big problem. I have a 1976 280Z that has a 'flat spot' when hitting the gas when the engine is cold, accompanied by back fires through the throttle body. I just had my mechanic replace the cold start valve, clean out the throttle body, clean out the mass air meter, and clean off the spark plugs. When the car warms up, it runs like a champ!

Please someone help meeeeeeee!! I want to get back to driving my Z!!!!!

Thanks!

later!
Sean

 

Having to clean off the plugs makes it sounds like they were maybe getting fouled, which might indicate it's running rich? That kind of matches your description of the behavior when it's cold. Things that could cause that would first be the cold start valve, which you've wisely already replaced, so it's not still that. The fact that once it's warm it runs fine implies maybe the temp sensor (front of the head) isn't right, or it's plug/connection is bad. So, you might want to check, clean, or replace that. The flat spot could also mean a dirty or bad connection on/in the throttle position sensor, so checking and cleaning that wouldn't hurt, either.
The whole temperature induced behavior thing, though, could mean it's some intermittent electrical problem on some piece about to fail, as basic as an alternator, or as obscure as some relay or piece of wiring. Once you've got the known and easy to check issues done, if it's still not right you're probably going to have to spend some quality time with the FSM, which has an awesome amount of info and diagrams on how the whole system works. It's time consuming, but you can always get it fixed that way.
Dave

 

Dave,

thanks a million for the advice! We had to replace the alternator as well, so my next guess would be the temp sensor (this is called a "water temperature sensor" I think???), then i was thinking it could be a bad mass air meter. Will check on the TPS as well while doing the rest of these checks.

Thanks again & hope to be on this site some more with good Z stories, pics, etc.

Later!
Sean

 

Hi Sean,
Glad to help. There're a lot of great people on this board, with almost scary amounts of Z know-how.
Yes, the temp sensor and the "water temperature sensor" are the same thing. It's on the front of the head, along with the thermotime switch (which could also be the problem, but less likely than this first round). Basically, they both help account for fuel-curve enrichment during warm-up, a solid state equivalent of a choke.
Funny you already replaced the alternator. I've twice had S30 Z problems due to bad alternators. It stops charging, which kills a fuel pump relay, which slowly starves your engine of fuel, resulting in the sputtering and dying out. One way to kind of check out that whole branch of problems is to temporarily splice in a fuel pressure gauge on the inbound line of the fuel rail (right after the fuel filter). You're going to need one sooner or later anyway, for tuning, maintenance, and diagnostics.
The good news is that with that gauge, a multimeter, the manual, and your beverage of choice , you can completely repair, tune, modify, and diagnose your whole EFI system.
good luck,
Dave