1978 280z misses at off-idle speeds

[Lyttek]

1978 280z

idles as smooth as could be, cold or hot. problem only occurs when warm, and seems to appear after stopping at a light.

replaced plugs, wires, cap/rotor, have recently replaced injectors. Thought the TPS sensor connector looked corroded, so cleaned the contacts in both the plug and sensor.

It *feels* like it disappears if I tromp the accelerator when it's happening, but not completely sure.

The other day I drove it for about 35 minutes, no problem. Sat for about 10 mins, then drove back into town, another 30 minutes. Got almost home, stopped at a red light, as soon as I took off, it starts missing, intermittently, and feels like it's only one cylinder. Doesn't clear up until it's sat for awhile.

[grantf]

Hmmm Seeing that you say it is temperature related it could be the coolant temp sensor or it's connection. Just throwing it out there. Easy to test and cheap to fix if that is the case. You should check you're fuel system also, How is the pressure? good current to the pump? ect.

[Lyttek]

I assume there's only the one temp sensor? The temp gauge appears to be working fine... Not sure about fuel pressure, I'll have to look and see if I have the tools to test that.

[Lyttek]

Here's a little more info to deepen the mystery...

Drove 40 miles one way last night, no problems. Car sat for about 45 mins, then headed back home. Got about 20 miles and noticed it started again. Here's what's weird:

driving in 5th gear at about 35mph, approx 1500-2000 rpm, I can really feel the missing. On a flat level stretch, I can push in the clutch and let off the accelerator ever so slightly to maintain the same rpm, and it's as smooth as silk. As soon as I start to let the clutch out, it starts missing again.

When I parked the car, I was able to reproduce the same effect, and basically it starts missing when it gets any load whatsoever.

[Lyttek]

More info:

Have replaced the temp sensor with no change. With a little more testing I've found it starts missing:

A) when it reaches the higher end of the normal operating range; when cold/cool/warm it runs just fine

about 1500 rpm, load or no load

C) under load, it starts a little under 1400rpm

D) unplugging TPS has no effect

[Zed Head]

Sounds lean. Have you measured the fuel pressure yet?

[Lyttek]

Haven't had time to get stuff to check fuel pressure... but did manage to fix part of the problem...

At some point in the past, someone "adjusted" the TPS so that as soon as the idle contacts opened, the full-throttle contacts closed... which may explain part of the poor mpg it was getting (I just figured it was an old car, ) Drove it quite a bit yesterday after getting the switch back to normal operation and it was nearly perfect. There is still a little stumbling when its warm, but the only way I could tell was a very faint sound that followed the same pattern as when it was missing badly. It may have poor fuel pressure to begin with, the the TPS "fix" may have been the previous owners solution, .

Having said that, I don't know why unplugging the TPS previously had no effect, nor why this missing problem decided to appear now... I've had the car a couple of years now and it's never done it before.

Edited December 30, 2011 by Lyttek