Engine problems

[jmead]

I've got a 1976 280z w/ auto transmission

Since I installed my upgraded throttle body (60mm 240sx), I've been having a problem. Sometimes the car "pops" a little bit, you can feel it very clearly in the gas pedal, and my girlfriend said she could feel it from the passenger side, too. It used to just happen once when you pressed the pedal hard from a stop, but now it seems to do it all the time under 30mph, even with the slightest acceleration. When it happens, the car lurches a little bit, and acceleration isn't even close to what it normally is. In addition, since this started happening, the car has also been stalling. The tach jumps up to 3500-4000, flutters around a little bit, along with a loss of power, and once the car slows it just shuts down completely. Normally if you re-start the car its fine, but on my way home it happened several times, when it normally wouldn't. Does anyone have any ideas? It’s to the point where I can't even drive my car now, so I'd really like to get it back into tip-top shape. Thanks for any help.

[JEFF]

Start with a compression check. If all the compression readings are within 10 psi of each other then OK. If not then I would check the valve clearances between the cam and the rocker arms, it may be tight on the lower compression cylinders. It may be that the intake valves are not seating fully, thus the 'pop' and able to feel it in the pedal,(the 'pop' causes the throttle plate to jerk). If the valve clearances are correct, have a 'leak-down' test done on all 6 cylinders to check for burnt valves.

You could narrow it down to a cylinder by driving with one plugwire off. alternate untill the 'pop' goes away, That's the one!

The jumpy tach ??? check the ground wire that grounds the intake manifold to the drivers fender well. That ground is part of the whole bundle of wires that run the fuel injection system. A loose ground can do VERY funny/irritating things. Replace the cap & rotor. Clean carfully inside the distributor. besure the magnet on the pickup is CLEAN. Besure that all wires are tight. Check all the ground wires that you can find. Besure that all wire connections are CLEAN.

Hope this helps. Good luck!

- Jeff

[sjcurtis]

jmead,

I would like to offer a few additional tips to what Jeff is already saying. If you changed the throttle body, could you have upset something else when you did the job. It may be as simple as a wire loose on a plug, I think your full throttle switch could do with investigated along with the airflow meter. I think your pop and associated shut down could be a very lean mixture and faulty air flow meter operation. You may have to do some work on the airflow meter to increase the fuel to air ratio allowing a better burn rate. let us know what you find.

my 2c

Steve:classic:

[JEFF]

JMEAD - sjcurtis has good advice in his post- ("I think your pop and associated shut down could be a very lean mixture and faulty air flow meter operation. You may have to do some work on the airflow meter to increase the fuel to air ratio allowing a better burn rate. let us know what you find"-sjcurtis)

If airflow and/or fuel mixture are in fact contributing to the "pop", then it could have started to effect the valves.(hope not).

You can "read" the burn on the spark plugs. If they are really light tan to chalky white - the fuel/air ratio is way to lean. The plugs should be a light chocolate brown, not sooty, not oily,and certainly not white.

- Jeff

[jmead]

I found that the ground wire that connects to the throttle body was just hanging loose, that cleared up the majority of the problems. There is still a slight pop sometimes off of idle, but it just happens once, and isn't nearly as obvious as it was. I'm still having a problem with it stalling, with the flutter tach and everything, but it seems like that has gotten better too. I can make it happen now, just by putting it in neutral and reving to 6k it will happen every time, even with no load.

More problems now, I found a small leak in the water pump. Now its running slightly hot(er).

I guess I'll check the spark plugs, see what color they are. How long would a lean condition need to exist before it colored the spark plugs? It has only been doing this for a day or two, is that enough time?

One other question, to anyone with a 280z and an auto. It seems like the low end torque is a little lacking (maybe related to a bad AFM or something). If you punch the throttle off idle, do the tires spin? It seems to me like the engine would have enough power to spin the tires from a stop, but mine can't. I know its not a very good way to measure the low-end, but its the only test I can come up with.

Thanks for your help!