What else, another engine problem

[seehunt]

My 75 280 started running rough about a month ago, after checking everything i know how to I still coudn't find the culprit, I then decided to start replacing all the usuals, ended up putting in new plug wires, distributer cap, rotor, fuel filter and cleaned the plugs [ had less than 5000 miles on the plugs ], the end result was that it ran a tad better but nothing significant, after thinking it over i decided to try a new set of plugs [ NKG's ] and i was not to hopeful about the outcome...well, i was surprised, it started right up and ran like a top. Forgot to tell you that when i pulled the plugs to clean them they were black all over. Back to the current situation; I was quite pleased that i had found the problem and fixed it...or so I thought. After a couple of weeks went by [ I only drove the car a total of about 20 miles ]

I started it up and there was the problem again, i couldn't believe it ! Now why would it start running rough again when i had changed all those parts, that got me to thinking it must be fuel related but on a hunch i decided to have a look at a couple of the new plugs to see what they showed...yup, they were all black again, all 6 of them. Now you have the picture, with the above info can anyone tell me what the problem might be.... I suspect valve seals or rings or maybe something else, I hope it is something else like the flowmeter. When it runs rough i get blue gray smoke but when it was running well i just got a little blue gray smoke on startup...... I am stumped, any of you ever have the same problem and if so what was it ?

[GTZZZ]

Sounds like your carbs are running a rich fuel to air mixture. Read these two threads.

http://www.classiczcars.com/forums/showthread.php?s=&threadid=8059

http://www.classiczcars.com/forums/showthread.php?s=&threadid=8089

[seehunt]

Hey Robb, thanks for the steer but the car is a fuel injected 75 280 Z. Got any steers on Fuel injected cars ?

Bobby

[GTZZZ]

The symptoms you are describing occur more often on carburated engines. Even though your car is fuel injected I think the black plugs are still telling you your car is running rich. Fuel mixture on fuel injected cars is constantly being adjusted by a computer. It changes the amount of fuel being delivered based on readings it receives from sensors on the engine going to the computer. My best guess would be a bad O2 sensor. This sensor is in the exhaust and when working correctly tells the computer what fuel mixture is burning in the combustion chamber based on the burn temperature. If it is not working correctly the computer is basing how much fuel to send into the engine on an incorrect reading and will supply the wrong amount of fuel, causing the engine to run rich or lean. If this is the problem changing the O2 sensor will fix it. It could still be something else, another sensor (airflow at intake), or a bad injectors (doubt all would go bad at same time) among other things. I would change the O2 sensor first, based on the symptoms it would be my best guess and is also a cheap place to start.

[seehunt]

Hey Rob, that sounds good to me, I'll get an 02 sensor and change it out and see what happens, will take me a while until i get into town though. I am wondering of you have ever pulled one ? I checked my z and it looks like the original one and that got me to thinking about how hard it would be to remove. Thanks for the info, it's much appreciated

Bobby

[2ManyZs]

Unless I'm mistaken there is no O2 sensor on any of the 280's other than CA models. They didn't start using them on all 50 states models until 79 when they put the catalytic converter on.

If you don't have an Factory Service Manual, you are walking blindfolded when trying to diagnose a problem with the FI systems. The FSM will give you all the resistance values for the sensors such as the water temp sensor which controls the cold start valve and other parameters of the FI. This would be the first thing you should check, if the water temp sensor or the air temp sensor are not working, they will more than likely cause you to have a rich condition.

[seehunt]

Hey guys, I do have a FSM and it shows the air sensor to be in the airflow meter so that is out for now as they one i have is fairly new. As you said tomanyz's, it might be the water sensor. The FSM shows it to be on the front of the block and it shouldn't be to hard to change, also it shouldn't cost to much. I live way out in the country and go into town about twice a month, when i get the water sensor installed i will post the results. Thanks a lot to the 3 of you for helping me out, that's what makes this site the best one on the web for Z cars.

Bobby Hunter

[seehunt]

Hey Rob, I do believe you are right about the car running to rich and the plugs are the evidence, lets hope the water sensor fixes it, if not i will try again, maybe change out the thermodine sensor.

Bobby