280Z runs rough when warmed up

[KZZman]

I was excited to purchase my first Z after years of admiring them. I just purchased a 77 280Z with only 10,000 miles on it. I did a full blown tune-up plugs, wires, cap, roter, fuel filter, before I started putting miles on it. I brought it home today, and it started to run rough after it was warmed up. It started backfiring  and was running at 500 rpm at idle. I looked around on past posts and noticed quite a few owners have had the same problem. I see quite a few suggestions, but I don't want to start throwing a bunch of parts at it if I can help it.  Could anybody give me some solid advice. I see possible solutions as replacing o2 sensors, coolant temp sensors or start messing around with the AFM. 

[S30Driver]

The best advice I can give you is not to drive a 10,000 mile Z in the Wisconsin winter, early spring.

While it's off of the salted roads, spend the time to clean every electrical connection & ground under the hood.  (no o2 sensor to worry about)

[rossiz]

+1 on cleaning connections - there are a set of bullet connectors about midway on the intake manifold that connect the sensors at the thermostat housing to the harness, i'd start with these and the connections at the sensors themselves. the coolant temp sensor is used to enrich the mixture at cold temps and if not functioning it will cause problems. check your plugs - i'm guessing they're black. 

do NOT mess w/the AFM to start with - usually the last thing you need to touch and many have chased their tails by starting here only to find the real culprit in other areas. once the AFM is out of adjustment it can mask other issues and make it difficult to diagnose. the efi system on these cars is pretty simple, but it requires multiple components to all be working together and connected correctly - and no vacuum leaks.

btw: where did you find a 10K mile car? are you sure it's not 110K? the odo rolls over at 99K...

[Captain Obvious]

I second the skepticism on it being a 10K mile car. I suspect it's a 110K mile car.  Pics or it didn't happen.  

Redardless, however... Welcome aboard!

 

[rcb280z]

Or 210k. Not impossible. Download the FSM and FI bible. It will help you get started on your diagnosis. Then you can post what you have tested and the results giving everyone a starting point.

[siteunseen]

Take a look at the rubber on the pedals.  They'll give you an idea of milage.  

Welcome aboard and a hardy handshake to you. 

[Captain Obvious]

"As chairman of the welcoming committee, it is my privilege to extend a laurel, and hearty handshake."

Edited March 23, 2016 by Captain Obvious
Fixed frontier gibberish

[Zed Head]

14 hours ago, KZZman said:

I was excited to purchase my first Z after years of admiring them. I just purchased a 77 280Z with only 10,000 miles on it. I did a full blown tune-up plugs, wires, cap, roter, fuel filter, before I started putting miles on it. I brought it home today, and it started to run rough after it was warmed up. It started backfiring  and was running at 500 rpm at idle. 

Check for vacuum leaks.  Ripped, disconnected or loose hoses.  All air that enters the intake manifold should pass through the AFM.  The "backfiring" might have actually been popping back through the intake system.  Did it dothis when revving the engine or while it was just sitting?  The low idle is a sign of a vacuum leak also.  Small leaks cause the idle speed to increase, then bigger ones cause it to drop.  Really big ones cause the engine to die.  If it was actually backfiring through the exhaust then it might be a timing issue.

It's not clear if it ran well warmed up before and you did something to it or if you never heard it run when warmed up.  Also, you said that you tuned it up first, then brought it home.  Did you drive it home?   Anyway, welcome and don't do anything drastic unless you're sure.  And mark or record all of your starting points.  Easy to get lost.

[siteunseen]

4 hours ago, Captain Obvious said:

"As chairman of the welcoming committee, it is my privilege to extend a laurel, and hearty handshake."

Blazing Saddles!!! 

"The sheriff is BONG

What did he say? 

The sheriff is near

No dad blame it...the sheriff is BONG"

 

 

[Captain Obvious]

Haha!! Exactly! I knew my bud has my back when I throw that stuff out there!!

One of the greatest movies ever. As a matter of fact, I was considering Waco Kid as my nickname here, but didn't want to continuously field the question "Are you originally from Texas?" So I went for something a little more realistic. OK, a lot more realistic.  

I also considered Mongo, Black Bart, and Governor Le Petomane (you should look that one up...)

[siteunseen]

12 hours ago, Captain Obvious said:

So I went for something a little more realistic. OK, a lot more realistic.  

I should have used A B Normal with this avatar.  That's my reality.  

 

[siteunseen]

Try unplugging the cold start vale. It could be fouled up somehow and continues adding fuel after the motor warms up. Too much fuel would cause backfire through the tailpipe. Some people don't have them at all, they go bad and the car will run without it so they just unhook them.