Slight back fire and stumble!

[ZSaint]

I just bought a '78 280Z that had been stored for 7 years. I changed fuel lines, checked and/or replaced vacuum lines, changed plugs, drained the gas tank, bought two injectors, had the injectors cleaned (two plugged) and started the car. It ran fine... With one exception: It back fires when I give it small amounts of throttle. If I give it a little more throttle it takes off and accelerates like crazy. Where is this "leaness" coming from? I have never played with EFI. I have changed the computer and the air sensor and it made no difference. I checked for vacuum leaks and it has none. Something is not working and I need to find it and replace it. What do you recommend?

Edited December 5, 2011 by ZSaint
forgot step

[FastWoman]

Zsaint, have you done everything in post 8?

[ZSaint]

I think this may be the problem. After setting so long w/o running, I think maybe the advance mechanism may be frozen. I will check it out today.

I do not understand the "blowing in the intake". Aren't the valves open to let the pressure off? How do I get "pressure" with valves that are always open? (valve overlap?) Where do I set the engine to avoid an exhaust and an intake patially open? What am I missing?

"thermotine sensor"? Where is that locatied? Cost? Dealer item?

Sorry to be so stupid but all of my experience with Datsuns is with Webers, Solexes or SU's.

[Willoughby Z]

Thermo time switch is next to CTS (coolant temp switch) at front and below thermostat housing.

$75-100 through various suppliers.

Have you downloaded FSM?, it's free and a required reading resource.

Edited December 5, 2011 by Willoughby Z

[ZSaint]

I have searched Google w/o success. I will now try to get info from CZC. You guys always bailed me out when I did my '71 240Z resto. I am learning new stuff here with this 280Z.

[Zed Head]

There is something not quite right about the "solution" in Post #10. The thermotime sensor only controls the cold start valve, which is only powered when the key is at Start. So the thermotime sensor should have nothing to do with your backfiring problem.

"Off-idle" leanness could be caused by the TPS not indicating "idle" to the ECU. According to the FSM, Engine Fuel section, there is an "after idle" enrichment. It appears this is added by the ECU when the TPS switch moves from idle to the middle or part-throttle position. The diagram is on Page EF-11.

[ZSaint]

I just downloaded the '78 280Z FSM. What a find! I will check out your page to see if I can figure this out.

The Z runs good after starting and it is cold. By the time I drive it a couple of blocks it starts snapping and backfiring when I take off with a little throttle. If I use a more agressive throttle, the Z accelerates normally. Something is wrong with this EFI system. Hope to get this resolved. This is a very quiet car that drives great. (a lot more pleasant to drive than my '71 240Z!)

[FastWoman]

ZSaint, in my experience, the valves rarely overlap, which would of course allow you to blow air directly from the intake to the exhaust. There will be one or two open cylinders in their intake strokes, but the corresponding exhaust valves will probably be closed. So part of what you're leak-testing is a couple of cylinders. That's OK. Just blow into the intake and you'll see. It should hold pressure, and it should feel just a bit like blowing up a balloon, except that it will be easy to blow at first and will get harder as pressure builds.

[LeonV]

I think this may be the problem. After setting so long w/o running, I think maybe the advance mechanism may be frozen. I will check it out today.

I do not understand the "blowing in the intake". Aren't the valves open to let the pressure off? How do I get "pressure" with valves that are always open? (valve overlap?) Where do I set the engine to avoid an exhaust and an intake patially open? What am I missing?

"thermotine sensor"? Where is that locatied? Cost? Dealer item?

Sorry to be so stupid but all of my experience with Datsuns is with Webers, Solexes or SU's.

The valves are not always open. There are 3 instances of valve overlap in one rotation of the engine (360 degrees). A stock 280Z cam has 30 degrees of overlap. This means that in one rotation of the crank, overlap occurs in 90 out of 360 degrees, or 25% of the time. Take the valve cover off if you're unsure, and make sure that no cylinders have both valves open.

[Zed Head]

Leon, you've corrected a statement I made in a different thread. I said 4% of the time (30 of 720 degrees) there was overlap, according to the cam specs. in the FSM, but I neglected the other 5 cylinders. 6 x 4% = 24%. Actually 6 x 4.2 = 25%.

25% is pretty good odds that there will be some leakage through the intake manifold to the exhaust.

Of course, this assumes correct valve lash settings. Looser lash would reduce the odds. You should also consider blow-by past the rings and valve seals. Still a test worth doing if you're stuck.

[LeonV]

Leon, you've corrected a statement I made in a different thread. I said 4% of the time (30 of 720 degrees) there was overlap, according to the cam specs. in the FSM, but I neglected the other 5 cylinders. 6 x 4% = 24%. Actually 6 x 4.2 = 25%.

25% is pretty good odds that there will be some leakage through the intake manifold to the exhaust.

Of course, this assumes correct valve lash settings. Looser lash would reduce the odds. You should also consider blow-by past the rings and valve seals. Still a test worth doing if you're stuck.

Happy to be of service!

[FastWoman]

True, but these engines only really stop in one of 3 positions, somewhere towards the start of a compression stroke. I don't know how that would correspond to valve overlap on other cylinders. All I'm saying is that the odds are very slanted towards the engine stopping at exactly the same sort of valve relationship every time (just with different cylinders). In the case of my own engine, it's without any valve overlapping.