Throttle jerking at lower speeds. 78 280z

[gr878280z]

I have a fuel injected 78 280z with a performance cam. It starts and runs great, and accelerates and cruises smoothly while cold. But after it warms up a bit the throttle is jerky. When making very tiny changes to the gas peddle (just to maintain speed), the throttle jerks on and off. Its great while accelerating or decelerating but just doesn't maintain a steady cruising speed without jerking in and out. This is very obvious at 50 mph or less but doesn't seem as pronounced at highway speeds.

Maybe a Throttle Position Sensor?

Air Flow Meter?

Vacuum Leak?

[siteunseen]

Could very well be the TPS, here's adjusting tip http://atlanticz.ca/zclub/techtips/tps/index.html.

Here's AFM info too, click on EFI and FUEL. http://atlanticz.ca/index.php/tech-tips.html

Has it just started the jerking or is it a problem you've had? I've read FI cars don't run good with bigger cams is why I ask.

Old Yeller's a fine looking car!

Edited February 19, 2014 by siteunseen

[Zed Head]

How did the engine get this way and did it run right before?

Could be that your fuel mixture is off. A "performance" cam typically requires more fuel. Word around the internet is that the stock EFI system does not handle modified cam profiles well. You might try the potentiometer on the coolant temperature circuit to see if adding fuel will smooth things out.

[240260280z]

Cleaning and adjusting throttle linkage helps. There are some geometrical changes that some have made to improve mechanical advantage,

[superlen]

I'm also of the opinion your leaning out. Lean mixtures can be a little bouncy as the engine hunts around (although typically this is described when idling, not so much as cruise, but it would be possible).

Like Zed Head said (LZHS - I need an acronym for this as it seems I just go around repeating what he said a few posts later. ) the cam is letting the engine breath now & of course you need more fuel. The stock AFM should account for it, but for whatever reason, it or the ECU isn't quite getting it done leaving you somewhat lean. Have you heard any backfires or had other signs of leaness?

I don't think its the TPS as in the condition you're describing the TPS should be in the NOT idle or NOT WOT, thus the ECU does nothing. Since our TPS is not really an anlog signal, it won't cause a lean condition while cruising. If, however you're experiencing a rich condition in cruise, the at TPS showing WOT (erroneously) could be the culprit.

It certainly doesn't hurt to adjust it per factory specs to see if it make a difference.

Len

Edited February 19, 2014 by superlen

[gr878280z]

Thanks for the help.

I am a bit confused by the TPS diagram.

Is it saying that the center should not be touching either side as Cruise?

Bought the car 1-1-14 and it had the cam in it already.

Been working on it since. I think the cam was done in 08.

[gr878280z]

Thanks for your help.

No back fires.

Seems to be rich.

We have adjusted the TPS by bending the contacts and rotating the unit and it seemed to help some.

[superlen]

gr,

I think the next step might be to investigate your AFM. Blue has a good writeup on them on his site. If you pop the cover off you can play around with it while the car is idling and see the effects of tightening/loosening the spring. You don't really do anything with the spring while it's running, just move the vane a little more/less with your finger simulating more/less airflow in the engine.

If you are indeed to rich, you should hear the engine RPMs raise a bit when you move the vane a little less than what the actual air is making it swing open, simulating less air flowing in the engine. This would collaborate your rich assumption as by playing with the AFM you're telling the ECU to dump less fuel.

Lenny