Stumped... Intermittent, uneven running. Please help.

[NiiChan GT-R]

I've tried my best to read up/ research and be thorough, and I'm still failing with my diagnosis... Here goes.

1975 280z, all stock l28e.

Valves adjusted, all filters replaced, new plugs, coil, efi temp. sensor, all vacuum lines replaced (Inc. under the dash), except for the evaporative system, which I've disconnected and plugged from the manifold and vacuum advance for the time being *to remove that variable.

I use the car as a daily driver. About a month ago, the car suddenly started running poorly (bucking, loss of power, weak idle), and I found that tweaking the mixture cog in the afm richer by about 45 deg (as per the Atlantic z-car site) improved the car to normal.

I assumed a vacuum leak initially but found none; I then assumed that possibly one or more injectors had previously been stuck open.

About two weeks ago, the car's running became erratic again. Random bucking, bad idle, lack of power... I turned the mixture screw another 45 deg richer, which cleared up the problem... But only 50% of the time now. The mixture in the afm seems to like me to vary it's settings by 20 deg or so every few days, either direction, still subjecting me to the above condition.

As I drive, it randomly switches from running well to roughly, often in the rundown from vigorous acceleration, or on the way up in moderate acceleration, the accelerator feels "clunky" for a moment before clearing up.

Disconnecting thermotime switch does not improve things, nor does blocking off all the climate control vacuum tubes at the manifold.

Egr lines all check out, brake booster was replaced, evap system disconnected as mentioned, good valve cover gasket and cap, aux air valve fuel pressure regulator, lines, pump *vac advance *yadda yadda all seem to have no visible problems...

I'm thinking maybe my injectors are toast? Poured some cleaner in, no real effect. *I can't think of anything else that could cause this condition. It runs strong when it does, then it doesn't.

I had the car in the shop back in march, they apparently replaced the temp sensor as mentioned, is it possible that it's already bad again ( I don't want to do business with that shop again, don't ask).

Ecu? Admit I haven't checked the timing lately because it was fine before. The ignition never seems to cut out, when it's idling roughly I can usually touch the mixture and see it improve til the car changes paradigms again.

Is there anything I'm missing? Thanks.

[240260280z]

What do plugs look like now?

[NiiChan GT-R]

At work at the moment, when I get home I'll check out their condition/gap at this point, probably post pics at that point as I suck at judging things like that...

[Zed Head]

I think that the "evaporative system" contains the vent to the gas tank. You may have plugged that vent which might cause fuel pressure or flow problems. Just a thought.

[grantf]

Just a thought, could it be a bad water temp sensor or connection? this will affect the mixture. You might be compensating for it with the spring adjustment. I see you replaced yours but you could still check it with a meter.

Also have you checked the AFM flap to see if it is sticking?

I am just suggesting these because they are problems I have run into.

edit.. The more I reread youre post and think about it it seems the problem is the AFM.

"often in the rundown from vigorous acceleration, or on the way up in moderate acceleration, the accelerator feels "clunky" for a moment before clearing up." this sounds like a sticky flap.

I could be way off, but it makes sense to me.

Edited August 12, 2011 by grantf

[NiiChan GT-R]

Hi,

As for the afm flap, when the car is running clunky I can blip the throttle, and see it move forward, as the car winds down I see the wiper reel back to the lean side, go a bit too far to the point where the car idles about 100 rpm lower, before stabilizing... The wiper seems to move smoothly, as if it's affected by air flow rather than sticking...

I should add that sometimes the car will idle correctly and still have hesitant acceleration, other times not idle but spin up decently, sometimes both probes at same time, other times it will be fine.

I'll figure out how to check the temp. Sensor once I'm home as well.

Thanks, will report back. Any other ideas?

I

[NiiChan GT-R]

I think that the "evaporative system" contains the vent to the gas tank. You may have plugged that vent which might cause fuel pressure or flow problems. Just a thought.

I plugged the port from the intake, and vac advance, left both ports on the charcoal can open... Will that affect things? Also, I should add that I did this yesterday, with apparently little effect.

[FastWoman]

Check your gas cap's vacuum relief valve. As long as that's working, you won't get a problematic vacuum in the fuel tank. That said, I think you probably should have your charcoal canister connected up. There's no reason NOT to, and tampering with it is actually illegal. (I'm imagining a conversation in a jail cell: "So what are you in for?" "Tampering with a carbon canister." "Oh god! Stay away from me! I don't want NO TROUBLE from you!")

A problem that suddenly comes and goes sounds VERY ELECTRICAL to me. I'm always suspicious of crusty wiring/connections. You might learn a lot by pulling off the ECU connector (WITH THE IGNITION OFF!) and going through all the measurements in the factory service manual (which you can download for free at www.xenons30.com). By measuring at the ECU, you will not only be determining the state of your various EFI components, but also the state of the wiring and connectors. If your connectors are anything like mine were, they're pretty crumbly and corroded. You can buy new 2-connector Bosch-style connectors (the style on your injectors) on Ebay from a seller called "f0rrest."

[NiiChan GT-R]

Ok... Sorry for the lack of response from me lately, I've unfortunately had little time to either work on the car, or post on the Internet, but here's status:

-Reconnected evap. system. Also, plugs looked good still, have proper gap.

-Replaced all six injector connectors and hit the injectors themselves with contact cleaner. This was clearly an issue.

Car runs fine after that, previous issues seem gone with a BIG if; if I drive sedately, the car is fine. If I accelerate vigorously through the gears, it scoots along... No roughness or jerking on the way up.

... But for a minute or so after that acceleration, the car runs roughly for a few moments, sometimes the gas pedal has no effect, and if I come to a stop it will often die, or at least idle not so smoothly; running seems overly rich from the smell, but i cant be certain yet, and fingering the afm wiper during these periods of rough running seems to have no positive effect in either direction.

The condition is similar to running the car with the fi temp sensor disconnected. So when I get home today I'm going to replace the connector and hit it with some contact cleaner.

In the meantime, what else could generally cause that condition? I'd need a fuel pressure gauge to check for sure, but could it be the fuel pressure regulator? Vac line seems ok.

Thanks for dealing with my headaches

Edited September 3, 2011 by NiiChan GT-R

[FastWoman]

That's a stumper!

Is your AFM's vane moving freely, without binding? I know you say fingering it has no effect, but I'm grasping at straws here.

Maybe you have a bad ground somewhere that becomes a problem when the engine is rocking? Is the ECU firmly bolted in?

Question: If you accelerate briskly, create this problem, come down to an idle, and then just sit there (with no actuation of the accelerator pedal, no movement, etc.), does the problem resolve itself spontaneously after a couple of min? Or do you have to do something, like work the throttle, for the problem to resolve?

What happens if you KEEP running the car briskly? Does it only run briskly for a short while, before it starts running badly?

[NiiChan GT-R]

I checked that the ECU is bolted very firmly in, but chickened out when it came to removing the connector.

There are two wires dangling next to the ECU, disconnected from anything, each with a round female terminal connector. I can't seem to figure out from the service or Hanes manual what those must be, or if they're necessary. Another ignorant question, where are the grounds that I should be checking?

Maybe a strange thing, the car only has these issues under load; I can rev the car freely, it runs smoothly in neutral.

The problem has become more intermittent, sometimes if I let the car idle the idle will dip to about 300 and the car will die. Sometimes it idles steady, and sometimes if I rev the engine the revs will dip 200rpm below idle on the way down, then return to normal. The AFM again wants something different every time I touch the car.

I'd say I'm back to square one I'm fairly certain that this is some sort of electrical issue, when the car "resolves" (which doesn't depend on anything I do), it sounds like I hear a bit of an electrical "click" from behind the dash, or firewall. Any idea what that could be?

I did use contact cleaner on the temp. sensor connectors, no dice. It does appear to be a used sensor. Since it's only $30 for a new sensor, I'm thinking this may be my next course of action, I guess I should take some resistance readings first...

I'm ready to give up and take the thing to a mechanic again, as I think I've reached the limits of my skill. I did try Mike's Z shop in Whittier, but to put it lightly, I got ripped off and they couldn't fix the issue. I gave them another chance to fix it, and all they did was accidentally (I think) disconnect a bunch of vacuum hoses, and give me a car that worked worse than it did before.

Does anyone in SoCal actually know how to work on these things?

I'm stuck here. Anyone want to buy it?

Edited September 5, 2011 by NiiChan GT-R

[Zed Head]

The clicking behind the dash is probably an effect, not a cause. When the engine RPM drop the fuel pump contact switch in the AFM opens, killing power to the fuel pump relay. Then the mixture leans out, engine RPM goes up, the contact closes again, and repeat. Usually good for a few clicking cycles before the engine dies or it smooths out.

Have you measured fuel pressure? It is one of the most important variables in an EFI engine.

Another possibility is your ignition module. Next time it starts running poorly, with bucking etc., stop and turn the key off. Then restart it. If the problem goes away temporarily that's a good sign that the ignition module is going bad. I could get my 76 module to do that when it went bad just by taking it to about 3200 RPM. Turn it off and restart and I could drive around all day below 3000 RPM.

The dipping idle might be because your air/fuel mixture at idle is all out of whack now. If fuel pressure or the module turn out to be the problem hopefully you can get back to where you started.

And the two white wires are apparently for some sort of diagnostic test. Everyone has them and they're all disconnected.