strange issue with 78 280z running - I'm at the end of my rope!

[gheiser70]

Hi, I've got an frustrating issue with my 78 280z. I bought it last fall from the original owner ( a very nice lady). It had been parked in her barn since the early 1990's. She said she was afraid it would leave her stranded. Any way, she had a lot of work done to it before it was parked, new AFM, fuel pump, injectors ect. I dropped the fuel tank, cleaned every connector, put new plugs, wires, tune -up parts, cleaned the 35 pin connector and reflowed the solder connections in the ECU, fuel filter and pressure gauge. I'm getting 40 PSI with the FPR disconnected and 32 or so with vacuum to it. I don't believe I have any vacuum leaks. I was pulling between 17-18 in. Vacuum. All FI components spec out per FI bible. Valve adjust also done. Timing set at 10 BTDC.

After all that I was still fouling plugs with what seemed to be a rich condition. I then found that my master cylinder was leaking into the brake booster (it was full of brake fluid). I thought I might have been burning some brake fluid causing my "rich" condition. So, I replaced the booster and M/C, then I found a new issue. When running down the road, the car would cut out under hard acceleration, like some one flipped a switch, when I back off the gas it kicks back in and bucks like a bronco. After reading information here and the Atlantic z site I installed a 5K pot in line with the water temp sensor. It helped some but I still need to adjust for the best condition.

Now though, my idle has become unstable and I'm only pulling about 15 in vacuum. The real brain teaser is that when I pull off the vacuum line for the air con. control to hook up my vacuum gauge, the idle increases and smooths out. So the car idles better with a vacuum leak - what gives???

Any input would be welcomed. Sorry the post is long, I just wanted to give some background.

[Jeff G 78]

The resistor in the water temp sensor circuit will make the mixture richer, so that should have made the situation worse, not better. A vacuum leak will lean it back out making it run better. As for the original problem, I'm not sure what to try next. What kind of plugs are you using?

[Zed Head]

I'm getting 40 PSI with the FPR disconnected and 32 or so with vacuum to it.

All FI components spec out per FI bible.

You said everything checked out but you're reporting fuel pressure that's 10% too high. The proper number is in the Fuel Injection Handbook and the FSM. High fuel pressure will cause things to be rich. With the potentiometer, you have rich on rich.

The cutting out sounds like a problem with the TPS. Did you wash the engine recently? They get wet and do what you're describing.

[siteunseen]

I'd clean the AAR. Maybe get a new PCV valve? Sounds like it needs more air, maybe a varmint got in the air box?

[gheiser70]

Thanks for the replies. I'm using NGK plugs, but I probably need a new set. No water has gotten into the TPS and it checks out. I did replace the PVC valve. I don't get any black smoke from the tail pipe. My in line FP gauge is just after the filter and was cheap from jegs so it may be off a little bit. The reason I wanted to try the pot was for the cutting out problem. Many posters (from my searches) seem to think this is caused by a lean condition. I have good hot spark, but if my plugs were slightly fouled from before I changed the brake booster, I suppose they could be failing to fire under a hard load when accelerating. The Pot did seem to reduce the cutting out under load, but the idle went to crap. It was a 5 dollar attempt and no harm was done, as I just wired in in the engine compartment, without cutting the harness. I'll get a new set of plugs and try from there. It has a new air filter, but I'll take a look for varmints. I had the AAR off and I seemed to work well. The fast idle works when the car is started and settles down as it warms up.

Edited May 18, 2014 by gheiser70

[Zed Head]

Good luck. There are so many components that work together, that "close enough" just doesn't work very well. You'll end up adjusting something else instead of the actual problem. The best method is to make sure that everything is exactly as it should be.

If you report numbers in your posts, instead of looks good and checks out, you'll probably realize that the numbers aren't quite right. An ohm here, a psi there, a small vacuum leak...the tiny things add up.

If you find yourself deciding to adjust the AFM spring, make sure you mark your starting point. So you can go back.

On the ignition module - often they don't fail completely. They show signs of failure, then come back to life when they cool down a little bit. So, spark testing won't tell the whole story. Watch the tachometer needle next time it cuts out and starts bucking. The tach sees what the module is doing.

Edited May 18, 2014 by Zed Head

[FastWoman]

Your high fuel pressure reading could be an inaccurate fuel pressure gauge. You might try confirming with another gauge. You might be able to borrow one from an auto parts store. (You leave a deposit with them that gets refunded when you return the gauge.)

It's entirely possible your engine was running lean, and then you added enough resistance to make it run rich. Or perhaps it was running rich, and you made it run richer. Any of that is possible. You really have to adjust the potentiometer to maximize the vacuum and performance -- and to give the plugs a nice mocha color.

The cutting out issue does sound like the TPS. Perhaps the previous owner/mechanic rewired it incorrectly, or perhaps it's broken. It sounds to me like it's sending a throttle-closed signal to the ECU when you step on the pedal. If the engine is turning above 3k or so, the ECU will then cut fuel. You should check this switch very carefully to make certain it's doing what it should. You should check it both visually and with a meter at the big ECU connector.

Another thought: When you reflowed the solder on the ECU PCBs, is it at all possible that you introduced a short between traces leading to the TPS? It might be worth a look.

[240260280z]

Data:

Fouling plugs means you are running rich

Vacuum leak experiment adds unmetered air. The improved performance with the extra air makes sense due to the rich condition... you are compensating for it.

Fuel pressure seems in the ball park and FPR is working

Questions:

Are all plugs the same? This shows that spark and injectors are working relatively the same.

Most probable problems causing richness:

1. Water Temp sensor and wiring to ECU. Measure resistance at sensor and also back at the ECU. Check to ensure it is nominal for the water temp. (do this on a cold engine).
   
2. Incorrect injectors. Check their published flow rate against part number.
   
3. AFM spring too loose. Did anyone adjust it before (look for signs of entry). If the spring unwound by a previous butterfingers mechanic then it may still be too loose.
   
4. ECU failure.
   

Edited May 18, 2014 by Blue

[240260280z]

BTW great report and work/checks that you have done to get the car up and running!

IMPORTANT. Be sure to clean the plugs with solvent and a wire brush once they are fouled or further attempts to resolve will be hindered by the fouled plugs. No need to buy new ones as once the air fuel gets close to nominal, the engine will clean up the plugs. Just ensure they are not carboned-up before trying something new.

I like to put fouled plugs in a bowl then hit with carb cleaner spray and wire brush.

Edited May 18, 2014 by Blue

[240260280z]

280Z stock injectors are 188cc units

Injector list: http://www.atlanticz.ca/zclub/techtips/injectors/injectors.pdf

Water Temp Sensor Resistances vs. Temperature

[TABLE=class: grid, width: 500, align: center]

<tbody>[TR]

[TD=align: center]C[/TD]

[TD=align: center]F[/TD]

[TD=align: center]Kohms[/TD]

[/TR]

[TR]

[TD=align: center]-30[/TD]

[TD=align: center]-22

[/TD]

[TD=align: center]20.0-33.0[/TD]

[/TR]

[TR]

[TD=align: center]-10[/TD]

[TD=align: center]14[/TD]

[TD=align: center]7.6-10.8[/TD]

[/TR]

[TR]

[TD=align: center]10[/TD]

[TD=align: center]50[/TD]

[TD=align: center]3.25-4.15[/TD]

[/TR]

[TR]

[TD=align: center]20[/TD]

[TD=align: center]68[/TD]

[TD=align: center]2.25-2.75[/TD]

[/TR]

[TR]

[TD=align: center]50[/TD]

[TD=align: center]122[/TD]

[TD=align: center]0.74-0.94[/TD]

[/TR]

[TR]

[TD=align: center]80[/TD]

[TD=align: center]176[/TD]

[TD=align: center]0.29-0.36[/TD]

[/TR]

</tbody>[/TABLE]

[gheiser70]

Thanks again. I'll recheck the TPS, because it does seem that the fuel (or spark??) is completely cut when the car acts up. I have considered the ignition problem as well (module failure or coil) when the car has run for a while. At this point the problem is not related to how long the car runs.

Anyway I have my niece's baptism today, but I'll try to get to the garage this evening.

I'll remove the pot as well and try again with clean plugs. I'll also check injector P/N. I don't think the AFM was open It was replaced < 1000 miles before the car was parked. The AFM P/N is A31 625 000 - Is that correct for 1978? It looks like there is clear sealant around the black cover.

Edited May 18, 2014 by gheiser70

[240260280z]

What are pn's on the injectors?