1977 280Z Stuttering

[BayAreaZ650]

I got back my 1977 280z from the body shop a few days back. My Z was there for the past 10 months while they worked on restoring her. When I got the Z back, I noticed that it idled roughly and would take 4 starts to get it going. When driving the Z, it would randomly hesitate when stepping on the gas pedal and it would have trouble holding the RPM’s steady. The Z ran perfectly and was gone through (new spark plugs, injectors, fluids, spark plug wires, timing and valve adjustment, etc) before sending it to the auto body shop. I went through two new tanks of gas, cleaned all the electrical connectors, changed the water temp sensor, fuel filter and put fuel system cleaner in. What could this issue be?

 

in the video, I’m holding the pedal to rev at 3000rpm. I’m not releasing my foot on the pedal when you see it dip to the lower rpm’s.

[Dave WM]

could not get the .mov to work, but generally speaking you should start by reading the plugs. running rich or lean will be a data point. Something to always look into is simple stuff like firing order and timing. don't know why they would change but once its out of your hands you can't be sure of anything. I see you changed the water temp sensor, did you check the resistance values at the ECU connector? This is important to make sure the wiring from the sensor to the ecu is correct and the values the ECU are correct for the given water temps. You mentioned fuel filter, have you checked the fuel pressure?

Another problem can be air leaks, a disconnected hose missing o ring on an injector, split hose on the AFM all kinds of issue  can be air leak related. Smoking the intake is the best way to check, just make sure you block off the front of the AFM, smoke can be added at the brake boost port.

Speaking of brake boost, I had a poor running problem that turned out to be a brake booster (poor brakes made that an easy find), but the point is you have to look at all place where air leaks can be an issue.

One more place to check, if so equipped, check the EGR valve, a stuck open EGR is the same as an air leak.

[siteunseen]

The tps might be out of adjustment? Trying to think what the paint shop could have done. Agree with Dave, check the simple stuff. Vacuum gauge and fuel pressure gauge would be good. Auto parts stores loan them out around here. Here's a good write up on the tps.

https://www.atlanticz.ca/zclub/techtips/tps/index.html

[S30Driver]

Add a bottle of good injector cleaner in the fuel tank.  Even with new injectors, sitting in the shop for almost a year with that wonderful quality California ethanol fuel is not good for them.

Another easy thing to do is, with the battery disconnected, remove the  ECU cover and unplug and re-plug the large harness connector a few times to clean up any corrosion on the contacts.  If you have some contact cleaner, even better.

With the car warmed up and idling, disconnect each injector one at a time to see if it firing.  You should notice a distinct change in idle with one cylinder disabled.  If you don't, that injector is suspect.  

[Zed Head]

You should put your videos on youtube.  I wonder if the guys at the shop took it out for a spin.  Did you record mileage before it went in?

Might just be something disconnected.  Did they work in the engine bay?

[Zed Head]

And check for the simple stuff like rags or mouse nests in the air intake system.  

[BayAreaZ650]

I appreciate the replies so far.

Here is the link to the video: https://youtu.be/H6bVZI4WoxE

In the entire video, the gas pedal is pressed to hold 3000 RPMs. Sometimes, the Z drives good but it would start acting like in the video and buck during acceleration. 

I checked the spark plugs and they were black when this issue was happening 100% of the time. Now it happens roughly 50% of the time and the plugs look better.

I did record the mileage and it stayed the same. They didn't work in the engine bay.

Where is the ECU located? I cleaned all of the electrical connectors besides the ECU.

Edited February 18, 2021 by BayAreaZ650

[Zed Head]

14 hours ago, BayAreaZ650 said:

The Z ran perfectly and was gone through (new spark plugs, injectors, fluids, spark plug wires, timing and valve adjustment, etc) before sending it to the auto body shop. 

 

19 minutes ago, BayAreaZ650 said:

I Sometimes, the Z drives good but it would start acting like in the video and buck during acceleration. 

I checked the spark plugs and they were black when this issue was happening 100% of the time. Now it happens roughly 50% of the time and the plugs look better.

Bucking during acceleration sounds like the typical lean AFM problem.  But the black plugs suggest a rich problem.  The video sounds like an ignition miss.

Here's another simple check - the FPR vacuum hose.  If the FPR diaphragm blew that would be a sudden problem.  Check it for fuel, it should be dry.

1977 has the old weak ignition system with the ballast resistor.  You should check it carefully.  They fail on a consistent basis. Old.

Forgot to say - check your fuel pressure if you can.  Might be something simple like a failing fuel pump or a clogged filter.

Edited February 18, 2021 by Zed Head

[Dave WM]

ecu behind kick panel driver side.

[siteunseen]

Yeah, the bucking is how I explained my coolant temp sensor problem before I installed a potentiometer. It would buck until about 2,500rpm and then pull good.

Make sure these connections are good and clean, tight. You can crimp them down a little too.

https://atlanticz.ca/zclub/techtips/tempsensorpot/index.html

Edited February 18, 2021 by siteunseen

[BayAreaZ650]

Thank you for all the replies so far. Here's what I have planned this weekend.

- Add an inline fuel pressure gauge. Is it best to add it after the fuel filter?

- Clean the ECU connector and AFM

- I will be replacing the FPR this weekend as well since I'm redoing some of the old rubber lines in the fuel rail.

- Is the EGR valve worth replacing? 

[Dave WM]

you really should be only replacing if there is an issue. The FPR and the EGR maybe fine, you just need to test or check. the reason being you don't want to introduce more variables into the situation. Re the fuel pressure you can just check it no need for an inline but it that is what you want its ok and should be after the fuel filter as the FSM shows it that way.

The power test will help find the issue (disable one cylinder at a time while the event is happening. disconnecting the injectors is the best way, find the problem and focus on it.

you need to address fuel/spark/timing/compression and see which one or combo is the issue.