Odd Stumbling/Hesitation

[DuoWing]

Well I've been trying to solve this problem for a while now with no luck. I have a 1983 280ZXT Auto that's running an 84' 300ZXT ECU and MAF setup. Anyway here's my issue.

 

When going from a cold start my car will start right up, idle decently, run well and everything. After a short period of time maybe a minute or two of driving, when I come to a stop my idle tends to get kinda rough, and I have hesitation/stumbling on acceleration. The car will start to go but not without stumbling. Then as I'm cruising along the O2 sensor will kick on and I'll pretty much have no issues, except for a little bit of a rough idle, but the car will run fairly well, accelerate fine and so forth. At full throttle or higher throttle my A/F gauge displays about 2-3 bars rich which I hear is normal for the turbo Zs and runs fine.

 

The other problem that I have is generally after I get to a destination and stop the car, when I come back usually after 15+ minutes or so. The car won't be completely cooled down and I'll go back to having a whole lot of hesitation until the O2 sensor comes back on then the car goes back to normal.

 

 

I've checked for vacuum leaks many many times by spraying brakleen everywhere under the hood with the car running. Ignition timing is just about spot on at 20 before TDC, etc. I just recently blocked off the AAC valve, removed the VCV, found my air regulator to not be sealing when closed and replaced it with a home depot ball valve to allow for testing. My idle now sits alot lower at about 500 RPM and the car starts to get a bit goofy so I need to adjust my idle via the throttle stop to bring it up a bit. This has been a problem that has been plaguing me for a while now, with no luck in solving it. So I'm hoping you guys could point me in some sort of direction.

[naviathan]

Check the Cold Start valve for leaks, the thermotime switch for continuity after 30 seconds, test the cylinder head temp sensor and check the idle air bypass valve for leaks. You can get a copy of the factory service manual from here: http://carfiche.com

Hope that helps. I'm leaning towards the head temp sensor though.

[DuoWing]

Well being a turbo car it doesn't have the cold start valve or thermotime switches, and if the idle air bypass you're referring to is the air regulator I've put my ball valve in place of it so I can completely close it off as it was leaking when closed. One thing I may need to do is unplug the O2 sensor once the car is completely warmed up and see if the car still runs ok or goes back to bad hesitation.

 

Oddly enough with the Cylinder Head Temp Sensor, the thing has been replaced a couple times. The previous owner had replaced it with a new one which turned out to be bad, I then swapped a CHTS from a Z that ran well and that CHTS soon turned out to be bad, it wasn't until I bought another one straight from Nissan that I got one that worked correctly. Although I'll have to test it as per the FSM.

[jmmorriso]

Please let me know what you find out! Ive got a z31 ecu swapped into an NA and have the same problem at low engine speeds, but once its above like 1500 its awesome. Are you running a big throttle body or anything else non stock? Also what is your water temp when fully warmed up? I think these cars are super sensitive to water temp with the swap for some reason. What have you done about the fuel temp sensor and VSS?

[woldson]

My 83zx na is doing this as well lately. This weekend going to cleant the contacts on the afm, then the tps. I will drive after each cleaning. Next going to disconnect enrichment injector, then air bypass thingy. When stubbling it defenity smells rich. It will pop through intake. Once warmed up no problem. This happens most when the engine is a little warm after sitting then refired.

[Guest duckyz]

Hmmmmm...... When was the last time you changed the fuel filter?

[DuoWing]

Well a few things I've found is that my mixture is off. I've followed the 84' 300ZX FSM and checked my idle mixture, which basically has you calibrating it up to about 2000 RPM. Anyway that needs adjustment. My fuel filter is fine, I remember once thinking it was that, I pulled it and checked it to find that it was not clogged at all. Although I did put a new one in with no different results. From what I can tell the stumbling almost seems to be related to the ECU not having a good ground.

[jmmorriso]

I ran ground wires to my MAF, intake, and ECM, then to the battery and still have the problem. Im waiting on a reman MAF and ECU right now, maybe that will do it? my fuel pressure is perfect, so unless the regulator is doing something so fast my in car pressure gauge cant see it I dont think its a fuel problem. I will put the wideband back in the car as soon as I get the datalogger cable so I can chart RPM, fuel press, car speed, and whatever else I can hook up to it, MAF voltage chts, etc.

[DuoWing]

It seems to me like part of my issues was related to the ECU's main ground wires. After I played around with them, I've noticed that the problem is very minimal. It's not right at start-up, and it doesn't happen once the O2 sensor comes on. It will also have the issue after I go into the store or something, come back out and start the car. I think it has to do with that known lean surging problem during warm-up that the earlier ECUs have. I've noticed at least for me on days about 50+ degrees, my O2 sensor will come on quite quick pretty much eliminating this problem. Now the period of sitting and restarting still exists, but again happens for a much shorter amount of time.

 

The only conclusion/difference I can come up with is: I'm running a non-heated O2 sensor, even the early 84-85 Z31 Turbos ran a 3-wire O2. Also for me, I've followed the FSM and checked my idle mixture ratio: http://xenonz31.com/AFMCalibration.html it's off and needs adjustment. One thing this page leaves out is that in the FSM they have you also check your mixture basically all the way up to 2000 RPM. So this alone may make a difference.

 

On this page: http://www.redz31.com/pages/fuel.html at the bottom of that page he mentions about the lean surging, and says this can be remedied by switching to FTS I assume means Fuel Temp Sensor signal. Although I don't know how you'd accomplish this.

 

You and I appear to be running primarily stock setups, rather than running larger injectors, an aftermarket ignition/coil, etc. So that alone may help to balance out or someway make up that extra difference. I also think that by going to the later ECU or a JWT tune may eliminate this issue. I added some stuff to your Z31 swap thread over on Zdriver, I don't know if you saw that or not.

 

I'm not sure what the VSS is, but as for the Fuel Temp Sensor I don't have that connected. My ECU will throw out the error code for the FTS and the Knock Sensor, but I'm not getting any other codes. When fully warmed up my car will sit at right about half on my temp gauge. It's the stock gauge, so I'm not really sure what that temp is. Like I said, I almost think the biggest things to do would for the 3-Wire O2, and make sure your mixture ratio is correct.

[jmmorriso]

Sorry I dont have AIM. Thanks for the info on the ground wires, I will go back and clean all my ECU contacts and fiddle withmore stuff. I have to play with my electric fan temp switch and possibly replace it with a thermostat to keep my car in closed loop. I put a 195 thermostat in, but the fan kicks on at 185! I will also disconnect my fuel temp wire, I have a resistor in line to simulate about 80 degrees right now. I also need to readjust the idle mixture and maybe replace fuel pressure regulator. I swear my fuel pressure was stuck at 40lbs at idle when I started the car today, although the sending unit seems to be very sensitive to air bubbles. I had to bleed it to get it to work right when new. Also I will plug the air regulator and cold start valve(damn stock fuel rails)

[DuoWing]

Also I don't know how much this helps, but did you rewire your injectors? Or I should say switch the wiring for the #2 and #5 injectors? From what I've read and what the FSM states. The 300ZX uses a batch fire system below 3000 RPM, not full throttle, etc. Where it will fire the injectors in two sets injectors 1-2-3 then 4-5-6. Once it gets up to higher throttle it fires all 6 simultaneously so this isn't an issue then, but at idle, lower throttle etc, it's firing to match up with the firing order of the VG30 which is 1-2-3-4-5-6, but on the 280ZX the firing order is 1-5-3-6-2-4, but the Z31 ECU is firing 1-2-3 then 4-5-6, so you're having the #2 and #5 injector firing out of order. I switched this on my car, I want to say it made a slight difference, but I'm not sure. This may be more noticeable if my idle mixture were properly adjusted. As for putting a resistor on the FTS wire, that's not a bad idea. At least that way it wouldn't throw a code, I don't know if this would cause an issue with the ECU or not.

[jmmorriso]

I did rewire the z31 injector harness to match the l28 firing order. I havent had the time to do anything yet, but I have a new MAF and a rebuilt? ECU on its way. I never checked the part number on the ECU I have to make sure it was correct for the wiring harness I have. Im sure its the same problem you had, ground wires and a leaky air regulator+calibrating the MAF, but I have to switch a bunch of parts anyway. Also, I think I might have a leaky or sticky injector, but I will hold off on pulling the fuel rail until everything else is done.

[DuoWing]

Although I haven't actually respliced the one major section of grounds for the ECU, I did pull it apart and mess with it. It does seem like ever since then my stumbling hesitation has really calm down alot. Before it use to really pop through the intake and have some bad hesitation, where now it's only a slight hesitation before it goes. So I need to redo that, but I'd almost think that getting the mixture to match up right should correct the rest of the issues. Although for me since here in Ohio temperatures get a good bit colder it would probably be beneficial to put in the heated O2 sensor.

[woldson]

Wow congrats on all the hard work and trouble shooting:)!

[DuoWing]

It's a rather long process. Have you made any progress with your issues? At times I sorta wished I had waited and made sure my ZXT was running with no problems before I converted over the 300ZX ECU/MAF, but ahh well.

 

It seems like the biggest issue with these issues is primarily linked to old connections. Also before I swapped to the Z31 Setup my Z would have problems generally with restarting which I found out was linked to my VCM being hooked up incorrectly, which would be the equivalent of the BCDD on the N/As. After the swap it's no longer really controlled properly so I unhooked it, plugged off the EGR, removed the AAC, the VCV, etc. This all dropped my idle and made the car run a bit better.

 

I came to the conclusion about the ECU wiring due to an issue that would happen once in a while. On two separate occasions my car upon startup would start acting real goofy, the idle was real unstable, but it would idle. Upon any throttle the car would start popping through the intake and nearly die. If I'd continue to give throttle the car would just stall. When warmed up it would run a little better, but my A/F gauge was not registering as if the car wasn't getting fuel correctly or if the O2 was dead. After looking at a bunch of things trying to make adjustments to correct it. I figured I'd shake the wires coming out of the ECU. I've heard of a lot of problems related to the ECU wiring. Upon shaking the wiring instantly the idle smoothed out, sounded better, my A/F gauge started bouncing around like normal and it seemed like I had no problem. Every so often I mess with the wiring and it seems like ever since then my initial throttle hesitation has been alot less. It sounds like there's a known lean hesitation during warm-up with the ECU, but now I just get maybe like a quick spot of hesitation before the car goes. Where before it would pop and sputter and barely move and then after a moment it would go fine.

 

I'm not sure on the N/As but the Turbos on one the ECU plugs has a spot for the main ECU ground wires. It's two ECU ground wires that goes into a little open metal connector then spreads out into 3 other ground wires. It doesn't seem like a very good design and I wouldn't be surprised if everything improved upon making sure all the wires are just connected or twisted right together rather than relying on them touching through this metal piece.

[jmmorriso]

The stumbling is almost all gone with the air regulator plugged. I changed the MAF and ECU, but still need to calibrate the MAF. I couldn't get the lights to blink at all with the rheostat turned all the way left. I dont think the new ECU is faulty, but since it is for an 84 and everything else is 85, maybe? I looked at the diagrams and everything seems the same. I will swap ECus back later and re install my wideband to see where it is at.

[DuoWing]

Glad to hear you're making progress, I still need to adjust my mixture ratio. I'll have to go drill out that plug. I almost think my issues are coming from running too rich. I encountered a few problems when restarting the other day. When the car was fully warmed up and I stopped for gas on my trip back home, the car started up fine. I had the ball valve that I put inplace of my air regulator completely open. I closed it off some to bring the idle down as I hadn't yet adjusted my idle at my throttle body. The next time I went to start it would turn over then die, unless I gave it some throttle. It reminded me of my 76 280Z when trying to start from cold with the way the air regulator is faulty and stays closed. So I'm thinking I'm running a bit too rich.

[jmmorriso]

you dont have an idle adjuster bolted on your intake? I wonder If I could eliminate mine? I have heard of guys drilling a small hole in the throttle plate. I dont think cracking the throttle open with the 60mm TB to adjust the idle will be very consistent.

[DuoWing]

No unfortunately the stock turbo throttle bodies didn't have an idle adjustment screw, it can be adjusted, but you have to loosen the nut and tighten the screw for the throttle stop. I've upped my idle some and got around to drilling out that plug on the MAF so I could get at the mixture adjustment screw. So now I gotta go drive around, warm the car up, check my idle speed and see if I can get the lights to match up in how they blink and what not. Although just from that information about the 84-85 ECUs having a lean surging during warm-up it sounds like we're never going to completely get rid of the issue. So it sounds like if you're encountering just a spot of stumbling or hesitation it sounds like you've got it fairly sorted.

[DuoWing]

I wonder how much of a role the ignition plays in this setup. I know that people say using something such as a mallory or MSD ignition and coil alone is definitely an improvement over the stock 280ZX ignition, I also know that people say the Z31 ignition was much better. I need to get my mallory and balster coil in there and see what kind of a difference that makes.