Acceleration Stumble

[Zed Head]

I don't think that anyone can really tell what you guys have installed on your engines.  Ogs said he put his hand over the AFM, things changed, he installed the stock air box, things changed.  "Normalized".  Not sure what any of that actually means.

The vacuum line reference to the FPR is a basic of ALL electronic fuel injection systems, not just early 70's Datsuns.  The computer programs are designed to work with fuel pressure that varies with intake manifold pressure.  You need that vacuum line.  Just because the engine runs doesn't mean that it will run right.

The best way to learn about early EFI is to get everything spec'ed in the way that the engineers intended.  Then mess with it and see what happens.  Messing with a messed up system just makes more mess.  No offense.  ckurtz2 wrote a whole page describing his experience with early EFI and you guys just blew right past it.  Everything that he wrote is true.  The fine details make the difference.  There are many many stories on the web from people who just gave up and put carbs on.  It's a challenge.

Edited September 6, 2022 by Zed Head

[ckurtz2]

Here is a link to Z when it was in really poor shape. Kind of acts like yours. It also was running incredibly lean. I had rebuilt the whole thing and was super confident it was done right with all of my "checks". Well turns out it had several sneaky vacuum leaks, and the OEM rebuilt injectors were unbalanced/undersized even though they were cleaned. However, in order to find that out I went through every single diagnosis check in the EFI bible ignoring the simple things and making my life miserable. So I encourage you to go through the entire EFI bible, but don't just skip over something because you checked it once. Just my 2 cents. 

I also would go through every single forum related to your issue. Soak up every detail, the more you know the easier things are. Also drives you crazy though🙃

Here is some of the process I had to go through on my EFI system. As you can tell it was a mess. Don't be lame/lazy and swap to carbs, stock EFI is a good system. 

Do your RPMs bobble at idle?

[chaseincats]

35 minutes ago, Zed Head said:

I don't think that anyone can really tell what you guys have installed on your engines.  Ogs said he put his hand over the AFM, things changed, he installed the stock air box, things changed.  "Normalized".  Not sure what any of that actually means.

The vacuum line reference to the FPR is a basic of ALL electronic fuel injection systems, not just early 70's Datsuns.  The computer programs are designed to work with fuel pressure that varies with intake manifold pressure.  You need that vacuum line.  Just because the engine runs doesn't mean that it will run right.

The best way to learn about early EFI is to get everything spec'ed in the way that the engineers intended.  Then mess with it and see what happens.  Messing with a messed up system just makes more mess.  No offense.  ckurtz2 wrote a whole page describing his experience with early EFI and you guys just blew right past it.  Everything that he wrote is true.  The fine details make the difference.  There are many many stories on the web from people who just gave up and put carbs on.  It's a challenge.

Normalized meaning the car no longer has a stumble when you hit the accelerator and revs/falls how a normal car should. 

[chaseincats]

7 hours ago, ckurtz2 said:

Can't say I am an expert, but I just got my car running near perfect after about 2 years of woes like this. I learned the little things are super important.

          First things first, make absolutely sure that your car has no vacuum leaks like you mentioned. Trust me on this. Even if you think you got them all, triple check. Even the smallest vacuum leak can cause huge issues. Sounds like you smoke tested it though. Second, if your plugs are fouled I strongly encourage getting a brass brush and cleaning them with brakekleen. Plugs make a huge difference, and often times when you are dealing with a sick engine they can get fouled pretty quick, so while you are trying to fix the motor, make it a habit to pull the plugs to inspect/ or clean if necessary. 

        Ok, now this is critical. Don't just have "cleaned" injectors. I learned the hard way that even if they are clean, the motor can still run like crap. Make sure that they are "flow matched" so that each cylinder combusts evenly. In my case, it made a huge difference on part throttle and idle response. 

        I am assuming that you checked your EFI coolant temp sensor. Definitely run the resistance check on this, because it can be a problem. Make sure the TPS adjustment is good too. You seem competent enough to have done/ know how to do this. 

      Make sure each cylinder is firing at idle. Pull each plug wire one by one and see if it is, there is always the chance that your problem could lie in one particular cylinder.

      Check timing, I know 10BTDC is what the manual calls for, but sometimes it likes just a little more. 

      Also I agree that your FPR regulator seems to not be working right, at idle it should be much less than 36psi due to vacuum pulling on the diaphragm and limiting the flow.

      Now last look at the AFM as the culprit. Very Rarely, especially on a stock engine, is it considered bad. In my case this was the very last thing I fiddled with on my motor. It was a rebuilt unit from zcarsource, and the tension cog was ridiculously out of spec (waaay to loose). 

     Now a backseat diagnosis that could be wrong so don't take my word for it. your AFR at idle should be around 13.6. Apparently the L engines like to be rich at idle. You can fine tune this with the AFM mixture screw once you find the real issue on why the car runs poorly. Next, it seems odd that when you give it throttle that the AFR is shooting all the way lean. It is possible (considering the black plugs) that the AFR is reading lean because the cylinders are running so rich the mixture is missing causing the AFR not to get a reading. In my opinion (because it happened to me) if it was running that lean when given throttle it would pop like a SOB out the intake. Yours just sounds like it's bogging/ missing. I also (with the same AFR setup) have seen numbers showing my car idling at the infinity mark. It turns out it was so rich my engine was missing considerably, and the AFR gauge couldn't get a proper reading.

     If it truly is running lean at throttle, then I would once again make sure there are no air leaks, fuel restrictions due to dirty gas tank etc. Read the EFI bible. Once you get a full understanding of how the system works you realize there could be dozens of possible reasons for your issue, but only a few plausible. The more info you provide the more we can help!

Thanks for writing this all up, I'll try and answer them individually

No leaks - we've smoked and sprayed it multiple times

EFI temp sensor is brand new and in spec

Every cylinder is firing spark and fuel (pulled spark plug wires 1 at a time then did the same with the injector power connectors

Last time we checked we set it at 10 but even when it was higher as we were setting it the stumble remained

The car sounds like its much more similar to your situation since its not bogging/missing it just doesnt want to rev and WILL pop loudly out of the intake if you press the accelerator all the way down and hold it at idle

[ckurtz2]

Ok. What are the injectors you are running?  On my car I had the old ones rebuilt by AUS injection. Turns out even the cleaned units were unbalanced and also still not flowing enough. They had the OEM late 280z green body and part numbers. So I would advise you pull the rail and do a volume test to see the flow rate. I had to buy completely new injectors from MotorMan on Ebay.

Could the stock filter be restricted and thus the engine runs richer and it seems like your issue is fixed? Or, funny enough maybe you had a vacuum leak there and while replacing the filter and piping it was corrected. If you could, take a video of what the AFM is doing when running, revving. I am curious what it is doing with the cone setup. The cone filter and stock filter should make absolutely zero difference. Both are considered cold air intakes, and both flow well.

Edited September 6, 2022 by ckurtz2

[Zed Head]

Don't overlook the EGR system.  Sometimes it leaks internally.  The smoke would just head down the exhaust pipe.  Even if you removed it and put a plate over the holes (I assume that you did, if your engine's had it), the passage walls sometimes rot out.

ckurtz2, you could be here for quite a while.  If they won't even put the vacuum line on the FPR, flow-matching injectors seems quite a stretch.  Good luck though.

[Zed Head]

14 hours ago, chaseincats said:

WILL pop loudly out of the intake if you press the accelerator all the way down and hold it at idle

This is the typical sign of a lean situation, except that it happens under load.  If it happens with no load that's pretty bad.

Worn out cam lobes also cause popping back through the intake manifold.  I've had it on V8 engines.  That happens with no load.  You might pull the valve cover and see if you have any loose rocker arms/damaged cam lobes.

[siteunseen]

3 hours ago, Zed Head said:

This is the typical sign of a lean situation, except that it happens under load.  If it happens with no load that's pretty bad.

Worn out cam lobes also cause popping back through the intake manifold.  I've had it on V8 engines.  That happens with no load.  You might pull the valve cover and see if you have any loose rocker arms/damaged cam lobes.

That was my thought on clamping off the return line for a test. That's how I figured out my cts was off to the ecu and did the RadioShack volume knob tweak. Easy to do and cheap. I ran the tune knob in the cabin and had a lot of fun running different mixtures. 

There's tons of info on classiczcars.com but here's the basic straight to the point info...

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

 

 

[chaseincats]

Is there any possibility this is ECU related, and if so how would he test it?