Acceleration Stumble

[chaseincats]

Moving this over from the "strange intake noise" thread since that's solved.

 

My friend's 1975 280 is having a strange problem where it stumbles every time you hit the gas.

We installed an air fuel gauge and when you hit the gas it will go into 'off the chart lean' numbers and then stumble up to catch it.  Once it's in a high enough rev range it's responsive, but getting it into the 2.5-3k range is difficult to say the least (see video below).  It's also worth noting that it idles fantastically, but it's literally just when you hit the accelerator that the jetronic EFI doesn't throw enough gas at the engine to keep up.  The only way to get it to run well(ish) at all is to richen it WAY up using the AFM gear.

Another interesting thing is when you pull the plugs they are black but the idle number on the air/fuel ratio gauge says 13.5 when the big gear is set back to the stock tooth (we marked it before messing with it).  Mid-13s is the number I have on my '78's afr gauge and my plugs are clean.

Here is what we've done so far:

• Gone through the entire fuel injection bible testing everything it recommends and all values came up properly
• Cleaned all connections/connectors/grounds according to atlanticz's page
• Changed EFI temp sensor (it was dead)
• Change EFI connectors on injectors and temp sensor
• Cleaned injectors using carb cleaner and a 9v battery to push the cleaner through
• Put a fuel pressure gauge on it between the filter and rail (36psi)
• Put a new fuel pressure regulator on it (the old one was hissing, the new one doesn't)
• Tried a different throttlebody
• Tried a different AFM (with stock settings)
• New head & intake manifold gasket (unrelated past problem)
• Smoke tested the engine for vacuum leaks and capped them all then sprayed the engine with carb cleaner and the idle didn't change indicating we got all the leaks
• Completed entire 'engine tuneup' chapter of the fsm including timing and valves
• New fuel filter

Here's a video of how it's behaving here - any ideas?

My friend is making an account and will take it from here

Edited September 4, 2022 by chaseincats

[SteveJ]

Let's start with the obvious. The car isn't getting fuel at tip-in on the throttle.

Has your friend thought about getting the injectors cleaned? What is the fuel pressure during this time? Is the vacuum line connected to the fuel pressure regulator? I couldn't pause the video in time to see.

[Zed Head]

Where is initial timing set?

Is the AFM vane moving immediately with the throttle opening?

Is the 36 psi at idle with engine running or engine not running?

You or your friend should write down the basic essential tune-up numbers as a starting point.  The video kind of looks like there is still a big vacuum leak and the AFM vane is not reacting quickly. 

By the way, I don't know how those "phone size" short videos are made and posted but they suck.  You can't enlarge them to see detail.  Better to just create a full size video.

[SteveJ]

YouTube does that to videos under a minute now.

[siteunseen]

Cheap and easy. Put rubber hose over the ends of needle nose vise grips and clamp the return line to "none shall pass". That will increase fuel psi to the rail/ injectors. If it runs better there's your sign.

[Ogs Classics]

Hi y'all,

its my car. 

Let me answer some of this. 

Timing is at 10, afm vane is moving as soon as you hit the throttle. 36 psi engine running. Injectors are cleaned, vacuum lines are all connected.

What do you mean by "none shall pass"? PCV?

 

LMK if I missed any of your questions?

[SteveJ]

14 minutes ago, Ogs Classics said:

Hi y'all,

its my car. 

Let me answer some of this. 

Timing is at 10, afm vane is moving as soon as you hit the throttle. 36 psi engine running. Injectors are cleaned, vacuum lines are all connected.

What do you mean by "none shall pass"? PCV?

 

LMK if I missed any of your questions?

@siteunseen was referring to the return side of the fuel pressure regulator.

[Zed Head]

1 hour ago, Ogs Classics said:

36 psi engine running.

Should be about 29 - 31 psi with engine running.  That would mean that the vacuum reference is not fitted to the FPR.  Which mean the engine is tuned lean and when you open the throttle the fuel pressure does not increase like it should, leading to a lean stumble.  You need that vacuum reference on the FPR.

[SteveJ]

1 hour ago, Zed Head said:

Should be about 29 - 31 psi with engine running.  That would mean that the vacuum reference is not fitted to the FPR.  Which mean the engine is tuned lean and when you open the throttle the fuel pressure does not increase like it should, leading to a lean stumble.  You need that vacuum reference on the FPR.

I actually saw that helping a friend with his 280Z. He bought a fancy FPR from the company that employs Corky Bell. The fancy FPR was supposed to work with the stock FPR to increase the fuel pressure even at higher vacuum.

Well, when my friend got someone to install the FPR, the installer didn't bother reading instructions (Sometime instructions aren't for sissies.) and removed the stock FPR. When my friend brought the car to me, he had Corky's number, and I confirmed what was needed. My friend got a stock FPR, and I installed it, but I didn't put a vacuum reference on it at first. On the first test drive, the car stumbled at tip-in, and I could even see it with the AFR meter. I found an available port for the vacuum reference, and the car ran a lot better.

[ckurtz2]

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!

Edited September 5, 2022 by ckurtz2

[Ogs Classics]

I went through all that, fuel pressure was like that before I changed the regulator. Haven't measured it after the new one was installed, but it did the way car runs. Also, I was playing around with the afm, once I put my hand over it to lower  the air intake...issue with stumbling/hesitation disappears. I have installed the OEM air filter and car normalized almost completely...does anyone has any idea why? 

[chaseincats]

Seconding @Ogs Classics's question.  My stock 280 has the same cone filter his had and didn't need the stock airbox to run properly.  Why would that be necessary for his car?