Cold Start Issues - Engine sputter (77 280z Auto)

[MH77280Z]

Hi Friends,

Clean gas tank, new bench tested injectors, all new injector connectors, also changed connectors to themo time, temp sensor, cold start valve is new and so is the harness connector. The car runs well after it warms up in 1-2 mins of idle.

Cold start is giving me issues, the engine sputter and try to die. If you pump the gas it is uneven, sometimes it suddenly jumps to very high rpm, i mean u really have to keep pumping the paddle to get it running.

We are thinking AFM? TPS? or anything else? any experiences you can share?

Edited February 5, 2022 by MH77280Z

[S30Driver]

There are many possibilities, but one thing I would do is connect a vacuum gauge  and see what that is doing after cold start vs warm up.  As the motor gets heat in it, thing tend to expand and seal up better.  I don't think the problem is with the afm or tps.  The IAC functionality  and throttle body cleanliness and others I would investigate.

Edited February 5, 2022 by S30Driver
addition

[MH77280Z]

What is IAC?

I have a feel it runs a bit lean at cold. The cold start valve is brand new and so does the connector to it. 

[S30Driver]

Actually I should have said AAR, aux air regulator.  It sits on top of a heating plate with 2 coolant lines going to it and speeds up the idle when the motor is cold.

[siteunseen]

The AAR is a good suspect. It opens when cold to allow more air to pass, increasing the RPMs. When the warmer plate gets hot the opening inside the AAR will close, lowering the RPM.

Just for the fun of it I would unplug the BCDD wire and see what it does. That thing causes fluctuating RPMs too.

The butterfly in the throttle body could be gummed up and stick when it's cold allowing more air through raising the RPM. Then when it warms up the sticky goes away. You can clean that with an old toothbrush and some lacquer thinner.

Here's a picture of the connection where you can unplug the BCDD and a thread that talks about function. Good luck. I hope you figure it out.

 

[Zed Head]

15 hours ago, MH77280Z said:

Cold start is giving me issues, the engine sputter and try to die. If you pump the gas it is uneven, sometimes it suddenly jumps to very high rpm, i mean u really have to keep pumping the paddle to get it running.

Pumping the pedal will only have an effect if you get the TPS to full throttle "full enrichment".  This will increase the fuel through the injectors by 27%.

Kind of sounds like you have low fuel pressure or a leaking FPR or fuel pump.  Pumping the pedal adds some fuel, keeping the engine going until fuel pressure builds up to where it should be.  You could probably just hold the pedal wide open and get the same effect, except it would be scary when it finally takes off.

The AAR will be just like holding the throttle pedal slightly open, except that you'll lose "idle enrichment", which probably isn't much.

[MH77280Z]

So today I took the AUX AIR Regulator off and inspected at cold, it was closed. I then bypassed the regulator and connected the hose directly into throttle chamber and the car fired up much better and no sputtering. Idle was fast though. I shut it down.

Plan to get this rebuild from fuel injection corp and test it again and adjust the idle speed. Will report back in about 1 week.

Really appreciate you guys hangin in there with me.

There is one more question: I plan to change some of the vacuum lines coming from the vacuum reservoir.

1. There is one thin vac line going into the firewall next to the battery, where does that go?

2. Also, in front of the fusible links towards the car front there are two round switches, vacuum lines also go into those. What are those?

 

 

[Yarb]

Vacuum lines for your AC system. Check out the FSM under AC controls and you will see the routing of the lines. 

[MH77280Z]

What is the cold start idle RPM range?

I think normal engine temp idle rpm range is 700to900?

[MH77280Z]

The spare cold start valve I have is about 1/3 open at cold. Will that work or does it have to be fully opened? Planning to test that While I am waiting for the rebuild to arrive.

[Dave WM]

1/3 sounds about right, hook it up to a 12v source and check it again in about 10 minutes,  should be closed. Note its not a precision part, will still pass air even closed, just some restriction.

unplug it and check it again in about 1 hr, should be back to the original opening of 30%-40%.

It uses a  bimetallic strip with an built in heater to  open and close, so its takes some time, as designed.

Make sure you use a 12v source (to test) that can flow a decent amount of current with out to much voltage drop i would guess its prob 1/5 amp or so.

 

[MH77280Z]

One more dumb question:  the idle screw on the throttle body, what is the default position of that screw? or how to set it to factory default?