Cold Start Low Idle - Any guidance?

[LBEEMING]

Hi Team, 1985 911 3.2

On cold start, around 20 deg or 68 Fahrenheit, this is what happens;

Car starts fine, and idles around 1100 for about 15 sec, starts to fluctuate about +/- 100 rpm for the next 30 seconds.

It then reduces to 800 rpm (Base Idle) but over then next 20 seconds slowly reduces RPM to 600RPM.

Once the car if fully warmed up it will idle at 800 nicely, with only minor fluctuations.

Car drives fine and starts hot fine, maybe some jerkiness in lower rpms at lower speeds.

I have changed out the O2 sensor and adjusted base idle. PO had the AFM at 14 turns out on the adjustment screw, not sure why he needed to lean it out so much. The O2 Voltage at warm idle shows a slightly rich AFR (0.7 to 0.8V), which is ok for idle.

Now I am wondering if I have a failing Fuel Pressure Regulator. Perhaps too rich at cold start up when open loop, but as the car is adjusted for warm idle when o2 kicks in, it seems fine.

Not sure if anyone can help out?

BTW AFM is clean, did smoke test, no vaccum leaks, new rotor and cap and fuel filter.

Thanks.

[proporsche]

i`d start with disconnecting the oXy sensor a go from there....
Ivan

[David Inc.]

Idle at 800? Isn't it supposed to be higher (like 950 +/-) or is that only on the SC?

[917_Langheck]

Originally Posted by David Inc.

Idle at 800? Isn't it supposed to be higher (like 950 +/-) or is that only on the SC?

800 on the 84-86 3.2 on original ecu program. RPM was raised to 880 on 87-89.

[wazzz]

800 for 84-86 models, then it became 880 from 87 on.

[wazzz]

Originally Posted by 917_Langheck

800 on the 84-86 3.2 on original ecu program. RPM was raised to 880 on 87-89.

Faster typing.

[917_Langheck]

Originally Posted by wazzz

Faster typing.

Time delay on your side of the Atlantic...

[911obgyn]

Double check idle switch, clean IAC valve?
Plug gap also.

[LBEEMING]

Originally Posted by 911obgyn

Double check idle switch, clean IAC valve?
Plug gap also.

Thanks for the tip, IAC valve is brand new and tests 22 ohms either pin. Idle switch works and gap to throttle arm is correct. Going for a 3 day trip in it, so will check spark gap when back. Will also pull the fuel pressure regulator hose off and check for fuel in the vacuum line.

[dfhtrhjn]

I'm wondering if the WUR has anything to do with this since it adjusts air fuel when cold.

[911obgyn]

3 day trip in the porsche with a jug of techron would fixit😉

[proporsche]

did you try it? to disconnect the oxy sensor and see how it is behaving?
Ivan

[LBEEMING]

I don't think 3.2s have WUR. I think the DME controls cold start.

@PROPORSCHE I thought the o2 sensor didn't do anything until the engine was warm. Thought it was open loop until about 180.

I will unplug the O2 sensor at cold start and when warm and report back.

Before I leaned the idle out though, if I unplugged the O2 sensor the rpm would rise up to 800. When I plugged it back in, it would drop to around 650.

[LBEEMING]

@proporsche, I disconnected the O2 at cold start, and nothing changed. Think this is expected. Once warm I pulled the sensor and I didn't perceive anything to change. This new O2 was installed 5 days ago. I measured the voltage with O2 disconnected and it was sitting around 0.8 Volts cold start when the rpm was aprox 650 and when warm around 0.7V when at 800rpm. Note I measured warm voltage when we were up the mountain and temp lowered about 10 degrees Celcius.

Can I ask if the AFM bypass screw only affect idle? Does it have any affect on partial throttle? Or does the Cog in the AFM adjust partial and WOT.

[proporsche]

increase the idle to 850 a add more fuel 1/4 of a turn on the fuel mix screw

[LBEEMING]

Quick update, I made those adjustments. It's improved but not 100 percent with the idle enriched and increasing idle. It still won't sit nicely at 1100 rpm at first start. It also drops to 800rpm after 45 seconds but then slowly drops to 600 rpm like before during next 30 seconds. What has changed though, is that it only takes about 5min of driving now for the idle to go back to a steady 850rpm whereas before it took about 10min.

I also checked fuel pressures. Engine off 36psi, idle 30 psi, afr hose removed 36 psi. This seems good. Residual pressure also good after 20min.

I also checked vaccum pressure and I am getting 14 in Hg. I've heard it should be around 18. Any comments on this? Also pulled all sparks, and they look good. Not rich, not to lean.

[Coastr]

Originally Posted by dfhtrhjn

I'm wondering if the WUR has anything to do with this since it adjusts air fuel when cold.

Only if he pulled out the 3.2 and put in a 3.0 CIS

The whole point of buying an 84 or 85 is to lose the WUR!

[proporsche]

quick q..did you clean up the idle control valve?
Ivan

[LBEEMING]

Yes, Brand New. Actually ordered two by accident, will try the other just to make sure I am not chasing my tail.

[LBEEMING]

Just adding on to this, I checked the ICV valve, and it seems to be forcing the rpm down when cold.

I started the car and when at 600rpm while it was still cold, I pulled the idle control valve plug. The ICV was completely shut.

So question is, if the DME reduces fueling until the o2 sensor is at 14.7 AFR, why would the ICV not open up to raise the rpm to 800? Can't understand why the DME is pulling fuel out and then also forcing the ICV closed?

Is the mixture so rich that the DME is trying to use both the ICV and Fueling to get AFR to 14.7?

As stated before, after 10min of driving idle is fine again.