Rouh Idle at start up 86 911Carrera need help

[Racecarguy1968]

Hi I have a 1986 Euro spec 911 Carrera cabriolet. after it sits 12-24 hours or more I get in and turn the key it cranks but no start, if I step on the accelerator a couple of times it fires off, it chugs for about 15 seconds then smooths out. I find myself letting it warm up a few minutes before I go out and drive. once warm it runs great! if I turn it off and come back when its warm it fires back up.
I have replaced the Throttle Sensor, the fuel filter, the idle control valve, cleaned the throttle body.
It seems like it is starving for fuel til it warms, Any ideas? What does the the Thermo Valve do on the back of the throttle body?
Thanks for any help

[Racecarguy1968]

Any Thoughts?

[ischmitz]

A 86 is DME and not sure what you mean by "thermo-valve". There is the ICV (idle control valve). It's a silver canister that bypasses the throttle body with metered air.

What do you mean by throttle sensor? The 3.2 has an idle switch and a WOT switch but no other sensors on the throttle body.

I would start with the usual suspects:

- make sure idle switch engages when throttle closed
- air leaks in intake track (gaskets)
- CHT sensor (if one-wire replace with newer 2-wire version)
- O2 sensor

Regards,
Ingo

[JAR0023]

Not an expert but I own an 86. My thoughts...

You shouldn't have to touch the accelerator pedal to start the car, ever. When cold the car should start and jump to around 11-1200 rpms then drop back down as the engine warms up. Correct idle for an '86 with factory chip is at or just under 800rpm.

You did not mention the CHT, cylinder head temp sensor. Bentley manual has the numbers you need to test the CHT before you pay for a new one. CHT tells the DME if the engine is warm or cold in your case. This should richen up the mixture to help with cold starts. I've been starting my car in cold temps for the last few weeks. It actually starts better cold b/c of the richer mixture. If you don't have a Bentley and can't find the numbers let me know, I'll dig out mine for the info.

Early 3.2 cars came with a single wire CHT that grounded to the engine. Tribal knowledge here says to replace these with the later two wire sensor that is a straight bolt on and performs more consistently.

There is an air temp sensor in the AFM that talks to the DME. I'm not sure I've ever read much at all here about that sensor going bad. If it were my car I'd function test the CHT with a meter and go from there.

The thermotime switch on the back of the throttle body should not be affecting your cold start. This is part of the Aux Air and was added to prevent icing of the throttle body but I can't see it affecting cold start. TT valve is connected to vacuum from throttle body. Below 50* or so TT valve opens. This allows vacuum to pull on the Aux Air valve. AAV is right side of engine bay ahead of oil filler. One AA valve is open warm air is pulled from oil filler neck through one way valve and into the airbox upstream of the air fuel meter. If you start your car in the cold and hear what sounds like a cicada in the right rear of the car it's probably your AAV. The diaphram dries out and won't seal. Mine has made noise the first handful of cold days each fall, then it quiets down and works fine for the rest of winter.

You mentioned a new ICV. Make sure you reset you idle correctly with the jumper wire. I know my ICV is a bit finicky and if idle is off I have had some surging and sub-idle dip until I got it set correctly. -J

[Racecarguy1968]

Thanks I'll try the CHT first.

[Racecarguy1968]

The condition is not temp sensative as far as air temp it is the same way cold Below 50 and above 50
Thanks

[JAR0023]

Sean - Here's the Bently CHT info. I pulled it from another thread. Also, some link to a little light reading on CHT and hard starts. -J



3.2 Carrera CHT sensor question:

CHT testing question

3.2 sensor question

[ischmitz]

The CHT can significantly change (richen) the mixture for cold-start:

With the engine cold (at ambient) the intake path from the injector is very cold compared to the 260 deg F at running conditions. Fuel condensates on the inside walls as liquid and the atomized fuel recombines to larger droplets. As result the effective mixture is way too lean to let the engine run properly. A lot more fuel needs to be injected to compensate. As the cylinder heads warm up the effects gradually disappear.

The 3.2 DME has no ability to recognize a faulty sensor like later DME do. It simply goes very rich if the sensor is open and lean (normal) if the sensor is shorted. A shorted sensor behaves as if the engine was operating temperature. An open sensor behaves as if you are trying to start the car at the south pole.

The one-wire sensor relies on GND from the mounting stud to the cylinder head. With age and corrosion this ground becomes less stable. The effect is that the 1-wire can permanently read a little off (to the rich side). The O2 sensor can compensate for that to a certain degree.

Keep in mind that a very lean mixture can appear as rich due to a cylinder not firing at all times and spitting out the unburned fuel instead. So to determine whether you are rich or lean is not always trivial, even with a wide-band sensor.

Ingo

[stlrj]

It could be caused by vapor lock due to loss of residual fuel pressure. The check valve in the fuel pump might be bad.

[ischmitz]

Originally Posted by stlrj

It could be caused by vapor lock due to loss of residual fuel pressure. The check valve in the fuel pump might be bad.

Not if you read his first post: "if I turn it off and come back when its warm it fires back up". Vapor lock usually happens when the check valve can't hold pressure in the fuel rails while residual heat causes the fuel in the rails to boil. The boiling point of fluid is a function of temperature and pressure. At increased pressure the boiling point goes up. A good check valve keeps the fuel rail pressurized and prevents boiling.

In his case that is exactly what is happening when he warm-starts the car.

ingo

[Racecarguy1968]

Thanks, for the info. Looks like I have a Busy weekend checking CHT values

[Determined]

Bump for my reference

[ClickClickBoom]

Originally Posted by Racecarguy1968

if I step on the accelerator a couple of times it fires off, it chugs for about 15 seconds then smooths out.
I have replaced the Throttle Sensor, the fuel filter, the idle control valve, cleaned the throttle body.

The odd thing is if you step on the "acc pedal a couple of times it fires off".
My guess would be that by stepping on the pedal you might be helping the idle switch send a signal to the DME.
After a short warm up the O2 sensor adds it .02 cents worth.
Check the stuff you messed with, and new parts can be bad out of the box.
Diagnostics over the internet is challenging.

[Racecarguy1968]

Well I checked the CHT this weekend, and it OHMed out at 4.68Kohm at 68 degrees in my garage, way off the charts you sent. I put a 330ohm resister in line in place of the CHT and it seemed to smooth the idle out. I need to start it with the resister in, but the winter storm blew in and had to let the engine cool down.
If I totally unplug the CHT while running the engine surges, so the DME sees something from the CHT.
Which is the idle switch? The TPS on the side of the throttle body, I thought was just for WOT. am I missing a sensor?

[JAR0023]

Idle switch is separate. Standing looking in, the WOT switch is on right side of throttle body and idle switch is on rear left with wiring that runs around back of TB. The plug is on right side of TB just above WOT switch.

The switch is triggered by the throttle lever when the lever is closed. Actually throttle level is two piece and there should be 1mm or so movement of front side before back side pulls up off idle switch. With car on but not running you should be able to hear a click as you raise the throttle lever up off the switch.

To test switch unplug harness and with car off test for continuity across the two posts in the switch plug. There should be continuity with throttle plate closed, none at all other positions.

Here's a photo I cribbed showing switch from firewall side. -J

[Racecarguy1968]

Thanks I'll check it out

[Racecarguy1968]

OK starts a little better but I still have to pump the gas pedal at least once to get it to fire, and still stunbles for about 15 seconds then smoothes right out!
Things done to date:
New Fuel Pump & Check Valve (fuel pressure 38psi idle/ 32 psi 2500rpm) Fuel pressure holds steady when running. Could not do leak down test, the M12 fitting doesnt fit 100% so it bleads off a tiny amount after turned off.
New Cyl head temp sensor (new 2 wire one)
Known good idle control valve installed
New WOT sensor on the side of throttle body
Checked Idle switch have continuity and turns off when throttle open
R&Red MAF sensor and completely cleaned it, moves freely (it was dirty)
Cleaned throttle body and boot, was nasty with caked on dirt.
Cleaned idle piece on the front of throttle body ( looks like a brass nut and when unscrewed looks like a threaeded brass bullett, about 9mm)
DME replaced 1 year ago but car acted like that before the replacement.
Fuel is good

Does anyone know if unplugging the MAF will put it in a default mode to test that? there is suppoed to be a temp sensor in the front of the MAF.

I don't think it could be a fuel pressur regulator since it holds still at 38psi.

Any suggestions or help would be great
Thanks
Sean

[ischmitz]

The barn-door AFM has an integrated temperature sensor. Not likely the culprit. If you unplug it there is no "emergency mode". The engine will simply die or run like crap. The 3.2 DME doesn't have any of the emergency or limp modes like later DME.

The fuel pressure regulator can be bad. It not only maintains constant pressure but changes fuel pressure when you step on the pedal. There is vacuum line from the manifold to the regulator.

The fuel pressure has to changes with manifold pressure so the pressure difference across the injectors remains constant with changing manifold pressure. Check by having the car idle. Then pinch the vacuum line and the idle note needs to change as the fuel pressure rises.

Ingo

[Racecarguy1968]

The arrow points to where the part is underneath, its a 8mm bolt head.
Pic Attached

[T77911S]

does the ICV vibrate with the key on

air leaks