3.2 Cold Start idle

[Mr.Doback]

Good morning Pelican Parts -

I have a 72 911T with a '87 3.2 swap that runs quite beautifully - except at cold start/idle. I have just replaced the idle switch, ICV and did a tune up (plugs, fuel filter, air filter, blah blah). The previous owner had already replaced the CHT and reference sensors.

The initial reason for the tuneup and idle switch was because I was unable to set a base idle - the car idles high - during cold start she sits right near 1200 like it should then the idle slowly drops to about 1050 and holds. Once I start driving if I put the clutch in to shift to second, she tends to stall or barely stall then catch and continue running. I jumped BC and the idle drops low so I adjust to what looks like 880rpms but when I pull the jumper, the idle goes right back to 1000.

During the tuneup, I deleted the entire vacuum assembly that would normally run to the brake booster since this car does not have power brakes, capped off all unused vacuum ports, leaving only the FPR lines to the manifold. In addition I replaced the Intake manifold gaskets and insulators and ensured all surfaces are flat as well as changed the seal between the two sides of intake manifold. So I am assuming I should not have any more vacuum issues as I checked all connections prior to reinstallation.

After the tuneup - the car remains unchanged. Still have the idle drop at cold start, still idles high after trying to set the base idle. The car does not have an 02 sensor and uses an autothority chip that has been in the car since purchase. Prior to all of this, I had an idle surge issue which was solved by adjusting the base idle, but as I set it to 880rpms - the surging disappeared but this issue appeared.

The last bit of information is that I have not had the A/F ratio checked, that was planned for Saturday.

If your still with me after reading all that and assuming you understood what I wrote - does anyone have any recommendations on where I should start next?

[CCM911]

Excellent description of your issues.

The only thing I see you did not do is replace the O2 sensor. But before you run out and buy a new one, just unplug it. See if that makes a difference.

I'm sorry, but that is all I can think of at the moment.

I am sure that greater minds then mine will jump in soon.

[Mr.Doback]

Thanks Chris - and I agree, however my car does not have an O2 sensor at all - perhaps that could be the source of my issue, I might need to look into how the 02 was removed.

[kodioneill]

The dme needs feedback from the exhaust you need to install a bung and a o2 sensor. I think that pertains to USA models not ROW. Not sure though.

[CCM911]

Originally Posted by kodioneill

The dme needs feedback from the exhaust you need to install a bung and a o2 sensor. I think that pertains to USA models not ROW. Not sure though.

You may well be spot-on with the RoW thrown into the equation. But for US, the system is built to have an O2 sensor in the mix.

Mr. Doback - did this just happen, or has it been this way since the engine upgrade?

[JAR0023]

You need to know what your idle mixture is (rich/lean). I recently went through and replaced intake gasket and basically eradicated all vacuum leaks. Once back together my car was significantly richer at idle without the air leaks.

You could also swap your DME into another car to see if it is the DME causing the issue.

-J

[BE911SC]

Does driving a 3.2 without the O2 sensor harm anything? Confuse the DME or?

[timmy2]

Originally Posted by JAR0023

You need to know what your idle mixture is (rich/lean). I recently went through and replaced intake gasket and basically eradicated all vacuum leaks. Once back together my car was significantly richer at idle without the air leaks.

You could also swap your DME into another car to see if it is the DME causing the issue.

-J

+1, what is your exhaust CO at idle when fully warmed up?

[Mr.Doback]

CCM911 - I bought the car in the summer with the swap, it had a surging issue/ high idle which I eradicated with adjusting the idle with B/C jumped, but when i set it to 880rpms, thats when this began.

Next step sounds like I need to get my CO measured - any competent emissions shop should be able to do this, correct?

[kodioneill]

Originally Posted by Mr.Doback

CCM911 - I bought the car in the summer with the swap, it had a surging issue/ high idle which I eradicated with adjusting the idle with B/C jumped, but when i set it to 880rpms, thats when this began.

Next step sounds like I need to get my CO measured - any competent emissions shop should be able to do this, correct?

You can do it with a afr meter. you can purchase one for about 200.00.

[Mr.Doback]

Originally Posted by kodioneill

You can do it with a afr meter. you can purchase one for about 200.00.

I would purchase one if I had a bung for 02, the car has autothority chip and cat delete pipe.

[E Sully]

The Authority chip changes things. I would try to find out what it is programmed for as far as O2 sensor and idle rpm. 1986 was still 800 rpm, increased to 880 in 1987, but that is stock chips. When I swapped an '86 engine in mine I ran it for a few hundred miles before I installed the O2 sensor and it ran and idled well with the stock chip.
As for removing the vacuum lines, I would think this would have some effect. The engine was originally designed with the air from this line bypassing the throttle body through the venturi, but it is still metered by the AFM. I just capped the end that went to the brake booster but left the rest attached. Some adjustment to the throttle body might compensate for this reduction in air flow at idle. There is another thread on eliminating this line and possible effects. Does removing the cap from the oil tank while idling cause a change in rpm?

[Mr.Doback]

Originally Posted by E Sully

The Authority chip changes things. I would try to find out what it is programmed for as far as O2 sensor and idle rpm. 1986 was still 800 rpm, increased to 880 in 1987, but that is stock chips. When I swapped an '86 engine in mine I ran it for a few hundred miles before I installed the O2 sensor and it ran and idled well with the stock chip.
As for removing the vacuum lines, I would think this would have some effect. The engine was originally designed with the air from this line bypassing the throttle body through the venturi, but it is still metered by the AFM. I just capped the end that went to the brake booster but left the rest attached. Some adjustment to the throttle body might compensate for this reduction in air flow at idle. There is another thread on eliminating this line and possible effects. Does removing the cap from the oil tank while idling cause a change in rpm?

Removing the cap from the oil tank does cause a drop in idle, but it holds. I had the issue where it would stall, but it was fixed quickly by adding the vacuum restrictor.

[kodioneill]

Originally Posted by Mr.Doback

I would purchase one if I had a bung for 02, the car has autothority chip and cat delete pipe.

Stick the probe (sensor) in the tail pipe.