Guys,
I have an interesting problem on my race car.

Here is the situation.

Cold idle is horrible. This means when I start the car cold it will not idle well. It revs too low and will stall easly. If I give it some revs (1500) it works and then gets better when it warms up. Once warm it will idle a bit high at 1100 rpm or so.

I have an issue with my O2 sensor so I unplugged it. This means I have an occasional stumble when hot. Most of the time the stumble is ok, but sometimes coming off the track after 20 min the car will stall when idling as I roll throgh the paddock in neutral.

The issue just came up after swapped motors. However I am using the same fuel injection from my old motor which did not have this issue. What is new is the DME and block temp sensor. I did have an issue with my throttle cable being too tight thust not giving a clear "throttle closed" signal. That made it stall easly on the hot idle stumble. Once I losened the cable I could hear the TPS click closed/open.

So I am thinking the issue could be the ISV or maybe the block temp sensor. The motor itself runs strong and the mixture is good Not too rich or too lean based on looking at the tail pipe (I have no cat btw). The issue is mostly an annoyance issue since cold idle is in the pits and I don't need to wait at traffic lights. Still it would be nice to figure out what it is.

 

Is the ISV working at all? Try disabling by jumpering the diagnostic port and see. I'm going to guess bad ISV or bad TPS (I've had one that "clicks" but still wasn't making the electrical connection - multimeter that thing and be sure).

 

I wonder if the engine temp sensor (NTC II) might not be telling the dme it's cold (no cold enrichment, no ISV adjustment).

 

I was considering an engine temp sensor as is the only new part in the sytem after the enigne swap. The temp gauge works fine, but I know it is a seperate plug. Does any one have method to test it? I assume is is a resistive element.

 

Have you tried bypassing the ISV by using a jumper on pins B and C @ the diagnostic plug. If your LED is still there it should light up Green.

 

I have an 84 chassis and I think that plug may still be in the car as it located right in near the firewall on the driver's side. However I have no idea if it still hooked up to anything. One issue is that the 84 uses a different method to stablize the idle than later cars. I do have a late DME harness and have been running a late ISV for 8 years so a late ISV in an early car is not the issue. However this may mean the dianostic port does not work.

 

As I just posted on another cold start issue thread--

I have had similar, maybe worse, cold start problems on my 89 951 the 4 years I've owned it. A couple of weeks ago I replaced the DME Temp Sensor and it seems to have solved the problem. $20-ish part, average level 944 install difficulty. I haven't started it much since then, so I'm not 100% sure its fixed or not, but I am very confident it is fixed.

Grant

 

Yes, it's a thermistor (Negative Temperature Coefficient or NTC).

The resistance for Engine Temp (NTCII) on the S is this (might be close for your car I just don't know those numbers):

0°C = 4.4~6.8Kohm
15°C~30°C = 1.4~3.6K
40°C = 1.0~1.3K
80°C = 250~390ohm
100°C = 160~210ohm

Same values for the air temp sensor in the AFM ("NTC I")

 

My car does the same, just until the engine warms. I found that tightening all the hose clamps fixed my high idle, from about 1200 to a normal 600 rpm. Still have to sit with my foot on the gas for 30 seconds before it'll warm up though :P

 

Ok so when cold resistance is high then goes down as the engine warms up.

That helps even if the numbers may not be exactly the same.