3.6 conversion wont start with O2 sensor plugged in??
 

My 1975 with a 964 motor wont start with the O2 sensor plugged in. It cranks fine but wont start. On occasion it will catch, and start if you time moving the key from START to RUN just right. Unplug the O2 and it starts perfectly. I know the easy answer is “swap the O2 for a new one and see if it fixes the problem.”

My concern there is that I cannot figure out why having the O2 plugged in would prevent starting. My only thought is that if it’s shorted out to read FULL lean, maybe there’s too much fuel gong in as it cranks. I also thought the O2 is not “in the loop” until the engine is running and warmed up? That would mean the O2 should have no effect on starting.

Make sure that the 12v source for your conversion wiring gets 12v while cranking. There are a number of fuses in the fuse box that get power cut during cranking.

Am I wrong to be surprised that the O2 sensor is even doing anything during cranking? Typically dead cold at that time of course.

The 12v is a good though. The car has been running great, and ran great with the O2 hooked up for w while…..then this started. I too am questioning why plugging in the O2 would have any effect on cranking and the Start process, even with a hot engine.

I am going to test the car by starting it with out the O2, and then plug it in while it’s running and see if something changes that gives me a clue.

Duc: You filled in some information above, please describe any differences when starting cold, or warm, or immediate warm restart. I agree that the O2 sensor should not make any difference during warm or cold start. The ECU should be open loop, so O2 should have no effect. Try starting without O2, then plugging it in when running, as you mentioned.

Next diagnostic procedure: Squirt starting fluid into the intake during a cold start and then during a warm start to determine whether it's lacking fuel or spark.

Do you hear the fuel pump running when the key is on? I don't think the 1975's had the CIS fuel pump safety switch, so the fuel pump ran whenever the key was on. BUT, your installation of the 3.6 and ECU may have changed that part of the wiring.

@PeteKz - Cold start W/O2.....cranks and doesn't want to start.
Cold Start W/O O2....starts like a champ
Plug in O2 and car runs well/ Idle starts to hunt a little bit, but not too bad. My WBO2 I could see the mixture go lean as RPM went up and then rich as it went down.
Warm start w/O2 car starts, though cranks a little more than w/o O2

The car recently had the battery disconnected, which means the DME lost its memory. I let the car run at idle, O2 sensor plugged in for 15 min to see if the 964 DME learned the idle circuit. During the tile the hunting got a little better but never went away. Yes the car has a LFW, and it has a Steve Wong chip with his LWF fix. I will do more testing tomorrow.

One can always connect a Porsche tester, e.g. PST2, Hammer, or the aftermarket Durametric, and read the actual values data.
That's assuming that the install included OEM wiring and connectors and not some Mickey Mouse install as is typical.

I’m thinking it might be the heating circuit for the oxygen sensor is robbing power or ground pathway from another circuit in the DME?

Some seem to overlook this, e.g. and point to wiring!

It was done with OEM connectors. I know it's running a 3.2 style DME relay though, not the 964 style. I think they are the same relay, with only the with from round to blade connectors the difference.

The key thing for me is that the car ran well on the O2 for a long time, then it started not cold starting with it connected. I believe I swapped in a new O2 and got the same result....but that was a long time ago. Keep in mind the car did almost 8,000 miles before this started to happen.

I have not gone back out to do more testing yet.

The other thing I am chasing, and I think is unrelated is.....if I stab the throttle hard, the car will almost stall. In fact if I go from idle to full throttle you can stall the car. You have to be aggressive with eh throttle to do this though....like more aggressive than you would driving it. If you don't treat the throttle like a light switch (think grabbing it by hand in the engine bay and stabbing it hard) it works fine. This seems not right, but even my Indy didn't have an answer.

Check through the wiring diagram. At least on my -87 with 993 varioram (with Timmins wiring kit) there is a fuse for fuelpump. Fuelpump and 02 sensor are under same circuit. When i had a short at 02 (wiring melted on exhaust) it was blowing the "extra" fuse for fuelpump. If you dont have this fuse in your loom it might cause some weird issue when having short at 02 circuit.

so an update on my situation. I replaced the oxygen sensor with a new one. On the first start it cranked and fired almost imefdiately and seemed to run well for 30 seconds. I shut the car off and cranked it again, and it cranked and cranked and cranked and cranked… And did not want to start. I did it again, it cranked forever and didnt catch, and as I moved the key from the crank position to the run position it caught and huffed a little and stumbled and didn’t start. I unplugged the O2 sensor, and the car starts perfectly almost immediately.

The old oxygen sensor was black, which is rich. I have another ECU that’s a known good ECU from Steve Wong with a stock chip in it, and today I’m gonna swap ECU and see if it makes a difference. The ECU I have been running on is also a known good ECU from Steve Wong, with his chip in it specific for the mods on my car, but not custom tuned. I cannot believe the new oxygen sensor got fried right away. Also, the cylinder head temperature sensor is very new, the wires look brand new so I’m ruling that out as the culprit for now. Especially since the air temperature today is 75° here in Florida. Frustrating.

OK, so now I’m really stumped. But my problem may be solved. I swapped my DME for another one I have. Both are known Good DME‘s from Steve Wong, the one I was running had his customized chip in it for my car (3.6 with headers etc, not a customer Dino tune), the other one had his stock 3.6 chip in it. There was no difference in how the car ran. So I swapped them back. Then I remembered I had a fancy solid state relay from Focus 9 Tech and thought I would give it a try. I have no idea why it might change anything, but I thought “what the hell.” Once I swapped the relay out suddenly the car starts and runs wonderfully. Once warmed up it idles at an air-fuel ratio of between 12.8-13.4. Before swapping the relay out, I noticed that my air fuel ratio would creep to over 15 or the gauge would just go to —- because it was too lean to report. That also seems strange given that the O2 sensor I pulled out was black, which would mean rich. Anyway I am happy. It’s running well.

I took the car for a drive and everything seems wonderful. I’ll report back if that situation changes, and I am still at a loss as to why this worked. I have actually changed that relay to a New’ish factory relay before, and that was what was in the car.