Seems like my oxygen sensor has been purposefully disconnected...

[GiantRider]

So I’m trying to diagnose a crank no start and I’ve been looking at everything and noticed the connection to the oxygen sensor disconnected. It’s possible that the electric tape was meant to keep it together but just came apart but it sort of looks like it was purposefully disconnected and taped up.

Is there any reason to disconnect this particular part?

‘76 911 w/ ‘80 3.0.

[GiantRider]

[tirwin]

The male end of the connection is known to disintegrate over time due to age and heat cycles. I guess some people don’t know how to repair it so they just leave it.

Other people have misdiagnosed an engine that “runs fine” but probably has some neglected maintenance. Sometimes the complaint is a “hunting” engine. They disconnect the O2 sensor but it is just addressing the symptom and not the real problem.

Or they didn’t know what they were doing.

Take your pick. ��

[tirwin]

I answered while you were posting pics. Looks like my first guess was the right one.

Contact forum member Louie_85 for replacement part. I may not have his screen name right. A search should turn it up.

[911tracker85]

Contact forum member Louie_85 for replacement part.

got that replacement from him when mine just broke apart doing the intake refresh.

[Dave Kost]

The USA 1980 911 SC has a Lambda CIS fuel injection system. To pass USA emissions, Porsche put a Lambda O2 system starting in 1980 and thru 1983 911's.

There should be a ECU and O2 relay under the passenger seat and a frequency valve in the engine compartment. If, the Lambda system isn't working you will have a hard time starting but it will start. Usually what happens is that it will rev up then immediately shut off. This is a sign that the relay, ECU or wiring is bad. It will still start if the O2 is unplugged and the Lambda (Frequency valve) is operating.

Please confirm that the Frequency Valve is working (buzzing).

Can you describe in more detail "won't start" ??

[tirwin]

The O2 sensor itself has nothing to do with starting. It doesn’t come into play until the engine is warmed up.

It sounds like someone else did the ‘80 engine swap into your ‘76. Hard to know how they did the wiring. Can you tell us more? Do you have the lambda box (normally under the passenger seat but who knows in a transplant)? If not, not much point in repairing the O2 sensor... Is there an O2 sensor installed on the exhaust outlet before cat or muffler?

If the starter turns the motor, does it fire and then die? If it fires and then immediately dies look at the fuel pump relay. This symptom points to a ground on pin 85. Can be one of 3 things: 1) faulty alarm 2) bad rev limiter 3) air pressure plate switch is open. This means you’re getting spark, fuel and air to fire but it won’t stay running so that’s why go to the relay first. Something is likely tripping the relay and shutting off the fuel pump. But that depends on if someone did the wiring right for the ‘80 or took shortcuts.

My guess is it’s an ‘80 motor that is wired like a ‘76. Cars with Lambda have a separate 12-pin harness. The plug is on the engine bay firewall to the right of the left shock tower when facing the back of the car. If that isn’t there and there is no ECU in the cabin, then you have a bastardized system.

If starter is spinning the motor and it won’t fire at all then check fuel, air and spark. For fuel delivery, put a jumper wire across pins 30 and 87a on the FP relay. Turn ignition to ON (not START). Lift the arm on the air plunger in the airbox. You should hear injectors squeal.

[GiantRider]

The battery seems good, I get a reading of 12.6 volts although the battery tender I have usually indicates its finished charging with a green light but after 16 hours it still remains on charging mode.

The starter is turning the motor to 4k RPM and as far as I can tell, in the on position everything sounds like it always has and from lifting the air plunger I can definitely hear the injectors squeal.

I thought to first rule out the battery and now I'm going to check for spark. The distributor has also been swapped for the pointless version. A little embarrassing but honestly I don't even know how to go about testing the spark.

Also, I do have the ECU under the passenger seat.

One last thing, this happened once and drove fine 5-6 times in between then and now but at one point I was driving and lost power and the motor died. I was able to start it again quickly just like normal but then on the same trip the car backfired and the motor died and the car was completely dead, no starter or anything and I had to roll start it to get it going and was able to make it home. It was a bit low on oil and was getting hot and ever since I put some oil in it it was driving just like normal.

[tirwin]

Here's the easy way to check for spark. Do you own an inductive timing light? If you don't borrow one from a friend or you may be able to rent one from the local auto parts store. Basically, you put the inductive lead on the wire going to spark plug #1 and then try to crank the car. If it flashes you have spark. No flash, no spark.

If it flashes, then it's still possible you have a spark that is too weak. If you're unfamiliar with how to test the spark safely, then maybe find a friend to help. It's high voltage so be careful. Maybe watch a few YouTube videos. The basic idea is to pull the wire from the coil to the distributor and ground it while a helper tries to crank the car. If you don't like the person helping, you crank and have the helper hold the plug wire. (kidding) If the spark is bright blue, then the spark is good. If it's yellow or orange, it's too weak.

Starting to sound like you have a bad coil or CDI (ignition box).

[Alfasrule]

Runs richer, but no harm. Me Spider runs way better with it unplugged, cause of the set of better cams I put in her.

[tirwin]

Originally Posted by GiantRider

The battery seems good, I get a reading of 12.6 volts although the battery tender I have usually indicates its finished charging with a green light but after 16 hours it still remains on charging mode.

The starter is turning the motor to 4k RPM and as far as I can tell, in the on position everything sounds like it always has and from lifting the air plunger I can definitely hear the injectors squeal.

I thought to first rule out the battery and now I'm going to check for spark. The distributor has also been swapped for the pointless version. A little embarrassing but honestly I don't even know how to go about testing the spark.

Also, I do have the ECU under the passenger seat.

One last thing, this happened once and drove fine 5-6 times in between then and now but at one point I was driving and lost power and the motor died. I was able to start it again quickly just like normal but then on the same trip the car backfired and the motor died and the car was completely dead, no starter or anything and I had to roll start it to get it going and was able to make it home. It was a bit low on oil and was getting hot and ever since I put some oil in it it was driving just like normal.

After re-reading this post, this comment stood out to me.

A backfire could've damaged the airbox. Might be a clue. Might be irrelevant. Maybe you have a big vacuum leak now. Need to look the airbox over and make sure there are no obvious cracks. Are the rubber boots on the intake runners ok? Do you have a pop-off valve installed?

The fact that the starter didn't engage at all when you tried to start the car again is unexpected. Now I'm starting to think you could have a problem at your ignition switch. The starter is engaged at the switch. There is a plug from the ignition switch to the firewall. I'd unplug it, spray the male and female contacts with Deoxit D5 and then plug it back in making sure it's snug.

Next verify you have spark at the coil.

If you have fuel and spark, the only thing left is air so that would point to a big vacuum leak.

[GiantRider]

Thanks for getting back to me; initially when it happened I did some research and inspected the airbox and everything seemed okay (it also does have a pop off valve installed). Also weird that it started up fine multiple times after that.

I'm going to check for spark but in reading about the ignition coil I think that that is a good thing to test as well. Is the standard protocol to test that just checking for resistance on the primary and secondary coils?

[tirwin]

This thread my help too:

ignition coil and CDI test

Do you have the Bentley book? I don't remember the coil specs and I'm not at home where I can look it up.

[GiantRider]

Thanks for the link. I do have the manual, I'll check the specs.