84 cranks, but doesn't start
 

I just swapped out my headers for stock exhaust for emissions check... now she won't start. She cranks fine, but never catches. I had to move oil lines and install a O2 sensor. I checked:

• Ohmed the speed and reference sensors
• Swapped DME relay
• Sprayed carb cleaner down intake--it didn't fire
• Pulled spark plug and it was wet (not as wet as I expected)
• Verified there is spark by removing spark plug and watching it while cranking. It sparked but it was orange-ish red (weak battery)
• Verified the Idle Control Valve is humming

I'm trying to figure what I could have bumped during the exhaust swap. I know the reference and speed sensors are close to the driver's heat exchanger. any ideas?

Well, I just went out there and she started--yay!!! However, I am not sure why it didn't start the dozen times before. I was planning on autocrossing tomorrow, but not sure I trust her. Off for a test drive...

looks like you covered them all. While there could be other causes, the ref (and speed )sensor lines do dry out and can give intermittent readings. If you read them again, move the lines a bit and see if it changes. if this happens, or if the insulation is crumbling, you're due.

The DME can be intermittent, the flywheel sensors are known to fail with intermittent cutouts sometimes before they go completely.

For what it's worth the spark should be strong and blue. Even with a weak battery. As long as it turns over the DME will increase dwell to fully charge the coil and the spark should be blue. Try tapping onto the DME with a wooden tool handle and see if it causes problems. If yes the DME might be at fault.

Good luck,
Ingo

Check the CHT (cylinder head temp) sensor. They have a new two wire that's more reliable.

Steely, I think you are right. I suspect the cause was an intermittent speed sensor. I am pretty sure I bumped the wire hard when doing the muffler swap. The reference senor failed and was replaced about 2 months ago, so the speed sensor is probably due for a replacement. I'll go wiggle the wires and see if I can get it to fail the Ohms test.

Since I passed emission testing, the stock exhaust will be coming out. I think I'll replace the speed sensor when the exhaust is off the car, since it will give me a little better access.

Trust Ingo on this...he saved my bacon a while back on this. Based on his suggestion I eventually determined my speed (or reference...can't recall which) sensor was failing...intermittently. It would test fine at one point then later totally out of spec. Ordered new one and started perfectly ever since. He also had repaired some broken solder joints on my DME control unit a year or so ago.

I just went through this same crap with my 84 Targa. In my case, it was the ECU. I am still planning on changing out the crank sensors this spring.

Ingo fixed me up very quickly. Listen to what he tells you. These cars will absolutely require ECU work, and new sensors, as they are now over 30 years old.

Carb cleaner isn't something I would use to try to get it to start. Good for finding vacuum leaks but not starting if verifying a fuel delivery issue. Quick start/ether should be used. Sparingly and when cranking. A two person job.

That's not true in all cases! Yes, some will require replacement of the sensors, but NOT
all ECUs will fail over time. Let's not mislead owners of 911 3.2s.

Actually, carb cleaner works well for a no-start situation when fuel delivery maybe an issue.
Ether maybe too volatile in some situations.

Yup, it was the speed sensor

Intermittent DME relay, cold solder joint common.

Cool - congrats!

Not misleading at all. I am quoting my tech, who has over 40 years experience with 911s and also drives a 3.2. The ECU will absolutely become intermittant and fail. There are countless posts on this site alone that will attest to that fact.

Another absolute in life besides death, right? Please!