My car has just recently developed an intermittent hard starting problem. When cold it sometimes refuses to start, yet at other times it starts up instantly and runs just fine with no problem. When it won't start, it just cranks but won't fire. If I let it sit overnight, it sometimes starts perfectly the next day. So far I haven't seen a warm start problem. I've replaced the DME relay but that didn't fix it.

When it doesn't start, I notice the tach shows no movement and sits at 0 rpm. I understand this could be the sign of a bad reference sensor. Is that correct?

 

Yes. I have seen sensors die after the car having spent some quality time on the dyno. Fairly easy to replace and good to do before you get stranded.

 

I don't know about the tach symptom but my 951 does this. A quick rattle of the sensor harness at the top rear of the engine gets it going. I should replace the harness (cracked from the heat, I think) but am too lazy.

 

could be the sensor, could also be the gap on the sensor is not correct.

 

If your tach stays at 0 and doesn't bounce, its likely your speed sensor and not your reference sensor. The reference sensor wil only send a single pulse to the DME per every revolution of the flywheel - every time the flywheel goes back to TDC, the reference sensor sends a signal to the DME. The speed sensor sends a constant signal to the DME based on how fast the flywheel is spinning. If you've seen your flywheel, you've seen the multiple set screws positioned around the flywheel - these are the reference marks the speed sensor uses to output its signal to the DME.

A bad sensor is probably your problem as the DME must see at least 200 rpms before it will allow the coil to fire. If it is not reporting the correct data, the car won't run. The best way to test this is with an oscilloscope on pins 8 and 27 of your DME harness, but if you don't have an oscilloscope, a multimeter on the same pins will tell you the nominal impedance. It needs to be between 600 and 1600 ohms.
An o-scope is the better test as it will allow you to see the waveform. The best test is an oscilloscope capable of overlaying dual inputs onscreen. Put one input on pins 8 and 27, and the other on 25 and 26, then crank the motor. You should see a wave with peak to peak voltage of greater than 2.5 volts on input one and a regular single pulse of greater than 2V to correspond with TDC. If either are outside of their impedance tolerance or if either do not display the correct waveform they need to be replaced.

Replacing the sensors is pretty easy. If you remove the speed sensor take care to put the shim back in (if so equipped) as you can ruin an improperly gapped sensor pretty easily.

 

Also check the 3-pole connection for the sensors to the wiring harness, the sensors themselves could be fine, but thanks to age and heat, the female terminals (for the dme) could have cracked/split rubber boots and internal damage, which happened to my knock sensor which i just finished 'refreshing'.

Good news, they are relatively cheap and don't take much time at all to replace.

 

You are very correct. To make it easier though, just do a thorough visual inspection of the entire length of the wiring to and from the sensor and typically you will see an open wire, melted shielding, etc. Also, if you use the DME pins stated above you can disconnect the DME and the sensor connector and measure resistance across the wiring harness and you should not see more than 1 ohm of resistance - this is much easier to do if the car is not starting. In fact, I prefer to do this anyway as I hate letting my car run at idle with any problems while I diagnose it.

As far as the gap being off, unless you had the sensors out recently there is no way the gap would have changed, so that is not likely your cause.

Typically though if you do a thorough visual inspection of the sensor wiring you will find if there is a problem or not, however if you do have an oscilloscope or multimeter it would definitely be a good idea to ensure the cause so you don't waste money on replacing a good part.