Intermittent starting problem

[aria]

Hi,

I have an 850 turbo wagon that recently has an intermittent starting problem. The engine turns over but no spark at the plugs so it is an ignition problem (fuel pump is fine). If I leave the car for hours, it often starts. The problem is not necessarily linked to a cold or hot engine.

I checked the fault codes and the only thing that was related was "intermittent missing signal from RPM sensor". My thinking was either coil/rpm sensor/crancshaft position sensor problem.

I took a look at the connector that sits on top of the transmission (automatic) where the RPM sensor wires go and the connector looks clean. In fact, I replaced the gear position sensor located in the same place about a year ago.

Has anyone goe any ideas as to what else I can check. I wasn't able to see the RPM sensor itself. It looks like the wires for it ran down the front of the engine so I didin't get a chance to dig in any further.

Where exactly is the RPM sensor?
Could this be a coil problem? If so how to I test the coil when it is failing?

Thanks in advance.

[dpauto.com]

There is a cam position sensor and an RPM sensor. The RPM sensor provides engine speed and cylinder #1 top dead center information to the ECU. A defective RPM sensor will disable both fuel injection and ignition.

If you have fuel pressure AND injectors pulse at the rail ... but no spark ... then very likely you have a bad ignition power stage which is next to the coil or the coil itself is defective.

Note: your fuel pump gets energized for a split second REGARDLESS of the RPM sensor whenever you turn your key on. The correct way to check your fuel pump is when engine is cranking, along with a test light and pressure gauge.

Regards,
Dave

[aria]

Dave, thanks for your very helpful reply. I know I don't have any spark because I have checked this when the car does not start. However, as far as fuel delivery goes, I have only listened for the fuel pump powering up by listening for it in the rear of the car. The fuel pump is energized. I didn't check for fuel delivery however. Am am unsure what is the best way to do that. I could crank for a bit and then pull a spark plug to see if there is fuel saturation on it or not.

From what you are saying, it seems that the RPM sensor may not be the problem. Is there a way I can test the crank angle sensor or the coil?

Thanks.

[dpauto.com]

There are many ways to skin this cat.

If you had no spark, and "assume" the fuel injection part is functioning... the easiest way is to check for ignition pulse.

Do this with a test light: clamp your test light to ground. With the key on, your test light should be bright when it touches the red wire and the blue wire at the coil. Now while touching the blue wire, have someone else crank the engine, your test light should be "blinking". That means the coil is getting ignition pulse and the coil is defective. If the test light does not blink, then you either have a bad ignition power stage (next to coil) or simply the whole system is shutting it down due to defective RPM sensor or even the Cam position sensor.

Regards,
Dave

[aria]

Dave,

The car would not start again so I had a chance to do your suggestions. With ignition key in 'on' position, both red and blue wires on the coil are enegized and the light is on.

When I crank the engine, the light dims (expected) but does not blink. It remains on when it is connected to either the red or the blue wire.

I didn't have a way to check for pulsation on the fuel injectors. I am not quite sure what to touch while cranking the engine.

So, based on what you said the problem could be the coil power stage, the RPM sensor or the crankshaft position sensor!

Should I start changing parts one at a time or is there a way I can test each part?

I appreciate your help and hope that I can send you a token of my appreciation once I have this nagging problem resolved.