Cam sensor question

[mfuld]

After much searching and reading I'm still not 100% clear on the cam sensor's definitive purpose. I found a few places saying its used to synch everything up timing-wise when cranking the engine. What I want to know is, once the engine has started and is running, does the EEC or the DIS continue to use its input or can you unplug it with no other effects except the lack of tach and a CEL?

The reason I ask is I'm still trying to diagnose the cause of a miss. For example, if i start the engine then unplug the cam sensor, while its still running, and the engine still misses, can I rule out the cam sensor?

Thanks guys.

 

[Rockledge]

It is important to keep in mind the difference between "sequencing" of the cylinders and what is customarily referred to as "timing and dwell". The former has to do with the order in which the cylinders are fired, while the latter has to do with precisely when the spark firing of each cylinder should occur while on it's power stroke.

In the DIS system, the cam sensor signal is used to properly sequence the firing of both the fuel injectors and coils (spark plugs). Once the engine has cranked over and is running, the PCM can "remember" the proper sequencing even if the cam sensor signal fails.

Regarding your specific question, just because the engine continues to run the same after the cam sensor signal is cut off doesn't necessarily mean the cam sensor is "good".

 

[rubydist]

While what Rockledge says is correct, it seems reasonable to me to assume that something else besides the cam sensor is causing the miss. As far as I know, the cam sensor can only cause a miss if its sick, and unplugging it eliminates that, so I'd say its highly unlikely that the cam sensor is the issue in the case you have described. The other side of the coin is that the cam sensor is not particularly expensive nor difficult to change, so you could just change it and then rule that out completely....

 

[SHOtime2511]

i had a cam sensor fail years ago on my 89 that cause backfiring.
cheap and easy fix.

 

[Mr Anonymous]

A KOER cylinder balance test may help you isolate the misfire.

As to cam sensor functionality, remember that the crankshaft rotates twice for each rotation of the camshafts (720 degrees of crank rotation to 360 degrees of cam rotation), so the PCM needs the camshaft position data to determine which TDC event the crank sensor is reporting.

Theoretically, since the mechanical timing of the crank and cams is fixed, the PCM only needs to synchronize the cam sensor to the crank sensor once and then it will know where the cams are in relation to the crank sensor signal for the remainder of that run cycle. Theory aside, the SHO PCM's continuously monitor the cam sensor.

 

[mfuld]

Alright thanks for the info guys.

Mr. A, the misfire only happens when its warm, so when I've run the cylinder balance test it either comes up clean or reports 5 and 6 because it starts missfiring near the end of the test. So I'm waiting on a new DIS and I'll see where that leaves me. I did my 60k not more than 3000 miles ago replacing everything except the camsensor so this is very perplexing.

 

[rubydist]

What do the plugs look like on 5 & 6? Any water or oil in the plug wells on the front bank?