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View Full Version : bad cam sensor causing a miss ?



thunderkid84
02-17-2009, 02:30 PM
i was just wondering if a bad cam sensor would cause a miss ?

i've been chasing down a miss for about a month. ive changed plugs and wires. i dont think its a bad DIS. the car was hard to start yesterday, took 3 tries to fire up.

it does not miss in boost (as far as i know). doesn't miss at light throttle but anywhere from 10-0 vac it will miss like crazy.

also, the cam sensor was replaced in november 2007 and only has about 2000 miles on it. its not a ford brand part eiter.

just wondering if you guys had any ideas.

Jason

Mike Puckett
02-17-2009, 07:04 PM
I cured a misfire by changing the cam sensor on my old 35th a few years ago. It also had the no start till the 3rd try syndrome. Both symptoms were intermittent and disappeared after replacement.

thunderkid84
02-18-2009, 12:38 PM
thanks for the reply mike

anyone else have any experiences ?

TbirdSCFan
02-18-2009, 02:35 PM
I've never had a misfire from a bad cam sensor. I've had them stop completely in which case the tach stops, the EEC+DIS compensates, and the engine continues to run just fine. Or the cam sensor output flutters rapidly and throws the EEC into a fit where the engine bucks, stumbles and the tach does a dance. But in both of those cases the problem is either there.. or not.

If you suspect a problem with the cam sensor, you can simply unplug it and run without it.

TbirdSCFan
02-18-2009, 02:37 PM
the cam sensor was replaced in november 2007 and only has about 2000 miles on it. I had a new one go bad, with the fluttering signal, with less than 2000 miles on it.

thunderkid84
02-18-2009, 03:17 PM
i think i will try unplugging it.

after that i think im gonna test the coil pack. after 150k miles that thing has to go bad sometime.

KMT
02-18-2009, 03:50 PM
>I've had them stop completely in which case the tach stops, the EEC+DIS compensates, and the engine continues to run just fine.

Correct. But only for a while...

The 'compensation' strategy (Failure mode and effects analysis) surrounding a connected bad sensor or disconnected good or bad sensor, includes periodic re-sampling (post start) for updated cam sensor info, which can involve at least a temporary hand off back to the cam sensor circuit. At this point (with a connected bad cam sensor or disconnected good or bad sensor), the engine will stumble - it may pick up if the strategy flips back to a default set of failure mode defined inputs, or it may require a restart to begin the process anew. Any conclusion about the reliability of the cam sensor at this point is speculation.

>If you suspect a problem with the cam sensor, you can simply unplug it and run without it.

If a bad cam sensor has already triggered the strategy that you first outlined, unplugging will have no effect. If the sensor is good but disconnected on restart, a failure mode will be triggered and the effect will again be the same as what you first outlined.

As you point out, cam sensor issues can be intermittent or constant. Troubleshooting by disconnecting with hard cranking (excessive spark advance from the failure mode)...multiple tries... on restart can indicate:

a good sensor that is disconnected
a bad sensor that is disconnected
a bad sensor that is connected
All three involve an active failure mode... Hard starts and 'runs well' do not make for a well operating vehicle as the seemingly well running car's condition is only temporary.

Reliable troubleshooting involves replacing the cam sensor with a new/known good unit. Hard starting should be absent over several restarts and the stumbling and erratic/non- functioning tach should not reappear - test drive the car to sufficiently warm the engine and test restarts again...both immediately and after letting the engine heat-soak for 15~20 minutes. Recheck codes and clear memory. Be sure to closely examine engine grounds and cam sensor wiring harness for any belt cuts when troubleshooting.

TbirdSCFan
02-18-2009, 04:07 PM
Reliable troubleshooting involves replacing the cam sensor with a new/known good unit. But of course. The suggestion to disconnect it is temporary to help troubleshoot the problem. On my SCs, my guiding principal is "It should work precisely like it came out of the factory." :D Well... mostly. There are a few exceptions, such as stereo equipment, solid engine mounts, fog lamps that actually DO something. :rolleyes:

KMT
02-18-2009, 04:28 PM
>The suggestion to disconnect it is temporary to help troubleshoot the problem.

:)

thunderkid84
02-19-2009, 07:42 AM
so, based on what KMT wrote, unplugging the cam sensor wont do anything and i should just swap it with a new one ?

i think this time i'll get a motorcraft cam sensor.

does this sound like a bad coil pack to anyone ?

the wierd thing is that i really hear and feel it missing between -10-0 vacuum. in boost i cant hear any missing but im sure its happening.

thanks for the replys

TbirdSCFan
02-19-2009, 01:46 PM
so, based on what KMT wrote, unplugging the cam sensor wont do anything and i should just swap it with a new one ? Not quite. What he was saying was disconnecting the cam sensor is not the long term fix. Rather, its a way to troubleshoot the problem.

does this sound like a bad coil pack to anyone ?

the wierd thing is that i really hear and feel it missing between -10-0 vacuum. in boost i cant hear any missing but im sure its happening.
I've never had or seen a coil pack go bad. Not to say it can't happen, but I've never seen it. There are no moving parts or electronics in a coil pack. Realistically, unless its suffered from an impact, there's nothing to wear out.

If the car idles... you should run the KOER cylinder balance test. Do a search here to find out how to. :cool:

KMT
02-19-2009, 04:22 PM
>unplugging the cam sensor wont do anything

...if the car's computer has already gone into failure mode because of a dead cam sensor, unplugging it won't do anything...the way the car runs will not change because the computer will continue in failure mode.

If the car is running poorly due to a bad cam sensor that is feeding wrong data, and the computer hasn't gone into failure mode, disconnecting will drop the car into failure mode and the wrong data won't exist. This can make the car appear to run better, for a short time at least.

The question is then about whether or not the cam sensor was bad (in such a way to feed bad data that was being processed), or dead, in which case it it would not feed any data for the computer to process, causing the computer to supply it's own data (via substitute default values) and guess at which cylinder needed to be fired when.

Unplugging a bad cam sensor is the same as unplugging a good one - the computer will drop into failure mode and after a hard start or two the engine will run fine for a while... Unplugging a dead cam sensor shouldn't make any difference since the car is already in failure mode and you should already be seeing hard starts.

If you understand how all this works, unplugging can be a harmless method of checking things out. If you don't, unplugging can perhaps lead to a false conclusion about the condition of the cam sensor. In that case, the best way to avoid confusion is to simply install a new/known good cam sensor. If the cam sensor was the (only) issue, the hard starts, tach dropping and stuttering will go away and the car will run fine - period.

thunderkid84
02-19-2009, 08:29 PM
thanks for clearing things up KMT.

my tax return comes in next friday. so im gonna go buy a motorcraft cam sensor then.

has anyone ever had terrible missing in vacuum only ?

in boost i do not notice any missing....so im confused there.:confused:

TbirdSCFan
02-19-2009, 09:20 PM
thanks for clearing things up KMT.

my tax return comes in next friday. so im gonna go buy a motorcraft cam sensor then.

has anyone ever had terrible missing in vacuum only ?

in boost i do not notice any missing....so im confused there.:confused:

You will usually experience a stumble with a vacuum leak. And of course, you can assume that if the IC tubes were removed recently... 90% of the time... there's where your problem will be. ;)

Oh, and I dissagree with KMT on this much.. if you unplug the cam sensor and the car runs better; dude.. replace the cam sensor. If no change.. reconnect it and look elsewhere. Its not that complicated. :rolleyes:

FMEM completely compensates for the missing CID signal and the engine will run just as well w/o it. Its one of the few things that the EEC can tolerate. What it can't do is figure out when to fire the coils upon startup. :)

thunderkid84
02-19-2009, 09:30 PM
You will usually experience a stumble with a vacuum leak. And of course, you can assume that if the IC tubes were removed recently... 90% of the time... there's where your problem will be. ;)

i've got a FMIC and my tubes are clamped with t-clamps and there are no other vacuum leaks. at idles it sits perfectly between 18-20.

fturner
02-19-2009, 09:57 PM
To clear one thing up about the EEC-IV's and failure management. Once the EEC has failed a sensor, it will NOT re attempt to use that sensor until a key off then on event has occured.

This is why people have successfully run the car with an unplugged cam sensor for months on end, with the only problem being numerous attempts at starting. As has been mentioned, if unplugging the cam sensor does not solve running issues, you have other issues, plain and simple. There is no magic voodoo with this system.

Adaptive strategy has nothing to do with sensor failure, and is a complete whole concept on its own in regards to fuel management.

Fraser

KMT
02-19-2009, 10:01 PM
Once the EEC has failed a sensor, it will NOT re attempt to use that sensor until a key off then on event has occured.

Adaptive strategy has nothing to do with sensor failure, and is a complete whole concept on its own in regards to fuel management.



Sorry you're confused. Maybe I missed it, but I don't think anyone mentioned 'adaptive' strategy. We're talking Ford's 'Failure Modes (and) Effects Management (http://www.sccoa.com/forums/showthread.php?t=103239&highlight=failure+effects+management)'.

To quote one of many online resources (http://www.ford-trucks.com/forums/112295-how-to-pull-codes-from-an-eec-iv.html):
"Failure Mode Effects Management (FMEM)

FMEM is an alternate system strategy in the PCM designed to maintain vehicle operation should one or more sensor inputs fail.

When a sensor input is perceived to be out-of-limits by the PCM, an alternative strategy will be initiated.

The PCM will substitute a fixed in-limit sensor value and will continue to monitor the faulty sensor input. If the faulty sensor operates within limits, the PCM will return to the normal engine running strategy.

Engine Running DTC 98 or 998 will be displayed when FMEM is in effect.

The Malfunction Indicator Lamp (MIL)/Message will remain on when FMEM is in effect."

An intermittent cam sensor can trigger an alternate operating strategy w/value substitution, but the system will return to normal, using that same sensor, if it the sensor is again operating withing bounds.

FMEM when the DIS & EEC work together in reaction to missing/out of spec data from, in this case, the cam sensor. Significant engine stuttering (without actual key-off) can cause various components to 'reset', including the EEC, at which time it will resample. If the cam sensor is condemned/ignored again, the EEC will guess again on cylinder ID. If it gets it right, the engine may recover. If it fails, the driver may invoke yet another restart.... and the whole string of events begins again.

If you're talking about which comes first...the stumble or the (re)sampling, keep in mind that the computer will know about it before you do, so it may seem like one happens before the other. What counts is that the computer knows :)

Talking about 'unplugging only relating to running issues'...maybe ya missed it when that the op stated "the car was hard to start yesterday, took 3 tries to fire up", which is a prime clue pointing to a bad/dead cam sensor....that's where we've tried to keep the topic & troubleshooting advice.

fturner
02-20-2009, 09:29 AM
Never mind :(.

I must have a large log in my eye, so therefore I must remove that before I attempt to remove the splinter in another's eye.

Fraser

TbirdSCFan
02-20-2009, 11:54 AM
Never mind :(.

I must have a large log in my eye, so therefore I must remove that before I attempt to remove the splinter in another's eye.

Fraser LOL.. ever the truth, isn't it? Tweezers anyone?? :D

Young-SC-Owner
02-20-2009, 03:58 PM
Sorry you're confused. Maybe I missed it, but I don't think anyone mentioned 'adaptive' strategy. We're talking Ford's 'Failure Modes (and) Effects Management (http://www.sccoa.com/forums/showthread.php?t=103239&highlight=failure+effects+management)'.

FMEM when the DIS & EEC work together in reaction to missing/out of spec data from, in this case, the cam sensor. Significant engine stuttering (without actual key-off) can cause various components to 'reset', including the EEC, at which time it will resample. If the cam sensor is condemned/ignored again, the EEC will guess again on cylinder ID. If it gets it right, the engine may recover. If it fails, the driver may invoke yet another restart.... and the whole string of events begins again.

If you're talking about which comes first...the stumble or the (re)sampling, keep in mind that the computer will know about it before you do, so it may seem like one happens before the other. What counts is that the computer knows :)

Talking about 'unplugging only relating to running issues'...maybe ya missed it when that the op stated "the car was hard to start yesterday, took 3 tries to fire up", which is a prime clue pointing to a bad/dead cam sensor....that's where we've tried to keep the topic & troubleshooting advice.
What if there is no stuttering to cause this "reset" your saying is going to happen with the cam sensor unplugged?

KMT
02-20-2009, 04:15 PM
What if there is no stuttering to cause this "reset" your saying is going to happen with the cam sensor unplugged?

If...maybe....might...suppose... Is this with a 100% good sensor or a dead as a door-nail sensor or a bad sensor feeding bad data that is sporadically going dead or a bad sensor that was feeding bad data but doesn't die.

With or without hard cranking? With or without other issues? With or without poor grounds? With cold engine or warm...or both...?

Rhetorical...because there is at least one issue and if it was known exactly - there wouldn't be any questions/troubleshooting.

By the time you ponder all these questions and chase/isolate an exact scenario that is repeatable, you could have simply swapped in another cam sensor...

If you think the cam sensor is the suspect....replace it and know.

Young-SC-Owner
02-20-2009, 04:26 PM
If...maybe....might...suppose... Is this with a 100% good sensor or a dead as a door-nail sensor or a bad sensor feeding bad data that is sporadically going dead or a bad sensor that was feeding bad data but doesn't die.

With or without hard cranking? With or without other issues? With or without poor grounds? With cold engine or warm...or both...?

Rhetorical...

By the time you chase/isolate an exact scenario that is repeatable, you could have simply swapped in another cam sensor...

If you think the cam sensor is the suspect....replace it and know.

But wouldn't running the car without the cam sensor plugged in rule out if the cam sensor is the problem or not, instead of me running out and ordering up a sensor that isn't faulty afterall

KMT
02-20-2009, 04:37 PM
But wouldn't running the car without the cam sensor plugged in rule out if the cam sensor is the problem or not, instead of me running out and ordering up a sensor that isn't faulty afterall

Did you read the thread? I'll wait :)

Again, depending on how the cam sensor is operating, there is one condition out of three where unplugging can reveal a valid condition. If the person doing the work understands all three conditions they might draw a correct conclusion (and these types aren't in need of troubleshooting tips). If they think the results are only binary, they then have a 1-in-2 chance of coming to the wrong conclusion and overlooking a valid issue.

Young-SC-Owner
02-20-2009, 04:41 PM
Did you read the thread?

I bow to your superiority :rolleyes:

thunderkid84
02-20-2009, 06:11 PM
guys, im gonna go on about a 1 hour drive here in a minute, and im gonna unplug the cam sensor to see wut happens. this is the way i've learned to troubleshoot things. "if it works better without the sensor unplugged, then replace the sensor"

KMT, ur threads are very informative but what you're basically saying is there really is no way to troubleshoot a bad/going bad cam sensor (or any sensor that feeds data to the EEC) and you should just try a new/good sensor in the first place.

am i right?

i'll let u guys know how the drive goes.

KMT
02-20-2009, 06:35 PM
guys, im gonna go on about a 1 hour drive here in a minute, and im gonna unplug the cam sensor to see wut happens. this is the way i've learned to troubleshoot things. "if it works better without the sensor unplugged, then replace the sensor"

KMT, ur threads are very informative but what you're basically saying is there really is no way to troubleshoot a bad/going bad cam sensor (or any sensor that feeds data to the EEC) and you should just try a new/good sensor in the first place.

am i right?

Almost - I didn't say there was no way.... I'm talking about good ways versus bad ways and ways that will help avoid making a mistake.

"If you unplug the cam sensor and the engine runs better, replace the cam sensor" Got it...let's see how that plays out...

1.) Unplug a bad sensor (sending out bad data; maybe constantly, maybe occasionally)...what will happen? Engine may run different/better because the computer dropped into FMEM rather than depending on bad data. Replace sensor everyone says.

2.) Unplug a dead sensor (sending out no data, ever)...what will happen? Engine runs same, because the computer has already dropped into FMEM. No change in running...don't replace the sensor everyone says.

3.) Unplug a good sensor (sending out good data)...what will happen? Engine apparently runs same, because the computer has now dropped into FMEM. No change in running...don't replace the sensor, everyone says.

What do you do?

#1...follow advice in blue...replace the sensor and problem fixed. Happy day.

#2 and #3 generate the same results - but what do you do about #2? Do you follow the advice in blue to not replace or do you ignore that advice?

- In the case of #2, if you follow the advice in blue not to replace, you leave a dead sensor in the system, thinking it isn't the issue and you begin to elsewhere look for other causes. Does not replacing a dead sensor sound like a process that will fix the problem? Sad day.

- If you ignore the advice....replace the sensor...you did the right thing and problem fixed. Happy day.

But in the end, what holds most true is...do whatever works for you :)

Keep us posted if you have time and good luck w/your issue.

thunderkid84
02-20-2009, 08:34 PM
ok....with the sensor unplugged i drove around for about an hour. did not detect any missing at light or partial throttle like it was doing. going into boost was the same as always.

now on my way back home i plugged the sensor back in and it pretty much ran the same. i really couldnt detect any missing.

my guess is i have an intermittently bad sensor, or #1 in KMT's post.

next week i will buy a motorcraft sensor and see how it works out.

fturner
02-20-2009, 09:01 PM
LOL.. ever the truth, isn't it? Tweezers anyone?? :D

Gonna need vice grips the way this is going.

KMT
02-20-2009, 09:50 PM
>with the sensor unplugged i

What was your starting like this time... p'gd vs. unp'gd?

thunderkid84
02-20-2009, 10:26 PM
>with the sensor unplugged i

What was your starting like this time... p'gd vs. unp'gd?

lol....took like 3 or 4 times to start, with alot of backfiring and stuff going on.

restarted first time after that, fired right up. restarted a second time, would not turn over after about 3 or 4 tries. plugged the sensor back in and fired right up.

KMT
02-20-2009, 10:40 PM
lol....took like 3 or 4 times to start, with alot of backfiring and stuff going on.

restarted first time after that, fired right up. restarted a second time, would not turn over after about 3 or 4 tries. plugged the sensor back in and fired right up.

And the tach....what did it do each time?

OK
Jumped around...sometimes
Didn't move....dead

thunderkid84
02-20-2009, 11:15 PM
it jumped while cranking but as soon as it fired up the tach would go dead....every time.

KMT
02-21-2009, 01:12 AM
Sorry if I missed it - does your car have a (working) VMM?

thunderkid84
02-21-2009, 01:05 PM
Sorry if I missed it - does your car have a (working) VMM?

yup, works great.

KMT
02-21-2009, 01:55 PM
yup, works great.

When you were driving around for the hour or so with a dead tach, did the VMM go off at all?

thunderkid84
02-21-2009, 03:14 PM
When you were driving around for the hour or so with a dead tach, did the VMM go off at all?

nope, worked like normal the whole time.

KMT
02-21-2009, 03:25 PM
nope, worked like normal the whole time.

That's interesting - without a tach signal (you said the tach was consistently dead), it should turn off after 15 minutes... do you know how to turn it into a virtual tach?

thunderkid84
02-21-2009, 04:47 PM
well i dont look at it every other second, i just glance at it from time to time but i would've noticed if it went out.

no i dont know how to do that. sounds cool tho

KMT
02-21-2009, 04:58 PM
well i dont look at it every other second, i just glance at it from time to time but i would've noticed if it went out.

no i dont know how to do that. sounds cool tho

Tony was gracious enough to contribute (http://www.sccoa.com/forums/showthread.php?t=103268&highlight=vmm) all the dirt on that...

Might try it and see if it responds while the tach is dead...that would confirm a signal and then perhaps put the tach itself in question - because again, with no genuine tach signal, the VMM will time out and shut down. I've had opportunity to confirm that much.