I had asked this question in an old thread but never really came to a final answer. My timing fluctuates up and down at idle (from about 6.5 to 19.5). It just keeps going up and then down and then back up all the time (usually by a few degrees at a time). The idle is not fluctuating more than about 20-30 rpm. I checked this on a Snap-on MT 2500 scanner. When I had questioned this before, someone said that this was normal at idle but that it should steady out at higher rpm's. I checkedthat today and it still fluctuates while going a steady speed on the road (about 1800 rpm). It was a little difficult to watch while driving but seemed to be ranging between 20-30 degrees. This does not seem normal to me. I know there are quite a few things that affect timing but I really am not interested in just throwing parts at it. Any ideas ? Anybody ever heard of this before ? I have been doing searches but have so far come up empty. Thanks.

 

Well, I know the stock timing chain was coated with something (I think Nylon)to cut down on noise but as a result they can stretchand throw timing off, but I would think that mightmake itconsistantlyoff, not a fluctuation.A faulty Camshaft position sensor might do that. You'd have to ask someone with a little more experience with that part to be sure though.

 

That much fluctuation does seem wierd. The only thing I can think of that would do it is a bad crank position sensor, or maybe a cam position sensor.

 

it's normal for timing to fluctuate on computer controlled engines, what you've seen is perfectly normal. don't start throwing parts at a problem that doesn't exist. the computer is constantly seeing changes that affect timing (intake temps, throttle position, coolant temp, etc...) and that's the variation you see on the scanner.

i have driving logs on my laptop from my grand prix showing timing vs rpm, vehicle speed, throttle position, coolant temp, trans temp, etc...and with the cruise on at 60mph, i'll see timing anywhere from 18 to 38 degrees without me touching anything, advanced timing at low rpm is how enginesget better mileage on the highway.

 

anybody else ? Does anybody know what symptons you get from a bad MAP sensor or a bad camshaft position sensor ? It actually seems to run OK but for some reason this does not seem normal. I also have a Mazda MPV and when I put my scanner on it - the timing does not fluctuate like this. The only thing is - it's a 1996 and my truck is a 2001 and maybe the MPV is not totally controlled by the PCM.

 

I'm leaning with the others about the crank positionsensor.
But it could just be a vacum leak. Check your hoses.

 

I just replaced the cyl.heads (I did the heads right after buying the truck so I'm not sure when this timing issue started) and I will double-check all vacuum lines tomorrow. Wouldn't I have other problems from a bad crankshaft sensor ? Does anyone else have a scanner that reads live data and can checkyour truck to see if the timing data does the same thing ? I would seriously appreciate it.

 

sometimes too much information can be a nightmare. the quest for the perfect running engine can cause alot of sleepless nights...

it could have to do with a weak cylinder due to either ignition or fuel. maybe your spray pattern is not good on it and it's causing the o2 signal to fluctuate.

I'd start by cleaning the o2's and then rechecking it. then if it continues, run a high concentration of fuel injector cleaner thru the motor and then recheck. after that... ignition & compression.

 

Again - I appreciate all the replies. I checked all of the vacuum lines I could find and all areOK. I re-did the compression test after replacing the heads to verify the rings and cylinders were all right and got good (anywhere from 150 - 165 psi) readings. The O2 sensors seem to switching normally on my scanner.I know there are many things that affect timimg (MAP - TPS - CPK - CMP) but do not really know what normal readings on these sensors are supposed to be. The TPS is new (replaced that because of a trans lock-up issue that I ended up having a re-flash to correct). The timing issue could have been there when I bought the truck - I don't know - because I just noticed it after connecting a scanner to it. I know there is some thought this could be normal (and I understand that some slight fluctuation could be normal) this amount seems excessive.I would appreciate if someone could connect a scanner to their truck- I would certainly be very interested what kind of readings they get.Thanks again.

 

As the IAC motor opens and closes the separate 'little throttle'
that controls the idle, the change in MAP would also cause the ignition timing to change.

In a similar way, when parked & idling the air filter inlet mouth can get quite different intake air temperature depending on how the wind is blowing, how warm the exhaust manifolds are, etc. With each change in IAT reading the PCM computer would also change the ignition timing.