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Chrysler "fuel sync"

Chrysler "fuel sync"

Chrysler "fuel sync"

I am looking for someone who "UNDERSTANDS" the fuel sync concept in Chrysler efi. Why? I am calling BS on virtually everything I read on the net about how the fuel sync "precisely" establishes when the injectors fire in relationship to intake valve opening. If that was so, the factory spec of +/- 8degrees would be in relationship to the stock camshaft opening point. Since there are three different v8 cams used from 93-98, it seems the sync would be at a specific number of degrees based on which specific cam was installed. All Chrysler says is "...These distributors are equipped with an internal camshaft position (fuel sync) sensor. This sensor provides fuel injection synchronization and cylinder identification ..." Nowhere can I find that the amount of sync degrees correlates with the amount of degrees related to intake valve opening. Additionally, any aftermarket cam or a different rocker ratio would alter the intake opening point. Add 20-30 degrees of duration to the cam and the intake opening is significantly earlier. I don't see any discussion of increasing fuel sync an amount equally proportional with the additional intake opening. In actual practice, +4 to +6 degrees seems to be the normal setting, regardless of camshaft. Consequently, I am not buying sync is based on valve opening but rather, something else. Could someone explain to me what fuel sync establishes? I have found posted by bigpaul "...Fuel sync is the overall injection start point delay measured in internal units." I don't really understand "start point," in relationship to what? I could accept the notion that the sync is tied to piston TDC but not valve opening. I would really like to understand this. Regards, dan...

RE: Chrysler "fuel sync"

Not Chrysler specific, but:
The ignition timing and fuel injection timing in most new cars is controlled by a computer.
The computer looks at a number of values that may include;
Engine temperature,
Oxygen in the exhaust,
Pre-ignition, (knock sensors)
Mass air flow,
Cam or crank position,
Ambient air temperature,
Throttle position.
The computer determines the optimum timing and then controls the ignition and injection timing.
Note that the computer may not provide ignition nor fuel injection on the until after the first cam timing signal is received.
Some of the first computer controlled diesel engines would make up to two complete rotations before the computer knew for sure where everything was and started fuel injection.
The signal from the cam shaft provides a reference for following ignition/injection cycles.

"Why not the best?"
Jimmy Carter

RE: Chrysler "fuel sync"

If Chrysler can install different cams, then they could also install different firmware settings into the ECU. But there are several other approaches that they could take, such as phasing the cam signal.

If someone later modifies the engine with a different cam and/or rocker arms, then that may call for an ECU software modification to re-sync things, if required.

I don't know for sure, but it's not something that's obviously "BS".

winky smile

RE: Chrysler "fuel sync"

I see your point about a software change for each camshaft, that is a good point. The only other problem is when a significantly larger duration camshaft is installed. When I've searched other peoples builds, and a dyno pcm tune is done, the fuel sync stays in the factory range, typically +4 to +6. This is when the cam has every bit of 30 plus more degrees of duration, half of which are on the opening side of the lobe. I just don't understand how sync wouldn't have to change the proportional amount if the sync is tied to valve opening. Thanks for all responses...

RE: Chrysler "fuel sync"

It's not tied to actual "valve opening" ... for the simple reason that the computer has no way of knowing it.

It may be tied to the leading or trailing edges produced by the flag on the cam position sensor. How that's related to the "valve opening" ... is built into the design of the camshaft.

A common setup, which is likely how this system really works, is that the CRANK position sensor has a ring with many teeth on it which actuate the CRANK position sensor so that the CRANK position is accurately known ... but the CAM position sensor is just one flag, the "accurate" timing of which is not important, as long as the cam position sensor is "on" during (for example) cylinder #1 passing through the version of TDC that is in between the end of the exhaust stroke and the beginning of the intake stroke, and "off" during the version of TDC that is passing through the end of the compression stroke and the beginning of the exhaust stroke. This is because the crank position sensor alone cannot know which "version" of TDC is approaching, being that there are two of them through the four-stroke cycle that both look identical to the crank position sensor.

When the engine first starts turning, the electronics won't immediately know whether a particular cylinder is on the intake or power strokes, until it gets proper signals from both the crank and cam sensors, and only then can it be "in sync".

Dunno how Chrysler in particular does it but I do know that Suzuki EFI motorcycle engines give an initial untimed fuel injection squirt when the system gets a non-zero crank position signal of any sort, then once they establish a TDC they operate in wasted-spark (a spark on every revolution, only half of which actually ignite something at the end of a compression stroke) and a fuel shot on every revolution until they figure out which version of TDC is the right one, then they operate with proper sequential fuel injection and ignition once it is in sync. It also allows the engine to continue to run in wasted-spark mode with a half-shot of fuel on every revolution if the cam position sensor fails.

If the engine has variable valve timing, THEN the cam position signal needs to be accurate with respect to the valve opening time, because then the cam position sensor is not only used for establishing which version of TDC is "intake" or "power", but also for feedback of actual cam timing.

But again, the phase relationship of the cam position sensor to intake valve opening is something built into the camshaft; the electronics have no way of knowing the actual motion of any of the valves.

It may perhaps be of interest that Honda single-cylinder EFI motorcycle engines don't have a cam position sensor. They figure out which version of TDC is the correct one by monitoring the crank sensor and figuring out when the crankshaft is slowing down due to the compression stroke. It again operates in wasted-spark with half-shot of fuel each revolution on initial start-up until the computer figures out what's going on. This doesn't work on multi-cylinder engines.

RE: Chrysler "fuel sync"

Earlier this year, my car's OBD set a 'P0016' code which informed us that the crankshaft to (a?) camshaft phasing had drifted outside the tolerance. So a new timing chain set of parts was installed; thankfully under warranty smile.

I learned from this experience that the ECU is keeping track of such tiny details, which is very nice. It even had something to say about a thermostat being out of tolerance, which was also replaced.

RE: Chrysler "fuel sync"

Hi Waross,

You listed as a possible measured parameter "Pre-ignition, (knock sensors)"

I don't think pre-ignition is the same as "knock" (detonation) although both are aberrant combustion, and when severe can coexist or trigger the other.


Dan T

RE: Chrysler "fuel sync"

The OP seems to be specifically concerned about the timing of the injectors to aftermarket camshafts that have increased lift and duration. These will always be high performance engines that do not need to meet any of the extensive emissions, noise, fuel economy, vibration and driveability requirements that OE engines have to comply with. As such, I submit that a few degrees of injector phasing are completely insignificant. These engines are already injecting more fuel than stoichiometric, a little hanging around in the intake manifold until the next combustion event will make no difference. Remember, a lot of early injected engines used simultaneous double fire, the injection timing was different on every cylinder. It's not ideal for emissions and fuel economy but doesn't really hurt power.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Chrysler "fuel sync"

A lot of early EFI engines simply batch-fired all of the injectors together once per revolution without consideration for the injection timing at all!

In many cases the duty cycle of the injectors at full engine load is considerably longer than the entire intake stroke, and there will be fuel injected against a closed intake valve.

There is always fuel wetting the intake port walls which hangs around from one cycle into the next.

In many cases the injection timing is deliberately against a closed intake valve, because it's better for the (rather hot) intake valve to vaporize at least some portion of the fuel prior to the actual intake stroke than it is to inject fuel as a liquid during the intake stroke.

Bottom line, unless you are concerned with meeting Tier 2 bin 5 EPA emissions, on a port-fuel-injected stoichiometric-operation engine, the injection timing is not important.

RE: Chrysler "fuel sync"

You're talking about an older system so the PCM is likely not that complex (at least the earlier ODBI versions won't be) so it's very likely the injection is just matched to the engine rotation. In other words, the #1 cylinder injector opening point occurs somewhere around the time of the #1 intake stroke. Using a cam signal allows this injection timing to happen at a certain crankshaft angle quite accurately. It also sounds like the distributor allows the adjustment and that's why it's only +/- 8 degrees. I'm not sure why you'd think that using a cam signal to match the injector firing to crank rotation is BS?

On new EFI systems, the crank and cam sensors tell the PCM where the engine is quite accurately. The PCM's are quite powerful and can do a lot of computing. So, it's not a small stretch for the programming to tell the PCM how far before TDC the intake valve opens. Given the right programming, the PCM can calculate when to fire the injectors very accurately.

If you have more adjustment then why don't you try adjusting it? I've read about the injector timing making enough of a difference on LS engines that some tuners will adjust it. It's been a while since I've read anything about it, but I recall the LS engines matching the closing of the injectors to the engine rotation meaning the opening moves forward in time depending on the injector duty cycle.

RE: Chrysler "fuel sync"

Thanks so much for all the responses. I am getting the impression that my original thought that fuel sync is actually tied to piston position rather than specific valve opening is possibly correct. That makes more sense to me but I'd really like a confirmation. I am embarrassed to say that I never thought about the cam sensor providing the reference for compression stroke vs. exhaust stroke, that was enlightening. And the Honda using time to determining power vs. exhaust, that's just too cool. I know OBD2 can detect the stroke time to determine misfire but I never thought about it determining compression. It's obvious once pointed out. Additionally, I know that batch fire has the fuel sitting in the port waiting for the valve to open, which is one of the reasons I could not accept valve timing to be the specific reason for the Chrysler fuel sync spec. +/- 8 degrees is directly tied to something so I guess it has to be piston position. Getting the maximum downward piston velocity to help draw the fuel charge into the cylinder, even with injector pressure, makes the most sense. I would still like to know this is true rather than assume it though. Thanks again for taking the time to respond. Regards, dan...

RE: Chrysler "fuel sync"

Just before the arrival of microprocessors that made the whole exercise moot, there was some activity in mechanically dithered systems that juggled ignition timing and/or carburetor mixture, and used instantaneous crank speed, as measured by a prox sensor on the flywheel ring gear, as a measure of 'goodness' of what were otherwise random adjustments, in order to make the engine optimize its own controls in real time.

There were a couple of detailed SAE papers on the subject, and I think maybe some patents, and some actual products, all associated with Harmon Electronics of Grain Valley, MO, USA.

The resulting systems did work, but suffered from durability problems associated with the mechanical components, as for instance a classical distributor's vacuum advance mechanism would not last long cycling at many tens of Hertz.

Where I'm going is that it's not quite trivial for microprocessors to dynamically adjust and optimize the things they can control electronically, without wearing out, and to infer quite a lot of information from the limited view provided by their sensors, so assertions of modern ECUs being able to adjust themselves to deal with a fairly wide range of engines without reprogramming or any sort of intervention (other than not being confused by driver input shortly after first start) are quite credible.

Mike Halloran
Pembroke Pines, FL, USA

RE: Chrysler "fuel sync"

Closed loop optimisation of AFR, spark timing, injector timing etc is very doable with modern electronic management. Problem is that engines today are tuned for emissions and need to run values that do not necessarily deliver optimal combustion efficiency.

je suis charlie

RE: Chrysler "fuel sync"


I am getting the impression that my original thought that fuel sync is actually tied to piston position rather than specific valve opening is possibly correct.

Does your engine have valve position sensors?

This should be clearly obvious. The PCM can't know about any change in valve timing unless the PCM has sensors for valve motion or the PCM is in control of the valve motion.

The PCM actuated valve designs by the Koenigsegg spin-off company Freevalve would be a case where the PCM knows when the valve is open and closed.

There is nothing wrong with the statement " fuel sync "precisely" establishes when the injectors fire in relationship to intake valve opening". IF you have both rotational angle and valve timing then you can precisely match injector firing to intake valve opening.

RE: Chrysler "fuel sync"

Given sensors for everything that could possibly be measured, it should be possible to write a map showing how outputs should respond to inputs from first principles. ... in theory, anyway.

But it turns out that you don't need sensors for everything.
Simple example; modern ECUs can infer that the fuel tank cap is off or leaky, by watching the behavior of the evaporative emissions system. No 'fuel cap loose sensor' is required.

Beyond that, a modestly fast microprocessor can easily measure the time interval between ring gear teeth using a simple magnetic sensor, and from that can infer the instantaneous crank speed. Using that and the crank and/or cam reference signals, it could compute the position of any given piston with reasonable certainty. Just from the time trend of the ring gear teeth, it can detect a misfire event. ... and it can make adjustments. ... and then it can watch and see how much the adjustments change the ring gear time pattern and in which direction, and make further adjustments.

In other words, a modern ECu can learn, and remember what makes a particular engine run best.

It can also apply an offset of sorts and run closed loop at a condition other than 'best', e.g. for emissions reasons.

Mike Halloran
Pembroke Pines, FL, USA

RE: Chrysler "fuel sync"

Again, thanks for the responses. I should have mentioned earlier that Chrysler v8 injection from 1992-2004 uses a cam sensor located in the distributor to determine compression stroke as explained earlier by another member. Additionally, the crank sensor is tied to the flywheel location. What I have been attempting to understand is the distributor adjustment of what is called "fuel sync." By turning the distributor, like adjusting ignition timing from pre-computer days, and reading on a Snap-on scanner or other tool, the fuel sync is +- 8degrees with +4+6 being the norm "REGARDLESS" of the cam installed, whether stock or aftermarket. All the precision that is being referred to doesn't really exist at that time. Fuel sitting at the valve is not a huge deal as batch injection has 20plus years of functional history, besides carburation also has fuel sitting at the valve waiting for it to open. But when fuel sync is out of spec by 4-5 degrees, the vehicle runs extremely poorly. So I think fuel sync is something different than tying the injector fire point to valve opening. BTW, I don't even know if the +-8 is really degrees, there is contradicting verbiage on that. Some sources indicate it is a range not degrees.

From what has been explained here, I have to conclude that sync is tied to piston position rather than specific valve opening. I think BrianPetersen and LionelHutz have nailed it, inferring to the lack of sophistication at that time. As has been stated earlier, "It's not tied to actual "valve opening" ... for the simple reason that the computer has no way of knowing it." I have to agree. The issue I have is I am looking for an absolute answer as opposed to believing it's piston position based on common sense. Sync could be something else entirely. One Mopar enthusiast insinuated it was tied to fuel quality, meaning burn rate. I don't accept that but it does have a small bit of merit, (I suppose it could change timing by a given amount for the quality of fuel, sort of like an software knock sensor, but that's a far reach.) Unfortunately, I do not KNOW what fuel sync adjusts and I'm hoping to find a Chrysler engineer or someone similar who really knows. If not that, I'm open to additional theory or opinion.

RE: Chrysler "fuel sync"

Which engine, exactly, are you looking at?

The 4.7 (which is OHC) and Hemi (pushrod, but a modern engine design) use distributorless ignition. The 5.2 and 5.9 are holdovers from the old days and used a distributor well into the EFI era although I'm not sure if it remained right to the end of production of that style of engine.

Ignition timing on these engines is based on the crank position. Because the cylinder ignition sequencing is handled by the distributor, the electronics know to fire the coil four times per crank revolution and the distributor takes care of which cylinder the spark goes to. But the ignition getting to the correct spark plug is contingent on the arm inside the distributor actually lining up with that cylinder's terminal inside the distributor. If the distributor is too far out of position, it may simply be that the arm inside the distributor is getting too far away from that terminal, giving weak ignition at best, and potentially it may even be firing the previous or next cylinder in the firing order instead of the one that it's supposed to fire.

The fuel doesn't have anything to do with it; the engine won't much care. The spark getting to the correct cylinder, it sure will care ...

This doesn't affect the engines with distributorless ignition, correct??

RE: Chrysler "fuel sync"

The mass of fuel delivered by an injector is mostly a function of PWM, since the rail pressure is kept constant. The intake valve open duration is a fixed number of crank degrees, but the time will vary with RPM. So the relationship between injection and intake valve open times will vary based on RPM and load.

RE: Chrysler "fuel sync"

We made both distributors and "cam position sensors" for Chrysler 6 cylinders (I think we made some 8 cylinder distributors too but all that stuff is long gone). The cam position sensors plugged into the block where the distributors used to go. They were driven by a gear on the cam and in turn drove the oil pump. The older distributors had the same hall effect sensor built into them. Sounds like the V8's had a similar scheme. It really has nothing to do with fueling, only properly telling the engine what stroke it's on and getting the timing between the crank position sensor and cam position sensor properly synched.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

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