Aftermarket Ecu warm up enrichment

[ChimpOnGas]

So if the purpose of a priming pulse is purely and exclusively only to purge the rail of hot gaseous fuel as suggested Simon, can someone tell me why every ECU has a temperature referenced taper that starts from stone cold coolant temps?

Cold fuel doesn't boil!

So irrespective of if the priming pulse occurs on 'Key On', or once the ECU sees a cranking signal, I'd love to know why a temperature referenced priming pulse taper is unilaterally used by all? Surely if priming pulse is purely and exclusively only to purge the rail of hot gaseous fuel as suggested, you would only need the option to make it happen at the kind of temps where the fuel is at risk of boiling???

I'm almost certainly missing something here, and I'm always happy to be educated when the lesson makes sense and is supported by fact, but for the above reasons and as it stands logic dictates the 'fuel rail purge only' theory simply doesn't stack up. Even if it's there just to clear air bubbles in cooler fuel why would the feature be configurable by temperature?

Forget weather it happens on 'Key On' or during cranking for a minute, as yet I can still see no logical or reasonable evidenced supported argument to convince me to accept the theory that priming pulse is purely and exclusively only there to purge the rail.

[spitfire4v8]

The initial large dose of fuel squirted from the cranking or key-on prime is there to start the engine.
Most injectors are nose-down, so by their nature they are to a large extent self-bleeding if , say, they do get very hot to the point of gaseous fuel in them / or air in them after a dry build .. the fuel vapour or air rises to the fuel rail where it is purged by the fuel pump initial run at key-on.

The issue I had was with the assertion that all ecus had an injection squirt BEFORE cranking, which is very much not true.

[Zener]

Of course you will get a certain amount of wall wetting Dave if the inj's have any kind of duration before the engine is running but in MS that function is best left to "Cranking Pulse" thats just my opinion maybe with your Canems that is the only way to Wall-Wet the runners personally I have never felt to or need to stray away from the default I can fire up no issue hot cold whatever, kind of interesting though dont recall this ever being discussed previously

[Belle427]

Megasquirts own explanation on the priming pulse page states that its used to wet the intake walls to provide some initial starting fuel.
Seems pretty clear cut to me even if you don't agree with it.
Anyway just for sts and giggles I gave mine 60 ms of priming pulse last night and it started on the button, within a second compared to cranking for 3 seconds beforehand.
Maybe tweaking the cranking pulse could also acheive this too but I've not tried as yet.

Probably a debate that wii go on forever.

[Dougal9887]

In MS, if you turn all the priming points to zero, it switches of the pump priming also.....

[Zener]

Dougal9887 said:

In MS, if you turn all the priming points to zero, it switches of the pump priming also.....

It does there is warnings about this on the MS forums IIRC

[spitfire4v8]

Can anyone suggest why injecting fuel when the engine isn't turning is a good idea?

I'm considering situations such as you're fitting a new stereo and repeatedly switching the ignition on and off whilst testing it out, you're also injecting fuel into the inlet every time ..

No major manufacturer to my knowledge injects fuel with the engine stopped .. if they did then they would open themselves up to all sorts of bore wash claims because people switch their ignitions on and off for all sorts of reasons with the engine stopped (in my focus for example you can't use the electric windows unless the ignition is on ..)

we won't even go over the issues Dave had with his ecu resetting over and over and constantly filling his intake with fuel

Surely injecting at first instance of crank turning is the only logical scenario ...

discuss.

[Dougal9887]

This has obviously been an ongoing conundrum.
Motorsport Electronics give the option and an explanation for it:

[Penelope Stopit]

spitfire4v8 said:

Can anyone suggest why injecting fuel when the engine isn't turning is a good idea?

I'm considering situations such as you're fitting a new stereo and repeatedly switching the ignition on and off whilst testing it out, you're also injecting fuel into the inlet every time ..

No major manufacturer to my knowledge injects fuel with the engine stopped .. if they did then they would open themselves up to all sorts of bore wash claims because people switch their ignitions on and off for all sorts of reasons with the engine stopped (in my focus for example you can't use the electric windows unless the ignition is on ..)

we won't even go over the issues Dave had with his ecu resetting over and over and constantly filling his intake with fuel

Surely injecting at first instance of crank turning is the only logical scenario ...

discuss.

I'm with you on this

Have never come across an ECU fitted to a mass produced vehicle that injects fuel when the ignition is switched on

Injecting before cranking makes no sense to me

[Zener]

spitfire4v8 said:

Can anyone suggest why injecting fuel when the engine isn't turning is a good idea?

I'm considering situations such as you're fitting a new stereo and repeatedly switching the ignition on and off whilst testing it out, you're also injecting fuel into the inlet every time ..

No major manufacturer to my knowledge injects fuel with the engine stopped .. if they did then they would open themselves up to all sorts of bore wash claims because people switch their ignitions on and off for all sorts of reasons with the engine stopped (in my focus for example you can't use the electric windows unless the ignition is on ..)

we won't even go over the issues Dave had with his ecu resetting over and over and constantly filling his intake with fuel

Surely injecting at first instance of crank turning is the only logical scenario ...

discuss.

No discussion from me I agree ^ cant see why you need to purge closed/shut inj's and a pressurised fuel rail keeps the fuel percolation gremlin away

[Belle427]

An interesting read.
http://www.vintagemodelairplane.com/pages/Snippets...

[ChimpOnGas]

What I'd love to know why they call it priming pulse

And if I'm to believe what others are saying, there's priming fuel you introduce when cranking, then there's cranking fuel you introduce when,... errrr.... cranking.

That's cranking fuel and cranking fuel then

As the name suggests I'm going with priming pulse being fuel used to prime the engine just as we did with a carb accelerator pump by stabbing the throttle, and cranking fuel is, again as the name suggests, fuel we introduced under cranking.

Why else would the two features be given different names?

Finally irrespective of weather it happens before or during the crank, priming pulse can't just be to purge the rail as suggested.... or the ECU software designers wouldn't universally go to the trouble of giving you a temp referenced declining table.

[Dougal9887]

I'm very pleased that this thread has come up since I have so far completely ignored the priming pulse settings and relied on the default MS curve - 5ms @-40c(!) and tapering off.
The engine starts nicely on the current cranking and ASE settings, BUT, always takes 2 or 3 secs of cranking before firing.
As soon as I get some time, I will try experimenting with some priming pulse settings to see if a quicker start can be achieved.
I expect it can and that is exactly what the priming pulse and wall wetting are there to achieve.
In MS, for sure, the priming pulse occurs at ignition on.

Edited by Dougal9887 on Tuesday 7th July 21:44

[ChimpOnGas]

Dougal9887 said:

I'm very pleased that this thread has come up since I have so far completely ignored the priming pulse settings and relied on the default MS curve - 5ms @-40c(!) and tapering off.
The engine starts nicely on the current cranking and ASE settings, BUT, always takes 2 or 3 secs of cranking before firing.
As soon as I get some time, I will try experimenting with some priming pulse settings to see if a quicker start can be achieved.
I expect it can and that is exactly what the priming pulse and wall wetting are there to achieve.

Also try squirting a small amount of fuel in the plenum before the crank, the results will be less cranking time before the start, it works even better if you let that fuel sit for 30 seconds and evaporate a bit as liquid petrol doesn't actually burn that well, but petrol vapor is extremely volatile. Without that pre-cranking prime of fuel it will always take a second or so of cranking to bring the mixture to the optimal combustible state, you could increase the amount of fuel injected when cranking but the balance point between too little and too much fuel is a very fine one indeed, applying a prime of fuel before the crank is far more forgiving and very effective indeed.

Back in the 1990's the old boys who taught me carb tuning used to laugh at fuel injection cranking times, their idea of a good clean start was to barely hear the starter motor at all, which they proudly demonstrated many times. Things are way better with modern cars, but 1990's injection cars always had very long cranking times compared to what these old mechanics could achieve on a carb and distributor, the right amount of air and fuel ignited by a strong spark at the right time barely needs any help from a starter motor at all to bring an engine to life.

The best car I ever worked on for what I called the magic start was a 1936 Talbot (of London) 105, retard the ignition on the steering wheel, apply some choke, then barely touch that starter button and she was instantly alive with none of this churning and churning nonsense. The reason for these magic starts was a good spark from the Bosch dual magneto and an ancient Zenith carb that was for ever puking fuel down the inlet manifold, even when the car was sat not running.

It wasn't especially sophisticated way to apply a priming pulse, but that leaky old Zenith did deliver a super effective priming pulse all the same

[Classic Chim]

Ive been reading this wondering if it’s actually important then realised all this wetting inlet walls etc must be like tickling carbs on bikes back in the day, bit freaking hit or miss if you ask me

I can’t help agree with the idea fine spraying fuel into an engine each time ignition goes on regardless of being cranked seems mad.

Surely this priming or whatever it’s called can be done the moment cranking starts, would it make any difference if this fine spray of fuel is injected before or as engine starts to turn. I’d have thought the introduction of air moving as it turns is a help not a hindrance. V8 are well known the world over for flooding easily on a cold day. My Jag was a nightmare if you started it cold then turned it off to soon so the idea it chucks even more fuel in every time you crank it would be a worry. What about the washed bores which is exactly what I’m talking about here!

[Zener]

You may also remember Dave if we are talking fixed choke carbs Webers etc that you definitely did not stab/hit the throttle a few times on a hot start otherwise you would flood the motor , this would explain this at least screen shot a warm/hot motor wont want a gob of fuel hence the much lower figures , which are pretty much what I have used for years out of interest pretty much out of interest

Edited by Zener on Tuesday 7th July 23:18

[Belle427]

Dougal9887 said:

I'm very pleased that this thread has come up since I have so far completely ignored the priming pulse settings and relied on the default MS curve - 5ms @-40c(!) and tapering off.
The engine starts nicely on the current cranking and ASE settings, BUT, always takes 2 or 3 secs of cranking before firing.
As soon as I get some time, I will try experimenting with some priming pulse settings to see if a quicker start can be achieved.
I expect it can and that is exactly what the priming pulse and wall wetting are there to achieve.
In MS, for sure, the priming pulse occurs at ignition on.

Edited by Dougal9887 on Tuesday 7th July 21:44

I've tried 60ms and can confirm it starts almost instantly.
Ive not tried the cranking pulse only yet as mine is already on 350% and I'm not sure how that relates to time.
I've seen figures as high as 450% on some other people's figures.

[ChimpOnGas]

Zener said:

The priming pulse is to evacuate the injector of any air (purc bubbles etc) not to wall wet, on the MS for every time you cycle the ign ON/OFF (no cranking) along with the F.P this function comes into play so using big figures (just not necessary) gives potential to hydro lock the engine unlikely but possible all the same, you want only want fuel inj during cranking and this is not the function of Priming Pulse

Definitely not just to purge the rail

With regard to this specific point it doesn't matter a jot if you're applying priming pulse to prime the engine on 'Key On' or during cranking.... which being frank is just cranking fuel (and as we all know there's a separate feature for that), if priming pulse really was just there to purge the rail you wouldn't need a declining temp referenced table to calibrate it.

Lets be sensible here, purging the fuel rail by opening the injectors would be a very wasteful strategy, it would also be very poor for emissions and very damaging to both lambda sensor a catalyst life. For all these reasons I guarantee no OEM engine management system would adopt such a strategy, as we know traditional feed and return systems have for a long time now been replaced with the now almost universally adopted returnless system, which in theory should suffer even worse fuel boiling issues.

Toyota adopted the returnless arrangement in the late 1990s with Honda following suit a year or so later, despite no longer benefiting from the fuel tank heat sink effect of the traditional feed and return setup I'd put money on Toyota and Honda still having nothing to do with the wasteful and damaging strategy of opening the injectors to purge the fuel rail of bubbles. Irrespective of when you apply it.... priming pulse is there to prime the engine, if it was to purge the rail it would for sure be called purge pulse.

As was proved pretty much right from the start of internal combustion engine development, an engine will definitely burst into life much more readily and with far less cranking if you introduce some fuel before cranking. Whether you're stabbing the throttle on your DCOE Webers or tickling the Amal on your old Triumph Bonnie the practice of giving the engine a sniff of pre-cranking fuel is as old as the hills, and was only adopted because it worked. If you then put that engine on injection the exact same rules apply and it'll still thank you for a prime (lets call it priming pulse) of fuel..... because at the end of the day its very much still an internal combustion engine.

[spitfire4v8]

There's only one reason I can think of why you would want to have the engine fire instantly and that's to save a precious couple of revs at starter motor speed for the camshaft. In every other instance who really cares if the engine takes one turn or 5 turns to start?

But I can see many downsides (a couple I have mentioned above) for having a pre-crank priming pulse which is why no manufacturer does it (to my knowledge anyway) - there literally is no real-world reason I can think of why you would want to inject fuel into a stationary engine.

If anyone can suggest why starting an engine on the first crank turn is important, at the expense of the many engine-damaging potential downsides, I'd love to know.

[ChimpOnGas]

spitfire4v8 said:

If anyone can suggest why starting an engine on the first crank turn is important, at the expense of the many engine-damaging potential downsides, I'd love to know.

• Because its more pleasing

• Because it adds refinement

• Because its what many of us always saw as a success when setting up a carb/dizzy

• Because its what engine management designers have strived for years

• Because its what they've now achieved

• Because it's what we've all become used to with our modern daily driver cars

• Because the truth is in the real world, it's damaging nothing

Back in the day it was common to define a car that burst into life without excessive cranking as 'Turn Key', other statements such as 'She Starts On The Button' were also used, our American cousins often say things like "She Starts Right Up"

For what ever reason.... True petrol heads do seem to enjoy engines that are 'Turn Key' and 'Start On The Button', well I know I always have