Hot start problems - megasquirt

seight · Post by **seight** » Tue Jul 07, 2015 5:22 pm

I've got some logs and yes it does go a little lean when it eventually starts hot. It also oscillates quite badly for a while as the engine keeps trying to die. This is after ASE has stopped.

RPM oscillates between 550 and 1000 with AFR 14.5.
Followed by a period of AFR oscillating between 14 and 16.5.

When it does start to even out AFR is still around 14.5 at idle, CLT in the 80's and IAT in high 40's. With that Gair thing down to 91%.

Looks like MAT correction and/or sensor moving will help then.

Mike

edit to add: I'm starting to 'get' this stuff a little now, interesting info from TS man Phil Tobin:
http://www.msextra.com/forums/viewtopic ... 22&t=45275

SuperV8 · Post by **SuperV8** » Wed Jul 08, 2015 10:00 am

Looking at my logs I don't have GAIR listed anywhere I can see.
I'm using the latest MSII code 3.3.3

From an old megamanual:
Gair = airCorrection = aircor = This is the percentage correction to the fuelling pulse width used to adjust the pulse width calculated from the fuelling equation based on intake air temperature (IAT) density correction and the ideal gas law.

I can only find labels for 'Fuel:Air cor' in my logs. Fuel:Air cor It goes from a max of 100 to lowest of 98.5, the higher the MAT the lower the Fuel:Air cor.
I don't know how this differs to GAIR but I think I would be tempted to update it to the latest code.

Does your GAIR look anything like this MAT density correction?

Tom.

stevieturbo · Post by **stevieturbo** » Wed Jul 08, 2015 10:16 am

If you use a table with those values anywhere where the sensor gets any heat soak at all, the corrections will leave the fueling miles off.

As said before, they may be great in theory, reality is different

If the sensor is placed away fro heat sources then it would be fine.

Spongo · Post by **Spongo** » Wed Jul 08, 2015 10:42 am

Isn't that air temp correction used on boosted engines as the air temp will have a huge effect on the actual volume of usable air going into the engine therefore if its at a high temp then the fuel needs to be reduced as well to stop it running rich?

Not something that would really need to be used on a NA engine is it?

As an aside I have just had mine running whilst diagnosing another issue and the air temp was up to 65c (sat in garage idling) and it had no effect on the fuelling with an AFR of 14.5 at tickover and thats on an SBC.

stevieturbo · Post by **stevieturbo** » Wed Jul 08, 2015 10:48 am

Spongo wrote:Isn't that air temp correction used on boosted engines as the air temp will have a huge effect on the actual volume of usable air going into the engine therefore if its at a high temp then the fuel needs to be reduced as well to stop it running rich?

Not something that would really need to be used on a NA engine is it?

As an aside I have just had mine running whilst diagnosing another issue and the air temp was up to 65c (sat in garage idling) and it had no effect on the fuelling with an AFR of 14.5 at tickover and thats on an SBC.

That table is as it says, based on gas laws. It should apply correctly throughout on any engine where there is an accurate air temp measurement, and in order to maintain the same fueling.

But as said, the reality is most engines will not want this, and nor would you want mixtures leaning out as air temps gets very high.. In theory it should help to maintain a consistent AFR, I've never found that to be the case.

And then with the large negative fuel corrections with hot air, when you get heat soak mixtures end up being too lean

SuperV8 · Post by **SuperV8** » Wed Jul 08, 2015 11:42 am

It should be an easy check to just change to a flat line the MAT temp correction (so there is no correction) after its warmed up and try a hot start to see if that's causing the issue.

Tom.

DaveEFI · Post by **DaveEFI** » Wed Jul 08, 2015 11:52 am

I did my tune starting form the base that comes when you load fresh code. The ATS correction with that seemed just fine under all running conditions, summer and winter. But what it didn't cope with was high under bonnet temperatures which occurred when parked up with a hot engine - hence heat soak sounding right.

My guess is what the ATS reports are being the air temperature isn't actually correct when the engine is started, so it is correcting for an air temperature which is actually different from what it thinks.

seight · Post by **seight** » Wed Jul 08, 2015 8:12 pm

So it looks like later firmware has done away with the two settings I have:

1. a single % measure for how harsh it applies perfect gas law

and

2. a table based air temp correction

Which makes sense i guess. My temp corr table is all zeroes so I am just using the gas law %. I assume that has its own table which is probably setup like that in Toms screenshot.

The corrections in Toms graph definitely seem to match the Gair figures I logged eg. 91 @50c.

Just to confuse things (i.e. me) my cranking pulse adjustment is all default (i think) and scales down to 101% by 71c coolant temp. I hope that doesn't need changing too or i'll probably go mad.

Mike

SuperV8 · Post by **SuperV8** » Thu Jul 09, 2015 9:43 am

Isn't that air temp correction used on boosted engines as the air temp will have a huge effect on the actual volume of usable air going into the engine therefore if its at a high temp then the fuel needs to be reduced as well to stop it running rich?

Not something that would really need to be used on a NA engine is it?

Any MAP based ecu will use a table like this to calculate the air mass entering the engine. Uses Air pressure and temperature to calculate air mass. Obviously depends on having an accurate air temperature reading and not an inflated heat soaked reading.

seight · Post by **seight** » Sat Jul 11, 2015 2:40 pm

Update .....

Reproduced the hotstart problem

Altered MAT Correction table to add a few % around 50c -> started

Now I just need to work out what else to do with the MAT Correction table

Mike

DaveEFI · Post by **DaveEFI** » Mon Jul 13, 2015 9:22 am

Think you can assume in the UK the only time you'll get very high IAT is with the engine stopped. When moving, it will be much closer to ambient. So really, all you're doing is correcting for that one specific instance.