Tuning for cold air

[immortality]

Hey Guys,

Probably getting a bit ahead of myself here (as it's more of a winter issue) but when I installed the modified CAI on the Calais it effected fuel usage in a negative way when it's reasonably cold outside (ambient temps below 15-17°C).

Fairly basic scenario, doing return trips up/down the motorway. If I do said return trip on the motorway in the afternoon and it's reasonably warm (20°C or above ambient temp) car cruises nicely, engine temps are consistent with thermostat temp ~81°C, fuel usage is as per normal. Now do the exact same trip later in the evening when the ambient temps have dropped (engine running temp is still consistent at ~81°C, checked with dash in diagnostic mode) that fuel usage is actually going up. I base this on instant fuel used readout on the dash at certain parts of the trip when driving at the same speed (cruise control set). It does this consistently when cold and it seems worse the colder it gets.

One possible issue is the engine running temp (although the engine temps is totally consistent regardless of ambient temp) I'm going to experiment with this using a thermostat fitted in the lower radiator hose as per this thread Cooling systems. I will also be fitting a MAT sensor in the LIM so I can monitor actual AIT's rather than just what the air temp in the CAI is.

The question I have is what in the tune could I alter to get around this problem? In theory, with cooler air the engine should be making more power for a giving volume of air.

The car is a VX Calais L67 using The1's Enhanced tune. The lean cruise has been adjusted from the factory settings to 15.4:1 target AFR.

One thing I had though about was to try adding a little more timing when the AIT's drop. There is a table Spark Correction Vs Inlet Air Temp Vs mgc which is normally used to reduce spark timing as AIT's go up.

edit: one a positive note, when it's cold outside and you give it a boot full it's a different car, the exhaust has more of a bark and it gets up and boogies

[Holden202T]

I think you really need more data, logs to see if the timing and AFR's are the same at certain load points .....

I dare say though that you are on the right track, it will be something changing AFR or spark settings due to the colder inlet temps, and it probably never saw that cold before the CAI due to engine bay heat...

I agree though, my turbo cruze is the same, on a cool morning or later in the evening, its way more responsive and feels like its loving the air more! (which makes sense)

[vlad01]

All my stock VPs were much more angry when the engine was stone cold and even more so when the air temps outside were low. I looked at the data and nothing shows up so the conclusion is more dense air.

I noticed in the company car, dulalis, that is suffers badly from poor economy on top of already crap economy during winter/colder weather.

As for my own cars with CAI I can't say I have noticed any difference so its probably in the tune. I did have a similar issue with the thermostat running too cold in winter (90 odd rated, ran at low to mid 70s in winter and was fine in summer), took me a few years to work that one out as it was about 4 or 5 trident? trico? stats in a row that were all faulty and problem was solved when I fitted a genuine one.

[MAGP]

I think its a MAF and air temp sensor issue.

MAFs work on the principle that the air flow cools a heated wire in the MAF and the amount of power required to keep the MAF at a certain temp tells the PCM how much air is flowing into the PCM.
The cooler the ambient air the more power required to keep the MAF heater element up to temp, the more power required to keep the heater element up to temp the more air the PCM thinks the engine is getting. The more air the PCM thinks the engine is getting more fuel is injected as per the corresponding table.

The air temp sensor, in your case probably an IAT (Intake Air Temp) rather that a MAT (Manifold Air Temp), is used to tell the engine how to adjust the fuel requirement to suit the MAF. If your IAT is seeing cold air your PCM will give more fuel, if the IAT is seeing manifold temp, or engine bay temp, the PCM will give less fuel.

Find out what tables adjust the fuel to the MAF and to the air temp and I bet you'll find the culprit to your dilemma.

This is one reason why I prefer air temp sensors in the manifold.

[immortality]

More data would be good, unfortunately have had some issues when logging that all the STFT and LTFT are coming back with very weird numbers and it's an issue I haven't yet resolved The1 did say he'd seen it before with the later models and unfortunately I haven't had a chance to test his latest XDF/ADX to see if it was resolved.

Been MAF equipped I would have though the PCM would adjust for the colder more dense air?

When I modified the CAI it did make a big difference because the engine temps drop down to thermostat temp much quicker once you get some airflow going. Previously it wouldn't run at thermostat temp until you had been cruising on the motorway for a fair while, now it will get down to thermostat temp just cruising around town. Now it's getting to ~81°C where as previously it would always hover around the 85-86°C. I will try running it a bit warmer when I alter the cooling system.

[immortality]

I think its a MAF and air temp sensor issue.

MAFs work on the principle that the air flow cools a heated wire in the MAF and the amount of power required to keep the MAF at a certain temp tells the PCM how much air is flowing into the PCM.
The cooler the ambient air the more power required to keep the MAF heater element up to temp, the more power required to keep the heater element up to temp the more air the PCM thinks the engine is getting. The more air the PCM thinks the engine is getting more fuel is injected as per the corresponding table.

The air temp sensor, in your case probably an IAT (Intake Air Temp) rather that a MAT (Manifold Air Temp), is used to tell the engine how to adjust the fuel requirement to suit the MAF. If your IAT is seeing cold air your PCM will give more fuel, if the IAT is seeing manifold temp, or engine bay temp, the PCM will give less fuel.

Find out what tables adjust the fuel to the MAF and to the air temp and I bet you'll find the culprit to your dilemma.

This is one reason why I prefer air temp sensors in the manifold.

Definitely.

Currently I'm running the standard MAF and AIT sensor fitted in the MACE CAI box. Will be going to a MAT sensor in the LIM. The table that adjusts the spark in relation to air temp is all zero'd out in the lower AIT region and is only used to reduce timing as the AIT's rise.

I have just started pulling it apart to do LIM gaskets so the MAF will get a clean too (just in case) although it looks reasonably clean.

[vlad01]

I'm pretty sure its been discussed before in another thread that the IAT sensor in a MAF system is just used to adjust ignition timing and fueling is entirely done via the MAF and trimming via the O2 sensors

[vlad01]

Have you got a WB installed and verified the AFRs?

[immortality]

I'm pretty sure its been discussed before in another thread that the IAT sensor in a MAF system is just used to adjust ignition timing and fueling is entirely done via the MAF and trimming via the O2 sensors

I think so too. I'm looking at this table

[immortality]

Have you got a WB installed and verified the AFRs?

No, no WB fitted in this car (yet).