Poor fuel economy - The weather, or my repairs?

[ballo]

VW MkV Golf, with a 1.6 BGU engine (VW 8v rocker finger, FI, non turbo).

I recently did some repairs to my EGR and PCV systems. This happened about the same time as my Summer to Winter weather transition, So I'm asking for advice on what might've caused my new efficiency problems.

My highway efficiency has improved - 5.2 vs 6.3 L/100km but my local trips have become much worse.
I Used to get 6.8-7.2 L on one local trip. Hilly, about 10km (6 mi) at 60 kmh (37 mph) Now I'm consistently getting 9.8! (24 mpg)
Even on longer drives this figure doesn't improve much unless I get to 80+kmh roads (50mph)

Now, I'm trying to work out the cause.
A -Temperature: Since the repairs temps have dropped from 20-25°C to 8-15°C. As far as I know we don't get winter blend changes to gas in Australia, only to diesel near the snowfields.
B -Effects of my repair? Crankcase breather was missing a clip, so could vent under pressure instead of flowing to PCV. EGR Valve was sticking open occasionally, causing idle problems. This also caused intermittent loss of vacuum. Replaced with new about the same time.

Tyre pressures are all identical [manuf. spec], I've had no impacts or repairs that'd affect wheel alignment, there's been no mods to the car. Carried weight is identical, and all compared runs were w/o AC and with windows closed. Car starting from cold, Consumption measured by OE cluster display.

I've got a poorly substantiated suspicion my problems are due to poor efficiency at low load. As mentioned, hwy efficiency is good. Steeper hills at 60kph are matching what I remember but maintaining that speed on level ground seems worse ~6.3 vs ~4.5 previously. I'm hesitant to call that data though, as instantaneous fuel economy depends on when you look at the screen.

-	-	Old	-	New
-	L/100km	MPG	L/100km	MPG
Highway	6.3	37	5.2	45
Local	~7	33	9.8	24
Temperature	20-25c	68-77f	8-15c	46-60f
Hills, 60kph	10-15	23-15	same	-
Flat, 60kph	~4.5	~52	~6.3	~37

So, um, that was a long post. Any pointers? Things to check that I missed?


Is there any way to get my short-trip fuel economy back?

[ChopStix]

When I have these pop up mystery problems I normally start by clearing the cars computer by disconnecting the battery for a while. Then I look at maintenance due, like miles since last service, air filter, spark plugs, O2 sensors, etc. Also I run 50 psi in my tires which is 15 over the side wall rating.

But my old van always got worse mileage in winter than summer from the temps, and my winter tires, etc. I was consistently getting 27 - 28 mpg on the highway in the summer. Verses 16 - 20 mpg in the winter depending on the outside temp. The winter engine warm up time was also a contributor to my mileage loss.

These are the obvious things to start with. Then others will likely chime in with more specific suggestions.

[ballo]

Thanks ChopStix,
I have a VW specific OBD interface, and regularly check for codes etc.
I also tried 'Reset Learned Values' to bring fuel trims to default. No change noticed.

I'm due for a timing belt in about 10k, air filter is relatively new, as is the battery. Plugs were replaced recently, but prior to other repairs. O2 sensors test fine using diagnostics.



That sounds like a similar FE loss, how big a temperature difference do you have in your area?



Regarding general FE, I've avoided going over rec. pressure as my previous car (EG Civic) got a bit skittish on windy roads with +10psi. I'm already using the narrowest/lightest spec'd tyres. That said this car is heavier and has wider tyres so I might get away with it.
I was also considering extending the undertray and various aero tweaks, perhaps even a M/T gearbox oil change (car has 200,000km on it) but wanted to get any underlying problems fixed first.

[MetroMPG]

"Winter" and "summer" is all relative.

You're talking about a 40% increase in fuel consumption going 10-12C cooler, and still remaining well above freezing. I can't see temperature being the only culprit in a change that big.

Unless you also get a big change in prevailing winds/speed with the seasonal change, but you're talking mostly about short trips (low speed, I assume).

[ballo]

Yeah, I figured Chop's temperature swing would be much bigger than mine. It's very temperate here. Winds are fairly similar (low) and the suburban roads I drive are fairly sheltered from wind by trees and structures.

Relatively low speeds I guess. I'm mostly on the road when there's little traffic, so I stay pretty close to the 50-60k/30-40mph limits.

----

Did some thinking, perhaps it's the PCV Valve itself. Would that match the load-dependent economy changes?

The oil-cap-off test has no impact on idle, which I think is bad. It's integrated into the engine head cover, so I can't test directly.
Unfortunately it's not listed or noted in the official VAG service manual for my engine, and the VW parts diagram doesn't list it as a part either.

[Stubby79]

What tale are your spark plugs telling you?

[ballo]

Haven't had them out since they were replaced a few months/~2000km ago. They're a pain to get at in this engine, obstructed by the intake manifold.
Injector fuel trims are within spec according to ECU.

Old ones were eroded to the point of occasional misfire, but didnt show any signs of being run too lean or too rich, just lack of maintainance by prior owner. That said, that was before I fixed the egr and pcv systems.

[Stubby79]

If you have a MAP sensor, I'd say you have a vaccum leak or a pressure/vaccum issues caused by messing with the PCV valve.

There's also the possibility your new EGR valve isn't working, killing your partial-load FE. If it was stuck open, you'd still have your idle issues. The valve could be fine but the EGR passes could be clogged with carbon.

And, finally...your oxygen sensor could have simply gone bad around the same time. Or, I suppose, a cracked exhaust manifold, sucking in air. I'd think this would turn up on your fuel trim, though.

And it could be something else entirely! Don't you love diagnosing things?

(I'd put the old EGR valve back on and see what happens. Then try the PCV valve, if there was no change)

[ballo]

Yep it's a MAP sensor'd engine. And diagnosing things is fantastic. Think I've done more of it than actual driving in this car

No messing around with the PCV valve itself yet, rather, the breather tube was blowing itself off under pressure due to a missing clip. That relieved pressure by venting outside the engine I guess.

When I was diagnosing the idling issues caused by the old EGR I thoroughly checked all the vac lines, vac check valves and brake booster. No leaks or cracks there.

Old EGR isn't an option. I had to disconnect the cable when it happened to stick in a closed position just to get the car reasonably driveable past warm-up.
New EGR operates fine, can hear it move in diag mode, and the idling issues are gone.
The pipes are a bit of a labyrinth but I had them off to change the valve. Thin but consistent carbon buildup, carb cleaner did nothing to it but the tubes are unobstructed. (Blow through and flexible brush checked).

[Stubby79]

if your scanner/laptop shows live data, you can see the various readings of sensors, including oxygen sensor voltage and calculated MAP vacuum level, which might help point you in the right direction.