Diagnosing Poor Fuel Economy

[voney]

Hi Guys,

I know this question has been asked before (ive spend the last couple of days reading other peoples threads) but I figure one for me can't hurt.

I have a 1998 VT Calais 3.8 Auto and in the last couple of months it seems to have been running poorly and feels a little gutless. I've just started a new job that requires me to drive my car every day and what was just a nagging doubt about the efficiency of my car has become rock solid conviction.

This last week I've been driving in city traffic between work and home in a combination of free flowing and start stop and my average fuel is around 18l/100Km which is much more than it used to be the last time I drove this car regularly which used to sit arout 14-16L/100Km with similar driving patterns.

While I don't drive like a grandpa, I'd call myself "average footed". Additionally it feels like i need to give the car far more foot to get the same acceleration/power that I used to.

Now here's what Ive observed:
- No Fault codes in the computer
- When accelerating anything more than gently my instant fuel jumps up above 50L, even as high as 80L if I need to get in front (but not planted)
- Anecdotally, the car feels like I've got 3 or 4 other people in it in terms of acceleration.
- The idle is a little rough, but no more so than "usual"

In the last 10000km I've replaced the plugs ad leads as well as gotten new tyres (i keep them inflated at 40psi), I also do a tank of total fuel system cleaner every 5000km or so.

I've heard the O2 sensor could be at fault, is there a way to check this with a multimeter or something?

Cheers,

Voney

 

[ephect]

How long have U had the car? Since u've had it have the o2 sensors been changed? Coil packs would be next on the list to replace IMHO.

 

[Cashman_309]

In this order..

1. Clean your MAF sensor.. search how to do this. Reset trip computer and go for a long drive.

2. Replace your spark plugs.. pretty obvious but if your dubious im sure there are guides. Reset trip computer and go for a long drive.

3. Replace Fuel Filter.. search how to do this. Reset trip computer and go for a long drive.

4. Replace Spark Leads.. Search how to do this. Reset trip computer and go for a long drive.

If you cant remember the last time any of these were done.. do them all, regardless of which one fixes the issue.

If none of these fixes the issue. You may have an injector or O2 sensor problem.. if it comes to that. Start searching the aforementioned parts.

 

[franklinfrog]

Do the free or cheap stuff: Clean MAF and throttle body, replace air filter, fuel filter.
Then probably O2 sensors:
Testing:
http://forums.justcommodores.com.au/vy-holden-commodore-2002-2004/60118-s2-vy-o2-sensors.html

Other reading:
http://forums.justcommodores.com.au/vy-holden-commodore-2002-2004/213951-what-branded-fuel-filter-oil-filter-commodore-vy.html#post2399299
http://forums.justcommodores.com.au/vt-vx-holden-commodore-1997-2002/39411-replacing-o2-sensors-vt.html
http://forums.justcommodores.com.au/vr-vs-holden-commodore-1993-1997/186959-anyone-used-these-o2-sensors.html

Also maybe disconnect and reconnect battery, to reset everything. And never trust trip computer.....actually check properly with a calculator.

 

[voney]

Thanks for the info guys.

ephect: I've had the car about 11months, Ive not done the O2 sensors or the coil packs.

Cashman_309: I've already done the plugs and leads a while back not long after I got the car to bring them into a known state. I have a fuel filter lying around that I could put in that I baught a while back and havn't gotten around to doing. I'll have a look at the MAF sensor thing tonight.

franklinfrog: I have a K&N air filter in there but it could probably use a clean, I did clean the throttle body/plenum a while back but it could probably go another... and my trip computer is pretty much dead on judging by the accuracy of the "fuel used" function and the speedo...

Right so my list for tonight is:
- Clean the MAF
- Clean the TB/Plenum
- Do that damn fuel filter
- Test the O2 sensors as per the link in franklinfrog's post

can anyone else thing of anything else that i can test/check/clean before i start throwing money at the problem? Any other sensors that i can use the ol fluke on?

 

[Cheap6]

I've heard the O2 sensor could be at fault, is there a way to check this with a multimeter or something?

Yes, with an oscilloscope. Assuming that you don't have one, some limited information can be obtained by checking the O2 sensors with a Voltmeter. You should see the output from them alter between ~200mV and ~800mV as you drive along at light-medium engine loads. What you really need to see is how quickly the O2 sensor Voltage swings high to low Voltage and back again. The LED light mixture meters are the cheapest way of seeing that. The sensors have to be connected when you check the voltage, wrt earth. The Voltage should change, rich <-> lean at least a few times per second. If it's slower than that or consistently high or low V, the sensor is suspect.

I don't know the wire colours for the O2 sensors off the top of my head but I'm sure that they have been posted before and there are VT wiring diagrams around too. It may help to know that VT and VS have almost identical (not quite though) PCM terminal assignments and wiring colours.

 

[Pollushon]

O2's should be treated like any consumable IMO. You can't really test them for efficiency only faults. Over time the sensor will degrade and clog reducing its ability to detect gases. One sure sign of efficiency loss is stinky exhaust.

Personally I replace O2's at maximum every 100k.

 

[voney]

Righto. Some more information.

I just cleaned the TB and it was ultra filthy, too filthy considering I cleaned it only 5 or 6 thousand Ks ago. Specifically, it was very dirty on the engine side, the filter side looked fine. The plenum wasn't as dirty as the TB butterfly which is strange since I cleaned them both at the same time last time.

Also the IAC valve was quite dirty/gunked so I gave it a good clean too.

MAF looked fine but I gave it a good squirt with contact cleaner anyway so we'll see about that.

Finally I'm cleaning my K&N filter at the moment, was quite dirty on the outside but still looked clean on the engine side. I'm waiting for it to dry as I write this.

Fingers crossed that this helps. I'll check back later.

 

[SJM38S]

Over oiling the k&n can cause issues when the excess oil builds up on the maf. Just a though.
Oxygen sensors are recommended to be change every 60,000km by Bosch (who invented the oxygen sensor...), so likely they're due.
Inlet manifold gaskets are common, and can gunk things up with a small amount of coolant as well as causing an air leak on the wrong side of the maf. Maybe try tightening the inlet manifold down a little, often does the trick.
Maybe have an auto trans service done (filter and pan oil change should be enough to see a difference).
Lastly, make sure you're still using the correct grade engine oil. Don't go thicker with age unless you're looking for a quick fix for oil consumption while saving for a rebuild

 

[Drawnnite]

just going to put this out there. you can use a multimeter to check if your Oxy sensors are working.
your Jaycar/Bunnings/Ebay-Specials multi's wont pick it up though.
in most cases they are just way to slow with picking up the very short change/spike

how do i know this.
i used it to check mine. wont show everything, but it was fast enough to pick up the occasional changes in voltage.
good for basic fault finding, and giving a rough guide, but wont tell you everything.
its a Fluke Multimeter though if that says anything.