Awful Fuel Economy

[taninecz]

Howdy everyone.

First, apologies for any rules I might break. I am new here, and about at my wits end with my 9-5 Aero, so I wanted to rattle this off on a break from work. I did try a few searches for similar issues, for what it is worth. Nothing seemed to exactly match my problem.

A few months ago I purchased a 9-5 Aero that I absolutely love(d). Hugely practical wagon, great power, beautiful car, etc etc. Feels like you know something other drivers don't.

However, the car doesn't seem to love me back. It has needed a heap of work, but more worrying to me is the fuel economy. I'm in Seattle, so obviously it isn't going to push upper 20's crawling around the city, but I *regularly* see 14-16 MPG for trips around town. Even coming back from the mountains on the highway (all downhill) I barely pushed 24.5. I don't use Sport Mode at all anymore, and the car has:
1. a freshly cleaned MAF sensor.
2. the tranny fluid is apparently fine.
3. thermostat and coolant sensor are new.
4. new alternator and brand new top of the line battery.
5. New tires that are properly inflated.

I am pretty shocked at these SUV-like numbers, and don't really see it as financially feasible to keep putting premium into the vehicle if this can't be improved. Honestly I am getting passed by garbage trucks and still seeing these numbers, which is astonishing. I don't even wind it out anymore as it almost feels like you can see the gas gauge dropping in real time.

Any pointers? Could the SID possibly be totally off?

Edit:
I should add the car is right at 100,000 miles, and is an auto. I've also tried to use an OBDII scanner to watch the fuel trim. It seems ok, though I don't really know what I am looking for.
I had the thermo and sensor replaced today, and am hoping this will fix the issue.

[swisssaabist]

Welcome here

[taninecz]

Just brought it back from the shop, 7 miles in mixed driving. Drove "normally", no hard pulls but not sitting in the right lane either.

Averaged 15.7mpg according to the SID, which I cleared before the short drive.

Obviously small sample, and I'll want to track it more.

[Digger]

Just brought it back from the shop, 7 miles in mixed driving. Drove "normally", no hard pulls but not sitting in the right lane either.

Averaged 15.7mpg according to the SID, which I cleared before the short drive.

Obviously small sample, and I'll want to track it more.

Ignore the SID mileage for now and figure it the old fashion way with miles between fills. When you know you have a mileage figure you can trust let us know which engine you have and we can maybe give you some good help.

[taninecz]

Apologies. I thought the 05 Aero only had the 2.3T engine in it. That is what I have. I'll plan on tracking the mileage myself for this tank of gas, but I was just curious if other people had had erroneous SID mileages show up before.

Thanks.

[GeoffR]

Some areas to look at:
The vacuum system, there are a number of vacuum hoses around the engine bay and, unless they have been replaced, they will have hardened and cracked by now. Replace all of them with 3mm silicon, not forgetting the short length from the manifold to the fuel pressure regulator. A 3M (10 foot) length will do the lot.
A leaking Compressor bypass valve (incorrectly called a dump valve on forums) the diaphragm splits after a while but I have never had this on a 9-5, doesn't mean it can't happen.
Fuel pressure regulator, though the vacuum hose to it is more likely
Leaks in the induction system, look for splits in the pipes and at the terminations.

Obviously don't know where in Seattle you live but "enjoying" the Aero whilst climbing hills will ensure a steady thirst for fuel and Seattle isn't short of hills. After a while with the Aero you won't notice most of the hills. Seattle isn't short of traffic either and that doesn't help.
I doubt the temp sensor or thermostat are making any difference but they are easily and cheaply changed.

[SteveJ]

You mentioned MAF what about throttle body cleaning, new spark plugs (verify correct plugs)?
Pull each plug and label it for future reference. Look for any dark plugs (cyl 3,4 could be loose head bolts)
Random missfires don't always show upon obd2 if they are far enough apart in time,

[GeoffR]

You mentioned MAF what about throttle body cleaning, new spark plugs (verify correct plugs)?
Pull each plug and label it for future reference. Look for any dark plugs (cyl 3,4 could be loose head bolts)
Random missfires don't always show upon obd2 if they are far enough apart in time,

Steve, after driving something over 200,000 miles in Saabs powered by the 2.3 engine my experience is that vacuum leaks are:
1. Inevitable with original Saab vacuum hoses
2. Always associated with poor fuel consumption
3. A very easy fix needing only a sharp knife
4. Cheap to fix

Spark plug problems in my experience almost always manifest themselves as poor performance before the deteriorating fuel consumption becomes apparent. Not that they can't cause poor fuel consumption but simply that I would expect the poor performance to be the major symptom.

Yes check that the correct plugs are fitted and torqued correctly, loose plugs will cause a loss of performance.

Throttle body coking doesn't actually have much effect on fuel consumption, it tends to be manifested in erratic idle and, again, loss of performance. However, cleaning is easy and doesn't require removal so it is worth doing, although the 9-5 suffers far less from this problem than did the 9000.

If the Fuel pressure regulator isn't getting the manifold pressure through the short vacuum hose it will always be delivering excessive fuel, as the vacuum hoses are a known weakness of this engine I would expect that replacing them would be beneficial irrespective of other considerations.

[yazdo]

I'm on mobile: I agree with the person who mentioned the vacuum system.

I have a 2002 9-5 Aero. My issue was the vacuum system and, after thinking I needed another throttle body, I pulled the old one off. Half of the gasket was just gone.

I replaced the gasket and my fuel economy was saved.

So respiration from a gasket could be another thing.

Sent from my Nexus 6P using Tapatalk

[Saab 9-5]

Any pointers? Could the SID possibly be totally off?

They are always off in some way and never gives the accurate consumption, never. They always gives you lower consumption than the actual. Fill her up, reset the trip and fill her up again after say half tank. Gas poured in divided with the driven miles and you have the exact consumption. Average on the highway with a Saab 9-5 OG auto is 9,0 liters/100 km (26 mpg) with moderate speed and never over 100 km/hour and no aggressive driving at all. With really long driving well below the speed limit on open roads you can get to 8,0/100 km. An this is all actual consumption and no sid readings.

That's for me that have three OG 9-5 and all auto. And there is big difference in the consumption if the car has a manual gearbox or auto. The manual is far better in that way.