Hi all,

My '98 V90 (153,700 miles and counting) recently experienced a relatively sudden drop in gas milege. I bought it amost three years ago with 74,000 miles, and it has typically gotten about 23-25 mpg in mixed city/hiway driving, and 26-28 on the highway. I use Mobil 1 synthetic oil (since about 100,000 miles), changed the fuel filter at about 100K miles, changed the air filter about every 35,000 miles (not as much dust in the air here in the wet Pacific Northwest), and changed spark plugs at about 100,000 miles. The plugs that came out were a tri-electrode plug, not severely eroded, and the color looked right. At the recommendation of FCP Groton, I put in Bosch copper-core plugs, a standard single-J-electrode plug, and very inexpensive. At the time I thought I could detect a slight reduction in mid-range throttle response with the new plugs, but general cruising and gas mileage didn't change noticably.

In September, my wife drove it to LA and back, and averaged about 25.7 mpg for the whole trip, with individual tanks ranging from about 24.5 to 26.4. That wasn't quite as good as the 27 and 28 mpg readings I have previously recorded, but I figured with almost 150,000 miles on the clock and sustained speeds of 75 to 80 mph, it wasn't bad. In-town mileage stayed at 23-25 when she got back.

We drove to LA again last week, and the week before we left, I saw that the in-town mileage was down to 21 mpg. In preparation for the trip, I had the timing belt, water pump (do it while the timing cover is already off; minimal extra labor), serpentine belt, air cleaner, and spark plugs changed, and changed the oil. My independent mechanic doesn't like Bosch plugs, so we installed an NGK lazer-platinum-iridium plug with a single ground electrode (spendy, about $15 per plug). The car worked well all the way down and back, but mileage for all 6 tanks of freeway driving down and back was 23.2 to 23.5 mpg, regardless of whether 87 or 91 octane was used. The one tank driven around the LA area was 20.0 mpg, the lowest I have ever recorded.

Any suggestions on what might cause this relatively sudden 15% drop in gas mileage? Thanks for any info.

Mike L ('98 V90, 153K; '82 245, 422K)

I agree about vacuum leaks-you may want to carefully spray carb cleaner while the engine is running-a jump in idle speed means you've sprayed near a leak.I have to check that vacuum tree poolman is always preaching about.I like the original tri-elecrode plugs-a full set only cost me about $30 at the dealer.My mileage at 100K+ is still about 20 mpg local,25-27 highway.Premium helps-and prevents chronic pinging and improves performance in these cars.
sredwine V90 has 104K,940T has 167K

Whoa!
I would love to get your kind of mileage.
Is that on regular gas or the recommended premium?
The best I ever got was on a 370 mile RT on interstate highways, no stops, 65 mph steady. Got nearly 23. Typical mixed gentle driving is less than 20 mpg. On regular, but the engine is never worked hard at all.
It has new timing belt and has passed NJ emissions test easily.

S'V'

I'm getting 25.5 at 70 mph and if I behave and limit the car to a maximum of 65 mph I'm getting high 27's-low 28's. It is a 1997 with 72K that runs all synthetic and has been Auto-Rxed. Around town the mileage really goes to the dogs 17 to 18 mpg normally with short trips. I run 89 when limited to local driving and run premium on the long highway trips.

I hate to admit it in this forum, but I run regular (87 octane) most of the time. As I recall, my best mpg readings (28+) were on premium, but I've gotten consistent 26 - 27.5 with regular during freeway driving, and mixed town-hiway driving (we live in the suburbs) has consistently returned 23 - 25. Maybe I shouldn't complain. I do stick with major brands (Shell, Chevron, 76) but I really can't tell any difference with premium. Maybe it would return better performance under heavily loaded/up-hill conditions?

I'll report back after checking vacuum lines and oxy sensors.

Mike L

Mike, excellent description.

I would agree with Poolman and look for vacuum leaks Check every single vacuum hose (especially the pre-molded rubber 90 degree elbows) and that infamous "O" ring that Poolman has found at the intake manifold vacuum tree.

And check the output of that old O2 sensor. They get lazy over time and can really hurt your fuel mileage.

DEWFPO
--
1998 S90 071,245 and 1995 964 154,100

There could be a number of things that could cause the increase in fuel consumption,you stated that you just had a timing belt change. How is the power in the vehicle-is it as strong as before-does it climb hills well.
I'm wondering if your mechanic has the belt dead on time like it should be-I once had an indy put mine on and it was off by a tooth and a half on the exhaust side.
It's not much trouble to take the belt cover off and check these things.
This is the first thing that came to mind-may be okay though, replace your ends on your vacuum hard lines and check for leaks-a vacuum line leaking will cause havoc with fuel mileage. While your at it take the vacuum tree off the manifold,held on with a 10mm bolt,then change the O ring that is there.
I don't know how many miles you have on your O2 sensor but it could possibly have gotten week and start causing you mileage-but with what you have stated I would look at the first two points I brought up first.
Hope that helps
Poolman

Thanks for your reply, Poolman; I've been monitoring this board since I bought this car, and it's clear you have a lot of experience with these cars, and you provide excellent advice. I will check the things you listed, but I expect to find the cam timing to be OK. My guy is pretty good, and the mileage drop showed up for several tankfuls before the timing belt was changed. I had hoped that replacing the air filter and spark plugs would make a difference, but it didn't. My mechanic also said that we'd look for codes implicating the O2 sensor, since the sensor is the original as far as I know -- at least I haven't replaced it since I bought the car at 74K. Also, the spark plugs I took out looked great for color, with no indication of either rich or lean conditions.

Can you elaborate a bit on your suggestion to replace the ends on the vacuum hard lines? Is this trimming the ends of rubber hoses (perhaps hardened with age) and re-plugging them onto hard lines?

Thanks again for your help. I'd like to see if I can make this one go as long as my dear departed '82 245, killed in the line of duty at 422,000 miles (no internal engine work at all), protecting my son from from someone making an oncoming illegal left turn in front of him.

Mike L

Hey Mike-The vacuum fittings on the hard lines is exactly whay I'm pointing at-don't simply snip the ends off and plug them back on-replace the suckers.
Those fittings can look OK and still leak air-if they are the original fittings I'll bet you will tell a difference once they are renewed-If you have the origanal 02 sensor,go ahead and replace the thing , like Dewfpo said they get lazy over time-I probally need to replace mine now for the second time-damn they are expensive-170 buck at the min. Oh yeah, don't forget that o ring in the vacuum tree.
Hope that helps
Poolman

Which O2 sensor are you talking about, aren't there two?

The front one is the normal one to replace-I have replaced the rear one once-but from the indications gathered from the brickboard, the front one does the most on regulating gas mileage.
Hope that helps
Poolman