1997 850 Wagon NA Still getting crappy MPG

[dairyqueen814]

Book idle is 840 rpm.

So if YOU were 150rpms high on your idle what would you do? When I did my PCV system I didn't clean my IACV. But if that were dirty wouldn't that make me idle lower or stall? Or is this just something I should leave alone?
If my math and reasoning are correct:
150 extra idle revs is almost 18% higher than average. That means the engine is consuming 18% more fuel sitting in traffic than it should be.
Would a high idle effect the rest of my gearing? I'm usually at about 2500 rpms at about 60-63 mph

[xHeart]

Try this simple fix if you haven't already https://www.matthewsvolvosite.com/airbox-thermostat-failure.html

... and make sure this vacuum line is connected... here are two photos...
upper nipple at this cover...

...to the connection at base near fresh air intake.

[rspi]

How are you calculating the rpm? The needle on the cluster can be off. Check it with a OBD2 tool. The ECU is suppose to control the idle.

[dairyqueen814]

Try this simple fix if you haven't already https://www.matthewsvolvosite.com/airbox-thermostat-failure.html

... and make sure this vacuum line is connected... here are two photos...
upper nipple at this cover...

...to the connection at base near fresh air intake.

Yes ive already removed my pre heat hose and thermostat and sealed off the hole. the hard plastic line is cracked a bit on mine but it was my understanding that the only purpose it served was to activate the air flap

[dairyqueen814]

How are you calculating the rpm? The needle on the cluster can be off. Check it with a OBD2 tool. The ECU is suppose to control the idle.

I'll check with my OBDII tomorrow when I'm home

[dairyqueen814]

Try this simple fix if you haven't already https://www.matthewsvolvosite.com/airbox-thermostat-failure.html

... and make sure this vacuum line is connected... here are two photos...
upper nipple at this cover...

...to the connection at base near fresh air intake.

Also one question has been plaguing me since I first popped the hood on this car.
The two vacuum nipples on the back of the air box.... Which one is supposed to go to the vacuum tree and which is supposed to go down to control the flap? I've people say both ways.. or does it not matter?

[xHeart]

top goes to the airbox base, nipple next to the fresh air intake.
bottom travels to the vac tree.
check for tear then seat/plug the elbows gently and properly.
that is your 18% with 850rpm!

airbox - upper vac line connection

MVS_3938x.jpg (452.14 KiB) Viewed 1023 times

airbox - lower vac line connection

MVS_0252.jpg (210.46 KiB) Viewed 1023 times

[dairyqueen814]

top goes to the airbox base, nipple next to the fresh air intake.
bottom travels to the vac tree.
check for tear then seat/plug the elbows gently and properly.
that is your 18% with 850rpm!

well I checked with my scanner and I'm right at 840 for my idle. I replaced the top vacuum line with a rubber hose just because it looked a little cracked. We will see

on another update..... I won't know until I fill up again but now it almost looks like I'm getting worse mileage since my T-stat change...

[dairyqueen814]

ALSO I bought my car with an after market CAT and Muffler... is there a chance that it didnt include O2 sensors? or would there be a code thrown? how would I go about checking it?

[erikv11]

Without O2 sensors a code would throw.

You can see the front one very clearly through the engine bay, look down from near the brake fluid reservoir it will be sticking out of the side of the downpipe. The rear one you have to pike your head under the car.

It is possible the aftermarket exhaust was accompanied by aftermarket O2 sensors. For the rear that doesn't really matter as long as it doesn't throw codes, but for the front it should be Bosch.