Diagnosing excessive exhaust backpressure

[Reed]

Is there any way to check whether or not a catalytic converter or a muffler is clogged without cutting the item out of the exhaust line and looking through it?

The reason I ask is I have been trying to get my 86 NYer to run decent and get passable gas mileage and I am at my wits end. I now have 14-15 inches of vacuum, steady, at idle, good compression in all the cylinders, a slightly rough idle, but it smooths out as soon as I open the throttle. Acceleration is okay, but I am averaging 12.2-12.3 MPG around town.

I am thinking that the cat might be plugged or my muffler might have rusted out and clogged.

Is it just time to bite the bullet and replace the components?

[guyd_15]

Sure, just disconnect the downpipe from the swingvalve housing. Then none of the exhaust is hooked up. It's been my experience that a converter will flow until it gets hot.
Guy

[PUNKslip]

Does your car have a turbo? Your milage estimates are pretty acurate, 12.2 and 12.3, I wish I could guess my milage as well as you. I would look at your ignition first, plugs, wires, timing, before you cut up your exhaust. Hell, a plug wire can cause your milage to dive, as quick as any cat., or rotted muffler.

John O.

[Reed]

Yep, i have a 86 2.2 with the factory TI turbo. My car came equipped with an onboard mileage computer so while I can drive I can look at either average fuel economy or instant real-time fuel economy.

I am frustrated because these cars ae supposed to get 17 MPG in town and 27 ont he highway, and I have heard reports of much higher figures from drivers.

Cap, rotor, wires, are all new, plugs have been cleaned and re-gapped.

[allmotor]

you take out the Oxygen sensor and install a back pressure guage.

[phantomrt]

^^^^^^^^ agreed. You can easily hack up an old, junk oxygen sensor and attach a vacuum hose to it. Remove the sensor from the exhaust, unplug it, install this makeshift vacuum adapter, hook the vacuum line to the boost guage and go for a drive.

A good flowing exhaust will actually develop the venturi effect and you can get some vacuum readings under heavy throttle.

[mpgmike]

Most exhaust shops have a back-pressure tester. They punch a hole in the pipe, then rev the snot out of it while monitoring presusre. The Meineke was charging $35 for the test, but would waive the fee if you had a repair done.

Mike