1990 V8 Fuel Injection issue

The fuel economy on my car has gone approximately in half. Went from around 22 mpg at 70 mph to around 10 to 11. I replaced the temp sensor, checked
the mass air flow sensor, checked the idle speed motor? and have cleaned the throttle body. Also replaced the air filter all to no avail. No codes in computer. What am I missing??
Many thanks Bill
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Bill Mote wrote:

Winter fuel blends.
May have dirty injectors with poor spray pattern. Try some Techron injector cleaner.
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I run Techron about every 10th tank or so. I should add that this issue occurred very suddenly, and is also intermittent. I can be driving at 70 mph show 12 - 14 mpg on the level and then suddenly the mileage jumps up to 21 -22 mpg. Overall average for the past 200 miles has been in the 11 - 12 mpg range, prior to that it was always in the 21 - 24 mpg avg. for freeway speeds.

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The T44 series had a problem with leaks in the steel gas lines that run under the car on the passenger side. The problem is that the plastic clips that hold it to the underside of the car hold moisture and cause rust.
If you smell fuel when walking by the right side of the car in a closed area with no wind, that may be something to check.
Does the V8 have CIS or fully electronic injection. I am more familiar with the CIS from the I5s.
I seem to remember that the V8 has an electronic multi-function plug somewhere on the top of the engine, possibly in the front. When these get wet or corroded they cause a number of problems.
Is the engine temperature running at the normal level?
Bill Mote wrote:

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The return and supply line are in good condition with no signs of rust. The V8 has full electronic injection - Motronic. I will look for the multifunction plug and check for corrosion. Engine temp both oil and water are right on normal. Other than the injection system running full rich, all seems normal. "TonyJ" <"tonyjnospam at nospam visi.com"> wrote in message

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I talked with a friend who has a lot of recent experience with these and his thoughts are (in this order of importance):
- O2 sensor is covered with carbon due to running rich. The only reason that the O2 would throw an error code is if its internal heating element is bad.
- Normal tune-up issues - Caps, rotors, wires and plugs
- Log the fuel pressure over a low and high mileage cycle to find if the fuel pressure regulator is on the way out.
Me. I bet on the O2 sensor.
Let us know what you find.
Bill Mote wrote:

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Ordered the O2 sensor today, should be in tomorrow. Thanks for the help. will keep you posted "TonyJ" <"tonyjnospam at nospam visi.com"> wrote in message

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Installed new O2 sensor and the situation improved. Checked the fuel pressure and found at all times the pressure was greater than the specification. Have new regulator on order.

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Good to hear its improving. The regulator might be an issue too but the O2 was probably most of the problem.
Let us know the result.
Bill Mote wrote:

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Finally got around to installing the new regulator. Fuel economy @ 70 mph is now around 24 mpg. You are correct, most of the issue was the O2 sensor. Changing out the regulator helped to the tune of about 2 mpg. Thanks for all of your help and suggestions. Bill "TonyJ" <"tonyjnospam at nospam visi.com"> wrote in message

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Good to hear that you got it sorted.
Tony
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