Fuel Gremlins???

92 OCC Following has been done:New wires,plugs,fuel filter,temp sensor,pcv valves,air filter,one injector,cat and muffler,resonator delete....Sounds like good maintenance right? Problem is I only get 8.5-11.5 miles to the gallon! Idle runs just a bit high at times....Any thoughts?

Do your injectors have a good spray pattern?

Have you checked for codes? The mpg would indicate that it is not entering closed loop

It might have a vacuum leak. Check the system by squirting some water, with a couple drops of dishwashing liquid in it, on the top of the engine while running. Any leak will suck the water into it. Hoses are the main culprit, with loose components next. It is best to check it when it is acting up at idle. The port for the PCV in the TBI may be clogged as well. You can clean it out with a paper clip, and some carb cleaner.
Did you clean the ground at the thermostat housing? You have to remove the stud, and clean it and the threaded hole too. Do it while the engine is cold so that you do not have coolant all over the engine. I used a brass gun barrel cleaning brush, and some PB blaster to clean the threaded hole, and the stud.
The ground is for the computer, and the O2 sensor, so it can affect the mixture.

Spray pattern seems fine...nothing to compare it to...

Changed the O2?

Have not...

Egr stuck open???

Bill

If the EGR was stuck open, the engine would not run.

Fred Kiehl wrote:

If the EGR was stuck open, the engine would not run.

Should have said stuck "partially" open. Had a TBI react the same way as this with the EGR hanging up part way through its travel. Valve would stay partially open, not enough to kill the engine, but caused the same symptoms. Would not close completely.

Bill

I also have a '92 OCC, and after my '96 Roadmaster, the OCC mileage was crummy. I replaced the O2 sensor, and my mileage went way up. Change it. There is only one, it's not hard to do.

-Mike

81X11 wrote:

I also have a '92 OCC, and after my '96 Roadmaster, the OCC mileage was crummy. I replaced the O2 sensor, and my mileage went way up. Change it. There is only one, it's not hard to do.

-Mike

You can reach it from the top with an open end, or adjustable wrench. The connector is attached to a plate on the pan. Unless you have really long arms, you have to get under the car to disconnect the connector.
Cleaning the ground at the thermostat is almost free, and I would do it first.

Mileage dropped to 6.5mpg WTH....Work has me tied up,So I had to surrender her to a shop. Starting to go thru withdrawal symptoms. Cue sad music...

Hope you have a locking gas cap because it could be someone stealing your gas.Agree with the O2 sensor replacement.

I will have them give it a look...thanks guys!

Andebe wrote:

Mileage dropped to 6.5mpg .

wow,.i,d think the motor would drown with that much fuel running thru it,.
I had a thirsty old 468 cube olds with a huge overlap cam in it with an equally big old 3 barrel holley,.
and it,d get near 10mpg cruising easy,..and it had 5:13's in it,...with L50-25's on keystone classics,.(wow there's a memory i just dredged up)

nick

Leaking sender? Does it ever smell like gas around the back of the car, but you don't see a drip?

A partially stuck open EGR valve would cause a drop in manifold vacuum at idle and create a rough idle along with a rich running condition. The ECM would see the drop in vacuum (MAP) as a load on the engine and therefore add fuel. The EGR is typically open at highway speeds to help cool the cylinders down to prevent NOx formation.

One thing that was not mentioned as being replaced, but mentioned as a possible issue is the O2 sensor not going into closed loop. The ECM would default to open loop operation. (TBI engines typically run in open loop at an idle because the O2 sensors are non-heated.) Also, the O2 sensor not swinging fully from rich to lean could also result in poor mileage and this is referred to as being biased.

convert2diesel wrote:

Fred Kiehl wrote:

If the EGR was stuck open, the engine would not run.

Should have said stuck "partially" open. Had a TBI react the same way as this with the EGR hanging up part way through its travel. Valve would stay partially open, not enough to kill the engine, but caused the same symptoms. Would not close completely.

Bill

Update: Shop found that my idiot light had been tampered with causing a problem with the computer. While inspecting computer they found a couple of bent prongs making nearly or no contact. Put it all back together,much improved idle and power. It really woke it up! Cant wait to see the mpg difference. Thanks again for all the help guys.

eh.......???

Phantom 309 should enjoy this one....O2 sensor now replaced,no known vacuum leaks,Still has an erratic idle at start up(when cold) The idle surges unless warm. Still stumbles when floored. Mileage 11mpg on a good day. Now what? Straighten some more prongs? Unplug and replug in computer? Do computers go bad?

Checked the EGR? Didn't see any mention of that.

Diaphragm looking thing back by the firewall?

Yes that's it. If you have a vacuum pump you can test the egr valve very easily. Just hook the vacuum pump up and start pumping. Get it up to a couple of pounds then stop pumping. The valve should not lose any vacuum. If it does then it's bad.

Also a simple thing to check, look to see if the heat exchange pipe is attached to the intake. If it's not attached and the valve is still in the intake it will cause the motor to surge. I don't know if this would affect MPG or not.

Check the vacuum hoses to the egr controller, and there's another one that comes off the back of the TBI that goes to a controller that is mounted close to the passenger side firewall. I forget what it is but when I broke the vacuum hose to this on my 93 RMW it started surging when Idling.

Then there's also the Idle air controller. Sometimes when it starts sticking it can cause rough idles.

You don't even need a vacuum pump to check that type of EGR. With the engine off and cold, just disconnect the vacuum line, push the diaphram in with your thumbs and then cover the port with a finger so it can't inhale. If the diaphram doesn't return, the EGR valve diaphram is still intact, and the fact that you can move it with your thumbs means it isn't stuck.

scoffman wrote:

Yes that's it. If you have a vacuum pump you can test the egr valve very easily. Just hook the vacuum pump up and start pumping. Get it up to a couple of pounds then stop pumping. The valve should not lose any vacuum. If it does then it's bad.

Also a simple thing to check, look to see if the heat exchange pipe is attached to the intake. If it's not attached and the valve is still in the intake it will cause the motor to surge. I don't know if this would affect MPG or not.

Check the vacuum hoses to the egr controller, and there's another one that comes off the back of the TBI that goes to a controller that is mounted close to the passenger side firewall. I forget what it is but when I broke the vacuum hose to this on my 93 RMW it started surging when Idling.

Then there's also the Idle air controller. Sometimes when it starts sticking it can cause rough idles.

The EGR valve requires exhaust gas pressure (engine running) to "unlock" it. It is not supposed to move with just vacuum.

Fred Kiehl wrote:

The EGR valve requires exhaust gas pressure (engine running) to "unlock" it. It is not supposed to move with just vacuum.

This statement depends on which type of EGR valve you have, either a Positive or Negative EGR Valve. Testing is different between the two. The statement is eluding to a Positive-backpressure EGR operation.

To test a Positive-backpressure EGR valve, bring the engine to 2000 rpm to create exhaust backpressure, then apply vacuum to the valve. The valve should open and cause a drop of 100 rpm or more. Exhaust leaks or a modified exhaust system can cause problems with this type of EGR system. This EGR Valve can be identified by a letter "P" stamped next to the part number.

To test a Negative-backpressure EGR valve, apply vacuum with the engine off. The valve should open and the diaphragm should hold vacuum. Have an assistant start the engine. The valve should close immediately. This EGR Valve can be identified by a letter "N" stamped next to the part number.

There is also the single diaphragm EGR Valve out there too. To test a single-diaphragm EGR, use a hand held vacuum pump and apply about 8 inches of vacuum to raise the EGR diaphragm. The diaphragm should hold vacuum and raise the pintle straight up. With the engine idling, apply vacuum to the EGR Valve, the engine should stall. Assuming all is in working order.

Keep this in mind, even though the EGR seems to functioning properly, as described above, the spring in the valve may have lost tension (weak) or there may be partial restriction in the EGR passages or cracked vacuum hoses, etc... If you have any question as to whether the spring has lost tension replace the valve with a factory type part and not one of those universal fit EGR Valves out there.

[You must be registered and logged in to see this link.] is some more information on these EGR Valves, it opens in a PDF format.

Pull the distributor cap and rotor again and take a look at the 8 pointed "star" reluctor wheel. There is magnet sandwiched in there with 4 rivets and has a tendency to crack near the magnets creating multiple N/S, S/N magnets within itself. This weakens the magnetic strength and sends a strange signal to the pickup coil. Typically this fails more so when warm than cold, but it is worth a look. Surging can be symptom of this fault.

Also, try disconnecting the distributor bypass connector to see if the engine straightens out, this takes the computer out of the loop. If the engine quits surging the computer (ECM) may not be managing the timing correctly. Also make sure base timing is set to 0° TDC while in bypass mode. With ECM in control it usually ramps timing up to 20° BTDC at idle. As an example if base timing is set to 10° BTDC ECM then adds 20°, now timing at idle would be 30° BTDC @ Idle. Or just the opposite would be true, 10° ATDC, ECM adds 20°, now idle timing would be 10° BTDC @ Idle. Either way this is not good for engine performance and mileage.

Andebe wrote:

Phantom 309 should enjoy this one....O2 sensor now replaced,no known vacuum leaks,Still has an erratic idle at start up(when cold) The idle surges unless warm. Still stumbles when floored. Mileage 11mpg on a good day. Now what? Straighten some more prongs? Unplug and replug in computer? Do computers go bad?

Last tanks mileage 9.4

gmtech wrote:

Fred Kiehl wrote:

The EGR valve requires exhaust gas pressure (engine running) to "unlock" it. It is not supposed to move with just vacuum.

This statement depends on which type of EGR valve you have, either a Positive or Negative EGR Valve. Testing is different between the two. The statement is eluding to a Positive-backpressure EGR operation.

To test a Positive-backpressure EGR valve, bring the engine to 2000 rpm to create exhaust backpressure, then apply vacuum to the valve. The valve should open and cause a drop of 100 rpm or more. Exhaust leaks or a modified exhaust system can cause problems with this type of EGR system. This EGR Valve can be identified by a letter "P" stamped next to the part number.

To test a Negative-backpressure EGR valve, apply vacuum with the engine off. The valve should open and the diaphragm should hold vacuum. Have an assistant start the engine. The valve should close immediately. This EGR Valve can be identified by a letter "N" stamped next to the part number.

There is also the single diaphragm EGR Valve out there too. To test a single-diaphragm EGR, use a hand held vacuum pump and apply about 8 inches of vacuum to raise the EGR diaphragm. The diaphragm should hold vacuum and raise the pintle straight up. With the engine idling, apply vacuum to the EGR Valve, the engine should stall. Assuming all is in working order.

Keep this in mind, even though the EGR seems to functioning properly, as described above, the spring in the valve may have lost tension (weak) or there may be partial restriction in the EGR passages or cracked vacuum hoses, etc... If you have any question as to whether the spring has lost tension replace the valve with a factory type part and not one of those universal fit EGR Valves out there.

[You must be registered and logged in to see this link.] is some more information on these EGR Valves, it opens in a PDF format.

The LO3/5 engines have positive pressure EGR systems.

Yikes I thought i was doing bad at 12-14 mixed and 17-19 highway. Best of luck figuring this out.

The egr seems to be the last piece of my replacement parts puzzle:scratch:. At this point,wth...I just want to get that mileage up for the trip to wagon fest! (and that whole daily driving business!)

Andebe wrote:

Phantom 309 should enjoy this one....O2 sensor now replaced,no known vacuum leaks,Still has an erratic idle at start up(when cold) The idle surges unless warm. Still stumbles when floored. Mileage 11mpg on a good day. Now what? Straighten some more prongs? :suspect:Unplug and replug in computer? Do computers go bad?

your assumption on subjects that might give me enjoyment,.is baffling.

phantom 309 wrote:

eh.......???

Was I wrong to think you found the mechanics answer to my problem questionable? I simply ment to suggest you were right in your doubt. Forgive this B-Body owner for suggesting I comprehend your reading preferences...

Andebe wrote:

phantom 309 wrote:

eh.......???

Was I wrong to think you found the mechanics answer to my problem questionable? I simply meant to suggest you were right in your doubt. Forgive this B-Body owner for suggesting I comprehend your reading preferences...

Ah,..

yes i did think your mechanics diatribe was indeed dubious,.

scoffman wrote:

Check the vacuum hoses to the egr controller, and there's another one that comes off the back of the TBI that goes to a controller that is mounted close to the passenger side firewall. I forget what it is but when I broke the vacuum hose to this on my 93 RMW it started surging when Idling.

MAP Sensor

I had similar issues on a couple of cars.
In my case, the cars never went into closed loop operation.

One of them had a bad O2 sensor AND a bad temp sensor.
The O2 sensor was an easy guess; the wire and connector were melted.
The coolant temp sensor never told the ECM that the engine was up to operating temp.
Also make sure you have the right thermostat in there. If it's too cold, the engine might NOT get to operating temperature.

The other car also had a bad coolant temperature sensor; eventually got a "Check Engine" light.


I'm not referring to the temp sensor that provides the signal to your coolant temp gage or light; it is the sensor that feeds coolant temp data to the computer.

Thanks for the help....Mileage is now between 9-11mpgs. Still sucks. Gotta get a LT-1,lol