Poor gas mileage/Sensors, please help a bit!
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HT4100 4.1, 4.5, 4.9 Discussion, Poor gas mileage/Sensors, please help a bit! in Cadillac Engine Technical Discussion; I have a 91 Seville 4.9, and here's my problem. It's eating gas. I busted a sensor on the drivers ...
  1. #1
    wikked_magick45 is offline Cadillac Owners Member
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    Poor gas mileage/Sensors, please help a bit!

    I have a 91 Seville 4.9, and here's my problem. It's eating gas. I busted a sensor on the drivers side of the rear head when I was doing spark plugs, I was told it was the temp sender, and was also told this is what's causing me to get less than 10MPG. Realistically, I have 151,000 miles, if I get it running really good, what type of mileage should I expect? Also, I have a *slight* miss my dad said the temp sender is causing. Since I have re-timed it, I don't know if it gets any worse, haven't driven it long enough to warm it up. Before, it ran great (with slght miss) and when it reached operating temp, it would stall and act like the timing was off if i didn't keep on the gas at a red light. When I put the computer in timing mode, it was so far advanced, I'm surprised it ran. I retimed it to 10 degrees, seems to be better. What kind of sensors other than 02 can cause the slight miss and God awful mileage?

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    wikked_magick45 is offline Cadillac Owners Member
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    Re: Poor gas mileage/Sensors, please help a bit!

    http://www.youtube.com/watch?v=NLVUdO81brI


    Video of the car, in this short drive it used 0.3 gallons of gas. If this explains just how poor my mileage is. I forgot to mention there aren't any codes for anything BUT the sensor on this, and something about the ISC (i *think* it was code 73).

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    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    Re: Poor gas mileage/Sensors, please help a bit!

    Most modern cars operate in two modes. The one that is in use most of the time is a closed loop, where the car looks at things like temperature, emissions, and other stuff, then tweaks itself accordingly. The other mode is stupid startup mode, where it basically relies on hard defaults, and it also tries to get into closed loop asap by doing things like using hot emission gases to help warm up the engine. Without the temperature sensor your car is in stupid startup mode all the time, will run like shit all the time, and will suck up gas like no tomorrow. Once you get your temp sensor straightened out you'll do a lot better.

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    lenny lincoln is offline Cadillac Owners Fanatic
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    Re: Poor gas mileage/Sensors, please help a bit!

    Man I should start charging you guys for my help. First lets fix the slight miss. Recheck your plug wires and firing order. I dont know the firing order off hand of a 4.9 but you can find it and the rotation direction of the rotor and figure it out. You have Two wires misplaced. The item you broke is a $12 dollar item, replace it. Very important that the computer knows what is the operating temperature of the motor is to determine how much gas it is going to need. When you timed it did you follow the FSM ? If not go to your local library and copy the proper procedure and folow it with the inboard computer and jumpers. My 4.9 is getting 13.5 city driving 2.5 miles then shutting off and 27.3 at 70mph long cruises.

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    wikked_magick45 is offline Cadillac Owners Member
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    Re: Poor gas mileage/Sensors, please help a bit!

    I just did plugs and wires very recently, and the firing order is labeled on the intake and the dist. cap, I'm 250% sure that's right. I'll get right onto replacing that sensor. Would those EGR tubes in the trottle body have anything to do with? I haven't even checked those out yet.

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    lenny lincoln is offline Cadillac Owners Fanatic
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    Re: Poor gas mileage/Sensors, please help a bit!

    a miss is a spark plug not firing. I have been turning wrenches for 20 yrs and still I had messed up on firing order and produced a miss. recheck. If it comes and goes its in the distributor. if its all the time, you have a bad plug, wire, or wrong routing of the wires

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    wikked_magick45 is offline Cadillac Owners Member
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    Re: Poor gas mileage/Sensors, please help a bit!

    In my case with MPFI having a separate inj. for every cylinder, one might be messed up. I had this on an Eldorado 4.5, it had a bad computer so one injector refused to fire.

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    wikked_magick45 is offline Cadillac Owners Member
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    Re: Poor gas mileage/Sensors, please help a bit!

    Okay update I managed to fix my sensor, and it changed my mileage drastically. I'm getting 14 city and 23 hwy, vs 4 city 10 hwy. Still missing a bit but the milage is way up. I opted to just repair my sensor, all I did was bust off the plastc housing that holds the connector in place. A little 60 second epoxy did the trick. Now since I've fixed it though, it don't take right off and start. It has to crank 5-7 seconds before hand, then fires right up.

    By the way, thanks for the help guys. You have no idea how much I appreciate good people like you!

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    ehall's Avatar
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    Re: Poor gas mileage/Sensors, please help a bit!

    Replace the sensor. Your patch will fail.

    Your poor start and running problems might just be a dirty fuel system. Run a few tanks of really good gas (Amoco 93 at a BP station for example) and run a bottle of system cleaner on your third or fourth tank. Don't use something too strong unless you go through a tank in a week or less of normal driving.

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    wikked_magick45 is offline Cadillac Owners Member
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    Re: Poor gas mileage/Sensors, please help a bit!

    The patch job did work, surprisingly well! Now I fix that, and I drove a while, it was great. Today, it began to miss horribly, had 4 cylinder power, and you could smell raw gas. Only code I have is E071, Intermittent MAP sensor signal. Would this cause the miss to come back?

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    NickySantoro is offline Cadillac Owners Fanatic
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    Re: Poor gas mileage/Sensors, please help a bit!

    Quote Originally Posted by lenny lincoln View Post

    Man I should start charging you guys for my help
    I'll pay you or anyone else for a correct diagnosis. Check this out and advise the cure. Abandon hope all ye who enter here....

    http://www.cadillacforums.com/forums...even-idle.html

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    ehall's Avatar
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    Re: Poor gas mileage/Sensors, please help a bit!

    http://en.wikipedia.org/wiki/MAP_sensor

    It could be that the MAP sensor is flooding the system. You might also have a busted fuel pump, or maybe an injector

  14. #13
    wikked_magick45 is offline Cadillac Owners Member
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    Re: Poor gas mileage/Sensors, please help a bit!

    Hmm... Well, I WAS going to replace that MAP sensor, till about a week ago the car ran fine. Only had a barely noticable miss once it warmed up and you were either at a light posted up or you were in overdrive under 55MPH. I drove the car to familys for christmas today, ran good. Started up and drove home, it ran like total SHIT again, cept this time absolutely NO codes. Same symptoms as before. Horrible miss, lack of power. But no codes. What should I check? It's cold out, I don't want to spend 4 hours weeding thru it in sub zero weather.

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    Re: Poor gas mileage/Sensors, please help a bit!

    What did your old plugs look like? One a different color than the others? Did you keep track of which cylinder each one came from? That would be a good indicator of FI or ignition problems.

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    wikked_magick45 is offline Cadillac Owners Member
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    Re: Poor gas mileage/Sensors, please help a bit!

    I dug into the computer the other night and it's reading 75 degrees n diagnostic mode when the motor is at 165-195 degrees. I'm stumped due to whatever is wrong with this not throwing a code. Whatever it is, it is missing full time, hot or cold, and the hotter the car gets, the worse the miss gets in my opinion. Here's the odd ball part - I put $5 worth of 87 octane in it last night because I was out of gas almost, and it actually seemed to run just a hair better. Wierd... I just want to get this running better so that when I get my lawsuit settlement soon I don't have to waste my money on it... I'm gonna take it to the dealership and have it serviced later on, and I don't want to pay them to replace things I can replace myself.

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