4.1 Running Bad Again
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HT4100 4.1, 4.5, 4.9 Discussion, 4.1 Running Bad Again in Cadillac Engine Technical Discussion; My 87 Eldorado with the 4.1 engine is running bad again and getting poorer gas mileage. 56,000 miles. Here is ...
  1. #1
    PeteC is offline Cadillac Owners Member
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    4.1 Running Bad Again

    My 87 Eldorado with the 4.1 engine is running bad again and getting poorer gas mileage. 56,000 miles.

    Here is what has been done: recent plugs, wires, dist cap, rotor, module, coil, oxygen sensor, EGR valve, air filter, fuel pump. Timing has been checked. Replacing the coil and wires had the biggest positive effect. After all this work was done the car ran OK for a few months, but the gas mileage was not good: 15 MPG.

    Now the car is running worse and MPG down to 13.

    What should I check next?

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  3. #2
    ehall's Avatar
    ehall is offline Cadillac Owners Connoisseur
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    Re: 4.1 Running Bad Again

    O2 sensor. The computer uses the O2 sensor to tell it when the catalytic converter is working (temperature) and if the O2 is bummed the car just keeps running rich.

  4. #3
    carnut is offline Cadillac Owners Master
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    Re: 4.1 Running Bad Again

    Also verify engine reaching normal temperature. Somewhere close to 195 to 200 degrees. If running too cold it will delay going into closed loop and will run richer longer. The oxy sensor delays monitoring fuel trim till the engine gets to temperature. All these reading are available thru on board diagnostics.

  5. #4
    Po Pimp is offline Cadillac Owners Fanatic
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    Re: 4.1 Running Bad Again

    On a similar note you might want to check the thermostat. It could have broken open, stuck or might not even have one in there. In the hot months you car could still be getting to operating temperatures and the problem not as noticable. In the cold months with no thermostat it may only hit operating temperatures when your idle after the car has ran for awhile and drop quick as soon as you start to move. The heat may still work. The coolant itself will still get hot just not hot enough to trigger the computers response.

    Of course that could be something more serious but might as well start with the easy stuff first.

  6. #5
    PeteC is offline Cadillac Owners Member
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    Re: 4.1 Running Bad Again

    Quote Originally Posted by ehall View Post
    O2 sensor. The computer uses the O2 sensor to tell it when the catalytic converter is working (temperature) and if the O2 is bummed the car just keeps running rich.
    The original O2 sensor had a broken wire so it was obviously not working. When I put the new O2 in it didn't really change the way the engine ran. So maybe the new O2 sensor is bad. Can I check this using the onboard diagnostic?

  7. #6
    PeteC is offline Cadillac Owners Member
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    Re: 4.1 Running Bad Again

    Quote Originally Posted by carnut View Post
    Also verify engine reaching normal temperature. Somewhere close to 195 to 200 degrees. If running too cold it will delay going into closed loop and will run richer longer. The oxy sensor delays monitoring fuel trim till the engine gets to temperature. All these reading are available thru on board diagnostics.
    The engine warms up quickly to 195 degrees. I will check my manual to see how to check the O2 sensor operation.

  8. #7
    Po Pimp is offline Cadillac Owners Fanatic
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    Re: 4.1 Running Bad Again

    Where was the wire broken? If it was part of the new wiring you got with the new sensor I would not worry about it. If it was further up you could have an issue there.

    You can check the operating temperature from the OBD. The oxygen sensors to, although, not as acurate as testing manually. I personally can not remember the exact numbers but something like a Haynes manual would be accurate for this job. Sorry been awhile since I had a 4.1L.

  9. #8
    awadecki is offline Cadillac Owners Enthusiast
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    Re: 4.1 Running Bad Again

    Well, first of all, it depends on what kind of driving you're doing. If you're stopping at a lot of lights, and driving 100% city, 13mpg is reasonable. In the cold weather the car has to burn a lot of gas when it first warms up until the engine reaches operating temperature.

    I live in a town where I stop at every light for nearly a minute, and I get about 13 mpg if I don't drive on the highway at all. This is a V-8 after all. On the freeway, I usually get about 25-26 (my timing is set at 12 degrees base). Even my friend's 4-cylinder 2002 Grand Am only gets 18 mpg around town!

    However, if you're mixing in some freeway driving, then you definitely have a problem. Check the 02 sensor by watching the "Econ" light in diagnostics mode (hold Off and Warmer together for 4 seconds). It should flash on and off when the engine is warm and your foot is on the gas.

    You can also check the 02 sensor voltage in diagnostics (don't remember which it is). Make sure it is varying above and below 0.5 volts.

    You can also try decarboning the engine with Seafoam. Early 4.1s had exhaust valves that sometimes stuck slightly open, but by 87 it was pretty much resolved.

  10. #9
    PeteC is offline Cadillac Owners Member
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    Re: 4.1 Running Bad Again

    Thanks for all the suggestions. I will check the O2 sensor from the diagnostics panel. I have a Factory Service Manual and the little diagnostic codes book.

    Decarboning the engine is a good idea too. I have never done this before so any suggestions on how to do this are appreciated.

    I will report back on the results, probably this weekend as I am working 10-12 hr days.

  11. #10
    awadecki is offline Cadillac Owners Enthusiast
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    Re: 4.1 Running Bad Again

    Decarboning the engine is very easy. Buy a bottle of Seafoam and pour it into one side of the throttle body while holding the throttle with your other finger to keep the motor from stalling. Empty half of the can slowly into the motor (don't stall it), then switch to the other side of the throttle body. After you're done, shut the car off for 5 minutes and restart, holding the idle at about 1500 rpms for a minute or so. Be prepared for TONS of white smoke...I mean a big plume. This is normal. Then, drive the car a little aggressively to get everything else out. Make sure you buy a small plastic funnel to stick in the throttle body, and hold the Seafoam can with a couple paper towels otherwise it'll drip everywhere.

    GM used to have a top end cleaner that they recently discontinued which looked--and smelled--like Seafoam (wonder if it was the same?), and their directions were to empty 2/3 of the bottle, then dump the last 1/3 in fast to stall the engine. Wait 30 minutes, then restart.

    Either method works fine, but I usually use the Seafoam method because I can't dump the remaining 1/3 of a can into both sides of the throttle body simultaneously.

  12. #11
    awadecki is offline Cadillac Owners Enthusiast
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    Re: 4.1 Running Bad Again

    What kind of driving are you doing? Does the car run fine, or is it missing/lacking power? Swab the tailpipe llightly with your finger and see if it comes out very black...a little black is normal. If you can't see your fingerprint through the black, you're running rich.

  13. #12
    ehall's Avatar
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    Re: 4.1 Running Bad Again

    There are different parts of the engine that get carbon and gunk build-up, and you have to address them separately.

    Pouring seafoam through the throttle body will clean the throttle body, the intake manifolds, and portions of the combustion chamber. Pouring some more in your gas will clean the fuel system and portions of the combustion chamber. Pouring some more into the oil fill will clean your valves and the lower parts of the combustion chamber. If you want a good cleaning, get two of their pop-top cans and put half a can in the tank, half a can in the oil, and a whole can in the throttle body. You need to CHANGE YOUR OIL after 75-100 miles because it will knock a lot of crap loose and you want to get it out while it's trapped in the filter.

    If you have seriously major build-up (which I've got) you may want to start an Auto-RX treatment. It uses esters to soak into the metal and push the crud off. I'm going to start mine in the next week or two.

  14. #13
    PeteC is offline Cadillac Owners Member
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    Re: 4.1 Running Bad Again

    I did the Seafoam decarbon. Car still ran rough.

    I then found I had a vacuum leak at the little tube that attaches to the air cleaner. The rubber tube had broken off the plastic tube and I had replaced the rubber tube with one that was too large. When I put a smaller diameter rubber tube on the engine ran better.

    I also found that the Idle Speed Control (ISC) motor has a switch in it that makes contact when the throttle closes at idle. I found this was making intermittent contact, which would possibly explain why the engine occasionally would not return to idle when you took your foot off the throttle. This scared my wife several times. I took the ISC motor apart and cleaned the contacts and I have not had the problem since.

    So the car is running better, though the idle is still a little rough. Hopefully the mileage will improve.

    I will try to check the O2 sensor output when I get a chance.

  15. #14
    awadecki is offline Cadillac Owners Enthusiast
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    Re: 4.1 Running Bad Again

    Good thing you fixed your closed throttle switch. 02 sensor would be a good thing to replace. Are you sure your timing is correct? Did you check the EGR yet (just tap it with a wrench)?

  16. #15
    PeteC is offline Cadillac Owners Member
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    Re: 4.1 Running Bad Again

    O2 sensor has been replaced and timing is correct. EGR was replaced a couple years ago but I may not have cleaned the passage like I see others suggesting here.

    I hope to check these this weekend.

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