Page 1 of 2 12 LastLast
Results 1 to 15 of 18

Thread: 86 Century T Type 3.8 Engine Roughness on Acceleration

  1. #1
    Junior Member
    Join Date
    Aug 2017
    Posts
    6

    Default 86 Century T Type 3.8 Engine Roughness on Acceleration

    My 86 Century T Type 3.8 SFI with 120,000 miles engine gets quite rough and lacks power under moderate or full throttle acceleration. The transmission shifts normaly, there is no stalling. With light acceleration and steady speed it is smooth as it should be. The check engine light is not on and I checked for codes and there were none. I replaced the spark plugs, wires and air filter. The catalytic converter is 2 years old. EGR valve is functioning. I listened to the injectors with a stethascope and they're all clicking as they should. Any suggestions what I should check next? ( a desription of any test procedures would be appreciated as well).



  2. #2
    Senior Member CorvairGeek's Avatar
    Join Date
    Dec 2014
    Location
    The Gem State
    Vehicle
    1992 Century Special 1996 Century Custom
    Posts
    609

    Default

    Has it ever had the camshaft timing gear replaced yet?
    Jerry

  3. #3
    Senior Member Keiths1976's Avatar
    Join Date
    Dec 2013
    Location
    Florida
    Vehicle
    1995 buick century custom
    Posts
    1,180

    Default

    Does tha still have the magnavox ignition coils pack or was it converted over ? I would check that u can ohm test them to make sure . Ohm test the injectors as well make sure you have a steady 12-14 range. Like covair stated that too all needs to be addressed . Age of the car and not sure the mileage crank sensor is getting old and shifted since when it's in demand of ignition and fuel and it's stumbling . Fuel pump and filter get a fuel pressure tester make sure the fuel pressure regulator isn't sticking . Vacuum leak go through all the hoses .

  4. #4
    Junior Member
    Join Date
    Aug 2017
    Posts
    6

    Default

    I've checked and rechecked all the vacuum lines. Cam timing gear is original. It has the Magnavox coil pack, I checked the secondary ohms were all the same and within spec. Crank sensor is about 10 years old but wouldn't a code show up if it's bad? It looks like it would be a PITA to replace it. I will check the injector ohms.

  5. #5
    Senior Member Keiths1976's Avatar
    Join Date
    Dec 2013
    Location
    Florida
    Vehicle
    1995 buick century custom
    Posts
    1,180

    Default

    Ready one more thing cheap fix replace and ohm test the coolant temperature sensor which if t gets old can cause more fuel and air mixture between open and closes loop . Ohm on the cAr and See the value and break it down from ohms to Celsius to F and see if that's about what the car is in s cool car . Some will trough out values the car is thinking it's warm and not and it will mess things up with no engine codes . And bug or electrical gremlins change cts but u really should check timing gear and chain it's nylon bases so it could have teeth damage causing it to jump timing etc . Till that's checked I would keep her grounded from being driven or replaced with a steel set .

  6. #6
    Junior Member Century86T's Avatar
    Join Date
    Mar 2016
    Vehicle
    1986 Century T-Type
    Posts
    5

    Default

    Mine acted this way when the MAF sensor was going bad - it would stumble bad under load on the highway like when I was going uphill or something. Sometimes I could see some black smoke too. There was no check engine light for about a week and then it progressed to where the car would barely run at all and the light came on with the MAF code. I don't know how to test the sensor but I found that if I tapped on the MAF sensor while it was running at idle the engine would stumble.

  7. #7
    Senior Member turbokinetic's Avatar
    Join Date
    Dec 2007
    Location
    Northport, AL
    Vehicle
    1984 Buick Century Olympia 3.8SFI Turbo V6
    Posts
    5,699

    Default

    Hi there. Welcome to the forum! My first A-car was an 83 Century T-Type.

    As for your 86, the LG3 has the direct ignition system. One of the first things to check is the ignition coils. They are easy to check. Measure the resistance of each and see what you measure. Each of the 3 coils should be similar.

    Here is a video on how to test them: https://www.youtube.com/watch?v=NmGLQg5Fsro

    Look me up on Facebook and CarDomain and My Webpage

  8. #8
    Senior Member
    Join Date
    Jun 2010
    Location
    West Haven, CT
    Vehicle
    1988 Ciera Sl Coupe FE3 & 1988 Ciera International Coupe
    Posts
    319

    Default

    Quote Originally Posted by T Type View Post
    I've checked and rechecked all the vacuum lines. Cam timing gear is original. It has the Magnavox coil pack, I checked the secondary ohms were all the same and within spec. Crank sensor is about 10 years old but wouldn't a code show up if it's bad? It looks like it would be a PITA to replace it. I will check the injector ohms.
    Maybe not directly related to your issue, but if, and someone on here might know better, that cam gear is the original and it's one of the aluminum nylon ones they were using in the 80's, I would urge you to change that out with an all steel timing set at your earliest opportunity. Those timing gears are 100k timebombs.

  9. #9
    Junior Member
    Join Date
    Aug 2017
    Posts
    6

    Default

    I ohm tested the fuel injectors and the ignition coils and they are all within spec. Tapping the MAF sensor has no effect on the engine running. The engine is rough on acceleration (and idle) both when the engine is cold and hot. I'm going to rent a fuel pressure tester and see what that shows. Also is there a way of testing the crank position sensor on the LG3 3.8?
    Last edited by T Type; 08-19-2017 at 06:54 PM.

  10. #10
    Senior Member Keiths1976's Avatar
    Join Date
    Dec 2013
    Location
    Florida
    Vehicle
    1995 buick century custom
    Posts
    1,180

    Default

    Coolant temperature sensor throwing off the fuel and air flow gm uses this as a base to adjust the idle it's a primary sensor the ecu needs to run right . That won't throw a engine light just make it run rich or lean . The Intake hose from the throttle body to air filter. Heck for cracks or splits in case it's adding more air or a off balance causing the idle ar control valve to go out of whack whic would throw off the rest .

  11. #11
    Junior Member
    Join Date
    Aug 2017
    Posts
    6

    Default

    I checked the coolant temperature sensor on the car and got 3500 ohms which converts to about 68 degrees which is about what the actual coolant temp is right now. It's only one reading but maybe it's OK? I also checked the fuel pressure and with key on it's 35 psi and my manual says it should be 40.5 to 47 psi. With engine on it is supposed to drop 3-10 psi. With engine on it is 28 psi. On my car the fuel filter is near the fuel tank and the nuts where the fuel line attaches are rusted but I'll have to change the filter somehow.

  12. #12
    Senior Member turbokinetic's Avatar
    Join Date
    Dec 2007
    Location
    Northport, AL
    Vehicle
    1984 Buick Century Olympia 3.8SFI Turbo V6
    Posts
    5,699

    Default

    Quote Originally Posted by T Type View Post
    I checked the coolant temperature sensor on the car and got 3500 ohms which converts to about 68 degrees which is about what the actual coolant temp is right now. It's only one reading but maybe it's OK? I also checked the fuel pressure and with key on it's 35 psi and my manual says it should be 40.5 to 47 psi. With engine on it is supposed to drop 3-10 psi. With engine on it is 28 psi. On my car the fuel filter is near the fuel tank and the nuts where the fuel line attaches are rusted but I'll have to change the filter somehow.
    You're probably onto something with the fuel pressure. These cars use a Bosch fuel regulator. Bosch made many different pressure settings for this regulator. They are NOT interchangeable. Some of the cheap DIY stores will try to sell you a "universal" one which claims to be for multiple models which had different pressure specifications from the factory. Needless to say this will not work well at all.

    I would try to locate a regulator with the correct 43 PSI setting. This is a good first step to getting things working correctly.

  13. #13
    Junior Member
    Join Date
    Aug 2017
    Posts
    6

    Default

    Quote Originally Posted by turbokinetic View Post
    You're probably onto something with the fuel pressure. These cars use a Bosch fuel regulator. Bosch made many different pressure settings for this regulator. They are NOT interchangeable. Some of the cheap DIY stores will try to sell you a "universal" one which claims to be for multiple models which had different pressure specifications from the factory. Needless to say this will not work well at all.

    I would try to locate a regulator with the correct 43 PSI setting. This is a good first step to getting things working correctly.
    I made a mistake. My apologies turbokinetic, I read the fuel pressure for the wrong engine. For my 86 LG3 the correct reading is 35 psi so mine is actually within spec.Sorry. I'm starting to think perhaps it is a worn cam timing gear that's causing the engine vibration/roughness on acceleration. Is the Ignition Control Module another possible cause as well?

  14. #14
    Senior Member Keiths1976's Avatar
    Join Date
    Dec 2013
    Location
    Florida
    Vehicle
    1995 buick century custom
    Posts
    1,180

    Default

    I would change that to rule it out but honestly grab a black light and a dark area start her up and shine the black light over the magnavox module she might ohm good but could have cracks in the plastic throwing out spark and it's grabbing metal near by or arching and giving that cylinder a weak spark so a rough idle and misfire symptoms .

  15. #15
    Senior Member turbokinetic's Avatar
    Join Date
    Dec 2007
    Location
    Northport, AL
    Vehicle
    1984 Buick Century Olympia 3.8SFI Turbo V6
    Posts
    5,699

    Default

    Quote Originally Posted by T Type View Post
    I made a mistake. My apologies turbokinetic, I read the fuel pressure for the wrong engine. For my 86 LG3 the correct reading is 35 psi so mine is actually within spec.Sorry. I'm starting to think perhaps it is a worn cam timing gear that's causing the engine vibration/roughness on acceleration. Is the Ignition Control Module another possible cause as well?
    No problem; it happens to all of us!

    If the cam timing gear has not been replaced and the plastic one is still in place, it is entirely possible that there is something going on there. If this is the case; it is very serious and will result in destruction of the engine if not fixed. Many of these cars have been scrapped for this reason.

    As for the ignition module and coils; this is an extremely likely cause of rough running, missing, and other bad performance. The LG3 (3.8SFI up to 1987) uses a different module from the LN3 (3800 1988-1990). They used the same module as the LG3 on certain LG7 3300 A-body cars. These have the Delco-II coils which work much better than the Magnavox.

Page 1 of 2 12 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •