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Thread: 86 Century T Type 3.8 Engine Roughness on Acceleration

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    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).

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    Senior Member CorvairGeek's Avatar
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    Has it ever had the camshaft timing gear replaced yet?
    Jerry

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    Senior Member Keiths1976's Avatar
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    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 .

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    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.

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    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.

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    Senior Member Keiths1976's Avatar
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    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 .

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    Junior Member Century86T's Avatar
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    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.

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    Senior Member turbokinetic's Avatar
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    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

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    Quote Originally Posted by turbokinetic View Post
    check is the ignition coils...Measure the resistance of each and see what you measure
    Interesting, with 2.8-V6 coils, I recall it being an open circuit on new/old coils when measuring from across spark tops, when coils were removed from ICM. Primary circuit showed an ohm reading of some 5 k ohms or so.

    When I change out spark wires, I'll take a second look at ohm reading for spark tops.

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    I have a new problem, possibly related to the first problem? I'm still having the engine getting rough on moderate to hard acceleration but I've never had a stalling problem till now. Today at a long train crossing I shifted to park and let the engine idle. After a few minutes it began to stumble and slow down, the CEL came on and it stalled. It started right up again and would not stall while idling in drive only in park. I checked for codes and got #44 lean exhaust. I checked the oxygen sensor wire and connections and it looks OK visually. Could a bad O2 sensor cause engine roughness on acceleration? Is there a way to test the O2 sensor with a multi-meter? If it is the O2 sensor is it very difficult to get it out?

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    Senior Member turbokinetic's Avatar
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    Quote Originally Posted by T Type View Post
    I have a new problem, possibly related to the first problem? I'm still having the engine getting rough on moderate to hard acceleration but I've never had a stalling problem till now. Today at a long train crossing I shifted to park and let the engine idle. After a few minutes it began to stumble and slow down, the CEL came on and it stalled. It started right up again and would not stall while idling in drive only in park. I checked for codes and got #44 lean exhaust. I checked the oxygen sensor wire and connections and it looks OK visually. Could a bad O2 sensor cause engine roughness on acceleration? Is there a way to test the O2 sensor with a multi-meter? If it is the O2 sensor is it very difficult to get it out?
    It seems these old LG3 cars all want to run lean at idle. It seems that a combination of small vacuum leaks, an aging MAF sensor, and aging injectors work together to cause this problem. The problem is going to be worse with less load on the engine. That's because the less load the lower the airflow and that is the region where the MAF is least accurate.

    The 02 sensor is easy to change. It is simply screwed into the collector at the rear manifold. There is no way to test it with a meter. It is a chemical sensor and would require calibrated test-gas and special equipment to test it.

    The "lean exhaust" code means that the ECM was trying to add fuel and correct the lean condition, however it reached the maximum allowable fuel trim without correcting the problem.

    A dirty air filter or dust on the MAF element will both exacerbate this problem.

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    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.

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    Senior Member Keiths1976's Avatar
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    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 .

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    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.

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    Senior Member turbokinetic's Avatar
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    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.

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