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81 SC sputtering/small backfires

 
Old 07-25-2009, 11:37 PM
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aadrew10
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Default 81 SC sputtering/small backfires

Hi Everyone,

My SC sputters out the tailpipe whenever I accelerate moderately. If I rev the engine up quickly it sounds like small backfires. Unlike the other posts, my sputtering happens at all rpms, only when I accelerate. It will run fine at every RPM, but will sputter whenever the engine comes under load, sometimes lurching slightly. The car seems to lack power as well. If I'm not accelerating, the engine sounds fine. It idles at a constant 900 RPM. I just changed the inline fuel filter, fuel injectors and sleeves, adjusted the valves, and changed the spark plugs. The old plugs had no oily residue and look to be in OK shape. Pop off valve is seated correctly. No loose ignition/coil wires, distributor cap/rotor appear good, fuses OK. Fuel pressure good. Compression is at 160 psi for cylinder 1 and 180 on the others.

I think this may be an electrical problem because intermittently my tachometer will start bouncing and will click all the way to the right. Sometimes when it's acting up it bobs around at the pace of my blinkers(whenever I'm making a turn). It is also sometimes affected by headlights. However, I don't know enough about the engine to be confident it's an electrical problem.

Maybe I have bad timing, bad valves, or am running rich? Should I try bypassing my O2 sensor relay? When I push up on the air flow sensor I get a buzzing sound coming from around the throttle body, which I think is the frequency valve working properly.

Again, the sputtering/constant small backfiring occurs only when the engine is under load, independent of RPMs.

Thanks in advance for your suggestions/ideas. This seems like a very minor problem but I just can't get my finger on it.

- Andrew
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Old 07-26-2009, 01:35 AM
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Not an expert but I put my money on timing and maybe too lean.
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Old 07-26-2009, 07:14 AM
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Chris M.
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The bounching tach and other strange electrical behaviors point to your alternator and/or voltage regulator. That's where I would look first.

Also, I had a new Bosch coil go bad after about 7000 miles, and it caused backfiriing under load. I doubt this has anything to do with the electrical problems but could explain the backfiring. How old is your coil?
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Old 07-26-2009, 10:20 PM
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Houpty GT
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Are you aware of the issue with broken head studs on these cars. I would suggest you pull a lower valve cover and use a telescoping magnet to check the barrel nuts that screw onto the studs and see if they are loose. My car made similar noise when it was under load regardless of rpm.

If the studs are not broken then I would suggest you get the fuel mixture adjusted.

As for your electrical problems, It sounds like your voltage regulator is bad. 1981 is the last year of the external voltage regulator. If you don't have an external regulator then your car has been updated and you can buy an internal regulator.
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Old 07-26-2009, 10:38 PM
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Interesting thing happened today. I sanded the grounds in the engine compartment and on the battery, so I thought I'd go for a drive and see if anything had changed. Halfway down the block the car dies. I pull over and check everything. Try starting it again, it would start, go up to 1k rpm then die. Had a few of my friends try pushing it back up the hill to my house without success, so I towed it back using another car and some interesting looking rope. It was quite the sight.

Anyways, when I get back to my garage I check for sparks, which were OK. I remembered that before I set out for the drive the battery voltage was 12.4 volts at 1.5k idle, so plugged my charger up to the battery, and bingo. The car starts right up. I revved it quickly a few times, and could not detect any of the sputtering I had heard the day before. With the battery off the charger, engine running, the voltage is at 12.4 volts. If I turn on the headlights and E flashers the voltage drops to 12.0 volts. This leads me to believe the voltage regulator is OK but the alternator is bad. Would this be a correct assumption and good place to start swaping out parts?

I am also having trouble finding the G2 ground, which may be causing the problems with the tachometer. The manual says it's on the passenger side right behind the heater controls, and I think it's brown, however I can't seem to locate it. If anyone could help me locate that I'd appreciate it.

- Andrew
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Old 07-26-2009, 10:46 PM
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Sounds like bad alternator .. sounds like you don't even have enough current to run the ignition.

I would start with checking and tightening all the connections and measuring the voltage again at idle. if your not over 13 volts you need a new alternator .
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Old 07-27-2009, 01:46 PM
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The internal voltage regulators have carbon brushes that wear out and can cause it to look like a bad alternator. I am not familiar with the external voltage regulators since my car was already updated. If you don't have the external regulator then I would suggest you pull the alternator out and check to see how worn out the brushes are. I have never had an alternator go bad in any of my cars and it has always been the regulators. These are frequently miss diagnosed as a bad alternator. I have never seen a parts store diagnose properly so don't trust their test.
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Old 07-27-2009, 08:12 PM
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Are those platinum plugs? My mechanic recommended against platinum plugs for the CD ignition. Could that be the problem?
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Old 07-27-2009, 08:54 PM
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I have never heard of an internal regulator having brushes....that is not the way it works. The brushes are part of the alternator not the regulator.
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Old 07-27-2009, 10:31 PM
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yeah it's all in the semantics .. the brush assembly is indded part of the alternator and would be calssified as a bad alternator. The only things that go bad in these tings are really the brushes and the slip rings. Occasionally you can have a diode bridge die to.
When you have your alternator overhauled by default they should put in new slip rings and brushes and replace the bearings.

That's all this thing needs really. Too bad there isn't a lister close yto you . I would have no problem sticking your regulator in my car and seeing what happens to the voltage. Is there not a guy with an old external regulator car that could help tis guy out .. it is like a 10 second swap.
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Old 07-27-2009, 11:03 PM
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I am not sure what those plugs in the picture are, however I am sure I had the same problems with the new copper plugs I put in two days ago.
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Old 07-28-2009, 01:06 AM
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A few more things... I read in the bentley manual "a burned out instrument cluster warning light may prevent the alternator from charging on some early models. " 81 is not an early model, but I can't find this warning light anywhere. Were the lights only on early model SCs?
Also, I don't have anything where the regulator is supposed to go, so I think I have an internal regulator. I'll know for sure when I get the pieces out this weekend.
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Old 07-28-2009, 01:36 AM
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The alternator light is in the oil pressure/level gauge, I think. It should come on when the ignition is on, and turn off when the voltage reaches charging level. This is how you know that the light is working. It sounds to me like your alternator light should be on. It also sounds like your alternator is bad. You can take it out to have it tested. The voltage regulators are an easy replacement, as already noted.

If the fan is stuck on the alternator or the alternator is stuck in the fan shroud, then use light tapping, no prying. The fan shroud is somewhat brittle, and more than somewhat expensive and a pain to replace.
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Old 07-28-2009, 11:20 AM
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Originally Posted by aadrew10 View Post
I am not sure what those plugs in the picture are, however I am sure I had the same problems with the new copper plugs I put in two days ago.
Right... show one thing, do another and then ask for help.
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Old 07-28-2009, 11:58 AM
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Back on point .....

You are in fact correct. The alternaor light is part of the " field circuit" which sets up the field for charging. you should definitely have that light on when your ignition is on . There should also be a diode in the circuit. In simplistic terms when the battery voltage is higher than than the alternator output the current flows towards the alternator lighting up the light. ( as the alternator has to be at least battery voltage to charge it ) . When the battery voltage is lower than the alternator , as it should always be when the engine is running, The diode stops the current flow and the light will not iluminate.
As stated this circuit acts as a dual function and also sets up the field effect for charging so start with that light bulb for sure.

Another good test to see if it is the root cause of your issue is fully charge the battery. Drive around the block and see if you have good power a fully charged batery should be more than enough to run the ignition. Caution - do not go to far. As the battery dies your car will start to splutter and miss as you do not have good spark.

I am a firm believer that you have to fix what you can see, before you can fix what you can't see. So get this alternator and battery cahrging circuit fixed , then we will move on from there. If in fact you do have an internal regulator ( send us a pic of where your relay should be and we will know for sure ) Be aware of the chineese manufactured regulators as I have heard they are junk. Guys in here who have the internal regulators should be able to guide you with this . I have one of the old external regulators .... you know .. the reliable ones
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