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Carburetors Carb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.

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Old 12-03-2003, 05:31 PM   #1
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why does my qjet have a low rpm surge?

It only seems to happen at constant speed ~ 40 mph, at light throttle. Feels like the engine is surging back and forth as it rides down the road. I have my IAB set to give me about 30-35 degrees dwell. My tps works perfectly. I've hooked up a dwellmeter while driving down the road and found that when this is happening, the dwellmeter is telling me that the mcs is going to full enrichment (least amount of dwell). If I hit the gas or let off the gas, the surge goes away.
Any ideas?
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Old 12-03-2003, 06:25 PM   #2
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Check the O2 sensor.

Either that, or your carb just doesn't like you...
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Old 12-03-2003, 06:54 PM   #3
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Nah, O2's barely a year old.

Stupid lousy carb... I'll fix you!

What else happens under those same conditions? Charcoal canister? EGR? I still have both of those things sitting under the hood, non-gutted. By the way, how does the EGR work properly if there's only one on one side of the motor? Seems to me that whatever benefit it provides would be lacking on the other bank.
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Old 12-03-2003, 11:51 PM   #4
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The EGR merely picks up exhaust gases from one side of the manifold. The port to which the exhaust is routed in the inlet charge is in the middle and goes to all cylinders equally.

Only being a year old doesn't keep the O2 sensor from being bad. I was thinking you could monitor the signal and the MCS at the same time and see what's going on.
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Old 12-17-2003, 02:09 PM   #5
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Well, I put my old sensor back in (I had replaced it for no good reason) and the car behaves the same way. I don't have a direct way of testing the sensor so I did that instead.

A thought occurs: while driving the car today I noticed that the surge doesn't always happen right away. Sometimes it takes a few seconds cruising at light throttle before it starts loading up. If I floor it right before I go into light throttle, the engine behaves normally for a long time before it loads up. Is it possible that at light throttle my oxygen sensor is cooling off? It's a single wire type, would it be possible to retrofit a four wire sensor in it's place? What cars came with those so I know what to say to the guy at the parts counter?
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Old 12-28-2003, 12:06 PM   #6
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Does anyone ahve a gm car with a four wire sensor? Can you tell me which wires do what?
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Old 01-01-2004, 08:33 AM   #7
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Use a 3 wire

Use a 3 wire the GM part # is 25176708. It is a heated sensor. I believe for a truck. You may want to find one at a local parts house first. I got this pic from a website sometime ago.

A long time ago I had an S-10 that did the same thing that you described and it turned out to be the MAP sensor. I know that the LG-4 in my T/A had one before I put the 355 in it. Just a thought.
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Old 01-05-2004, 06:32 PM   #8
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hmm... Map sensor. Might as well try that. Thanks for the tip.


Why does my car need a friggen map sensor?
If I wanted to buy a bunch of do-nothing sensors I'd have bought a fuelie car.
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Old 01-17-2004, 12:55 PM   #9
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Nope. Not the map sensor. Bought new map sensor. Didn't fix it. Replaced the baro sensor with the old map sensor. Didn't fix it either.
Now the computer gives me a code 41. "No distributor reference pulses to ECM at specified engine vacuum"

What is that mean?
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Old 01-24-2004, 04:46 PM   #10
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Replaced the baro sensor today. No dice.
How many other sensors are left on this damn car?

Now I see why everyone ditches their computer...
9 mpg is just not worth it.
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Old 01-24-2004, 07:01 PM   #11
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Have you checked to see if your torque converter is locking and unlocking at that steady speed?
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Old 01-24-2004, 07:06 PM   #12
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Good point.

How do I do that?
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Old 01-25-2004, 06:48 PM   #13
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Quote:
Originally posted by Homer
Now the computer gives me a code 41. "No distributor reference pulses to ECM at specified engine vacuum"

What is that mean?
Don't know, I will look it up in my manual tomorow (its at work). I would assume that the distibutor is not talking to the computer, maybe because the module is bad, the ecm is bad, or there is a broken unplugged wire, etc. Not sure on this one.
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Old 01-26-2004, 02:25 AM   #14
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Do you have an Auto Xray or something similar. Or could borrow one from someone?
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Old 01-26-2004, 09:43 AM   #15
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No. What does it do?

Here's another hint: I found out today that if I press the brake pedal just enough to turn the brake lights on, the symptom goes away. Does that mean it might have to do with the torque converter lockup?
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Old 01-26-2004, 10:26 AM   #16
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Could very well be. That's how you check TCC lock-up.
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Old 01-26-2004, 01:47 PM   #17
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Well.....

What's the next step then? I've ridden in tcc automatic cars before, I know this is not normal. And I know that you don't need full rich at constant cruise speed.

Should I be looking for a short circuit? Is my ecm bad? Does the converter clutch add nvh to the engine bay that would activate the knock sensor?
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Old 01-26-2004, 07:54 PM   #18
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Xray is basically a diag computer that you plug into the ALDL to see what sensors are reading what.

Actually, TC locking and unlocking at low speeds is fairly common. You can get a new chip burned with the TC lockup speed raised. Also, alot of the aftermarket chips have raised TC lockup speeds.

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Old 01-26-2004, 09:46 PM   #19
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Still getting code 41 reliably. "When the distributor reference line signal is lost, the engine runs full rich and with retarded (base) spark timing." I think that's what's happening when the tcc locks up. Why? Code 12 is only set when the engine not running, indicating that the code 41 is intermittent.
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Old 01-26-2004, 10:33 PM   #20
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You should still be able to pull the code 41 when the engine isnt running unless the ecm is getting reset. 12 should always be present as long as the ECM is getting primary and backup power.
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Old 01-31-2004, 01:14 PM   #21
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Now the damn thing won't even start today. Changed plugs (I never pulled a plug out of it that wasn't black) and it still won't start.

What the hell is this? Is this what carburetors are all about?
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Old 02-02-2004, 04:08 PM   #22
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Installed a new Accel hei module today. OK it's not the module. The module I installed was defective from the start. Carb loaded up, ignition timing retarded. Got godes 12 and 41 while idling (which means that the module is no good). Took the Accel piece back, installed the new one. Worked like normal. Bumped base timing up from 6 degrees to 10. Now I don't notice the problem. I'll clear the computer to see if the problem comes back. Maybe someone will learn something from this odyssey of mine.
Later,
Homer
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Old 02-02-2004, 06:20 PM   #23
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Factory ignition modules are known to go bad after 10-15 years.

They just don't make things like they used to. Why, back before 1975, we just replaced the points & condensor every year or 10,000 miles, whether they needed it or not. None of this stuff of only lasting 10 years...

(FWIW, I hadn't been checking this thread regularly. The reason it stopped acting up after unlocking the TC is you effectively lowered the cylinder pressures, making the air/fuel mixture easier to ignite. Your mileage should increase now, assuming the coil wasn't taken out by the bad module.)
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Old 05-20-2004, 06:08 PM   #24
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Finally threw in the towel. Took it to a shop. Guy says the ECM is faulty. $210 for a new ecm. Do I really want a new ecm? I'm getting tired of getting screwed by doing things "the right way."

If I decide to take the "meathead" way out, what else do I need to worry about besides carb, distributor, TCC, Evap? How do evap canisters work with nonfeedback carbs? that part confuses me.
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Old 07-22-2004, 04:18 PM   #25
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Is anyone still following this thread?

Found a new ECM at NAPA. Had to special order it from the factory. $100 plus core exchange. After replacing the black box, mileage jumped from 10 to 14 mpg. I don't really feel like I figured anything out, just by changing parts one at a time, but I guess sometimes that's the only way out.
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Old 07-22-2004, 04:18 PM
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