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Old 01-13-2003, 03:25 PM   #1
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what could cause the car to lurch forward when stopped?

My 89 Jimmy is a bit odd. While I'm sitting at a light in Drive, every once in a while I can feel the car try to lurch forward for an instant. The RPMs do not change at all. When I try to accelerate from a stop, the car is impossible to get moving smoothly unless I don't press the throttle until after I'm rolling. If I press the throttle from a stop, no matter how easily I try to do it, the vehicle lurches forward.

My guess is that I've got a torque converter going south, but since I've never had this problem before, I'm not sure. Anybody have a problem like this?
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Old 01-13-2003, 04:59 PM   #2
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Jim,

How it the trans oil level?
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Old 01-13-2003, 08:31 PM   #3
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Dont see how the lurch could be a torque converter issue really... The converter, until it hits 40 mph and locks, is a fluidic link between the engine and transmission. Engine turbine thing spins, moves converter fluid inside, which turns the transmission turbine...therefore they arent physically connected..which is why autos don't stall out coming to a stop. Can't see how that fluidic coupling would cause any of your problems, and there's no way the TCC could be mechanically locking it because the converter cannot lock in first gear...physically impossible.

Lurch at a stop sounds like you're not applying QUITE enough brake pressure to me...mine does it sometimes too, but it's from a rough idle..

Jerking from a low rpm acceleration.....how's it run once you get it up to speed? Rough or normal? How's your timing check out? If it runs fine in higher rpms, maybe your timing chain has slack in it? Check timing..if it fluctuates too badly, time for a new chain.

Hope any of this helps.
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Old 01-14-2003, 04:47 AM   #4
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Does it do it when the A/C is on? Cause my 99 Mirage does it when the air cond is on.. If I dont have the brakes on heavy enough, when the compressor clutch pulls out, the car will jump forward..
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Old 01-14-2003, 10:20 AM   #5
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My suburban does the same thing sometimes, but the idle goes up and down when it does. Still cant figure it out.
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Old 01-14-2003, 10:35 AM   #6
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Sounds like a vacum leak. My MOnte was doing the same thing with my L98TPI, it turned out a vacum leak was casuing that problem and a couple of obvious others like backfires etc. But I would do like Vader said,, how is the fluid level?
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Old 01-14-2003, 02:54 PM   #7
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it's not a vacuum leak. It's something mechanical in nature. The engine rpm does not change and it makes absolutely no change in sound when it happens.

The timing is at 0*, right where it should be. It does spark knock like a bastard unless I run 93, and I haven't found that problem yet. I don't think they're related.

Once I get going, the vehicle runs fine with the exception to the spark knock. I haven't had my scan tool on it yet. I even tow my snowmobile & trailer (heavy iron trailer) without problems. The engine is smooth. It's a 4.3 crate motor with only around 40k miles on it.

Vader, I haven't checked the trans fluid, but I will now. It doesn't seem to behave like a trans that's low on fluid, but before I burn it up, I'll check it anyway.

Thanks for the suggestions guys... and feel free to keep them coming.
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Old 01-15-2003, 06:35 AM   #8
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Knock......running lean?
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Old 01-15-2003, 08:26 AM   #9
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I suspect so, considering that the timing is spot-on. I just haven't figured out why, so in the meantime I've been dumping 93 octane in the tank.
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Old 01-15-2003, 11:22 AM   #10
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Quote:
Originally posted by Nixon1
Can't see how that fluidic coupling would cause any of your problems, and there's no way the TCC could be mechanically locking it because the converter cannot lock in first gear...physically impossible.

It is possible that the converter is locking up in first gear. My car did this. If I started the car and put it into gear it would jump forward and kill the engine. After getting a different converter it's never done it anymore.
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Old 01-15-2003, 05:18 PM   #11
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That converter must've been SERIOUSLY messed up..
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Old 01-15-2003, 05:18 PM
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