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Old 11-14-2004, 06:45 PM
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Acceleration Problem-Highway

Hello,
I have a question I have been trying to find the answer to. I have a 91 Caprice with a 305 TBI and a 700R4. I have good acceleration on part throttle, half throttle, and 3/4 throttle. When I put it to the floor at a stop, it peels no problem. Problem is on the highway when I give it full throttle it downshifts revs like mad, but doesn't really accelerate at all. No backfire, no weird noises, etc. If I just use 3/4 throttle it accelerates well. What is going on? Is this a transmission problem? Fuel problem? Advance problem? Please someone help!

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Old 11-14-2004, 08:41 PM
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WILD SHOT in the dark........guess answer.....



I had the same problem on a 90 Silverado ( 350 TBI ) and it was the fuel filter. Have you changed it lately? Changed it EVER ?


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Old 11-14-2004, 08:52 PM
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I'd really hate to even try to make a guess based on that disciption can you try to expand a little more?
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Old 11-14-2004, 09:36 PM
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I had a problem with the converter lock up. It had similar symptoms. I think the 700R-4 had a mechanical lock-up.
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Old 11-14-2004, 09:54 PM
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Power loss

I'm with Deuce, sounds just like a fuel filter. And when they start getting stopped up they do just like what your car is doing. And about 75% of GM pump failures is due to not replacing the fuel filter about every 30,000 miles.
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Old 11-15-2004, 04:08 AM
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Sorry if I was vague. I have gone through the fuel system within the last 1K miles, dropped the tank, replaced the pump and sending unit, replaced inline filter, new injectors. I have also replaced the trans filter. As far as what I have done to it; I have ported and machined the throttle body, 1 inch spacer, plus injector spacer, high flow filter, advanced initial timing, replaced coil (stock), wires (stock) cap(stock) rotor (stock). My car has 93K on the engine. Car has good power everywhere, except on the highway when you floor it. Then it just revs (smoothly) but does not accelerate. I will replace the filter again, but I think it is something else.

oh, forgot to add, it has an electrical lock up (stock).
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Old 11-15-2004, 05:16 AM
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I'm thinking the catalytic converter is getting clogged..............
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Old 11-15-2004, 05:28 AM
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Can't be, cat is removed and I run a flowmaster 40 series. Also, when I say highway speed I mean anything from 55mph up. Seems to be where it makes a difference. Could it be that when it drops into the lower passing gear, it just doesn't have anymore? I revs cleanly, no backfire or popping. Just no go.
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Old 11-15-2004, 07:16 AM
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Check 2 things now that we now the rest of the story as Paul Harvey said.
Base timing- put it back and see what it does. Remember timing gets controlled by MAP sensor after base.
Make sure you have a good vacuum to the map sensor and you are not losing vacuum under wide open throttle conditions.
O2 sensors still in place? Service engine Soon light on and does it work?
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Old 11-15-2004, 07:28 AM
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Car only has 1 O2 sensor on the downpipe prior to the cat. It is new, replaced it just 1k miles ago. I will check the timing check for good vacum and replace the fuel filter again. Anything else I can check while I am at it?? Thanks for the replies so far!
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Old 11-15-2004, 07:43 AM
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WOT

I would not bother with the fuel filter if it only has a 1,000 miles on it, unless for some reason you had bad fuel.
I feel pretty sure now the problem is with base timing.
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Old 11-15-2004, 08:20 AM
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When I started messing with the car I bumped the timing from 0 degrees to about 8 degrees before TDC, think this is messing up the timing or could it be a vacuum leak somewhere that is affecting timing, and I guess not advancing properly when I need passing acceleration?
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Old 11-15-2004, 08:43 AM
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It could be either one. That's why I would put the timing back to base so you know ecm take over controls are correct and I would check vacuum with a gauge. Make sure Map Sensor vacuum hook up is to a main source. I remember years ago people replacing Ignition modules with Electronic Spark Contol over EST problems when it was a MAP sensor at fault without setting a MAP sensor code, but sometimes it was a vacuum problem.
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Old 11-15-2004, 09:01 AM
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Thanks, I will check when I get home and post later or tomorrow evening whenever I get a chance to.
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