1986 Chey Silverado 4X4 stumbling... still!

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  #1  
Old 11-12-02, 07:33 AM
rgrogan
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1986 Chey Silverado 4X4 stumbling... still!

OK. I've done the basics. Complete tune-up. New fuel pump.
Carb rebuilt. This thing still has a lack of throttle response on
acceleration. Will usually pull itself out of the hole... eventually.
Has developed a new problem. At cruising speed(45-55), it has
also started to "stumble". Its like all of a sudden you drop an
anchor. It will usually recover from this also, but is getting
worse.

Can this still be a carb problem? Fuel pump failure?

This is getting pretty expensive, since I am having to get all
this done by a mechanic. In this area, all the mechanics want
to throw new parts at it because they get a commision on
parts. I don't have a problem with that if the original parts
are bad.
 
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  #2  
Old 11-12-02, 04:29 PM
Joe_F
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If it stumbles out of the hole on initial acceleration, a bad accelerator pump in the carburetor is a possibility.

Does fanning the gas pedal help at all? Will it do it flat to the floor out of the hole, partial acceleration, light blip of the throttle?

Was the fuel pressure checked? How about the exhaust for restrictions/stuffed converter? (If the carburetor was bad it will likely stuff the converter eventually).
 
  #3  
Old 11-12-02, 05:44 PM
davidf
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Remove air cleaner cover and look down the carb.throttle barrel and see if there is a squirt of gas when you open the throttle with the carb acelerator cam with the engine ignition off.This is to check if the acelerator pump is working.
 
  #4  
Old 11-12-02, 11:34 PM
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The symptoms you are describing lead me to think that your centrifugal advance weights in the distributor are sticking. This will cause erratic power loss, especially when trying to accelerate. I would clean them and replace the nylon bushings in them. Cheap easy job. This may be part or all of your problem, and if it hasn't been done in the past few years, it needs doing anyway.

Check the accelerator pump too, but this usually shows itself only when accelerating from idle for a couple of seconds.
 
  #5  
Old 11-13-02, 07:00 AM
rgrogan
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Joe,
Accelerator pump checked good with a solid stream into carb.
Sometimes it will clear up with fanning the throttle, sometimes it will only clear-up only if I floor it for a few seconds. It is steadily getting worse.
Haven't checked the cat; because this has been an intermittent problem(although now at about 90% of the time). What is best way to check cat, for a diy-er?
Haven't checked fuel pressure for same reason, plus it was a new unit, not rebuilt.

Cheese,
Are the distributor centrifugal advance weights something that should have been taken care of when my mechanic did the tune up, or is that something above and beyond?

Thanks to all!
 
  #6  
Old 11-13-02, 08:47 AM
Joe_F
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With the exhaust STONE cold (so you don't burn yourself), rap on the converter with a fisted hand. If you hear pieces jingling around in there, the converter is finished and must be replaced.

That means too much fuel poisoned the cat (rich running condition), and the cause must be corrected before installing a new cat, or the new one will die the same fate .

Don't assume that the fuel pressure is good because the fuel pump is new, defects happen. .

No, the weights and servicing of the same are not part of a normal tuneup, but are work a look/see.
 
  #7  
Old 11-13-02, 11:06 PM
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In addition, you may want to recheck your timing, check for an open HEI ground and also the ever pesky EGR valve.
 
  #8  
Old 11-15-02, 12:04 PM
rgrogan
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Just got call from mechanic; they rechecked the timing and said it was off. They just tuned it up last week, so I don't know what is is going on. Any way, they say its running fine now. At least there was no charge.

We'll see.....

Thanks again,
 
  #9  
Old 11-17-02, 01:44 PM
rgrogan
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No such luck! Picked up the truck Saturday morning and just barely got it home. What really gets me is that it runs great in the drive. It idles smooth and revs great. No hesitation in the throttle, skipping or anything abnormal. Get it on the street and it just will not accelerate. It is now starting to backfire under load. Back to the mechanic tomorrow.
 
  #10  
Old 11-17-02, 05:59 PM
Joe_F
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Mileage? Vehicle ever had a timing chain and gear replacement?
 
  #11  
Old 11-17-02, 06:47 PM
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Disconnect the cat convertor and muffler and take it for a noisy run and see if performance improves.
 
  #12  
Old 11-17-02, 08:52 PM
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The red wire that connects to the coil in the distributor cap may be loose/corroded and making poor contact. This will cause all sorts of funny problems. The timing chain and gears as Joe mentioned would be a good place to check too. Especially if they haven't been changed in the last 60 to 80K miles. Could also be distributor pickup, module, advance weights, or worn shaft bushings.

When you said it got a "full tune-up"...you did mean plug wires too, right?
 
  #13  
Old 11-17-02, 09:34 PM
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Has anybody done a compression check on it yet? Might have a worn or cracked valve. How about a worn camshaft? Just a couple of possibilities from personal experience with same symptoms.
 
  #14  
Old 11-21-02, 07:02 AM
rgrogan
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Finally! My mechanic finally solved this puzzle;
although with several calls to a regional GM service
rep.

Spark control module inside cab. Had to remove
glove box and most of dash to get to it. Apparently
this thing doesn't just go out; just has a slow death
that torments you for weeks with intermitent
problems. Runs like a screaming eagle now. Almost
as good as my old '68 Firebird 400. (Boy I wish I still
had that car!)

This thing should be good to go for many miles now
as almost everything under the hood is new. Of
course for what I've spent on rental cars during all
of this, I could have almost swapped in a late Vortech
350!?!

Thanks to all for your advice!
 
  #15  
Old 11-21-02, 10:11 AM
Joe_F
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Good job, glad it's sorted out!
 
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