1994 Suburban shakes until 2500 rpm

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  #1  
Old 03-20-09, 09:26 PM
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1994 Suburban shakes until 2500 rpm

Alright, I got a 94 Suburban 4wd with 5.7L engine and auto tranny. Recently it started shaking while having the throttle pushed in. Today we changed the plugs and wires and the shake became more noticable so I started paying attention to my gauges. It doesnt matter what speed I'm going as long as the throttle is pressed in and the rpm's are below 2500, it shakes. Once the rpm's reach 2500+ or the throttle is let up on, it stops shaking. What are my most likely causes?
 
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Old 03-20-09, 11:42 PM
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well, tough to be real specific but I suggest you were on the right track with the ignition parts. I would think you have a serious miss problem but it was not due to the plugs and wires.

not sure what ignition system GM used in that year so a couple other things to look at are, if used;

distributor cap
coil pack

if this is fuel injected, you could also have a fuel problem such as a bad injector.
 
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Old 03-21-09, 05:08 AM
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Could it be a sensor going/gone bad? I talked to someone who has a similar problem with a 90 model astro van with a 4.3L and they had to change a sensor but they don't remember what sensor it was.
 
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Old 03-21-09, 12:47 PM
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sure it's possible. I believe Auto Zone does a free computer scan. A bad sensor should throw a fault code in the computer.
 
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Old 03-21-09, 01:18 PM
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Did you analyze all the old plugs to see if it appeared if one cylinder was being affected, or not? That way you could now recheck that same cylinder's plug.

Did you remove the air cleaner to see if anything looked amiss there as far as dirtiness, oil, or any closed air mechanism?

If one cylinder seemed to be picked on, you could hone in on a particular coil, plug wire, secondary distribution contact point, or one particular fuel injector, let's say.

If it did not appear any particular cylinder was being picked on any more than the others, you could then check to see if some common fuel problem like pump or filter. Or an electrical problem that relates to ALL cylinders. Or an engine vacuum problem. Or a spark advance(retard under load) issue.

Does this vehicle utilize a distributor?, or instead uses a crank position sensor and cam position sensor?
 
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Old 03-21-09, 02:26 PM
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Could the EGR valve have something to do with it? We unplugged the vaccum line from it and it had no vaccum. Is it supposed to have vaccum on it?
 
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Old 03-21-09, 02:37 PM
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it shouldnt have vacum at idle the egr only works at light throttle and cruise speeds if you leave the vacum line off and drive the vehicle to see if it still shakes if it doesnt shake would replace the valve with a new ac delco valve.
 
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Old 03-22-09, 12:16 PM
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Alright, unhooked the EGR Valve vaccum line and still shakes. Put a new fuel filter on and still shakes. Air filter is relatively new because it's still pretty clean. Where should I look next?
 
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Old 03-22-09, 02:01 PM
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Well, I've already suggested a list of things.

It's hard to know just what to pick first, without use of an engine analyzer. Someone with the right equipment might hone into the right area quite quickly.

If you get frustrated trying to just guess at what component to look at, you might want to choose a garage that has a good reputation and equipment to quickly run down problems. Labor might not be too bad for such an equipped outfit. Sometimes one can be pleasantly surprised at the bill, considering the nuisance problem.

I'm curious if you ever looked at your plugs. Although, that doesn't always tell the problem, even if there is a problem - because unless you shut the engine off while the problem is occuring, the plugs can clean themselves up when it starts running right again, if it runs right again for long enough time duration. I know from my own experience in trying to catch problems in the act, and pulling plugs. I have seen black plugs clean themselves up to that nice tan color, all by themselves, after it starts running right again.
 
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Old 03-22-09, 02:06 PM
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I think I would try pulling one plug wire at a time and see if you get any change. When you pull one and get no change, thats where you start looking.

I've done the same with injectors.
 
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Old 03-22-09, 04:13 PM
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alright, I was running it down the road then the magical SES light came on. I ran the codes and came up with "Code 32 EGR Diagnostic Sw Signaling Incorrect EGR Valve Operation". Can anyone enlighten me on what exactly this means?
 
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Old 03-22-09, 05:18 PM
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is this with the vacuum line to the EGR unplugged? If so, it was most likely due to that. Plug the vacuum line back in. If you have a scanner, is that the only code?
 
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Old 03-22-09, 05:42 PM
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The vaccum line is connected; I do have a scanner and that was the only code that came up.
 
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Old 03-22-09, 09:06 PM
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I kind of wonder if the code wasn;t due to unplugging the hose previously. I would clear the codes and see if it resets.

I have had EGR's burn out and have had them plug up with carbon as well. The burned out situation would cause a miss but there are also other things that could as well. Your situation doesn't sound like a typical EGR problem.

It still sounds like an ignition problem to me.

is there any missing at idle?

can you leave the scanner active and drive the truck? If so, I would drive it and keep an eye on things, if you have a scanner that would show you anything in this manner.
 
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Old 03-23-09, 02:17 AM
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See, it only does it when its not in park, in park it runs fine. Its when you're in overdrive and drive that it'll do it. Sitting at a light it shakes a bit. When driving if you're trying to cruise at a certain speed and the rpms are below 2500 it shakes. When I first got it, it did it occasionally, but recently it started doing it all the time.
 
  #16  
Old 03-23-09, 04:15 AM
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Now lets look in a completely different direction. I am thinking you may have a torque converter clutch (TCC) acting up. The TCC only engages in drive and overdrive, and directly couples the engine to the trans input shaft, much like a clutch in a standard trans. equipped cars. With the TCC engaged if the engine pulls down too far it will cause a severe shudder, if you step down on the accelerator the clutch will normally disengage and the shudder goes away. Try this, the next time the shudder starts, lay your foot on the brake pedal, do not apply the brakes just use enough pressure to turn on the brake lights. This should also disengage the TCC and cause the shudder to stop. Bad news is if a trans. fluid and filter change doesent help, the only other fix is to replace the torque converter. Good luck.
 
  #17  
Old 03-23-09, 06:38 AM
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Torque converter could be a possibility I guess. When it didn't do it often, the only times it would occassionally shake would be when I was stopped at a light and would go or when I'd slow way down and try to accelerate.
 
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