1990 3.1 Chevy V6 stumbling problem - Hot Rod Forum : Hotrodders Bulletin Board
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Old 12-18-2007, 10:32 PM
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Brian Martin,Freelance adviser
 
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1990 3.1 Chevy V6 stumbling problem

Do any of you know anything about these darn motors? It has a horrible stumble when you step on the gas. Stomp it to the floor and it will smoke the tires. It will start at any time, stick my hand in thru the window and turn the key and it will fire up. But put it in gear and step on the gas and it falls on its face, then races of like nothing is wrong.

It has 35 lbs pressure at idle and 42ish when the motor is not running but the ignition is on. It passed smog, and if I run the idle up a little manually the thing will hardly have the stumble but the RPM is way too high.

Brian

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Old 12-18-2007, 10:38 PM
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What is it in? Automatic?
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Old 12-18-2007, 10:44 PM
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Brian Martin,Freelance adviser
 
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LOL, what is it in? Will you still help me? It is in a stone stock Celebrity Wagon And it's an automatic.

Brian
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Old 12-18-2007, 10:51 PM
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The intake gaskets are prone to cracking, and will cause the motor to run as you describe or worse. Sometimes they run a long while as you describe. Maybe a TPS or other sensor. Run a cooling system leakdown test. I've seen hundreds of these and 3.4's do the same thing. 9 times out of 10 it's been the intake gasket. They deteriorate at the coolant passage and then continue until they reach the intake port, maybe just a tad now but it will get worse, and you may end up ruining themotor if you don't find out that it is not your problem. Do the pressure check before you do anything else
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Old 12-19-2007, 02:26 AM
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x2 on Strokes suggestion

The lower intake manifold gaskets were very prone to failure on the 3.1's and 3.4's, often causing a nice coolant leak that is difficult to track down if you don't know where to look. While I've never seen one that made it's way to the intake ports, it is very possible. Pressure up on the cooling system and check the valley where the head/ lower intake/ block meet and see if there is any coolant present. If so, you have a gasket failure, and need to replace it before it gets any worse. On a brighter note, these motors have a number of vacuum lines, particularly the hard plastic ones that are prone to cracking. Check all the vacuum lines, especially at the inside of the bends, and make sure that none of them have developed a crack over the years. Also, some of the 3.1's had a large vacuum line, the pcv I think, that tied in to the upper manifold using a tee fitting just after the throttle body. This fitting plugs in to the manifold using a ribbed grommet that can harden and become loose over time. Because it ties in on top of the manifold, gravity and the tubing itself can make the fitting look like it has a nice, tight fit in the manifold, but on further inspection proves to be just sitting in the hole. If this is the case, it may seal when the motor is just idling, but when opened up a little ( enough to cause the motor to move a bit) it will break the weak seal and cause a pretty large vacuum leak, i.e. your stumble.

If none of this helps, it could still be a IAC motor or maybe even a dirty throttle plate. Hope this helps a bit!
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Old 12-19-2007, 02:58 AM
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The Intake gasket is a plastic gasket with o'rings. They are prone to leaking coolant out of the ends first. I have found worn out throttle cables, dirty throttle body's to be a problem. There is also a crank position sensor on the back side of the block (in the center of the engine a couple of inches above the oil pan on the firewall side). If the engine has a lot of miles on it, check the timing components for a lot of slop. They also have a transmission kick down cable that might need to be adjusted. (depress the lever and have someone push the gas pedal to full throttle to adjust)

I've also had the same problem being caused by the vacuum connection's above the throttle body. This caused a high, or erratic idle.

The TPS is non-adjustable on these engines. It could have a flat spot(most likely not).

A vacuum air leak can effect the mass air flow sensor.

If you feel a "catch" in the accelerator pedal right off of idle, take the cable off of the throttle body and pull the cable by hand to see if you can feel a bind.

Have someone push the gas pedal slowly and watch for a jump in throttle movement.
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