abrupt shaking/missing
 

Car is a 94 SC2. Here is the problem.

Start car and it immediately starts shaking and running like its missing. No warning lights, no ck eng lights, fluids are all fine, no over heating or anything. It's done this a few times in the last year and we just had another episode a few days ago. It's been driving perfect now but I'm sure it will happen again. It does not seem to be outside temp related. I checked for vacuum leaks or loose hoses and found none. I tapped on the cat converter with a rubber mallet and it sounds solid. I don't hear any rattles when I bang on it. What makes this hard to figure is it's so random. Any ideas?

update
 

See above post for the problem. This is on an 94 SC2 with about 105K.

It got to the point that the car was doing this daily so we stopped driving it. When you put the car in gear while keeping the brake on it's really a bad idle. Doesn't stall but lopes and the rpms are very low. Seems to be more pronounced on humid days.

Here's what I've done so far:

New plugs (NGK), new wires, new air filter, cleaned egr valve, checked the ignition coils and verified no vaccum leakes. It's still doing it HOWEVER, tonight I let the car idle and after about 15-20 mins it cleared up and ran like a new car. It did not gradually clear up either, it did it like all of a sudden someone flipped a switch.

One observation I made is that before the car cleared up the exhaust smell was noticeable and if you stood behind the car you could clearly hear it missing. Once it cleared up, no exhaust smell and no miss and perfect idle even in gear.

Even though when I bang on it I get no rattles I'm thinking a converter at this point. I'm not sure about the o2 sensor but I'm NOT getting a check engine light. I have no idea what codes if any the car has stored. Any suggestions would be great. TIA

Well - a trip to the local AutoZone (or similar) to get the codes read would be the first step, for sure.

".... tonight I let the car idle and after about 15-20 mins it cleared up and ran like a new car. It did not gradually clear up either, it did it like all of a sudden someone flipped a switch.

One observation I made is that before the car cleared up the exhaust smell was noticeable and if you stood behind the car you could clearly hear it missing. Once it cleared up, no exhaust smell and no miss and perfect idle even in gear....."

From that set of comments, I'd replace the Engine Coolant Tempersture Sensor (ECTS) and its associated electrical connector. Go to your local GM dealership, and get OEM parts (about $40usd). Make sure the sensor has a metal tip on it, not a resin tip.

This sensor modifies the air/fuel mixture ratio of the engine based on the temperature of the coolant. If it goes bad, the balance is lost, and the car will be hard-starting, sloppy idle, and stinky exhaust

Thanks for the reply. I should have mentioned that I already replaced the ECTS. Thanks to this forum for that. The car had a high idle when you would put it in park and the cooling fan wouldn't come on when it should. I replaced it with a Saturn part with the brass tip and the connector and all is well on that front.

I was thinking it may be the converter due to the fact that perhaps after it gets hot enough maybe the inside melts/gets soft to the point that the exhaust can pass without a problem. Again this is just a theory. I will start it today and if it's running well enough I'll take it to get the codes read.

Thanks again

Update # 2
 

Per Ocean's suggetion I took the car to Autozone but they apparently can't pull codes on a 94 car. It has to be 95 or newer. He showed me his plug and it wasn't a match for the car. Hmmm. Anyone else run into this?

Car started and drove perfect today. Yesterday I revved the engine and I wonder if that blew out a restriction in the exhaust. (ie cat convt pieces) I don't like to revv engines that aren't under load so I only was revving it to about 3K. Today while driving the car I kept it in 2nd (it's an auto) and took it to about 4-5K while zipping around in light traffic. I was able to floor it a few times and I did not see any smoke coming from the exhaust and the car seemed very responsive and smooth. Till today, I didn't realize that these are fairly quick little cars! I don't drive the car much as it belongs to my grandmother and I'm trying to fix this for her. So it's in my possession for now.

Top engine mount ? If you can stick a pencil between the top of the rubber and the steel strap, replace it.

The hissing noise is a dead givaway to a plugged Cat,
To confirm, take the exhaust loose before the cat and drive it( yes it will be very loud) but it should run better .

Oops, I should have mentioned that I already replaced the top engine mount thinking that was also contributing to the problem. The one I removed was original and even though it didn't look too bad I was able to bend the rubber part quite a bit with my hand. I didn't know about the pencil trick but I looked at the mount real close and I believe you could have easily put a pencil through the top of it. I already threw it out otherwise I'd go try that right now. I ordered the Anchor mount (frowny face mount?)and since then I've read these are not the best. I installed it anyway so when this one fails I will make sure to get a genuine one from Saturn. Easy to install.

I also did a valve cover gasket as this one was leaking a bit.

So far the car has been running perfect so we'll see.....

Thanks for the suggestions.

As to a plugged catalytic converter, I have had a few of those in my life including my 94 Saturn. They generally do not fail in such a way that by getting hot they suddenly start working. They fail by being plugged for a variety of reasons I guess, but once plugged always plugged. The symptoms in general become some form of engine overheating or running hot depending on how far driven and how badly plugged and running really poopy simply because exhaust gas can not get through them. While nothing is cast in concrete the generally will not suddenly start working once they quit.
The basics of an engine require fuel, electricity (spark) and air. In general of those three items the only one that can be intermittent for the most part is spark generation. Fuel can be a problem if an injector is intermittent but it is an electrical signal that operates the individual injectors. Other fuel failures do not usually show up as a failure of one cylinder not to work. Air is not an individual failure either. If the filter is plugged, it effects all of the cylinders, not one and it does not magically clean itself. That leaves electricity. A plug wire can give grief but then again it rarely fails intermittently. But spark generation can be a problem. And there is a sensor that controls that along with the coil curcuitry. Electronics are prone to intermittent failures and for that you need the capability of reading stored codes to help guide you through to a solution. At least as I see it. You may have to go to a GM dealer if your local autozone can not read your computor. And pay to have it diagnosed. If this were my car with the problem at this point I would consider spending the money to buy my own code reader that could both read this problem and the newer cars. Used once just may pay for itself.

You stated "no chk engine lights" there for No active codes . there may be history codes, maybe.

uncljohn: you are correct about the basics . well almost... air, fuel, spark all correct, but you neglected one item, at the right TIME.
And air and fuel reaching the cylinder depends on vacuum and compresson, that means the valves and associated hard stuff need to be right.

Sorry I guess I am being to absolute.

Bottom line if you have good compression, air and good fuel, and spark getting where they need to be at the right time. then all will be well. :)