Scratching my head on this one. My '97 Lt1 engine has this weird problem. When the outside temp is below deg. F. or so the car idles rough until it warms up. I cleaned the ICS pintle and throttle plate and ran GM fuel system/injector cleaner/top end decarbon stuff through it and no change. Press on the gas just a little and when the rpms come up, it smoothes out. The car had new plugs and wires 30,000 mi. ago and since there is no mis under acceleration at all, I'm not thinking ignition anyway. As soon as everything warms up, the problem goes away. Put up with this most of last winter and it went away all summer. Anybody got any ideas?

 

The PCM richens the fuel mixture during warm up. What if the PCM is reading the wrong temperature and thinks it's warm before it actually is?

 

It is worth a look. Thanks for the hint. Generally what happens in a sensor failure is the vehicle becomes hard to start because the mixture is too lean. I guess this could be enough to be too lean but not enough to cause a hard starting condition. I'll drag out the Autotap and see what is what temp-wise.

 

Oddly enough, the problem seems to be with the alternator. I started the car and had the usual symptoms, I put my hand on the alternator and moved the throttle just a bit and there was the same shaking and the alternator got hot under my hand. I guess the alternator is getting marginal when it is cold. The battery is new so I don't think it is a charging problem. This is a new one on me. I guess I'm going to need a new alt. soon.

 

What did Autotap show the coolant temp psensor reading to be on cold start? Any codes? At what "outside temp is below (??) deg. F. or so" does this happen?

If you feel its related to the alternator, what does the dash voltage gauge read on cold start? ....when fully warmed up?

 

I haven't time to hook up the Autotap with the engine cold, maybe tomorrow. Problem seems to occur when outside temp drops below about 72. It has been hotter than that here at night for months. Oddly enough the charging voltage looks just like it always did, perfectly normal. That's one reason I never suspected a problem there.

 

Im having the exact same problem...does yours do it also when you restart it after sitting for a few minutes ? say you drive it to a store, come out and start driving and seems like its missing on half the cylinders?, poping and carrying on for about a minute and magically corrects its self?

 

No, mine isn't really a miss at all it is just roughness. I guess I need to take the serpentine belt off the next time it does it just to be sure it is the alternator. I still can't understand why it would do this unless it is being dragged down by having too big a load on it. The battery is fully charged and less than a year old. I suppose it could be some kind of internal short that only occurs at lower temps. I could rev the engine with my hand on the alternator and feel it heat up under my hand, which I thought seemed not quite right.

 

Update. Still don't know what it is but more of an idea on what it isn't. Borrowed an AC/AD amprobe to test alternator function and the thing works perfectly. Started the engine after being parked all weekend, charging current 29 amps. Threw on the battery tester load and the alternator went up to 98 amps. Clearly nothing wrong there-I guess it is possible that there might be an intermittent alternator bearing problem but no way to test for that. I'm wondering now if the noise could be coming from the idler pulley. Usually they make more of an audible squeal, at least the last one did. More later...

 

These cars came with 120amp and 140amp alternators so I would say 98amps is low.

 

The 98 amps is a function of the load I put on it. If it will do 100 amps, there isn't anything wrong with it.

 

I gotcha.

 

can you take it anywhere to get it tested ? i know advanced does it, even local shops should have a battery tester and it will show if the alternator is charging. typically if the alternator gets hot ifs not good anymore, if cant keep up with the draw and is working too hard. I also think you have too separate problems but fix the alternator and see where that gets ya.

~greg

 

Back from the nearly dead. Found part of the problem, at least. I was at Advanced Auto and had them throw the computer tester/load on the battery and it's cold cranking amps was about half normal so they replaced it on the spot. This resulted in no change. I hate just throwing money at a problem but I was pretty certain that the alternator had something to do with the problem so I got a rebuilt one. Surprise! Most-but not quite all-of the problem went away. There is still a little shake at cold idle but it isn't very bad. Probably a passenger in the car wouldn't notice it at all but I do cause I'm tuned into it. I can only surmise that the old alternator had some sort of field short that was causing it to be hard to turn with the engine running. Go figure.

 

Since I solved the problem I thought I'd resurrect this thing one more time. The culprit all along was the balancer. For some reason the aftermarket balancer is temperature sensitive, I guess the rubber element is too stiff to work below a certain temperature. I confirmed this by heating it with a heat element and verifying that the shake went away. I replaced it with a used balancer off a '94 that I found and the problem went away. This was some detective story.