Hey all,
I've had this problem that I've been wondering about on my SL2, but my brother's SL1 has the same problem.
After the engine has been running for a bit and when the engine temp reaches operating or higher (1/4 on the gauge and moving, 1/2 when still in traffic), the engine starts sputtering a little bit and runs a bit rough, kicking, etc. Not enough for it to affect anything other than it being a little bit "unrefined" (that's the only description I can think of). I can't take a video of it right now, but think a small, intermittent missfire, although it does start out minimal and gets worse and worse with longer driving.
The question I had was does this happen to anybody else? I only have the sample size of two, and it's the same exact symptoms. It's worth noting that we both have aftermarket mufflers with better flow, but I very highly doubt that's the problem. My guess is probably spark plugs, as both of us have not changed it since getting our cars (his 2 years ago, mine 6 months ago).

If this isn't common or normal for this engine, what do you guys recommend to start with when troubleshooting?

 

I would check for any pending codes with a code reader. If you replace the plugs please buy the proper OEM spec plugs, they work the best in these engines.

 

Having put almost half a million miles between two mid-90s sc2s, I think I know exactly what you're talking about. My coupes would start to run ever so slightly rough as the engine temperature increased while sitting in traffic, usually when the temp gauge goes above 1/2 and the car is not moving. I always attributed it to the fact that it is not the most refined engine ever put out my GM, and the fine tuning was never really done. The engine has a horrendous vibration resonance which increases right around idle RPM. If your upper torque access mount is worn out, these vibrations significantly increase and get transmitted into the cabin. As such, any small imperfections in the way the engine is running gets amplified as far answer the driver is concerned because of vibration is coming through more strongly.

Once the auxiliary fan by the radiator kicks on and cools the engine down while sitting still, or you start moving again and the temp gauge shows around 1/2 or less, the vehicle usually returns to running more smoothly.

That's the best I can do.

If it were misfires, believe me these engines and the design of the ignition system would flag them like nobody's business.

I of course concur with Andy concerning checking for codes and using the stock NGK copper plugs. People put all kinds of expensive plugs that the ignition system was not designed for. Some cars don't seem to care, others puke on them. Just use the stock ones and cross it off the list of possible causes.

 

No codes (although I did just have the battery off yesterday, so I'll take a look again in like a week or so, but there hasn't been any SES light in the last couple of months either.

I did see that it's got the NGK Iridium plugs in it. They look like they might be sparking too hot. Went to the local o'reiley shop and they don't have the copper/nickel ones that the car actually wants, so I'll have to wait for some to come in from RockAuto along with some new wires, since these have a little bit of gouges and nicks in them, probably my fault from all the work I've been doing on it. Do you guys have any recommendations on what wires to use too, or just use whatever?

I did find that the spark plugs did have oil on the threads, is there any chance that this might cause some ignition problems too? I know for sure it means a new valve cover gasket is in order .

I'm not finding this to be the case. Just little itty bitty kicks just enough to feel inside the driver's seat even after the radiator fan cools it down. This appeared after a long trip a couple months ago. I did end up stuck in stop and go traffic uphill for about an hour due to an accident (looked like somebody's car had caught on fire ) so that might've caused this problem originally. I had to have the heater blasting and everything.

 

Looks like you got a Good Deal. And it will be worthwhile, giving it a major Tune-up while learning about your SL2.
Glad you found a oil leak down by the spark plug. This will need to be fixed before the new NGK copper plugs go in.
For SP wires many Sl owners use the Blue Color Wires and they are good reliable wires.
For the rough running, Clean or replace the PVC valve.
IT can be hard to find at first but it is in the valve cover and close to the Throttle Body air intake..
Remove the hose and pull it out with ( Needle nose Pliers )
(spanners) ( small vise grip) or what have you...Clean it out with Brake parts cleaner untill it rattles and moves when its
upside down or just buy a new one.
Next learn how to remove and clean the Throttle body (air Intake) with Brake parts cleaner...it needs it...cheers

 

It seems that my next step truly is new spark plugs, and wires.
I already did this. I had to replace the coolant intake on the stupid plastic intake manifold (Don't get me wrong, that has been the hardest thing I've done on a car so far) so I had all that off and everything. Pretty much anything that has to do with intake has been cleaned quite well.
The same story there. I did still take a look at it though and clean it again just now. I did the intake manifold repair about 20k miles ago.

 

I uploaded a video on how it's running right now. It comes across so much more in the camera audio than it does in person, but it actually has a pretty loud tick. And just ignore the "exhaust" part. It seems my phone couldn't handle the sick muffler I have on it . But you can hear it going "put put" in it in person as well.

The rocking is absolutely not due to mounts. Those have just been replaced 1k miles ago about 2 months ago.

 

Okay There could be, like you wrote before, "sound like a near misfire" . And it could be from the wrong Plugs
With this engine design the platinum plugs tips get fried too soon. So the cure is Oem Copper plugs.
And new wires come with dielectric grease Many people don,t know its for keeping water / moisture out
so than they end up with the grease on the Spark Plug contact to the wire causing misfire.
Check the order the wires are plugged into the coils. Cylinder 1 is on the LEFT when standing in front.
the order they plug in To the Coils is not 1234 it is 4123 On the SL1 SOHC I need to look it up for the SL2
.When changing plugs you don,t want oil on the treads. The spark plugs need to get ground from the engine block.
and a safety tip is never touch spark plug wires with engine running because they carry H.V. 20k
When you have time Clean the flap on the throttle body again. Because it's a common cause of rough idling.

 

Why would you need the heater blasting while sitting in stopped traffic? If everything works as it should, the aux fan under the hood will cycle on and off based on the inferred temperature from the ECTS resistance. The temperature gauge needle May go past the three-quarter mark near the red before the fan kicks on. This freaks out people new to the land of s cars, as intuitively, that's not what you want to see. Remember that that gauge response is not linear. If your cooling system is healthy and the fan motor is healthy and the fan relay is healthy and the ects is healthy, and the front O2 sensor is healthy then everything should be okay and it should cycle without overheating.

The only way you will ever convince yourself of the above is to let it idle in your driveway until the temperature eventually goes up high enough. Tougher to do if it is cold out but you can work the gas pedal to get the coolant temperature up there. Until you see it the first time heading right towards the red and then the fan kicking on and cooling down, you won't believe me.

If by saying that's not what you're seeing you are saying the fan never kicks on, that is a completely different issue.

What kind of mileage is on this engine?
The ticking is a combination of fuel injector noise and lifter noise. They all sound like this.

How much oil does it burn?
Is it the original engine?

If the air conditioning still functions, with the HVAC fan off, depress the AC button. Turn the fan to the lowest setting. The light should come on on the AC button and the AC clutch should engage with the compressor under the hood. At the same time, the aux fan under the hood should come on and stay on for the entire time the AC clutch is engaged. This is by design and is an easy way to test if there is an issue with the fan relay or the fan motor. But the compressor has to be able to kick in.

These engines vibrate at idle. They just do. They have a bad resonance around idle RPM. I wish my spark plug wire only vibrated as much as yours. Have you checked for vacuum leaks? Compression test?

I'm not just throwing random things out there. There are a lot of sources for tiny imperfections and how it runs. Just remember it's a 24-year-old engine and is likely not to run smooth as butter at this point in its life.