From the start, sorry about writing a novel. Trying to be as precise as possible.

I'm gonna start from the beginning. Everything is just as usual, I'm on the highway making my 30 mile drive to college. Just when my exit comes up, i start slowing down from 70mph... and feel something strange in the car's feel. Right away I shut off the stereo and vents to minimize confusion. It feels like its shuddering. I get to my first red light, idle is steady at 700rpm in D and brake applied... sounds normal and then again shudders on and off at random times. Now i'm freaking out, it feels like a "cammed" car, then smooth as ever. On and off. Pull in to the first service station I see and pop the hood. I'm staring at the engine and I can see it run smooth... then it jumps o so slightly, but noticeably, on the car's lateral axis. Now I might add it's snowing and I have a bad hand so I was pretty frustrated at this point that I can't check very much. So I check my fluids, coolant is low (surprised me) but engine was running, i'll have to check that tomorrow morning. Usually that has always been full to the MAX line and it was about 3-4 inches from the full mark. Engine oil was a little low, added some. Couldnt check tranny with my bad hand. Quick visual inspection, everything looks OK...

This went on all day, only seems to be around at low RPMs. Idle to around 1800 RPM. But the car doesn't feel like it lost power... I floored it on some open stretches and it feels fine.
No CEL(and it's not burnt). Gauges are normal. RPM doesn't vary with the "shudders".
Fuel filter is not even a year old. I always run 91octane. Spark plugs changed last year. COPs maybe?? but then why no CEL... Engine mounts?? vacuum leak? but then RPMs are steady.

Now. I just got home and it's a nice and quiet street. So I throw it in Park, no brake. Smooth... AH there it's still doing it. so I go to the back of the car and listen to the exhaust. Usual flowmaster sound and then when the vibration/shudder occurs it sounds like pop-corn in the exhaust. So when the "problem" happens, inside the car you feel a vibration/shudder and a sound like corn popping in the exhaust.

All this and no CEL.

I am stumpped and freaking out.
One thing that crossed my mind but I don't know how possible this is. Maybe the plastic coolant crossover finally gave way just a bit and coolant is getting sucked into the engine? that would explain low coolant and the odd running. I really have no idea.

Thanks in advance for any help/ideas.

Heres my two cents.

Check out the thermostat area thats intake area # 1
Then check out the Heater core hose nipple. Thats # 2

Lastly running 91 octane ( UNLESS YOU HAVE A TUNE )is pointless in our engines.Especially is yours as you have a knock sensor. The computer will retard the timing and all additional power will be null.

'98 does not have a knock sensor.

Coolant can't get into the engine from the normal cracked crossover or other coolant leaks that these cars experience. BUT... the coolant can get into the plug wells and cause misfires. Usually that would be accompanied by a flashing MIL for the misfire, but it would be a good first spot to look.

I bet you have a cop thats going bad, check and make sure you dont see any coolant by the #4 cylinder or any coolant by the thermostat housing area/alt. BTW i like your center caps nice and shinny

Tomorrow afternoon i'm going to my buddy's garage to get the car scanned, maybe there's a soft code that'll lead me in the proper direction. Then I'll throw the car in the garage for a bit to get out of the snow and check the spark plug wells. I'll let you all know how it goes.

How many miles on the car?

Seems like what you could be experiencing is a little bit of a stumble at idle and low rpms/decell. Not uncommon on a higher mileage engine, and to tell you the truth if anything running 91 octane is only making it worse. You should always run the standard 87 octane as that is what your engine was designed to run on. It can increase combustion temps forming carbon buildup inside the engine. It effectively raises your compression ratio and can cause the problems you are experiencing. I think at this point the 91 is making it not so noticeable. I would try some 87 and see how it runs.

If it was my car I would check the plugs and COP as mentioned above, and if it's got miles on it run some SeaFoam through it.

Car has about 120,000 miles on it. I always used to run 87 until I noticed a nasty knock at WOT. Changing the plugs helped, but it was still there a bit, so that's when i went to 91. Guess I'll go back to 87 for a bit to see what happens, until i get my tune.

So on to the main subject. Just to give it a shot, I popped in a bottle of lucas injector cleaner before my usual 60 mile trip to school(and back). Seemed to be better mannered on the way home. Got the car scanned, no codes. Diagnostic passed. Not doing it anymore... T-stat area dry, heater area dry, still gotta look around and under the cops.

I did find my coolant leak tho, water pump... pretty obvious. Coolant frozen under it and dampner covered.

So I guess it's gone for now? I'll keep the thread posted on what happens.

still got this darn problem

anybody got an idea how I can determine which COP is giving me problems?

Put it on a (I forget the name) scanner and watch the misfire counters real-time. These cars WILL MISFIRE BADLY without setting a 'soft' code or a CEL.
I left coils UNPLUGGED on my '98 and at idle for a few minutes the light never came on.
Misfired like a mofo and never set a light, had to watch the counter to see that #4 coil was bad. A week or two later #2 went, but that DID set a light as I was beating on the car, on the freeway.

I had the scanner on last week and OBVIOUSLY it decides to run like a charm when I scan!! Don't you just hate that...

Did you brake torque it to put a load on the motor?

It usually does it in every gear selection, even neutral with no brake, parking brake or load. I did try revving it up and hoping it would misfire as the revs dropped. Tried Drive with brake. Sometimes she decides to behave.

You will have to put it on a scan tool where you can graph all 8 cylinders and then power brake it and see what cylinder drops on the graph this will tell you what cop is giving you issues.

Well well well... I pulled the passenger bank COPs today and looky what I found. Heater nipple leaking...





Time for me to join the club.
Gives me a reason to get the PI intake on there

....and another happy ending!