Se7ix

I am trying to correct a bogging problem at 1700 - 2000 rpm when the engine is hot. I also have a pulsation that appears to be from slight engine miss (very pronounced pattern and sound at 48mph - 60mph at 1700 - 2000 rpm). I'm leaning toward engine miss and not driveline problems because rpms tend to "bounce" in cycle with the pulsation / vibration. Under high rpms (low speed or high), the engine doesn't bog and the pulsation disappears.

My 94 LT1 (111k) is new to me so I only have about a month of history. I have changed plugs and wires, flushed the radiator, changed oil and filter, replaced fuel filter, replaced several hoses and the pcv valve, and cleaned the throttle plates. There are several small things that I will be checking out in the car to get it back to tip-top shape. Right now, the priority is correcting this miss / bogging issue.

After the engine gets hot, I can pop the hood and hear a very noticeable rattle or ringing sound. I have identified the source as being the coil. Using a stethoscope, I can hear that the coil sounds like an old-fashioned alarm clock. I have also noticed that after the engine gets warm (actually, hot. it usually does it once it reaches is hottest temperature), the rpm needle will spike upwards. It appears to jump 150 - 200 rpm without the engine getting any louder (and seems to jump faster than the engine could rev leading me to believe it is electrical, not actually engine speed change) and then the iac idles the car higher to compensate. The rpm will spike sometimes several times in a row causing the iac to idle as high as 1500 rpms.

I know the pcm is fed by a low res sensor in the opti and in turn directly controls the rpm gauge. I have also read on this forum that the coil can get very hot and start to fluctuate pretty badly. Someone posted some information about relocating the coil (or mounting it off of the head) to reduce the temperature. At this point, I am thinking that the coil may be a little flakey and is causing some electrical problems (as related to the distributor).

I intend to replace the coil with some cheap aftermarket part to see what happens but wanted some opinions about my symptoms. All comments appreciated! :c)

Thanks guys / gals.

97FormulaWS-6

With what you've replaced... and with your milage I'd say swap the coil.... but I think it's more likely your Opti.

I swapped both out over this past summer... with a stumble that had gotten horrible.

bad95z28

usually the coil is high rpm miss instead of low. i'd check the O2 sensors. when i had a low rpm problem it was the O2s. as for the noise, i have no idea. make sure that you don't have a wire arcing or a spark plug loose. maybe even an exhaust leak.

zslates

Look at the coil in the dark and see if it glows. Mine glows and Im trying to figure out if that is causing my miss in the lower RPM. Read the "Coil Glow" post below and see if it sounds like what you have. Im going to try and isolate the coil with rubber or something (relocating?) and see if it still if it glows when cold.
Zack S.

Se7ix

What about the power / rpm surge? Anyone ever seen anything like that? Has anyone ever found any problems with the optical sensors in the opti that caused similar symptoms? Does anyone know if misreported resolution voltages could cause an engine miss?

Do you check o2s with a propane or butane torch?

zslates: I didn't notice any glowing last night. I would suggest checking the braded metal ribbon that attaches to the bolt that holds the coil on the head. Do you notice any corrosion on your battery or have you seen any unusual voltage fluctuations on your gauge in the car? Maybe when you turn on your blinker or headlights initially?

Thanks folks.

Se7ix

I just replaced the coil with a GP Sorensen model. It turned out to be a little bit more aggrevating than I anticipated, but it's over with. As for the problems, it will take several days of driving to determine what affect (if any) this will have on my surging and stumbling problems.

As for the ringing, it's gone. I used my stethoscope to listen to the new coil and it does "ring" but at a much lower volume. I suspect that the old one was weak and possibly going to break down soon anyway. For the first 30 minutes of driving (so far), the problem appears to be better. The vibration seems to have broken up a little also.

I will write again in a day or two to report the overall outcome of the swap.

Thanks for everyone's help!