Violent shaking, help please, Engine issues Options

[neisenkr]

Long time member, rare contributor.
I have a '99 celica that I have daily driven for 10 years now with very few issues.

About a week ago I notice that the engine seemed to have a miss or stall of some sort on my commute to work. I made a mental note to keep an eye on things, but it wasn't severe enough to pull-over.
Each day I have driven it since, the problem has gotten worse.

- The engine starts and idles fine.
- When below normal operating temperature it runs fine, though it is definitely down on power and seems to be rev limited to ~3,800 under load. With no load it will rev to 6,000.
- When the engine reaches operating temperature it begins to have the stall. It is as if the whole engine cuts, restarts, and repeats numerous times within a second. It does it so much, and so quickly that it creates a violent shake that I am worried is going to do serious damage. At this point I am driving as little as possible, but it is the only car I own.
- The problem only seems to happen when I am at constant throttle. The stop, start shaking does not occur during accelerating, coasting, nor idling (though it is down on power and weirdly rev limited at all times).


1. At first I thought it was a dying fuel pump that was becoming intermittent and not supplying enough juice, but the problem persisted after replacing pump and filter.
2. Years ago, I installed a secondary ignition control and coil (MSD), but left the stock parts in place with a bypass. The problem is identical running any combination of ignition control module and coil. The distributor cap passes any sort of visual inspection.
3. The air intake system passed a similar visual inspection, but I have not tried to look into the throttle body.
4. On a wild hair, I attempted unplugging the TPS. The car had the same issue, but with additional problems. Removing the TPS was the first time a dash light came on and is the only thing that shows up on diagnostics history.
5. There are no codes showing in current diagnostics.

At this point I am thinking the ECU took a dump, but I really don't want to replace it without some sort of lead.
I have also thought it might be the EGR, but that doesn't really add up.

Anyone that may know, please give any advice you can. I am near my wick's end and I can't afford to replace much more before I give up and park the thing for good.