So here is my first real question to the forum. I'm hoping someone can help.

I have a 1996 Disco I SE7 that has an intermittent misfire problem. When it happens it its usually after extended driving when the engine it at temp but only occurs about 20% of the time. If going along at 70 mph the truck loses power and drops down to 1000-1200 rpm. Any acceleration beyond 1200 rpm will lead to gross misfiring and the engine simply wont rev above this limit. When this happens the engine will idle fine and seems to display normal power and acceleration up to 1000-1200 rpm. If I let the truck cool down I can start it up and the problem goes away til next time.

Here's what I've done recently:
* Rebuilt engine w/ new main & cam bearing, machined head, new gaskets all around
* new cat and muffler system
* new magnacore ignition wires
* new bosch plugs

There's a lot of posts on this topic but no apparent resolution. The best I could find was that it could be related to the VSS and/or ECU failure. The VSS is relatively cheap so I ordered a new one from BA. I can't find an ECU anywhere.

Does anyone have a new info or insights into what could be causing this?

I appreciate your help! Thanks!

 

Try the VSS first. It has exactly the correct set of symptoms.

If you have an OBD II scanner, you could look at the freeze frame data for speeds above those you know you've been traveling. That indicates the VSS isn't working correctly.

 

After a thorough inspection we found that the crank shaft position sensor was damaged. The spindle on the CPS was badly bent. We replaced the CPS and while we were at it also replaced the coolant temp sensor, throttle position sensor, and vehicle speed sensor (basically every sensor that was suspect). Truck has 173k so it was probably due. Happy to report that the problem hasn't come back since after a week of driving in various condition, hot/cold, wet/dry, etc.

Unfortunately because we replaced all the sensors at once we'll never known exactly what caused the problem. I'm happy it seems to be fixed because the only thing left to R&R would have been the ECU and sounds like that would have been a more expensive PITA!