Huge misfire @ 4000 revs

[105e]

Driving along at 70 mph, suddenly speedo stops working, few minutes later it starts working, does this more than once. Then at a junction while accelerating a big misfire at 4000 revs, tried a few times before journeys end and everytime a misfire at 4000. Less than that runs sweet. Any ideas anyone, thank you.

 

[Rone]

4000 revolutions is probably the rev number of maximum torque, which means at these revolutions the engine pulls in the most air and thus the ignition coil voltage required to jump the spark gap is the greatest throughout the rev range. So the first suspect would be a traditional failing ignition coil, but if changing those doesn't fix it, the problem could also be a failing HT lead or a spark plug (I give the spark plug the least probability to be the problem).

But then about the failing speedo? Could it be just a coincidence? But if the problems are related you have to find a common nominator for the two problems. Maybe a bad earth connection. So check the engine bay earth connection between body<->engine, body<->gearbox, etc. I think there are around four earth straps at least? Maybe someone has more insight on this. Also, is the speedo working while the engine is misfiring? If it is working, then the problems are probably not related. But it could also be a bad power feed to the ECU and when the ECU loses power it shuts off the speedo and the spark?

 

[jamesycc]

sounds like what happened to my cup. Turned out it was an,injector after trying a new coil and the usual plugs etc. At 4k its trying to chuck a bit more fuel in and cant. Got an obd reader to still if its a fault code logged. I think I had a p0204 fault which meant injector 4 wasn't firing. Swapped it with 3 and sure enough the fault moved to p0203. About £40 from ecp for a new one if needed. Look out for my discount code thread regarding gsf and ecp weekend promos

 

[105e]

So, went out to try some of the suggestions, took it for a drive to warm it up first, no issues today.

 

[bloke]

if on eml then first port of call crank sensor.

 

[jamesycc]

So, went out to try some of the suggestions, took it for a drive to warm it up first, no issues today.

that's how mine was. Fine for a few days and then a random cut out and misfire. I had to leave the obd reader plugged in constantly and as soon as it misfired do a scan straight away. Took 2 weeks in the end as it was sooo random.

 

[105e]

OK, so to help me out, can I buy a fault code reader/ code reader fault???? . I have an early ( single lambda ) phase 1. Looking at some on ebay but they say 2001 onwards. thanks

 

[bloke]

If you get something half decent you should be OK. If anyone suggests torque or whatever the stupid thing is don't bother, it's vague to say the least.

Autel do one for about £130 that is good all rounder, will get model later for you. Will do airbags, abs etc as well.

 

[105e]

So do you know which model?

 

[bloke]

sorry forgot about this. think it's the md802.

 

[105e]

Cheers

 

[105e]

Did exactly the same thing again, so definitely a connection with the speedo not working.

 

[bloke]

first place i'd check is where the loom comes up and over the rack and to the main run. known a few with issues there.

 

[105e]

So to update this, same issue continued, it would be fine for a mile or 2 then no speedo/ lack of revs, then fine again, so i bought a new bosch crank sensor and while I was fitting it also fitted a spare used speedo sensor I happened to have. So far so good.