So after doing the head gasket my 4.0 GEMS idles beautifully and actually performs not too badly - up to about 2500rpm, at which point it start spluttering and generally won't rev beyond that. It's the same on petrol or gas. There's a fault code stored for the camshaft sensor according to my cheapy ELM327, but when I look at the appropriate pin with the sillyscope I can see the correct two short two long squarewave pulses, looking just like the picture in the manual.

One of the lambda sensors seems a bit lazy, but it was like that before I started working on the engine.

What have I missed here?

Check tps and crank sensor connections, go over all the other connections, especially temp sensor. I don’t think the problem is the cam sensor, it would be extremely rare for that to go, it’s most probably just a bad connection.
If you can, just spray connector cleaner up the plugs you can get to, including maf and airbox temp, you know what these buggers are like, one dirty or misreading sensor causes hell !

Perhaps you can persuade dhallworth to drop by with his Testbook?!
All I can suggest in addition to above is, as same on petrol or gas it has to be spark related, so go right through feeds to coil pack, HT leads properly plugged in both ends, HT leads damaged or contaminated plugs.

I'd start to suspect the crank sensor too. Although you've not been near it, I assume you've disturbed, if not disconnected, the wiring to it. As the crank sensor can't give a fault code it does often come up with a cam sensor code instead as the two aren't in sync.

I wonder if it's possible the connector is full of crud? With the crank sensor unplugged it won't start at all. I did have that cable unplugged to move the wiring loom out of the way of the head.

Orangebean wrote:

Perhaps you can persuade dhallworth to drop by with his Testbook?!

That's on the cards for this evening if all goes to plan.

I hope it's nothing valve related that's causing the problems.

David.

After much faffing and resetting of adaptives and poking and prodding I crawled underneath and hosed some IPA into the sensor connector and the plug on the lead, which seemed a bit damp. I managed to get some water out, so I flushed it with some more isopropyl and stuck it back together and lo and behold! Off it goes, right up to the rev limiter!

David mentioned that his dads exhibited similar behaviour when the flywheel sensor was on the way out, so possibly the signal was only clean enough at low revs or something. While he was here he reset the adaptives and I moved the throttle pot back to a sane position, so it's a lot smoother pulling away on both petrol and gas although I suspect I need to tweak the gas settings a little. It also looks like one of the lambda sensors is on the way out.

We had a quick look at his VSE too, while we had Testbook on in the shed. It's weird, it seems to be looking for downstream lambda sensors but they are not fitted. I wonder if someone's been fiddling?

That's like a different car since you did the head gasket. So much quieter and smoother :) I hope she continues to behave! At least you've got our spare P38 there incase anything goes wrong over the next few days.

David.

It really does have a lot more poke between 2000 and 3000 revs. That blow between pots must have been actually pretty severe!

Sounds like a win then?

Certainly seems to be. I now know the tappets are pretty dished (you could serve soup out of the back four) and a new camshaft is on the cards, but it doesn't need to be particularly soon.