Some time ago, I worked through getting my 2004 engine rebuilt by a now (apparently notorious) engine rebuilder. Hindsight is 20/20, but I think it is possible to get this vehicle out of the woods. I feel like I am close to getting this thing done, and hoping I can figure it out without too much more expense. Believe it or not, nearly all of my expenses have been covered by warranty, and I am trying to keep it that way.

The engine is getting really poor mileage compared to it's previous self (12 vs 17 mpg). I am very meticulous about tracking fuel economy, and have tested it under various conditions now with these consistently low results.

It also has a persistent misfire on cylinder 7 according to my Ultraguage.

I took the vehicle to my local shop to check out as much as I could before taking it back to the rebuilder to eliminate issues with injectors, spark plugs, wires, O2 sensors, airflow sensors, catalytic convertors, etc. Shop results seem to indicate that there is an issue with the ignition system, which I am learning was experimental and high powered. I will have a factory spec original type put in to replace it.

Now the contentious bit. The shop also noticed a lope at idle speed, and a big dip and jump in vacuum pressure (I am working to get the specific numbers and will update with those). The jump and dip in the vacuum pressure coincided with the lope. My mechanic suspects that there is a leak somewhere in the top end with valves or something else.

I mentioned these issues with the rebuilder. Ignition seemed to be an non-issue, and he would check and replace it if needed.

The vacuum pressure issues were met with skepticism, and a claim that my mechanic (who has replaced my engine three times and worked on a few other Land Rovers in town) wasn't knowledgeable on Land Rovers.

Can anyone give me the expected range for the vacuum level at idle? Or whether one would expect that level to jump around much at a steady idle with the engine warm?

As always, appreciate any help in advance, and I would be happy to know if there is further information that would be helpful. Thanks!

 

I think the first order of business is to address the "persistent misfire on cylinder 7". Once that is corrected then address the lope at idle if it still remains. The two issues may be related. JMO D*

 

17 MPG??? Man... I wish my Disco did 17 MPG. I'm sitting at 12 MPG for city driving.

 

I think the mechanic is right.

Vacuum on our trucks is pulled from the intake sucking in air to the cylinders, so it is related to the cylinders pulling air on the downstroke. If you have a cylinder (7), that is pulling air from somewhere else, it could lower vacuum when that cylinder is on downstroke, and then, if a valve was not seating/closing, it would cause a misfire.

I think you have poorly seated or bent valve or valves--or maybe a broken valve spring--on 7 and, looking into my crystal ball, it seems most likely that it is an exhaust valve not closing. If the exhaust valve doesn't close, then on the downstroke, the cylinder is going to pull some dirty exhaust (lowering vacuum). If the valve doesn't fully close, the detonation would send some of your compression that should be used to force the cylinder through the rest of the downstroke out of the leaky exhaust valve instead. That could result in a lope and misfire, and also burn your valve because, basically, that cylinder would be acting like it had blown rings, or a crack, that would result in poor compression.

The test should be simple. First, a compression test on the left bank (1,3,5,7). 7 will probably show low compression. Then, a leak down test, or forcing compressed air into that cylinder at TDC then listening at the intake, exhaust, and crankcase will, I think, reveal a hiss in the exhaust.

If it was a bent push rod, the valve wouldn't fully open; same for a rocker problem. I bet that valve is not closing all of the way.

Why do I think that? Because I am an IDIOT and somehow dropped a grade 8 BOLT in a cylinder, which completely screwed up a valve, bearings, and everything else, and it had the same symptoms (plus a loud crunching sound as the bolt threads were imprinted on the heads, just FYI). Idiot got smart. I may have posted a picture in the forum but if not, I don't want to re-humiliate myself. Trust me: the (freshly lapped) valve didn't close all of the way after the bolt got between it and the head.

In fact, aggressive cams increase the duration/stroke of the valves and usually cause a lope, especially where a bad spring (or too weak springs) allow the valve to "float"--meaning that it doesn't clamp shut quick enough--which can affect performance.

That alot to digest, and some of it might actually be rights, but the summary is that I think your lope is coming from a valve problem

Anyway, that is my THEORY.

I guess there could also be a huge crack in the wall between 5 and 7, or maybe a missing piece of head gasket between the two. I guess it could also be an intake valve that isn't closing, and on the upstroke, 7 could be pushing exhaust back the wrong way. But if the leak down test shows a problem, but does not narrow it down, you will be pulling the driver head anyway and will be able to see everything you need to see. Unless I am shockingly wrong (always a good possibility), having your driver head redone will solve the problem.

 

I'm sure this isn't your problem but just throwing it out there. I had misfires on one cylinder and also had an O2 that was not responding correctly. After months of trouble shooting and waisting money on parts It ended up being a short in the ECU sending bad signals to the O2. This was causing the misfire, the Bad O2 reading and also bad milage.

A mechanic traced the short from the O2 to the ECU. I bought a used ECU and paid a place $100 (1 minute of work with proper diagnostic computer) and they synced evertytihg up and it's been fine since.

 

Gentlemen (and ladies if you're out there), thank you for the responses. Very informative. There are many layers to this. I am very patient but also disappointed after going through this process with the product. Want to see it right.

The bad valve really starts to make sense. I found my paperwork, but it did not have the specifics on the vacuum pressure. So, talked to my local mechanic to learn some more about his troubleshooting. He remembered it being at about 15-16" at a steady idle, and then jumping down and up 5-6" from that ready with the loping. He said hear could hear and feel a rattling, rough noise on the top end that he says is most often a valve issue.

He was able to replicate the misfire as well on demand. I had observed it most frequently when the engine was a moderate load, such as pulling away from a light, at about 2200-2300 rpm. He was able to replicate it just by holding the breaks and adding some load to the engine in Drive while stopped. The ignition seems to be breaking down or failing somehow under load.

I am planning on taking it in on Monday. Hoping this has a positive ending.

 

You are not alone here. My main hobby is fixing an old Airstream, and I've had enough similar incidents, usually involving a bench grinder, air hammer or some kind of sheet metal to know the feeling exactly. All of these mistakes are painful, expensive, massively aggravating, and always involve bloodshed and copious swearing. I think my children have emotional scars, both from hearing me and seeing whatever grievous injury I have inflicted on myself.

I appreciate your thoughts a great deal.

 

As a point of reference, that vacuum measurement is a bit on the low side, but not necessarily horrible. The bigger issue is the variation you're seeing. 5-6" variation is way too much. A couple of inches is OK. Based on your comments so far, it sounds like you have not replaced the plug wires or diagnosed the ignition coils (I always recommend swapping banks with these to see if the misfire follows before just replacing them). These are the two primary causes of misfires on a D2. I can see an ignition-related misfire causing some vacuum variation, but that's a pretty big number. I'd be interested to observe it under load at various RPM vs idle.