I sense a disturbance in the force...

I believe I've sorted all the vacuum leaks. At least I no longer have any EVAP codes, and the Fuel Cap error doesn't come on any more.

I've done a hard reset, cleared all the codes and the car still exhibits issues.

Symptoms:

Cold start -> Starts quickly, and smoothly, however after the RPMs begin to settle to normal idle, the engine begins to lope and idle rough. Driving while cold the car accelerates fine, but when coming to a stop, the rough idle returns. For an example, it is 2 miles to the coffee shop, with four stops. At each stop, including parking for coffee, while the car is in gear and my foot on the brake, the RPMs drop so low, the headlights dim, and the engine attempts to recover, but it oscillates to the point it looks like I'm intentionally turning the headlights off an on. It never completely dies, but it gets really close.

Warm Start after getting coffee -> Starts quickly, and idles perfectly for the rest of the trip to work.

As a test, I drove the 13 miles to work without stopping for coffee, and at work the car stumbled and loped while parking at work, and while pulling into the parking lot, the Check Engine Light and Yellow restricted Performance light came on, and limited the engine to 3000RPM. The codes are "Too Lean Bank 1" and "too Lean Bank 2".

As long as I shut the car down and restart it once it's warm, it doesn't throw the codes or Check Engine Light, or Restricted Performance. I've driven it the same for a week, and it's been fine, no codes.

On most other cars, I would suspect there is something wrong with one of the sensors that flip the car from open loop to closed loop, as if it may be stuck in closed loop even when it's cold, because I assume when first starting the car, that the ECU is expecting an enriched mixture because one of the temp sensors says the engine is cold, but for what ever reason, the mixture isn't being enriched. The ECU sees this as a Lean environment due to the readings from the O2 sensors, and attempts to compensate, but eventually gives up trying to correct the problem and throws the DTC codes and Restricted Performance.

Were this a BMW, I would suspect both the engine temp sensor and the intake air temp sensor.

Again, I'm a novice on this car so any insight from those with more experience would be helpful and appreciated.

 

Okay, so you're getting P0171 and a P0174 ? How are the fuel trims ? I think when the long term gets to +25 on a bank it throws the code. I've had this. There are a lot of YouTube videos for leak checking and smoke machines and stuff. I think it's these darn plastic intake manifolds. I've replaced mine and the seals, but my fuel trims are still consistently high - but not enough to throw a code. Strange thing is, if I drive it steady, the trims get high. If I stop somewhere and it sits for a minute, then restart and go, the trims look better. Sounds kind of like your situation. But it never does stumble or run rough. How are your fuel trims ? On mine, bank 1 always is a little higher. I got a spare MAF off fleabay and it doesn't make any difference. Mine is not bad enough to throw a code, it usually stays below +10, but most things I've seen and read say it should be around +2 - -2. Here's a video on a cheap smoke test -

 

Classic signs of air leak(s).

+1 watch trims

 

I threw the OBD adapter in it and drove to work with my tablet logging using Torque.

Yes, the trims are way high +22 to +25 on the long-term meter.

Apparently I'm missing something with regard to vacuum leaks. I'll probably strip the intake off of the car and put it all back together with new gaskets, but the engine does produce really good vacuum, it collapses cheap vacuum hose, so I replaced the cheap rubber hose I had originally tried, with braided hose that won't collapse.

The exhaust also smells incredibly acrid.

I believe there is an exhaust leak somewhere near the transmission. It doesn't splutter, but the exhaust sounds hollow under load, from under the car.

This car keeps getting closer to becoming a parts car. I could probably get more out of the engine alone than I could out of the car at this point.

 

A smoke test would be a good idea before you start taking the intake manifold off.

 

High is not forced to be air leak - but is if the trims drop when you rev (hot engine, parked, say 2500rpm).

 

Have you checked the duct between the air flow meter and the throttle body?
Needs to be taken off and inspected carefully, especially the underside, as they are known to crack or break up.
Second the recommendation for a smoke test, the seals which jaguar uses on the inlet manifold seem to be quite robust; made of rubber and plastic, not paper. Have just had my motor apart and they were still good. (replaced them anyway)

 

I'll check that as well.

 

Yep, it's solid, although the mounting tabs where it attaches to the top of the throttle body have degraded. I've taken extra precaution from this attachment point being a potential unmetered air infiltration point by using a large clamp to insure everything is tight and won't work its way loose.

I went digging for the EGR valve suspecting that the EGR gasket was leaking, however, if this thing has an EGR valve I can't find it. There aren't any connections to either exhaust manifold.

It looks like I'm going to have to invest in a smoke machine. As a former smoker, (no smoking for 4 years now), with an addictive personality and a love for cigars, I don't want to risk using a cigar and length of hose to pump smoke into the engine.

I do have a couple of propylene glycol based fog machines... hmmm I wonder.

On a side-yet-related note, I am going to go buy a Black 2000 XJ8-L Vanden Plas, which according to the previous owner has a bad head gasket and radiator, however the transmission is "good", although I think that's being a little generous to the 5HP24 because I don't believe any of them were ever "good". It does have many of the little odds and ends parts I need. While looking for a parts car I also found a maroon 2000 Vanden Plas which according to the advertisement "Won't Start", however the car is in excellent condition.

Naturally, neither of these Vanden Plas cars are supercharged. The Black car is $700 and the interior is pretty rough, the Maroon car is $1100 and while it's in much better condition that's high for what I would consider a parts car. I'm going to take the time to pull the codes from the Black car to make sure there haven't been any transmission faults before I buy it.

 

Most early V8's did not have a EGR valve. I don't think propylene glycol would be harmful, since it is just another form of antifreeze, but I wouldn't want it getting on the MAF sensor.

I believe plumbers use some kind of smoke tablet, but have never done the test myself. Maybe someone with more knowledge will help.