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A couple years ago I had the dreaded lean codes and chased down a lot of leaks including replacing the eight upper plenum ring gaskets, the two lower intake manifold gaskets and other gaskets, o-rings, hoses, injector seals, etc. would think I finally get it sorted and running well again. I replaced the MAF (Denso), intake plenum, sparkplugs, coils (Denso), fuel filter, fuel pressure regulator, etc. etc. A leak developed at the throttle plate spring, and replaced the throttle body. All temp sensors appear to returning correct readings.
The latest return of the codes was a mystery because fuel trims pointed to a vacuum leak, but smoke testing didn't reveal anything. Then a couple of week's later oil started to drop on the garage floor once it had overwhelmed the capacity of the pad in the drip/protective tray. If oil is getting out, then air is getting in and pinpointed the leak to either the pan or filter housing. Replaced the oil pan gasket and the filter housing "figure 8" gasket.
Once the oil leak was addressed, thought I had everything sorted until my wife started the car the next morning. "I thought you fixed the car, why does run so poorly?" Ugh. It seems when started from cold (sitting overnight) there were multiple misfire codes and code P1316. Once warmed up, the car ran more smoothly.
Decided to try putting Techron concentrate in the half full tank, and took if for an extended drive. Ran great, so I cleared the codes and then drove it for another 45-60 mins. Afterward I checked for any new codes — no codes, no pending codes. Foolishly thought I had it…
Started it after an overnight cool down, and before the wife was going to use the car, and it ran like crap. Sigh. Drove enough to bring it up to full temp so she could use the car for the day. When she returned last evening pulled the codes. Misfires on all cylinders, and lean codes on both banks.
So all I know now is that fully warmed up, it doesn’t throw the lean codes or misfires and runs nicely so I think that eliminates candidates such as fuel pressure, oxygen sensors, plugs, coils, etc. Started from dead cold, it has or thinks it has a vacuum leak that goes away (thermal expansion?) or something is causing the ECU to over-fuel during open loop, but not once reaching closed loop.
Check fuel trims from a cold start this evening, short term was 25% waited for long term to reach 19.5% and raised the rpm to 2500 and LTFT drop to ~10%, which typically points to a leak. Noticed the last couple of times of starting from cold, I could a smell fuel — clue?
Any suggestions are most welcome.
Last edited by Mac Allan; Oct 24, 2025 at 11:59 AM.
My first thought was to recommend a smoke test... but you've already done that. Obvious question, since the issue is at cold start, did you do the smoke test cold? Seals that leak when cold won't show smoke leaking when hot. It's also possible the smoke test is just inconclusive. I would not expect any minor oil leaks to upset the fuel trims this much from phantom air through the gaskets.
My second thought is there's an obscure vacuum leak that smoke is not showing--brake booster is a possibility. I would say a quick test of this is to disconnect the booster vacuum hose and plug it, see if the cold start is normal. I also had a faulty Air INjector solenoid (front of engine) that was cracked and leaking vacuum, but that would not be a massive leak like you describe. Could also be a leaking EGR valve maybe? Not sure how to test this other than throwing parts (not recommended).
Lastly, it is possible that the wideband O2 sensors are faulty. The ECM determines trims from the upstream sensors. WHen it's showing this gross over-fueling, what do the downstream O2s show? If they are showing a massively rich condition, then you know the upstreams are bad or have some issue.
My first thought was to recommend a smoke test... but you've already done that. Obvious question, since the issue is at cold start, did you do the smoke test cold? Seals that leak when cold won't show smoke leaking when hot. It's also possible the smoke test is just inconclusive. I would not expect any minor oil leaks to upset the fuel trims this much from phantom air through the gaskets.
Thanks. My plan today is bring out the smoke machine and retest. It's possible there is a new leak since I last smoke tested, but I also found a post by @Don B that suggested some other points of smoke entry to test in addition to the intake at the throttle body, so perhaps I've missed a leak the last time I did a test. Will also set SDD and do some live reading of all sensors to see if I find something.
My second thought is there's an obscure vacuum leak that smoke is not showing--brake booster is a possibility. I would say a quick test of this is to disconnect the booster vacuum hose and plug it, see if the cold start is normal. I also had a faulty Air INjector solenoid (front of engine) that was cracked and leaking vacuum, but that would not be a massive leak like you describe. Could also be a leaking EGR valve maybe? Not sure how to test this other than throwing parts (not recommended).
Great suggestions and will check brake booster as suggested. Perhaps the Air Injector Solenoid and EGR can be tested using Starter Fluid and watching fuel trims
Lastly, it is possible that the wideband O2 sensors are faulty. The ECM determines trims from the upstream sensors. WHen it's showing this gross over-fueling, what do the downstream O2s show? If they are showing a massively rich condition, then you know the upstreams are bad or have some issue.
Doesn't the ECM ignore the O2 sensors at cold start while running open loop? I think it uses a fuel map instead of the O2 sensors, and since it runs fine once up to temp I thought (perhaps wrongly) that the sensors were fine.
Doesn't the ECM ignore the O2 sensors at cold start while running open loop? I think it uses a fuel map instead of the O2 sensors, and since it runs fine once up to temp I thought (perhaps wrongly) that the sensors were fine.
You're right, Mac. Until the HO2S's reach functional temperature (about 600°F off the top of my head), the ECM operates in open loop and uses a default map for fueling. The heaters in the Heated Oxygen Sensors are there to heat them to operating temperature more quickly.
My first thought when I see a sudden onset of multiple misfires is contaminated fuel. How recently before the onset of symptoms had you purchased fuel? Is it possible you got a little water with your gasoline? I don't know if Techron will help emulsify water with fuel, so it might be worth adding a couple of bottles of Heet fuel system dryer, then sit on the rear bumper and bounce up and down to agitate the tank to help mix the Heet with the fuel and possibly the water at the bottom of the tank. It may not resolve the issues, but for $10 it might be worth a try.
Set-up SDD and with misfires and restricted performance when cold are the selected symptoms the only codes it says are relevant are the misfire codes. The lean codes are shown in unrelated section. Under "highly recommended" is to run Transmission Control Module Adaption Clear. There are other codes in the unrelated section, but none of them have a mileage reference. There are a couple of battery low voltage codes.
I've put the charger on the battery and run the TCM clear, I'm going to let it charge for a bit then use the Start function on my charger to start and see if low voltage is the issue.
Will report back if it works, but I think it's unlikely.
UPDATE -- As predicted neither made any difference. Going to grab some lunch then commence smoke testing.
Last edited by Mac Allan; Oct 25, 2025 at 02:59 PM.
Found it! One of eight the upper intake plenum gasket/seals has failed.
Incredibly frustrating after having replaced these less than 10k miles ago. Back then, the only place I could find them, from what I thought was a reliable source, were the ones I ordered from Motorcars Ltd. The jaguarlandroverclassic.com site doesn't even list these or have a part number.
Does anyone have a better recommendation for these?
Thanks
Last edited by Mac Allan; Oct 25, 2025 at 04:18 PM.
I assume you are talking about the seals between the plastic upper plenum and the aluminum runners? I remember replacing those in the past, but like you, I can't find a part number at parts.jaguarlandroverclassic.com. If I recall correctly, you have to remove the entire plenum because the screws that hold the plastic to the aluminum runners are on the underside...
It doesn't sound like you looking for Part 4 in this diagram, but if so, it's AJ85392:
We've had some reports of cracked intake manifolds. Any possibility that is what you are dealing with?
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The Amazon link Jacuar gave us reminded me that I had to find these gaskets through the Land Rover part number, which is 9450A or 4526549. The gaskets fit the 2005-2009 Land Rover Discovery 3 / LR3 with the 4.4L V8. Here's the parts diagram:
Parts.JaguarLandRoverClassic.com shows these as No Longer Available, but FCP Euro claims to have genuine Land Rover parts available:
There was an improved design of both the plenum and the seal which is part number LR006678, but that isn't backwards compatible.
I found a seller who had 8 of the OEM Land Rover seals, which looks like they are also on there way to becoming unobtainum. There must be a Ford/Lincoln part number but I couldn't find it.
Mac, I have had a little experience with smoke-testing and have discovered that a vacuum leak does not always translate into a pressurized leak as in a smoke test. However, I do think it a good idea to do a smoke test on cold engine.
Tōb
2009 xj super port
Would just add that yes, on cold start the ECM does not use the O2 sensors for open loop operation. But, the system does enter closed loop pretty quickly after startup. It's not like the old days where the coolant had to get to 150F or whatever. It's more like within 30 seconds of starting. You can watch the OL/CL data point on any OBDII scanner to see this happen.
To close out this thread, I finally got time to replace the intake manifold seals and it looks like the aftermarket seals I got from Motorcars Ltd hardened much faster than the OEM ones. I replaced them with genuine Land Rover part number ones (which were very hard to find), and got everything back together. Started her up with the scanner on and the fuel trims were beautiful!
Oh joy I thought, but the Jaguar gods laughed and I have a new problem, which I'll put in a new thread.
Lean Code Whack-A-Mole At Wits End
