Nothing on the injector o-rings, I sprayed those down while trying to determine if it was the intake gasket itself leaking.

No fuel leakage out of the fuel pressure regulator, it holds vacuum fine. Car definitely doesn't smell like it's sucking up excess fuel either.

I have a big kit for testing fuel pressure, I'll look in it and see if I've got the right fittings to test with. The pump and filter are new, however. I also flushed the tank out with diesel before I got it running, and it hasn't run any differently in the time since I've gotten it running (I've gone through maybe 13ish gallons just from idling and some short test drives around the field?)

Cleaned the MAF as well.

I will check the EGR valve and solenoid when I clean the intake port.

 

You were onto something, at least partially. I was able to determine there was a vacuum leak from the EGR valve while the car was running. It did slightly improve things, enough to where it actually had enough power to spin a tire on wet grass.

Fuel pressure seems fine to me. Again, no fuel in the vacuum line either. Starts up basically instantly every time.

I cleaned the ground on the MAF, and took a couple of videos to see how it ran. Seems to be more happy to rev with the MAF disconnected, which... not sure whether to blame that on another vac leak (false air), or a MAF fault.

Here's a video I took of the car running:
Finally got it to throw a code 12... Which, yeah, no ****, thanks VCM

 

Finally had the time to remove the throttle body and EGR for proper cleaning.

Thanks to this write-up found over on Jag-Lovers, I determined my TPS was full of oil while I had the throttle body off, so followed the procedure for drilling a hole in the case and flushing it out


The EGR port was indeed completely plugged up, so I took care of that and cleaned the inside of the intake behind the throttle body as best I could.

I cleaned the MAF again while it was off as well.

Unfortunately, it made absolutely zero change in how it runs. I found the PDF for testing the MAF, so I will do that in a few days. Due to how filled the TPS was, I'm suspecting that as well, so I'll have to look up how to properly test that. I saw a few posts referencing the EMS relay that can cause similar issues, so I'll have to find and clean or replace that relay to see if it makes a difference.

Failing those two things, it looks like I'm buying a smoke machine... I can't find any broken vacuum lines, and it runs no differently with all the vacuum ports capped. If it is a vacuum leak, it has to be coming from somewhere not immediately obvious (valve cover, intake gasket, two halves of the intake, etc)

 

I thought I had found part of the problem. Above is the EMS relay, at least I think from looking at the diagrams. Red socket, far right, below/forward of the main relay shelf.

I removed and cleaned all the relays, which did solve my flickering park/aux lights.

I replaced the EMS relay with a spare one from another car. Unfortunately it made no difference whatsoever.

Back to testing the MAF/TPS I suppose.

 

Using the charts from the Jag-Lovers MAF testing, I made some interesting discoveries.


So.... Maybe some bad grounding points?

 

Corroded grounds are one of the most common causes of difficult-to-diagnose gremlins since even light oxidation adds considerable electrical resistance to every associated circuit. You can't tell their condition without disassembling them. I use a small brass-bristle brush and zero-residue electronic spray cleaner to clean every eyelet terminal, nut, start washer, flat washer and the threaded stud, then reassemble and snug the nut tightly but not overtight or you risk snapping the stud off.

The Electrical Guide shows the locations of the grounds. I've developed the habit of cleaning any ground point I happen to be working near. It's good to clean them all every few years as basic maintenance.

Cheers,

Don

 

Yeah, I'm going to clean the grounding points before proceeding.

I've already cleaned all of the engine bay and trunk grounding points, to no avail. None looked particularly bad.

The right A-Pillar ground is one I'm most interested in. With the condition of that relay, and the damp floor I get when it rains, I'm thinking there must be water intrusion somewhere on that side... If so, I can only imagine what the ground looks like (or ECU, if I'm unlucky).

Not looking forward into digging into interior and behind-dash grounds with near triple digit heat, but oh well.

 

Back with another update


Smoke machine arrived. I must say, I'm very impressed for the sub-$40 I paid for it. Definitely look forward to using this on some of my other old cars.

It did find two vacuum leaks, albeit small. Either way, didn't improve how it ran, unfortunately.

I cleaned all the grounds (thanks Don for the diagrams and pics over on Jag-Lovers). Again, no change, unfortunately. A few of them definitely needed cleaned though, the one at the middle of the firewall behind the cylinder head was particularly nasty. My flickering park lights w/ the engine running and the headlight switch off came back.

While rummaging around looking for tools, I found my old laser thermometer. After the car was running for a couple of minutes, I noticed that the outlet for the rear exhaust manifold was at ~315F, while the front manifold was at ~250F.

(Cylinder 1 on the right, Cylinder 3 on the left)

The front 3 plugs look... not great. The rear 3 look pretty good.

If it was still a partially clogged/leaking injector, I'd expect one of the plugs to be significantly different than the other, and it's not. The only commonality between the front 3 cylinders I can think of would be the upstream O2 sensor, no?

Although the VCM has never thrown a code for it, I'm going to see the best way to test it with the engine running. Maybe unplug the front O2 to get it to run on a base map and see if it improves?

 

Just thinking out loud, the difference in exhaust manifold temps could be due to the front manifold being closer to the radiator fan, and the rear manifold being closer to the catalytic converter. Have you had any indication that the cat is obstructed or partially obstructed? The next time you can run the engine to full operating temp, it might be worthwhile to check the temperatures at the cat inlet and outlet ends (on the cat itself, not on the heat shield).

I may be wrong, but my recollection is that both of our XJ40s had only one upstream O2S that the ECM referenced for fueling all six cylinders. Do those plugs look a bit rich to you? Or carbon fouled? Have you ruled out water in the fuel?

Cheers,

Don

 

I suppose the proximity to the fan could explain the temp difference.

I can't say it's given any any indication of the cat being clogged. The car will drive okay, if you're very easy on throttle application, and runs at higher in the higher rev range no problem. Just absolutely falls on its face with sudden throttle input or under load (at lower rpms primarily, makes moving from a stop on any sort of slope difficult). Depending on the difficulty, I could just disconnect the header downpipe entirely for testing purposes.

You might be right about having a singular upstream O2, I was thinking of my X300 that has 2 upstreams. With no code for the O2, or any codes in general, I'm kind of debating on just disconnecting the upstream to see if it runs any better? If the O2 is sending a bad signal to the ECU, I would assume the XJ40 just defaults to a base map with no sensor input at all?

I don't think it could be water in the fuel, I flushed the system quite thoroughly before getting it running, including removing the fuel level sender and shop-vacuuming all the liquid from the bottom of the tank. It has also ran through maybe 15ish gallons of fuel in my various testing over the past couple of months.

 

Before you mess with the oxygen sensor, try swapping the temp sensor. These are fairly short-lived components IME and drastically affect running if they go bad.

 

Wouldn't a temp sensor out of expected range throw a code 14?

I just went out and checked, and with an ambient temp of ~70F, I'm getting a reading of 2350 Ohms... Which is in spec according to the resistance values I found.

I'm not saying it doesn't fall out of spec when warmed up, it's possible, but my car runs absolutely no differently regardless of engine temp. From what I found on Jag-Lovers, if the sensor falls out of expected range, the ECU defaults to 80F and sets the FF14 code.

 

All very well and good, I don't remember having codes set when problems with temp sensors occurred but I do remember extremely erratic running conditions, and over the 100.000 kms it has happened twice. They run about $10 or so, and take about 5 mins to swap.

 

Okay, I swapped the sensor with the brand new one off of my XJS, after finding out they were the same PN#.

It unfortunately made no difference, regardless of engine temp. I also tried running the car with a paperclip instead (i.e. 0 Ohms resistance= Hot Engine) and it made the idle slightly weird maybe, but no change otherwise. I also ran it with the sensor disconnected completely and it didn't run any different, but did throw a Fuel Fail 14 code.

Cleaned the ECU connector while I had it off. The only two grounds on the car I've yet to clean are the ones fore and aft of the J-Gate.

I also disconnected the O2 sensor while the car was running, and it did make it run worse, so... It's obviously doing something.

Took a video of its current condition:

 

Sorry I can't remember, but when you cleaned the TPS, did you test its resistance over its range to see if there were any dead spots?

Cheers,

Don

 

I backprobed the connector with the key on & engine off. 5V Supply is good, ground is good, and the voltage when moving it is nice and smooth from closed all the way to fully open.

 

I was pondering your symptoms and remembered that back in the day there were a lot of XJ40s with collapsed fuel tanks. There were blanking plugs installed in the EVAP carbon canister that caused excessive vacuum if not removed. I can't remember if this applied to the 1990 model. There should be plenty of info at the Jag-Lovers forum if not here.

Another thing that occurred to me is that, while ignition timing is not adjustable on the XJ40, it is possible to rotate the distributor too far out of range for the ECM to properly trigger spark in each cylinder. Any chance this could be the case on your engine?

 

It's not the tank, if anything the evap system isn't venting properly and causing too much pressure buildup (imho). Doesn't run any differently with the fuel cap off anyway.

I suppose ignition timing is possible. It hasn't been touched in 25ish years, as far as I can tell (no marks on the hold-down bolt, fingerprints on the cap, etc).

I was reassembling the interior today and was thinking about my TPS. Although my voltage climb is nice and smooth with no dropouts.... What should the resting voltage be? I certainly didn't touch/adjust it while I had the throttle body off, and it maxes out at ~5V, but I've seen conflicting forum posts on the idle position voltage.

 

I adjusted the TPS down to 0.51 V from where it was at approximately 0.68–0.69 V.

It runs... no differently at best, possibly slightly worse, although that could just be my imagination since I haven't started it in a few days.

I loosened the distributor hold-down bolt and moved it to either end of its travel in the slot. No difference, again. I didn't see the point in pulling it out and moving it over a tooth in either direction.

I'm going to pull out the compression gauge and see what it says before ordering more parts or continuing further. Although, in my experience, if your question requires the compression gauge, you're not gonna like the answer. Fingers crossed — will report back tonight or tomorrow.

 

Fortunately/unfortunately, compression tested good.

1. 185
2. 185
3. 185
4. 183
5. 185
6. 185

So no closer to an answer, unfortunately. I'm going to order the proper injector rebuild kit with new pintle caps and everything and reflow them with some solvent and see if anything changes.... with my guess being no, as no one cylinder is different from the rest.