My 1987 5.3L V12 with new after-marked 3-wire Oxygen sensors is running (or at least sounding) great, but it's mileage is pretty poor...a recent road trip produced 11MPG.

I've taken the full throttle vacuum and micro switches out of the equation, no change. I measured the resistance of the coolant temp sensor, it's spot on. The air temp sensor is another matter...I was getting 1000 ohms resistance at 100 degrees F when I should have been getting 150 ohms.

Being the clever sort, I thought I'd stump the system by pulling the plug out and putting a 220 ohm resistor across the plug terminals. To my surprise, nothing changed. I then substituted various resistors ranging from 10 ohms to 10K ohms, and nothing seemed to change. By 'changed' I mean that my air-fuel meter continued to show that the engine was running way rich.

I then pulled the vacuum hose that runs from the outlet regulator to the manifold off the manifold, capped the manifold inlet, and sucked on the hose attached to the outlet regulator vacuum port as hard as I could. Once again, no change. FYI, my car doesn't have the inlet regulator, just the outlet regulator.

I then took it out for a drive. From just cruising at 30 MPH to putting my foot into it at 55 MPH and holding it there for several seconds, it stayed rich.

Any thoughts on why these changes that should have made some sort of difference didn't?

Thanks!

John
1987 XJ-S V12, 62,000 miles

 

Maybe there's an unaccounted for bigger problem that masks what you've been doing.

One quick thought: have you checked for vacuum hose at the ECU? Is the hose connected?

I'm curious about your air-fuel meter. What are you using?

Cheers
DD

 

talking of XJS using too much fuel watch this for fun..


Best quote ever from Clarkson re 'buy a brand-new house with triple glazing, or the 17th Century Manor house that costs a fortune to run, you always bump your head...I'd buy the Manor house'

 

Find some photos of my fuel system components attached.

The air-fuel meter is a Sunpro. I hooked it up to a desktop power supply, and (as in the photo) it shows "ideal" around .3 volts. It's not the most expensive out there, and I've heard that if I'm not using wideband sensors the readings "may" be meaningless, but certainly something isn't quite right if I'm only getting 11 MPG on the highway.

My Oxygen sensors are Bosch 3-wire sensors. There were about half the cost of the specific sensors for the Jag.

All the hoses seem to be intact and in good shape in the boot.

I then pulled the #1 plugs from each bank. Somehow I expected them to have more soot on them than they do.

So at the moment I'm scratching my head wondering if I am really running rich. Again, the only indication of a problem is the poor gas mileage.

This car only has the fuel outlet regulator, the inlet regulator has been removed.

Any thoughts on further testing I can do to narrow this down?

Thanks much!

John
1987 XJ-S V12, 62,000 miles.

 

I agree that the plugs don't show an over-fueling problem.

The Bosch sensors whould be fine. The Jaguar originals were nothing exotic to begin with. Back then O2 sensors were rather generic.

So, if we set aside "running too rich" idea for the moment.....

- How's your vacuum advance and mechanical advance? Troubles here are common and could really hurt fuel economy

- Any sense that there might be a brake system or transmission problem? Something that drags down the engine?

Cheers
DD

 

Well Doug, now that you mention it...:-)

On the recent roadtrip, I do remember that it had a tendency to pull right somewhat. I suppose that could be caused by lots of things, but having to always steer it to go straight can't help matters. I think I was just so happy it ran that distance at all, and reliably to boot, I put other things at the back of my mind.

Anyway, I just pulled the LH front tire and then attempted to measure the gap between the brake pads and the disk. The narrowest blade on my feeler gauge is 0.004 inches (0.006 mm) and it wasn't even close to fitting between the pads and disk on either side. While I can turn the wheel hub by hand, I can hear some scraping. The car is in Park and the parking brake (which I think works on the rear axle only) is off.

I don't have a repair manual, so I don't know what this gap is supposed to be.

The brakes were redone by a Jaguar factory trained mechanic late last year, but from what I've read in Palm et.al. that may or may not be worth much.

Other than the above, I'm not aware of any issues with the transmission. It's not been serviced since I've had it. It leaks little if at all, doesn't slip or show any other bad behavior.

Thanks!

John
1098 XJ-S V12, 62,000 miles

 

None, really. It's normal for the pads to lightly contact the brake rotors. There's no retraction springs or anything like that to pull 'em away.

If you can turn all four wheels easily by hand that's about as good as it gets. Naturally the rears will be somewhat harder to turn as you're also turning the drive axles, differential gears, etc.

Do check the timing and distributor advance, though. Quite common to find problems there.

Cheers
DD

 

Oh yes, forgot to answer your other questions...

I rebuilt the distributor advance per Palm's document, so it does now function as intended (or at least as far as I can test it). It was seized when I got the car.

On the vacuum advance, if I pull the hose that goes between the advance module and the manifold, block the manifold then suck on the hose I can hear the engine change, so that also appears to be working (I also replaced that module last fall).

Well, I'll slap the wheel back on and look elsewhere. I know in a previous thread you mentioned a grounded injector or two. If that were the case (and assuming the problem was affecting only one bank) could I expect cat temps to be higher on the side that's shorted? Just trying to figure out how to diagnose that one.

Time to fork over for a timing light I guess...

Thanks!

John