Terrible Gas Mileage
 

I know, it's a 12 cylinder 5.3 liter...

But still...

I get about 10-12 mpg driving around the city. On a good highway stretch I'll see as high as 14 mpg but that's about it. I've had the car over 10 years and that is pretty consistent.

According to Kirby's book I should be getting 14-18 mpg...

I overhauled the mechanical advance shortly after getting the car and check it periodically to ensure it isn't frozen. The vacuum advance modulates when I take a pull on it with my mouth (I don't really know how to check it beyond that). I replaced the O2 sensors about 6 months ago.

Now I'm kind of at a loss. I haven't checked the timing in quite some time. I do run 180F thermostats (not sure why I decided to do that... had them swapped out when the radiator was recored a decade ago), which isn't helping mileage I'm sure but nonetheless shouldn't be making *that* massive a difference.

I know you can check the ECU output voltage when it is on closed cycle, and I'm thinking of doing that next to ensure it is drifting around...

I'm also getting my data exclusively from the trip computer, which could be off I suppose. The H&E gas tanks are notoriously fiddly, and I can't always get it filled reliably.

Any other thoughts?

Cheers,

Will

About as good as it gets in city driving, IME






More like 16-20 I'd say, perhaps a bit more if driven very gently





Good that it works using lung power but there's a complicated control system ahead of the vacuum advance cannister itself. Try running your vac hose directly to one of the ported vacuum nipples at one of the throttle bodies and see if the situation improves. As I recall the nipples on top are ported vacuum, the ones are the bottom are constant manifold vacuum....but I'm a bit foggy on that point




Can't hurt to do so




I agree, as the cold fueling (via the coolant temp sensor) is all done by then




That's fine, although I'd look elsewhere first



My experience is that that the trip computers are surprisingly accurate as on-car gadgets go, within about 3%. But, still, you do need to verify!




Sure!


-One or both enrichment switches stuck "on"
-Coolant temp sensor skewed, giving a "cold" signal to the ECU
-Leaky fuel injectors
-Excessive fuel pressure
-Leaky vac hose to ECU
-Air temp sensor faulty

Individually some of the above might not make a huge difference but it isn't at all unusual to have a stack-up of faults, especially on older cars.

Others will chime in with more, no doubt.

Cheers
DD

Cannot hurt to take a plug out and look at its colour. Also to poke an analyser up the exhaust and see if it is running too rich.

Greg

a little bit of timing advance couldnt hurt.

also, i dont think doug was on the money with his comment about switching the vacuum lines around for the advance. IIRC the vacuum advance shouldnt be getting more than about 12 in/hg.

plugging it in to manifold vacuum will give you 25-30in/hg which could damage something.

maybe im just overly cautious.

I agree with Doug on the vac hose porting. The top ones are ported vac, and the lower are constant vac.

Mine has been plumbed that way since 1999, as in the under side of the RH throttle housing, direct to the capsule.

I have timed mine by "drive". Did it by the book in 1996, still ran bad, drank fuel, so simply set it to run, drove it and kept advancing the timing until it "just pinged", and then backed it off a smidge. Of course the vac advance was connected as it would normally be. It has not been adjusted since then. I also ONLY run our 98 octane fuel, no exceptions.

Before you get too worked up I would do a proper fuel mileage test. See if it matches the fuel computer. Then as others suggested check the tailpipe with an exhaust gas analyzer. You need to do this to set the CO anyway.

When I bought mine the vacuum advance system had been modified to bypass all the control devices. It ran fine but had a slightly rough idle.

Reading that one of the reasons that Jaguar went to such a complicated control system was to "stabilize idle quality" I decided to return everything to factory configuration. The result: same power, same idle, slightly less fuel economy.

Then I read that the "Australia" spec for the control system was the ideal configuration...so I made the necessary changes which, as I foggily recall, involved eliminating one component and changing a couple vacuum hoses. Result: no perceivable difference from USA spec.

So, I ended up returning it to "previous owner spec" with the control system bypassed entirely.

<shrug>

Cheers
DD

Doug -

1. What does the vac hose to the ECU do? Best method for testing?
2. How would you recommend testing the coolant temp sensor, enrichment switch, and air temp sensor?

Thanks!

Cheers,

- Will

Okay, so I've been gradually sorting this out. I did verify I get vacuum to the ECU (and the car idles like crap when you disconnect it).

Discovered that the vacuum line from the 45 second timer to the solenoid regulator was disconnected. So the vacuum advance was completely nonfunctional.

Connected the vac advance directly to the ported vacuum. Drive to work this morning got 15 mpg. Difficult to say if the car is more responsive or not; feeling HP gain by the seat of the pants is a poor metric.

Will keep an eye on the mileage and see what happens. Hopefully it improves and I've found the culprit.

Not sure why someone disconnected that circuit. Looks to me like it was deliberately disconnected, but I suppose it could have just fallen off. Maybe someone was fiddling with it to get it to pass California emissions and did not reconnect. In any case, will hook up a gauge to test out the regulator this week to ensure it is giving proper vacuum at idle. If it is working, I'll probably just bypass the 45 second timer and hook the regulator directly to the manifold.

Why wouldn't you connect the vac advance directly to the manifold? What's the point of connecting it to the ported advance?

Cheers,

Will