jakob27990

HI Guys. About A Month ago now I purchased a 96 LS400. I got this thing ridiculous cheep since it needed a bit of work to get on the road. I've been reading on this forum and on other sources that i should be getting way better fuel economy then what im getting. Im not sure about the mpg or how many l/100kms but i do know that with 50/50 city/highway im not even getting 500kms to that tank with the needle below empty and that seems kind of ridiculous for an 85L Tank costing me $90 to fill every week. I replaced the obvious air filter, all the plugs wires and coils, and had the exhaust leak fixed. What do you guys think i should do? I know it has a V8 but still...this seems kind of ridiculous.

Here is some pictures of this thing. I think its pretty clean for a 96 in my opinion. I definitely have a bit of work to do still!

Losiracer2

Coolant temperature sensor would be a good start. Its an inexpensive sensor, usually ranging from 18-22 at parts stores to about 40-45 at a dealer.

Its probably the number one thing that could affect fuel economy at this stage in most LS400's lives since by now, all the connectors and vacuum lines would be starting to develop corrosion and getting hard/brittle developing hair line cracks. Also check your intake tube rubber coupler behind the MAF sensor, mine was cracked in 2 spots allowing air to enter unmetered after the MAF doing weird things with my throttle input.

The first thing I had to do to my 97 was replace most of the vacuum lines under the hood since I had some weird idle issues. The next thing I did was the coolant temp sensor along with fluids and radiator hoses following a timing belt/water pump for preventative maintenance.

Banshee365

How does it start when hot? Any other weird drivetrain issues like other codes and such?

Just to get a baseline economy number, reset one of your trip meters when you fill up next. Then fill up again and divide the miles/km's driven by the amount pumped in shown on the pump.

Im currently getting about 19 mpg (8.08 km/l) with mixed driving. I think it's a bit low and I'm going to work on looking at any possible causes for overfueling on mine. My car otherwise runs great except for some cat efficiency codes. Those codes can be caused by overfueling as well as exhaust leaks.

Also, what octane fuel are you using?

jakob27990

It seems to start up fine when hot. Cold start sometimes I feel it takes longer to start then it should considering it gets driven every day.

I reset the trip meter to zero every time I fill the tank, and last time I got 510 but that was with a lot of highway and the needle was below empty. I really tested the waters with that tank.

If I do the math I tend to average 17 - 19L per 100kms and that's with the mixed highway. My tanks are usually about 50 / 50

I run 91 octane since that's all that's available for the most part here. I did experiment and try using 94 a few weeks ago and saw no difference in the way it ran or in fuel economy.

I have no check engine lights, no weird codes nothing. I had to replace an O2 sensor when k bought it to get it to pass emmisions. This thing currently has just over 270 000kms on the odometer

jakob27990

I should also add that i thought the car had a really bad exhaust leak, so I brought it to my mechanic. Turns out there is a pipe or a tube located above the right side front CAT I'm not 100% sure what exactly it is but it has a crack in it making the car sound like it has a bad exhaust leak. I think it might be an EGR pipe or something EGR related I'm not sure. I haven't looked at it myself. Just haven't had the time yet. Maybe you guys know something about it. He told me not to worry too much about it right now it's not gonna cause any problems if I leave it. And its not easy to get at

valex

A bad thermostat will result in poor fuel economy. It is a cheap part and as easy to change as an oil change, if not easier.
Start with simple things.

jakob27990

If my thermostat was bad, would it not show on the temp gauge or have a more obvious effect that it's a problem then just having bad fuel economy? I'll pick one up and change it and see what happens.

valex

When my thermostat was bad I think it was taking a while for the temp gauge to move up in winter. I guess it was running rich and wasting fuel.

valex

You mentioned you replaced an O2 sensor, just one , not a pair? Upstream or downstream?
Upstream is $40 at rockauto plus shipping (OEM Denso), I would replace them in pairs, correct me if I am wrong. The upstream ones would affect the fuel economy, downstream do not ,afaik.

Banshee365

17-19 L/100km? So that's 12-14 mpg. That is quite low. If you can get an official calculation it would be handy.

Losiracer2

The EGR pipe was also cracked in mine when I got it. I had it welded in between the accordian sections, so it'll most likely never fail again. The pipe new is about 130, so that why I opted to weld it for 20. Good luck trying to get access to it easily, it took me about 3 days off and on since I kept running into problems left and right with a seized cat converter nut. I ended up having to literally drill through the top stud since there was no way to get the nut off. I tried my butane torch, then propane, then mapp gas and it still wouldn't budge.

You're better off combining this service when you replace the starter since you'll get much better access to the top two bolts, as they're literally touching the firewall area behind the engine. .

oldskewel

iTrader: (1)

Thanks for doing the MPG conversion for the rest of us. I'll agree the 12-14 MPG is low, but at least for my 1991, I would not call it "Beyond Terrible" as it is in the title of this thread.

Given that, I would recommend just trying to get your car running well first, and hope the MPG bumps up accordingly.

OP, you mention 91 and 94 Octane. Just like the L/100km thing, is there some conversion between USA Octane numbers and Canadian? Here in Califonia, the highest widely available you can find is 91 Octane. The standard low Octane is 87. Same in Canada?

jakob27990

Canada is exactly the same as the USA for the most part, 87 being the standard low and 89 mid grade. 94 octane is just a higher octane gasoline and i think it's only available at certain Petro Canada Gas Stations. Aside from that I've never seen anything higher than 91 octane anywhere else in Canada. 93 does not exist here, or at least anywhere in Ontario.

I replaced only the defective sensor and it was a downstream sensor after the CAT's. I have no records if any of the upstream sensors have been replaced previously.

Im gonna try maybe patching mine somehow. Welding it might be the best option. Like i said i haven't had a chance to really look into it. It seems like a real pain to get at. i have no idea how bad the leak is or where its leaking exactly, but makes sense that it would impact the fuel economy a bit?

Banshee365

More than likely an exhaust leak that you can actually hear is your EGR tube. It is not very easy to replace as you have been told. You have to remove at least the right cat but I just remove both cats and the mid pipe together. After that it's just long extensions and a u-joint with a 12mm socket on the end to get to the tip bolts. The bottom bolts can get had with a ratcheting wrench, but if they're really stuck be sure to use a 6-point socket to break them loose.

This leak will generally not effect fuel economy a noticeable amount.

jakob27990

Last time i had a look underneath the previous owner had the flanges on the cat's welded. I guess it must have been leaking previously and that was the easy way out. This job looks like its gonna be a real pain. Is this something I should worry about in terms to other problems?