# Gas Mileage Problem



## MaxQ (Jun 3, 2004)

It seems like I am bleeding fuel. I have replaced my oxygen sensor and my fuel injectors, and detect no fuel smell in the engine bay while or after the car has been running. I am managing about 12-13mpg at this point, and was around 10-11 before the oxygen sensor swapout. The EGR appears fine, but I have also replaced the gasket just in case. 

Should I reset the ECU since changing the O2 sensor and adding the cat-back exhaust? I've had the problem for almost a year now, so the new additions aren't what's causing the problem. I've checked the timing and all seems fine. It's a rebuilt motor (<10,000 miles on it), but here's a hint--the problem existed with the old motor as well, just not as bad (I was at around 14-15 mpg).

New puzzle for the day!


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## z31 (Oct 19, 2004)

pull the vacuum hose off the FPR and see if it shoots fuel at you.


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## MaxQ (Jun 3, 2004)

I replaced the FPR 2 months ago with OEM from Courtesy Nissan. Still try this?


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## z31 (Oct 19, 2004)

MaxQ said:


> I replaced the FPR 2 months ago with OEM from Courtesy Nissan. Still try this?


I hate it when you guys punch holes in my easy solutions....

Is the ECU throwing any codes?


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## MaxQ (Jun 3, 2004)

LOL. I haven't checked the ECU yet, that's why I was also inquiring about resetting after the new parts that I've installed. I'll try to check it tonight or tomorrow morning for codes, though. Should I also reset?


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## Zen31ZR (Mar 24, 2004)

Is this a _new_ new engine, or just a newer old engine........


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## MaxQ (Jun 3, 2004)

This is a complete rebuild of the original engine--brand new heads, new pistons, rings, rods, etc. Top and bottom end rebuilt, less than 10k total on it. But same problems as when the motor was 178k old, so I tend to discount the motor as the source of the problem. Also, there were a lot of externals (hoses, etc._) that the shop didn't replace after the install, so even the CHTS is suspect, in my book.


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## Zen31ZR (Mar 24, 2004)

Yeah, you may have a lot of unmetered air coming into the system from leaky hoses, or your injectors might be partially clogged or simply weak. Clogged injectors will cause excessive use of fuel due to poor atomization and the resulting incomplete combustion. If you had O2 sensor failure before, this was likely the case, they don't like to be constantly doused with unburnt fuel..... Neither does the catalytic converter, it might also be partially clogged. 
EDIT: I see you replaced the injectors, so maybe you can eliminate those as a problem.
Definitely reset the ECU, it can't hurt.

If you can't raise the fuel mileage by at least a couple points, you might wish to suspect excessive driveline drag, or maybe brake drag.


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## MaxQ (Jun 3, 2004)

Cool. I'm resetting the ECU this morning and will track codes. I agree that it may be faulty vacuum--I've noticed a couple of unconnected ones that I can't track, including one directly underneath the TPS, attached to the collector, that I can't locate the other side of.

I'm pricing cats now. Will keep you updated. Thanks for the great advice, and I'll let you know if any of this stuff actually solves the problem!! Just a side comment--you guys on here (zen,az-bum, z31, james) are an excellent resource and I've learned a lot from asking and from reading....keep up the good work. I love my Z and will keep it until it packs a bag on its own and walks out of my garage, but every day I find out how much more there is to learn about tuning and maintaining it. Thanks for the patience.

I'll let you know if the reset/cat replacement change anything. In the meanwhile, do you have any idea where the vacuum hose directly under the TPS, leads? My hose appears to be original, and is marked with the number 5 continuously along it's length.


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## MaxQ (Jun 3, 2004)

OK, here's what I found........VACUUM HOSES. 

Apparently, when I had my engine rebuilt, the dumbasses at LA RIMS CARE CARE CENTER (FYI) incorrectly ran some of the hoses. Even the vacuum diagram on the jboykin site doesn't cover all of the hoses that are missing or incorrectly routed, so for now I'm stuck--until I can just compare my routings to another 300zx side-by-side.

There are two hoses underneath the throttle body--right (rear) larger one leads to a metal line that goes around the back of the plenum and eventually into it. That appears correct. But there is a smaller hose on the left that is completely disconnected. I think it goes to one of the four rails at the front of the plenum, but they are all connected to other lines.

Also, there is a questionable fuel line leading from the FPR. There's a small patch missing, but it doesn't appear to be leaking. Seepage, though? I'm replacing it tonight.

Any ideas on that errant Throttle Body hose and those around it?


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