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Discussion Starter #1
I need a bit of help, and don't know where else to look.

I'm getting about 12mpg city and 15-16 freeway. I've changed out my TPS, 02 sensor, fuel injectors, FPR and catback exhaust. I've replaced most of my vacuum lines, but a couple remain disconnected on the passenger side.

ECU codes read Throttle Valve Switch, CHTS and Fuel Temp Sensor. I've replaced the TPS and the FPR is fairly new (it included the sensor). I haven't had the time or tools to change out my CHTS, so it's still in my glove box.

Any ideas on this mileage problem? It's been dropping slowly but steadily while everything else on the car has been upgraded. Would a bad CHTS have any effect on the mileage? I also don't understand the Fuel Temp Sensor reading, as the FPR is only about 4 months old, but suggestions would be appreciated.
 

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A bad CHTS can give false readings for other components I believe if they are bad so try the CHTS next. Another thing to consider would be a fuel leak. Have you checked your fuel lines from injectors all the way back to the pump? You could have a small leak. You should be getting way better gas mileage so something is up. My 300zx gets about 19-20 in town and about 29-30 on the freeway. Good luck
 

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Discussion Starter #3
There's no gas smell coming through the vents, so how else would I check for a small leak in the fuel system? I thought about that, but don't know how to check it. FPR with gauge? I'll work on the CHTS tomorrow night if I can....seems like it'll only take an hour or so to swap.
 

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If you go through with the CHTS - ignore your manual and follow the steps in the link below:

http://z31.com/repairs/chts.shtml

Here are clear pictures of the CHTS if you haven't seen them already




Also, this may beone of those parts you want to get from the dealer if you can. Good luck!



Post Edited (Jul 13, 7:44am)
 

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Discussion Starter #6
Thanks, Stu....that diagram helps me out a lot more than the z31.com walkthrough--I can't see my CHTS or harness at all just looking at it from the top of the distributor like they suggested. I bought the sensor from Nissan for about $32, but may also buy the harness today prior to installation. So I remove the shroud, fan, clutch and timing cover and it should be right there.

Fa-kin-a....that's what I was thinking when I replaced the o2 sensor, but I wasn't aware that a faulty CHTS could cause a continuous open-loop mode as well.....we'll see what happens after I change this puppy out in the next couple of days though.

Any other thoughts on tracing a possible fuel leak?
 

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MaximumQ - before you go about replacing the CHTS the hard way - take a flash light and really look down underneath your distributor cap - you should be able to see it there. If you can get it to work in the manner that z31.com suggests it could potentially turn a 4 hour job into a 20 minute job.

I am in the processing of purchasing a replaced CHTS so I haven't actually done the replacement but domdogg here on zcar.com completed it recently via the z31.com method and said he was not only successful but did it in about 20 minutes.

Food for thought anyway - glad I could help!
 

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As for the fuel leak you'll actually have to inspect every hose. Remember that gas evaporates rather quickly, especially in the summer, and if it's far enough back you may not smell anything. A while back I had a fuel leak on my 280z and the only way I noticed was one day while I had it warming up I popped the hood to check everything out like I occasionaly do and noticed it luckily. In your case I would check all the rubber hoses in the engine. If they are old, hard, or cracked don't risk it and just replace them. The metal fuel lines that run under the car don't usually crack but just get under there and check anyway. Those should be easier to see a crack in. Lastly check all the clamps and connection points along the way for tightness. Again if a clamp isn't holding too well or jammed after all these years just get some replacement clamps.
 

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Discussion Starter #9
How likely is it that the fuel pump is partly the culprit? I've also noticed that I get great mileage for the first quarter tank or so, then it starts dropping like a brick. I've changed the sending unit by the way, so the gauge reads pretty accurately.
 

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Problem you might have is with all the carbon going thru the exhaust...your O2 sensor will probably be non-functional by now....after the CHTS is done....use some good injector cleaner to clean it?

Get the EGR working?


Ways to save some gas costs with your na Z....


* new plugs...wires...dist cap & rotor.

* check gas cap to see if it holds pressure...check EVAP system (charcoal can & lines).

* use synthetic oils in the engine...trans...diff...maybe 3% gain in mileage?

* new O2 sensor (if the old one doesn't work)...possible 10-12% gain in mileage...found this out by replacing it.

* a non functioning EGR might cost you 10% in mileage...found this out by disabling mine.

* tire pressure up to max (be SURE and check for good traction)....maybe a 3-5% gain?

* drive like your grandma.....

* for those with a sense of adventure....

http://www.lubedev.com/smartgas/ ...have gained 12% mpg with an na using acetone

* for those from outer space.....

http://www.kfor.com/Global/story.asp?S=3390503&nav=6uy5aHLq



Post Edited (Jul 14, 4:13am)
 
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