My 280z is only getting 14mpg...

I've replaced the fuel filter, and the filter near the tank which raised me from 10mpg to the 14mpg.

I've replaced the spark plugs and the wires with NGKs.

My engine holds idle at 800rpms with no problems except when my engine is cold it gets stuck at 600-500 if it hits 500, it wants to die unless I rev it until it warms up.

Sometimes when I start my engine when it gets stuck at 600-500rpm it will backfire at the intake when I rev it and then after roughly 2-3 backfires it will idle normally.

It also backfires at the intake without the AFM attached so I don't see my AFM being an issue.

I've already fixed my vacuum leaks and my fuel pressure is perfect (~35).

What should I start diagnosing?

Whow!

Check cts sensor and correct operation of throttle position switch, then check fpr.

First check the TPS, as a quick test you could disconect it and see if fuel comsumption improves. Check the water temp sensor (and also the coolant level). check you're new plugs and see if they are black (probably). Also do a search for mpg and you may find a rather long thead involving the same problem I had, lots of info there. Also check the timing though it probably is not the cause.

Just read a guide on my TPS, and it was at fault. The second I hit throttle, it was telling my engine I was WOT. I followed the instructions and got it spaced exactly where it needs to be.

Now I'm going to do a mpg test

Also, my distributor timing was off by A LOT, I fixed that and set it to 7deg / 800RPM

after my next mpg test if it's not acceptable, I'll test the sensors.

Check cts sensor and correct operation of throttle position switch, then check fpr.

What is the FPR ?

Fuel pressure regulator. but it seems you may be on the right track.

After a quick test run on the freeway and a couple red light stops; my car runs so much smoother, and it's accelerating much better. I actually said "Holy S**T" when I floored it from a dead stop to 65mph to get on the freeway.

Filled up my tank to full, and I hope my mpg is better now. it was only 14mpg before.

So just for future reference,

I should check my Water Temp Sensor and my Coolant Temp Sensor?

How would I check my Fuel Pressure Regulator as well as those other two?

It looks like you have found the culprit, but it is a really good idea to check all of you're efi system. To test the FPR you will need a fuel pressure gauge and a vacuum gauge. the water temp sensor can be tested at the ecu connector at the lower drivers side kick panel, you can test most of the EFI electrical systems there using a multi meter and the factory service manual:

http://www.xenons30.com/reference.html

check and clean all electrical conections and for sure inspect all vacuum hoses as a vacuum leak will cause the engine to not run optimally and could cause a miss diagnosis.

Good job you are saving money in two ways: less gas and understanding you're engine so as to not throw new parts at it or paying a mechanic that does not understand older EFI. let us know youre progress.

Edited December 30, 201113 yr by grantf

Timing should be at 10 degrees minimum. A lot of people will advance it to 13 or so. Running it at 10 was lower than the previous years and beliefs done to lower emissions at idle.

Timing should be at 10 degrees minimum. A lot of people will advance it to 13 or so. Running it at 10 was lower than the previous years and beliefs done to lower emissions at idle.

I was told I needed to be at 7deg for california.

I'm curious, where does this information come from that it needs to be at 10-13 ? And what would the benefits be

Also, I've been reading a bit more, and I see people are able to go to 20deg and such.

My timing won't let me go past ~12deg, but I can go the opposite direction way off the timing marker.

Could anyone explain why this happens?

Also, my engine ticks now at idle, but ever since I timed it to what I believed was 7deg(which I don't believe it truly is anymore) my engine has ran better than ever.

One last thing, I had about 2 instances where I heard a weird metal grind from the engine bay when I revved up the engine around 4krpm that last maybe 1 second. Any ideas as to what this could be?

Edited December 30, 201113 yr by Milenko2121

I was told I needed to be at 7deg for california.

I'm curious, where does this information come from that it needs to be at 10-13 ? And what would the benefits be

Also, I've been reading a bit more, and I see people are able to go to 20deg and such.

My timing won't let me go past ~12deg, but I can go the opposite direction way off the timing marker.

Could anyone explain why this happens?

Also, my engine ticks now at idle, but ever since I timed it to what I believed was 7deg(which I don't believe it truly is anymore) my engine has ran better than ever.

One last thing, I had about 2 instances where I heard a weird metal grind from the engine bay when I revved up the engine around 4krpm that last maybe 1 second. Any ideas as to what this could be?

Screw California.As far as timing goes, the figure is a base line,each motor will like what it's gonna like.Play with it untill it runs the best.

Screw California.As far as timing goes, the figure is a base line,each motor will like what it's gonna like.Play with it untill it runs the best.

how would you know what the best timing is?