1993 300zx rough idle when warm, bad gas mileage.

[flyhotstuff]

I recently bought a 1993 twin turbo Z with 55210 miles. has a lot of power. I am getting really terrible gas mileage like 8 mpg. if i could get at least 16 MPG on city driving i would be happy with my purchase. When the car reaches operational temperature it starts to idle at 500 rpm and really rough, kinda like it's surging. but it stays around that rpm mostly.
The gas tank doesn't appear to be pressurized as there is no whoosh sound when i open the gas tank. I am not sure if the gas cap is bad.

when i slow down and just release the clutch the car starts jerking and the rpm drops to somewhere are 200 rpm when it jerks.

I have the old style injectors if that helps.

[PapaSmurf2k3]

Welcome to NICO!
Are there any mods to the car?

First things you should look for are:
Bad MAF
Bad O2 sensors (they are probably all carboned up now because its been running so rich anyway)
boost leak somewhere (I'm not sure if your charge piping is slightly under pressure at idle, but if it is, and you have a leak, you'll be leaking metered air)

I'd check the last one first, as it's free.
One thing you can do to help with the poor idle in the meantime if you can't find the boost leak, is to induce a vacuum leak somewhere (via the intake manifold). This will suck in un-metered air and help correct the air fuel ratios. Note- this will also turn into a small boost leak when you are hammering on the car, but shouldn't affect performance too much. Let us know how you make out.

[flyhotstuff]

There are no mods other than a cold intake and an exhaust system. i haven't yet run any codes. i checked the MAF and it appeared pretty clean. that's all i checked today been a bit busy.

whenever the car starts it seems to hesitate a bit beofre fully coming to life, like the Tach who run to 600 then 100 and back and forth for a second or two then it starts up.

when i unplugged the MAF and started the car there was no of that hesitation issue. She came to life immediately but idled a bit higher than normal. when i plugged the MAF back in she dropped back to the old 500-600 idle. when i removed it again the car stumbled and shut off.

[PapaSmurf2k3]

I don't suppose you've checked the ignition timing on it have you?
Is the CEL on?

[flyhotstuff]

no CEL whatsoever. The Hicas light comes on now and then but goes away in less than a few minutes. i don't have a timing light to check the timing. is there any other way to verify? I will be checking the O2 sensors today. but wow an oem MAF is quite expensive lol. i hope that isn't the problem.

[PapaSmurf2k3]

Yeah I wouldn't pick one of those up unless it was a last resort. Swapping to a used one would save a ton of money, but still pricey.

You should make a boost leak detector and see if you have any leaking couplings.
http://www.nicoclub.com/archives/turbo- ... ester.html
I know the pictures are broken, but you should still be able to follow along.
The end result should look something like this:

or this:

[flyhotstuff]

thanks for the tutorial for the boost leak tester. will come in handy. i ran the codes and got a code 12. nothing more. keeps repeating only that code. Something with the MAF circuit? i can still run the car pretty hard. revs past 7k. pulls pretty strong.
also i am not sure but i started the car after pulling the MAF. That would set a code as well right. i plugged the sensor back up and drove a couple miles since then and ran the codes after that. got the code 12. not sure if the code may be related to that earlier event.

[PapaSmurf2k3]

Yeah it very well could be related to the earlier event. You can disconnect the battery for fifteen or twenty minutes to reset them.

[flyhotstuff]

i suppose you're right i disconnected the battery overnight and it cleared the code. drove around for about 2 miles and the code 12 has yet to return. keeps giving me code 55. car drives so well apart from those two issues. another thing is if you rev the car pretty hard going upwards to 5-6 and release the throttle it backfires/back pressure and black smoke comes out. no smoke whatsoever when the car is idling. could that be due to the rich mixture?

[flyhotstuff]

drove around the car today, i checked the codes again and got a code 13 coolant temp sensor. after driving for a bit the check engine light came on it went away shortly after. i ran the codes again and still got a code 13. I will attempt to clean the connector and if that doesn't work buy a new sensor. i hope that is the problem.

[PapaSmurf2k3]

Yup, black smoke is generally due to running rich, which a faulty coolant temp sensor can cause.
It can also be caused by a blow off valve vented to atmosphere. At that point, coming out of boost would blow off metered air, causing a temporary rich condition.

[PapaSmurf2k3]

Not sure how interested in learning all about turbo cars and a blow off valve's function, but this was written by a friend of mine, and is a fairly good read:
http://mattminer.com/?page_id=85

[flyhotstuff]

terrific thanks alot man. i went ahead and ordered a consult to usb adapter so i can read the ECU with my pc. I changed out the coolant temp sensor for a used one from a sr20. No difference. i checked the resistance of the coolant temp sensor at room temperature and got 1.4-1.5 ohms. decreased when it was heated. i plan to change out the vacuum lines for some silicon ones. hopefully i will get to the bottom of this problem. gonna go ahead and make that boost leak tester.

[PapaSmurf2k3]

Great. Keep us posted.

[flyhotstuff]

**UPDATE** i am still not sure if this has rectified my bad gas mileage but i solved the rough idle issue. turns out that i had a Vaccum leak at the clutch booster tank. it was cracked and thus made an audiable hiss. i removed the tank and condemned the hose. sure enough placing my thumb over the hole immediately caused the idle to even up. Thanks a lot guys. when i am back on the road i will know if the gas mileage has gone up.
does any have an ideas why my gas tank doesn't hold pressure? thanks

[Ace2cool]

Gas tank not holding pressure? They don't always pressurize. But you can get your gas cap tested, and that would be the first thing to look at. Could possibly have something to do with the carbon canister as well.