My 1983 280zx is making bad gas mileage, oil smells like gas and makes a funny knocking sound. Not sure what the problem is but would like some help. We already checked the valve clearence and changed oil.

 

Did you make sure you don't have a fuel leak anywhere? If that knocking noise is coming from in the engine then you're gonna need a rebuild. If the engine is worn out internally, then you're loosing compression, and the fuel may just be slipping right by the rings into the crankcase. In which case the O2 sensor will read a lean condition and just dump more fuel that gets wasted. Run a compression test. I was getting about that mpg, then I found out my injector hoses had slow leaks. Fixed that and now I'm up to 17mpg I don't have my overdrive though.

 

Thanks I schould be able to check the compression within the next week.

 

Hey J, 17mpg is terrible also. On the average I was getting 19-23 or so, going to work with traffic lights. You should be getting better than that. Unless you have beefed up your car a little. If so I will shut up now!LOL

 

I think it's just the fact that my rearend is geared to run with overdrive, and when I put the 4-speed in, I lost that overdrive. If your cars manual, cruise around on the freeway for awhile without using 5th and you'll see where my crappy gas mileage comes from I am looking for a 5 speed if someone wants to get rid of one cheap. I'll be cruising to VA later this month or early Aug. if someone wants to part with theirs I can pick it up on the way. I'm in South Dakota now, so anyone east of here....

 

Yeah I agree I knew it had to be something to lose your mileage! Good luck on finding a 5-speed, a lot better man. And like I said on a crappy day, my 5-speed would be at the least 19 or so. Not bad at all!

 

With a 3-speed, my 82 turbo gets 19-23 mpg. What rpm does your car run at 70, J?

 

After replacing the Fuel Pressure Regulator and O2 Sensor, my '83 went from 16 mpg to 22 mpg. And passed emissions easily. HTH...Robert

 

Yup, that was my problem at one time too. Just those two items. Oh, and I also had leaky injectors at about 150,000 miles. But not leaky enough to effect gas milage.

 

Since my speedo is way off,and judging by speed checks with my bro I'd say somewhere between 4k-4500 at 70. But remember, your diff is geared accordingly to work with the 3spd auto. And the gears themselves are different.

 

Yeah in my 5-speed the rpms were like 2- 2500 at 70-75. In fifth gear of course!

 

Does anyone know what the knocking sound is? It sounds like its coming from the cam. Another qestion how hard is it to get 200hp out of a N/A?

 

200hp wouldn't be hard at all. A little more compression would get you there. You could install a thinner head gasket for more compression or have the head decked. Be cautious though, if you take out to much you can have piston to valve interference. So have this checked by a machine shop before running a decked head. Othere than that free up the breathing; exhaust maybe some head work. The more efficient you make the engine the less compression you're going to end up needing. Does it sound like you have a diesel engine under your hood? Is that what you mean by the knocking sound?

 

Its idles really rough and it just a tick/knock from the front part of the engine/cam. When you rev it up it goes faster and then goes down it could be a bad valve something or other but we adjusted the height.

 

I have that same noise coming from my rocker cover too. I also checked the valves, but they were all in alignment. I'm leaning toward a bad valve myself. I did the ghetto paper on the exhaust trick and sure as spit, I've got a valve sticking part time. I haven't had any problems with the car running so I'm assuming the problem isn't anything mechanical in the engine itself and the sound is definitely coming from under the cover. I can't afford to have my head rebuilt so I'm just leaving it be for the time being.

Has anyone else had this problem. I know timing chains are noisey, but this doesn't sound like a timing chain. I don't know how the chain is, but my car doesn't show any signs of the cam being out of time due to a stretched timing chain. Could worn sprockets be causing this noise? Anybody chime in here who's had this problem.

 

So do you think this could cause bad gas mileage?

 

If you're talking about the noise: If it's what I think it is then I don't think it would affect your gas mileage to the point where you're only getting 15mpg. Is your car shifting into 3rd? What RPM are you cruising at on the hwy? If it's not shifting into top gear than that will definitely kill you gas mileage. Otherwise I'd check out some of the things mentioned above like the Pressure Regulator and the O2 sensor. If the O2 is bad, your car may be running really rich which will affect your gas mileage.

 

Ya it shifts into third im gonna go take it out real quick and check the rpm's when im cruisen along it used to make like 20 some but it sat for like a year or two.

It's at 3,000 RPM at 65mph. Is that Bad?

 

hows it running? Is it running good, or is it having problems? If it's been sitting and it's not running very well, then you may just need to give it a once over and see what needs to be replaced as far as ignition parts, fuel filter, O2 sensor, etc... Could just need a tune up. Let us know how the RPM's go. If it's not that then I'd start looking things over. 02 sensors are cheap if you get the ones without the terminal on the end, and that's something that's good to replace anyways if it's been on there for a while.

 

it runs pretty good i need to retime it again but its just a really rough idle and the constant tick. Plus there's some structal damage so it pulls really hard to the right(Previous Owner).

0-60 is about 11 to 12 I think the timing is off and i need a couple new spark plugs and I got a new O2 sensor today

 

Let us know if the O2 helps any. Another thing to check for with rough idle is vaccum leaks. Those will wreak havoc on your idle. And also, I noticed you said you're knocking noise is coming from the front of the valve cover, right? Well a timing chain in need of replacement would make noise and that noise would come from the front of the cover. This would also cause a rough idling condition. So that's something else you might want to look into. The noise I'm having is coming from the middle somewhere so we may not be having the same problem after all. I'm pretty sure it's just a valve on my car. I'm going to be replacing my timing set soon, and I'm going to inspect the valve train then.

 

Thanks for all the help I hope it's not my timing chian but it could be. Hopefully when it gets retimed and the o2 sensor it will get better gas mileage if it deosn't I will put up another post. Thanks again and good luck with your store! Hopefully it will be up and running soon.

 

Found my vaccum it was me EGR valve it was stuck open now its fixed kinda check my other thread.