97 Camaro Z28, 6 speed.

New plugs and wires. New water pump (and opti)

I've been getting about 15mpg per tank for the longest time. I can squeek out 20 on the highway....but it's rare.

It seems to be running rich (obviously)

I had a recent Smog check and everything was under the spec (waaayy under). If I had to be less than 90, I was at like 15 (not sure the gas/spec). If had to be under 450 for another of the 3 gasses (NOX?) I was at 125 or so. A really good smog check.

What could cause the low MPG?

I know this car has a "tune" in it, but I have no way to check what the tune is or return it to stock config. The stat is also a 160. It never gets to the 1st line the right of cold. (never gets hot either)

The rings seem good. The engine is quiet and doesn't use any oil.

MAF wire dirty?
O2 sensors?
ICM? <--don't even know what this is, but in a search I read about it....

EDIT: New info...
Just thought of something else...


When the car is cold, it fires right up. (.5 to 1 second crank and Vrooom).

When the car is hot and been sitting for 1 to 15 minutes...it starts right up.


However...

When the car is warmed up, but I let it sit for about 20 minutes to 2 hours....it cranks quite a bit then slowly catches (acts like it's flooded a bit). It takes about 4 or 5 seconds to start up with this much cooling off.

If you let it sit for more than about 2 hours, it starts up with a quick crank again.

Weird...?

Any advice?

 

start by getting the car scanned for codes. im not to familiar with smog testing so i really dont know what any of that means. hows the exhaust smell?

 

Any mods?

 

test evap system. https://camaroforums.com/forum/93-02...62/#post557634 test temp sensors air and coolant. http://shbox.com/1/4th_gen_tech2.html#ect have injectors cleaned and flow tested. that would also let you know what size injectors it has. should be able to pick up a stock pcm to replace the one in the car. do you know what has been done to the motor? if it has a tune the chances are something has been changed.

 

I believe they scan it when they smog test it, but I'll see if I can get my code OBDII code reader to pick anything up.

The internals of the engine are stock, however I found a receipt for $300 for a "performance tune" and a 160 degree t-stat that the original owner (apparently) put in it some years ago. I'm guessing it was some sort of PCM flash tune.

Since the internals and everything else are stock, I'm thinking to replace the 160 degree stat and return to stock tune (or a canned "stock type tune" from PCM for Less).

I will check the sensors as best I can and see about getting the injectors flow tested.

The exhaust doesn't really smell when it's warmed up, however when it's cold, there's that "the catalytic coverters are working hard smell (like catalyzed fuel). Not a burn smell or gas smell.... The exhaust seems to flow very well and the cats don't seem to get too (burning) hot. Based on the measurements from the smog check, I'd say the cats are working very well.

I have the two on the left exhaust, so I'm pretty sure I'd notice a change in tone if there was a restriction of some kind. It's got a great exhaust note.

One of my thoughts is that the car isn't getting hot enough to go from closed to open loop (sensing from the O2 sensors) and it's just running off the fuel map. Not sure if that makes any sense, but I've read that somewhere on here and on a tuners website. ---> here LT1 PCM Tuning - Tips & Tricks for DIY Tuning!

Thanks for the info. With fuel at $4.40 a gallon here and my truck getting 10mpg with a tailwind...I'm looking to the old Camaro for some savings.

 

lt1pcmtuning is solomon. i use him instead of pcmforless. he is the best i have found for tuning the lt1.

 

Thank you. I appreciate the feedback.


I'll describe the condition a bit more.

What it's NOT:
1. A rythmic pulse (like a bad plug, injector or lifter)
2. A surging (the RPM's are constant to what you'd expect them to be...no rolling idle)

What I'm concerned about is just a general roughness in the idle. It's almost like it's mildly cammed, but I know it's not (or at least it runs like absolute crap if it does! LOL)

It's a manual. When I get off the gas, the fuel cutoff is abrupt.

I'm also going to change the fuel filter and cut the old one open. It might be the original one from 8/1996! (who knows...) I'm hoping it hasn't fallen apart inside and messed up my injectors...but if it's that..at least I'd know.

The car has about 33k miles on it, but it's sat quite a while (2k miles a year for the last 13 years on average)

 

Just thought of something else...


When the car is cold, it fires right up. (.5 to 1 second crank and Vrooom).

When the car is hot and been sitting for 1 to 15 minutes...it starts right up.


However...

When the car is warmed up, but I let it sit for about 20 minutes to 2 hours....it cranks quite a bit then slowly catches (acts like it's flooded a bit). It takes about 4 or 5 seconds to start up with this much cooling off.

If you let it sit for more than about 2 hours, it starts up with a quick crank again.

Weird...?

 

Check fuel pressure that's what the starting issue sounds like

 

^^^ that would be a good idea. the fuel presssure regulator may be getting sticky. high fuel pressure could be it. i would not be surprised if the injectors insides are coated with gas varnish from each year of setting.