95Z28QFZ

here goes..... Just bought a 95z 5 months ago with 140,000 and have changed plugs bosch platinum 2, msd 8.8 plug wires, opti, coil, egr valve, k&n oil and air filter, mobil 10w30 oil, iat, map, water temp sen in waterpump, waterpump, 160 stat, cat del, magnaflow muffler,trans rebuild and have had it scanned for codes and everything looks good and she runs good but I am only getting about 140 to a tank of gas and a shi$ load of carbon in the exhaust. I was unable to find out who owned the car before me but with this mileage and times is it posible that someone may have changed the cam or rocker arms, even track times seem to be good 2.179 60ft, 9.266 1/8 et, 77.36 1/8 mph. Here is a list of options that came on the car as well.
AG1 ADJUSTER, FRT ST, POWER, 6-WAY DRIVER, SPLIT BENCH
AK5 RESTRAINT SYSTEM, FRT SEAT, INFLATABLE, DRIVER
AR9 SEAT, FRT BKT, EUROPEAN STYLE, PASS A DRIVER RECL
AU0 LOCK CONTROL, REM0TE ENTRY
A31 WINDOW, POWER OPERATED, SIDE
A90 LOCK CONTROL, RR COMPT LID, REM CONT ELEC RELEASE
B35 COVERING, REAR, FLOOR MATS CARPETED INSERT
B84 ORNAMENTATION, EXTR MLDG, B0DY SIDE
C49 DEFOGGER, RR WINDOW, ELECTRIC
C6G GVW RATING, 7600 LBS
DG7 MIRROR 0/S, LH & RH, REM CONT, ELEC, PAINTED
DL5 DECAL, ROADSIDE SERVICE INFORMATION
FE2 SUSPENSION SYSTEM, RIDE HANDLING
FE9 CERTIFICATION EMISSION, FEDERAL
GU5 AXLE REAR, 3.23 RATIO
G80 AXLE POSITRACTION, LIMITED SLIP
IFD TRIM, INTERIOR DESIGN (FD)
JIF BRAKE PROVISION SPECIAL TO SUIT FOREIGN REGULATION
J65 BRAKE SYSTEM, POWER, FRONT AND REAR DISC
KG9 GENERATOR, 140 AMP
K34 CRUISE CONTROL, AUT0MATIC, ELECTRONIC
LT1 ENGINE, GAS, 8 CYL, 5.7L, MFI, HO
MXO MERCHANDISED, TRANS, AUTO PROVISIONS, O/D
M30 TRANSMISSION, AUTOMATIC 4 SPEED, 4L60E, ELECTRONIC
NA5 EMISSION SYSTEM, FEDERAL, TIER O
NP5 STEERING WHEEL, LEATHER WRAPPED
NW9 TRACTION CONTROL, ELECTRONIC
N96 WHEEL, I6 X 8, CAST ALUMINUM
QFZ TIRE ALL, P245/50ZR16
STE PLANT CODE - ST. THERESE, PQ, GM OF CANADA
T96 LAMP, FOG
UB3 CLUSTER, INST, OIL, COOL TEMP, VOLTS, TRIP ODOM,
U1T RADIO AM/FM STEREO SEEK/SCAN, COMPACT DISC, HPS, CLOCK, ETR
U63 RADIO, AM, MTR U64 SPEAKER SYSTEM, 4, DUAL FRT DASH MTD, DUAL STD RGE
U73 ANTENNA, FIXED, RADIO
VK3 LICENSE PLATE, FRT MOUNTING PKG
VM3 LABEL, CONSUMER, CONTAINS BPR IMP STANDARD
V73 VEHICLE STATEMENT, USA/CANADA
Z28 MERCHANDISED PKG, SPECIAL PERFORMANCE
1AY ORDER MODIFICATION STOCK
1SA OPTION PACKAGE 01
12I INTERIOR TRIM, GRAPHITE (92)
122 TRIM COMBINATION, LEATHER, GRAPHITE (2) (92)
81Q MOLDIMG COLOR, BRIGHT RED (9
81U PRIMARY COLOR, EXTERIOR, BRIGHT RED (91)

Any help with this would be great.
Thanks, Mike

Laymaster

go to napa and buy this stuff called seafoam. let your vacuum hose suck it up little by little, and by the time you're at about 1/2 can left, let it suck enough up to stall the engine out. pour the rest into the gas tank. let it sit for about 20 mins then drive your car hard. it'll startup a bit hard, but that's ok and normal. there will be a TON of smoke pouring out of your exhaust. this means it's working. it'll clean out all the carbon inside your engine.

95Z28QFZ

Thanks for the response laymaster, I did forget to mention that I did use the seafoam and also cleaned the throtle body. I have also checked the fuel presure regulator and changed the fuel filter and tps. Oil presure seems to be good as well running around 72psi cold 42psi hot. I would like to also note that before all of this was done, I was getting about 220 miles to a tank of gas. Could it be posible that the cooler weather could cause the gas mileage to be bad?

Thanks

Terran2k

42 psi hot? at idle???

95Z28QFZ

Terran2k, I would say 38-42 @ 800 rpm in park. In drive @ 600rpm 32-38. It really depends on the engine temp.

Laymaster

o2 sensors?

NewbieWar

that doesnt sound right... when my car warms up it'll idle about at 5-10 somewhere

when its cold it'll idle about 40-50 tho

OBE1 95Z28

I second repacing the O2 sesnors! Also verify that you don't have any vacuum leaks and the PCV is not clogged.

slopokrodrigez

Curious about some things. You replaced the things like the Opti but didn't replace things like the O2's which are "tune up" items that should be routinely replaced

Second, why have the trans rebuilt? Was there a problem? Is there now a problem where you may not be getting into overdrive? What is the cruising rpm's?

Is your water temp getting to actual temp or is it somewhat lower?

Do you have too much oil in the air filter? A common problem with cotton filters.

JPSartre12

Agreed. I have a Melling high volume pump and I'm at about 25 psi at idle (925-1050rpm) when warm using Mobil 10W - 30.

Joe Bellman

I too would replace those two items.

You need to tell us miles per gallon not 200 per tank. use your trip meter to determine how many miles you drove between fill ups and divide that by how many gallons it took to fill up the car.

ex:
200mile/10gal=20miles per gallon

The normal mpg should be around 23-27 on the highway and 16-19 in the city.

95Z28QFZ

Thanks all for the replys......(NewbieWar) assuming (which I dont like to do) that 80 is the max on the oil pressure gauge and it has four increments I would think that they are 20psi each, Then again it may not be that accurate. The K&N oil filter (HP-3002) may allow better pres over the stock do to it being bigger than stock filter. (OBE1 95Z28) I think you are right about the O2 sensor, Have replaced the pcv valve and checked with a can of carb cleaner by spraying it around the intake and vacume lines. The reason I have not changed them is do to them looking new and no codes have been set by them. (slopokrodrigez) The transmision is doing great now. I lost 2nd and overdrive and had to get it rebuilt. Water temp stays around 175- 190 depending on the temp outside(fan mod)and no too to much oil in the K&N air filter. (Joe Bellman) I am getting 10.37mpg on this last fill up and that is mostly highway and very little city. Could it be possible that the sea foam could have coated the O2s in carbon? and if it is running rich wouldnt I see at least some black smoke?

Thanks, Mike

Joe Bellman

Or it could be the O2 sensors are original and just are bad. But yes, I have heard of seafoam taking out the O2s.

Is the engine shaking under load like a cylinder is not firing? 10mpg is horrible for mostly highway driving. There is definately something wrong or the car has internal mods you don't know about but I am not a good person to ask what those could be.

95Z28QFZ

(Joe) I put highway, but I live in the country and its not a whole lot of stop and go. I drive 10 miles to work every day and there are no stops. 65 all the way there. The car runs great. I can be at a complete stop and punch the gas and she will spin the tires and be gone with no hesitation what so ever. Performance wise it runs great! no complaints, but knowing what gas mileage this car is suppose to get, opposed to what it is getting, leads me to believe something has been changed internaly with the motor or something is wrong with the motor or sensors.

Thanks

slopokrodrigez

You said earlier that you checked the fuel pressure regulator. How? did you check for fuel leaks on the vacuum side? You say you live in the country, have you ever removed the snorkel from the air cleaner box to the TB to look for obstructions. I had a mouse get in one of my air filters and chewed the S@#$ out of it.

Have you cleaned the MAF or have you unplugged it to see if the PCM recognizes its gone? Since the vehicles can run with it disconnected, maybe you have a connection problem and the PCM is defaulting to the internal enrichment tables. Does the PCM give confirmation that it goes into closed loop?

Just trying to throw out some ideas for you to try.