Change In LT1 Fuel Economy

[hobrien]

I am offering my apologies in advance because I am not sure this is the proper forum.

I have a 99 FRC Corvette, but my daily driver is a 93 Pontiac Formula, 6 Spd. manual trans., 3.23 Rear Axle, and LT1 Engine. I have owned the car since it was new and it is driven daily. It has 146k (generally trouble free) miles. The spark plugs have about 20K on them and the Optispark was replaced at about 130K. I replaced it because the original unit failed at roughly 65K and the replacement was a preventative measure.

I had the fuel pump replaced at 144K. It had not failed, but it was getting very noisey and I was afraid of getting stranded somewhere. Before the fuel pump change, the car's fuel mileage, for its entire life, had been superb! Interstate mileage at 75 to 77 MPH was consistently 25 - 27 MPG.

Since the pump change, very careful highway driving produces about 23 MPG at the same speeds. The overall performance of the car is good. Throttle response is, perhaps, better than before the pump change. Idle is great, there are no stalls, misses, etc.

The mechanic who changed the pump is, in my opinion, quite competent. I took the car back and explained the problem. He peformed an engine scan which indicated normal fuel trim and nothing else amiss. A fuel pressur test indicated 41 PSI key on engine off and 46 PSI engine on.

As a practical matter, I would not be concerned, because 23 MPG is not the end of the world, but the change was so sudden that it suggests something is not quite right. Any ideas will be much appreciated

[froggy47]

Originally Posted by hobrien

I am offering my apologies in advance because I am not sure this is the proper forum.

I have a 99 FRC Corvette, but my daily driver is a 93 Pontiac Formula, 6 Spd. manual trans., 3.23 Rear Axle, and LT1 Engine. I have owned the car since it was new and it is driven daily. It has 146k (generally trouble free) miles. The spark plugs have about 20K on them and the Optispark was replaced at about 130K. I replaced it because the original unit failed at roughly 65K and the replacement was a preventative measure.

I had the fuel pump replaced at 144K. It had not failed, but it was getting very noisey and I was afraid of getting stranded somewhere. Before the fuel pump change, the car's fuel mileage, for its entire life, had been superb! Interstate mileage at 75 to 77 MPH was consistently 25 - 27 MPG.

Since the pump change, very careful highway driving produces about 23 MPG at the same speeds. The overall performance of the car is good. Throttle response is, perhaps, better than before the pump change. Idle is great, there are no stalls, misses, etc.

The mechanic who changed the pump is, in my opinion, quite competent. I took the car back and explained the problem. He peformed an engine scan which indicated normal fuel trim and nothing else amiss. A fuel pressur test indicated 41 PSI key on engine off and 46 PSI engine on.

As a practical matter, I would not be concerned, because 23 MPG is not the end of the world, but the change was so sudden that it suggests something is not quite right. Any ideas will be much appreciated

Most cars get less mpg in the winter, also regional gas formulations (ethanol content) can adversely affect mileage, wide high performance tires, etc to name a few factors.

Try to find out in your region which gas has the LEAST ethanol. That will probably be good for 1-2 mpg.

My COMBINED mileage (not much 6th gear hwy to be sure) is usually around 14 mpg which includes track.

Track only I have done a reset & see about 6 mpg, hwy cruise (with a reset & on cruise control @ about 75mph I see 26/27 mpg.

Lots of factors. You do mention the fuel pump chg about the time of the mpg drop.

Ask your mechanic what he thinks about getting the injectors serviced, either a cleaning or (better) new injectors. A leaky injector could be a factor in lower mpg. and those have seen a lot of miles.

This is just a guess so take it as that. I don't know as there is any way to test injectors on the car, you pull them & send them to be cleaned/repaired/blueprinted. The other thing is 02 sensor(s) they wear out/get lazy.

Some BG44 and or techron servicing never hurts.

:)

[hobrien]

Thank you for your suggestions. I did change the O2 sensors at roughly 115K just as a matter of maintenance. The Phillips 66 stations in our area do not add ethanot to their gas, and that is what I have consistently used for years. I have recently tried Techron, and I don't really know whether it helped. I believe your suggestion to change the injectors is a good one. Would you have a recommendation as to what brand? If possible, I would like to avoid damaging my bank account as much as possible.
Again, thanks for your help.

[LLC5]

Originally Posted by hobrien

Thank you for your suggestions. I did change the O2 sensors at roughly 115K just as a matter of maintenance. The Phillips 66 stations in our area do not add ethanot to their gas, and that is what I have consistently used for years. I have recently tried Techron, and I don't really know whether it helped. I believe your suggestion to change the injectors is a good one. Would you have a recommendation as to what brand? If possible, I would like to avoid damaging my bank account as much as possible.
Again, thanks for your help.

I wouldn't just throw parts at it. Before you replace anything, have a fuel pressure test performed and see if it is with in spec's. Let the vehicle sit over night with the gauge attached and see if you lose pressure, and if so how much. If an injector is leaking, it can usually be cleaned, either at a injector specialty shop or through the tank with BG44K.

Ethanol in fuel ruins mileage and power to about the same percentage of the amount of that crap in your tank (10% ethanol= about 10% lower power and mileage). This sounds like what you may be dealing with, don't always believe what the signs tell you as petrol prices are all over the place and you never really know for sure where the fuel came from. Try another high volume Top Tier station to be sure. I would want to be 100% sure that this is not your problem before spending any money. Good luck with it.

[froggy47]

I agree 100% with LLC5 about throwing parts at the problem. If it comes to injector service I use RC Engineering

RC Fuel Injection

There are also some Ford injectors that fit (injectors are somewhat generic) to avoid the Vette tax.

Pretty easy to change. I also suspect gas quality as the primary reason.

:)

[c4c5specialist]

HI there,

Here is something that is not always considered.

Look at your PERCENTAGES of your fuel trim data.

You see, GM states that plus or minus 8% is normal, but if you are at +8%, therein could be your fuel consumption change.

Please specify WHERE your fuel trims were and what scan tool you used to check it.

I have found that Tech 2 is the most accurate whereas Modus/Perceptor and other aftermarkets can be a bit off.

Allthebest, c4c5

[hobrien]

I am not sure which scan tool was used, but I will call the shop and find out.

Also, I mentioned that I had used Techron but was not sure that it did anything. I am now on the first tank of gas since using the Techron and I can tell there is an improvement. (I am something of a creature of habit and I fill the tank every week at the same place, having driven roughly the same number of miles. The guage has dropped much less this past week than previously.)

I am going to try another shot of Techron and see what happens.

Thank you again for your help.

P.S. I will let you know, just for the sake of information, if Techron solves, or partially solves, the probelm.

[froggy47]

Originally Posted by hobrien

I am not sure which scan tool was used, but I will call the shop and find out.

Also, I mentioned that I had used Techron but was not sure that it did anything. I am now on the first tank of gas since using the Techron and I can tell there is an improvement. (I am something of a creature of habit and I fill the tank every week at the same place, having driven roughly the same number of miles. The guage has dropped much less this past week than previously.)

I am going to try another shot of Techron and see what happens.

Thank you again for your help.

P.S. I will let you know, just for the sake of information, if Techron solves, or partially solves, the probelm.

If techron helps a noticable amount, you may want a more aggressive top end cleaning that really gets injectors & valves harder without being diluted into a tank of gas.

I am a religious believer in techron IN ADDITION to using top tier gas ONLY.

An interesting post I recently read said that even with top tier gas, the additive packages are different. The poster (a gasoline chemist?) said the even the best top tier gas leaves deposits & you should switch every few months from one to the other (say from chevron to shell) and that way the deposits from one tt gas are cleaned by the other tt gas and so forth.

Made some sense to me.

Wish I saved the post, but it's gone.

Don't forget to give it an Italian tune up every so often, these cars like to run wide open.

:)

[hobrien]

Thank you again for your comments. The suggestion to alternate fuel brands is interesting and something I would never have thought of. One of the reasons I have stuck with Phillips 66 exclusively is that in this area they advertise the fact that they do not sell any gas containing ethanol. Even Chevron and Shell, locally, add ethanol to their gas, which makes it less desirable from my point of view.

I am going to keep after this problem and see what it takes to solve it. I will let you know what the solution is assuming I find it!