Rebuilt my top end with all new stock delphi injectors, new slp intake, ported plenum, new egr valve and solenoid, new BB&K throttle body, new air intake sensor in plenum, new AC Delco fuel pump and strainer and filter. Set the TPS, fuel regulator and IAC to spec..No SEL is on...But...We have a 18 gallon fuel tanks right ? With a full tank of gas, I set my tripometer and it presently reads I've driven 44 miles and I've burned a half tank of gas. WOW what's that only 4-5 mpg....Help !!!

 

Forgot to mention new plugs, coil, caps, rotor, and wires...The engine idles smooth and runs good...I'm stumped...

 

15.7 Gallon tank. I typically get about 150 miles out of a tank and put 9-10 gals in at 1/4 tank. I also have 3.42's and do straight city driving only no freeway.

 

Ok...So you're getting roughly 10 mpg with city driving...Seems low doesn't it ? 3:42s are not that low of a gear either..

 

If you do the math, Will's getting about 15 in town. Not bad for these cars. Our fuel gauges are not terrribly accurate. Run the car down to 1/4 tank and refill it. Do the math to find MPG.

 

I did the math..Based on Will's comment, he says he gets 150 miles to a tank and tank capacity is 15.7...That comes out to just under 10 mpg...

 

Hey...If I can get 15 mpg, I'd be happy. If they are 15-16 gal capacity tanks then I'm only getting 6-7 mpg...Somethings wrong.....

 

He also says he fuels up at 1/4 tank, about 9-10 gallons. 150 miles / 9-10 gallons= 15+ MPG.

 

ASEDOC is dead on. I typically get around 15mpg straight city. I filled up today, 10.48 gal with 162 miles on the trip.


DJmarch, I'd go a full tank and check the mileage before you get worried. as stated the gas gauges aren't very accurate. when I fill up my gauge barely makes it to the full mark and after about 25 miles or so I am usually close to 3/4.

As for possibly causes? injector sticking open, bad Oxygen sensor, do you have an adjustable fp reg? if you do what is your fuel pressure set at?

 

Try also finding gas non ethanol if there is any around your area. Noticed a difference in my L98 with it

 

Yes, oxygenated fuels(10%ethanol) do run leaner causing your BLM to compensate by adding more fuel. One more way big oil screws the consumer. 3rd gen will offers a good list possible causes. However, most any of these would cause the motor to run poorly.

 

I have been stealing your fuel when your sleeping, I didn't think you would notice

 

Hahaha...Aren't you a funny one...Must be the best you can do in providing any technical expertise ?

 

Just to clarify, oil refineries are FORCED by government mandates to put ethanol in gasoline. It hurts margins to do this too.

 

i agree with Will, if your fuel pressure reg. is adj. ,what did you set it to? Where is your TIMING at? Also, did you hook your solenoid hoses back up properly? The hose to the egr valve itself is the rear hole on the solenoid. Sound to me your car is dumping a lot of fuel. I would check for codes anyway,even with no SES light. Lots of things could make it run rich, bad CTS, O2 sensor,etc. So, are you using an AFPR?

 

Pressure regulator set @ 39 psi...Timing at 6 degrees advance..New O2 sensor..Re-checked vacuum hoses...Read my initial thread for all other new stuff....I'm going to pull a plug and see if it's wet...But what has me puzzled is that it purrs like a kitten at idle and all the way through to 2,000 rpms when I open the throttle body by hand...No miss what so ever...No leaks under the car...What am I missing here ? Fuel gauge needle not acting eratic...When I filled up the nozzle at pump shut off, my gauge showed full. With 44 miles of driving I've watched the needle slowly drop to 1/2 tank.

 

my car gets 17 city and 20 on the thruway until my engine light comes on then 14mpg. i have the mods in my signature with a 52mm tb, stock tpi and microtech maf 3k on rebuild synthetic mobile 1 oil. used to get better years ago. i think the gas quality sucks now. you know there is a problem when you can put your cigarrette out in gas without it igniting.

 

Sorry, wasn't trying to poke fun at you. Was just trying to make a viable point in a funny way. Did someone steal fuel from you?
As already stated i would first fill up and see exactly how much fuel you have actually used as your fuel gage may indeed be wrong.
also you implied you did all these mods at once, the 44 miles you put on were at cruising speeds or stop light to stop light WOT trying out your new setup?
Do you have access to a scanner? if so have you checked your fuel trim, 02 sensor operation and if its open or closed loop operation. If your fuel regulator pressure is set too high or you have a leaking injector it will show up on the fuel trim #'s
You should also look at your coolant temp readings when warmed up, should be at least 180-190 deg and your intake air temp should be at least ambient air temp (will probably be more depending on how much heated engine compartment air your intake is ingesting). Inaccurate low readings on these two will cause a richer then needed fuel trim, coolant temp being more important.
You may also want to remove the vacuum signal hose to your fuel regulator and inspect for wetness as a leaking diaphragm will cause both high fuel pressures at the injectors and sucking unmeterd fuel into the intake via the vacuum hose.
As stated by a few others here there can be many causes of your problem.

 

Sounds like something going on with your fuel pump or sending unit itself. Why don't you get a 5 gallon gas can and fill it up and drive your car until the gauge says empty and see how long you can really go for? It may be your gauge itself going bad. That is unlikely but you never know.

 

Base timing is right, I have mine set at 9 deg btdc seems to run a tad better.

As stated a bad CTS can cause you to use extra fuel, since it can make the computer think the engine is cold so it's in cold start mode and throwing in extra fuel. Doesn't always cause an SES light either. The CTS I am referring to is the two wire one on the front of the lower manifold it is the one that tells the computer the engine temp.

 

The fuel rail should hold pressure even when the engine isn't running. Does the fuel pressure stay at 39 psi or does it drop quickly when the engine is stopped? If you have a scanner take a look at the BLM to see if the ECM is trying to lean/richen the mixture.
If the fuel rail pressure doesn't hold...leaky injectors probably, which will kill your mileage.

 

I won't argue that leaky injectors will kill your mileage, or that injector failure is common on high mileage TPI cars. However, again, leaky injectors will amost always cause drivability issues, most specifically, poor idle. The service limit for fuel pressure drop after shutting down the engine is 20 seconds.

Perform the fuel economy test as recommended earlier. Again, the fuel gauges on these cars are notorious for inaccuracy. Don't let a quick change of the gauge a few miles after fueling convince you theres an issue with fuel economy.

 

Well I replaced the CTS on the front of the intake manifold along with the Plenum temp switch. If I'm not mistaken They are the same switch. Like I said, I completely went through the top end on this motor. Even installed Dyno Don's headers. The other thing I replaced was the coolant fan switch that mounts on the passenger side block. I put in a 185 from TPIS to turn the fans on sooner as we all know how notorious these cars run hot with no front grill. The fans do turn on at 185, (according to my gauge). Could it be that I need to change my thermostat to 185 ? As it has the 195 in it now. Like I said, tonight after work I'm going to pull a few plugs and see if they are wet and post my results..

Thanks everyone for your input...

 

Plenum temp switch? You reffering to the one on the underside of the upper plenum for Intake Air Temp?

As for the Fan switch, the reason you want to have your thermostat coincide w/ your fan's is so your fans don't kick on before you T Stat opens, it's pointless wear on your fans and draw on the electrical system. I'd go to a 180 stat for sure.

The only other thing I can think of is an actual fuel leak at the lines or the O Rings on the rail.

 

Ill go 3rdgenWill one better. There should generally be 15-20 degrees difference between thermostat temp and fan on temp. Otherwise as Will pointed out, your fans will run almost constantly when the engine is at operating temp. This is especially important when the fan temp switch is located in the cylinder head since head temp is usually a touch higher that mean coolant temp. I would say that with a 185 fan on temp, you should run no higher than 170 thermostat. In fact, I have my DFI programmed to turn fans on at 180 and I run a 160 T-stat. The fan cycle seems about perfect.

 

Just throwing this out there:

Are you sure the trip odometer is working correctly and not sticking?

The reason I ask is last summer I thought I was getting horrible gas mileage in my 94 K1500, ie single digits on all highway mileage. Turns out my trip odometer sticks at around 15 miles, then will start turning again for a few miles, then stick again, then turn again, then stick........

Once I noticed that, I checked my mileage by writing down the starting and ending mileage on the actual odometer and got a more "reasonable" number.

 

The degree rating of the thermostat is the Full Open temp, I just know that you want your fans set to kick on at a higher temp than the thermostat or basically your fans will never kick off once your car get's warmed up. Your way is probably a bit more effecient though.

 

Thanks for all of your input guys..Yes I meant to say the air temp switch mounted under the plenum. I often wondered about the thermostat. So your thinking should be to install a 180 thermostat ? But I'm told the 195 is tuned into your ECM. If I run a 180 will it effect it ? Really don't want to burn a chip. And yes the fans run quite a bit. But I achieved my goal of these engines running 220+
On another note I pulled a couple of spark plugs and guess what ? They are wet. Looks like this is were my gas is going. But it runs so smooth..So what should I do ? Back off the pressure regulator ? These are new stock delphi injectors.

 

That's too bad. I wanted you to say those were the original 20 year old injectors...darn. Those injectors should be able to handle 39 psi...darn.
My advice...I would attempt to return your injectors and get a different set. The new set could be exactly what you already have or something entirely different.

I had excellent luck with FuelInjectorConection (FIC). I purchased some yellow bosch gen 3 III, that were adjusted to exactly replace our old TPI injectors. Here is the link... http://www.corvetteinjectors.com/sho...p?productid=22

I am not a salesman...just a very satisfied customer.

 

Have you verified actual feul mileage?

 

Are the plugs wet from Oil or Fuel? Smell the plug, does it smell like burnt oil or like Gasoline?

Yes I'd go with at least a 180 Tstat or something cooler. You have the Secondary (pass side) fan coming on at 185 (or with AC request) since you put that fan switch in the pass rear side of the head. It was basically a wasted effort at this point without a cooler T stat.

I have no factory part left of the Fans, I have a Taurus Fan on a controller, so you don't necessarily need to burn a chip to have the fans do what you want them to do. My car never gets over 180 (approx. according to the factory gauge) I have a 180 T stat and my fans are set to come on at 185 (Don't yell at me ASE DOC! HAHA) I read on here that running a 160 stat can keep you out of closed loop giving you crappy mileage so I went with a 180.

 

Yeah. I forget to take closed loop into consideration. Most guys who run 160 t-stats are either running programs that take this into account or, like me are running more extreme systems where closed loop isn't really a consideration.

 

I know from exp. that you want to set your fuel pressure to a "sweet spot". Not too lean and not rich. That is where my car makes the best power. You may have some faulty injectors too. A while back, i replaced my IAT sensor and started getting way better mileage. Did you try driving it around on empty and carry a 5 gal gas can yet? See how long it actually goes for.

 

Yea My buddy Tom's car running the Accel DFI we have a 160 stat in. It's a 355 w/ an F1a on it though def. not running a stock computer.

 

Thanks for all your input guys...You've been alot of help...Yea it's fuel on the plugs...I screwed my 0-100 psi fluid pressure gauge back on the fuel rail this evening and it was measuring 42 psi at idle with the vacuum hose disconnected to the plenum. so I backed it off to 38 psi. I hope I didn't damage the injectors. I did notice it held pressure for about 15 seconds after I had the wife turn the car off and then it slowly bled off. Don't know what happened when I adjusted the regulator previously. Obvious I didn't have it right..Took it out and toped off the tank. It took about 7.5 gallons so the fuel level gauge in the dash showing 1/2 tank tells me the gauge is working about right. I reset the tripometer and will see what kind of mileage I'll get. Didn't notice any performance difference when driving either. I'll keep you posted and thanks again everyone....

 

Good point...I'll check for that....

 

I need to clarify my previous post here...When I mentioned I got a 185 fan switch from TPIS, I meant that it turns the secondary fan on at 200 and off at 185. So with your guys recommendation, I'll go with the 180 thermostat because it gets it gets a little cold here in Virginia during the winter months.

 

how many miles did you get out of it? You wont damage the injector's w/ 42 psi either, so don't worry about that.

 

If you verify that you're actually getting poor mileage, the first thing to do is look at data. Since the engine runs well, it may be hard to narrow the problem down to any one cause without viewing live data while you drive. The first two items I would suspect would be MAF sensor and O2 sensor. O2 is the most likely culprit as it can cause major changes in Block Learn without setting a code. As far as fuel pressure goes, my position varies somewhat from the norm. I run my Bosch injectors at 70psi to fuel my 355 Beast. Higher pressure provides better atomization. Lowering pressure below 40 psi can cause fuel droplets to become too large and hurt atomization.

Viewing data requires either an OBD1 scan tool, not a code reader, or something like tuner pro and a lap top. You can find old used scanners that read OBD1 online or through tool dealers fairly cheap. It's a good thing to have with your 3rd gen.

 

I agree on the O2 Sensor causing the biggest change in fuel economy. Do you have an AC Delco one? I only run them because the other ones on the market don't seem to work quite as well. I have a factory style 1 wire AC Delco Oxygen sensor that I ran for about 500 miles before going to a 3 wire heater that you can have, I will ship it to you if you pay for shipping cost.

I also run my Fuel Pressure at 44 PSI for better atomization.

 

Yes Will....I try to buy AC Delco when ever I can...Including the new AC Delco O2 sensor. Along with the injectors. Thinking I was getting the best. GM told me to stay away from Bosch and stick with Delphi or AC Delco on the research I did. No plans to take it to the drag strip on weekends. Just want a nice cruiser hoping that with the mods I've done wil make it breathe better and pick up a few more horses. So let me ask you guys, (trying to understand the logic here), on how the O2 sensor is tied to my leaky injectors. Man if I have to replace the injectors, I'll be pissed. Not only the work to pull the top end off, but I paid 450 bucks for that set. Also..I totally agree with you guys in getting atomization of the fuel by increasing the fuel pressure. ..And one last thing..What is the advantage of going to a 3 wire sensor ? And how would I wire it to my single wire harness ?

 

Cool, well not everyone will spend the extra couple bucks for better parts. The brand of oxygen sensor has no bearing on the injectors leaking or not. I have read on here (no personal experience) that you will suffer a 1-2mpg drop in gas mileage with a bosch O2 sensor. I know from personal experience they will cause LSx motor's to run weird because they have different switching points also effecting performance and fuel economy.

There is a great 3 wire install write up on here. The short of it it you hook the signal wire up as usual, put 12v switched to one side of the heater circut and ground the other. It's more of a necessity with header since they don't retain heat very well like a cast log manifold and a 1 wire w/ headers can take longer to heat up and cool off easier (ie high speed driving, colder temps) If you have stock manifold's I wouldn't even go through the effort of the 3 wire install.

 

ASE doc...The MAF is the "only" thing I haven't replaced on this engine. Because of cost. It is a bosch that has been de-screened. How does one test one to tell if it's good or not. Came with the car when I bought it. Because of the wide range of price I wonder which one would be the best one to use. I'd like to go with AC Delco but the prices I find are in the 450 dollar range.
Regarding the scanner, I would like to take this topic off line if you don't mind. I'm going to pm you, because I have several questions on what kind, so on, and so on. This is something I've wanted for some time and I need to know what's going on inside these engines...

 

Yea I do have the Dyno Don's headers on it....I'll have to do some research on this site to find it...

 

The sensor to use is the AFS 74, it's a delco 3 wire sensor. I pulled my 12v switched from the pink w/ black stripe that powers mostly everything under the hood. Emissions and injectors and such, then just grounded it to the chassis.

 

Thanks Will...I'll look it up and try to find a 3 conductor connector with about 10" of lead on each wire that would mate to this new sensor assembly with pigtail.

 

Have any of you guys heard of the "Chevy TPI Fuel Injection Swappers Guide", or is familiar with the book ? Would this book be worth having ?

 

http://cgi.ebay.com/ebaymotors/ws/eB...=STRK:MEWAX:IT

How do you guys like this set up for the 3 wire O2 sensor. It's a NGK sensor. I've heard they are pretty good...Seems pretty steep in price though. I'm thinking I can tie into the 12v wire going to the fuel pump relay. Bury it in the plastic channel that runs on top of the left chassis rail between the fire wall and battery. I don't want the wire to look adhoc...

 

I wouldn't pick off the fuel pump wire, just because there is known voltage drop to the pump w/ stock wiring, I wouldn't want to put even more electrical draw on that circut. That is why I went w/ the pink/black circut, it is designed for multiple items and is ign switched.

the AFS 74 is $49 from www.rockauto.com and you can use the discount codes off here, (in the vendor review section) to take 5% off also.

as for the connector. get this and make the wires the length you need them

http://cgi.ebay.com/ebaymotors/MSD-8...Q5fCarQ5fParts

 

Sounds good to me.