Thoughts about my bad v8 fuel economy.

canadavan · Post by **canadavan** » Sat May 12, 2007 11:22 am

Hey guys. I'm experiencing terrible fuel economy and with prices the way they are these days ($1.27/lite cdn) its getting acute. My bad fuel economy has been around since I did my v8 swap. Here is my current setup:

tbi 350
192 thermostat
tbi chip from tbichips.com
mild cam from comp cams

I get 450 kms to a tank mixed city highway (mostly city) and I have a very light foot. Tank size is the standard 100 litre or so (25 gallon ?).

My air filter always has a strong odor of fuel when I take off the assembly (I think this means that it is dumping fuel).

Some thoughts: I sent the tbi chips guy my motor specs and asked for a mild performance chip. The sheet that comes with the chip insists on 91 octane fuel even though I specified to him I wanted to use 87. The sheet also says that a 180 or lower stat must be used. I never experience knock. My computer never registers codes. In the time since my swap I have replaced the o2 sensor and the egr valve and there has been no change in fuel economy. I was running a 185 stat for awhile with no difference in fuel economy. The only symptoms the motor ever shows is a slight stumble at idle for the first half minute or so from a cold start up. Otherwise, there are no performance issues that I can detect. One other thing thiough. During my last emissions testing, my NO2 values were very high, close to the failure line even though my full exhaust is only a year old.

Should I go back to the stock v8 truck chip? Should I go to a 180 stat and run 91 octane. I'm reluctant to do that because of the crazy fuel prices...

What else could I be missing? If the computer wants higher octane, could the computer be advancing the timing to compensate for spark knock which would increase my fuel consumption?

Thoughts?

canadavan · Post by **canadavan** » Sat May 12, 2007 12:31 pm

by my calculations I'm getting 10mpg!

Mr_Roboto · Post by **Mr_Roboto** » Sat May 12, 2007 6:27 pm

For starters, is it stock or modified? Have you checked your timing? Pull the air cleaner off and see if your injectors are spraying in a fine mist cone. If they're GP Sorensen replacement injectors (black colored with red plastic posts) you had one take a crap probably, and it's dumping a ton of raw fuel in. Go back to the stock chip and see what it does. Also, get a fuel pressure gauge and see what that looks like. The springs in the regulators can rust out and that would skyrocket the fuel pressure. I've seen it on a few with blown motors in the junk yard.

canadavan · Post by **canadavan** » Sat May 12, 2007 8:27 pm

Thanks for the reply. Injectors are fine. Nice even cone spray. They are stock from the suburban that the motor came from. Cam is a mild one, better torque numbers but nothing outrageous. TBIChips guy told me that he thinks its the cam that the motor should have come with stock. Timing checked and bang on.

Motor was professionally rebuilt before it was installed. Only engine mods are cam and throttlebody spacer. It was also bored 0.040 over and given flat top pistons.

Tried the stock chip tonight, idle is a fair bit lower, around 550 rpm when in gear. Has a slight, irregular stumble under throttle that isn't present with the tbichips chip. At cold startup, there is a stronger irregular stumble than with the aftermarket chip. Still not that bad. Seems to me that the tbichip is doing its job correcting for the different cam. Throttle response seems stronger with the aftermarket chip. I do like the lower idle on the stock chip though.

Mr_Roboto · Post by **Mr_Roboto** » Sat May 12, 2007 10:14 pm

What about headers, and are you running a 1 wire oxygen sensor?

canadavan · Post by **canadavan** » Sun May 13, 2007 7:49 am

headers and 3" exhaust. O2 is single wire and has been replaced since the swap. Thing is though, for the first year, I ran the stock truck manifolds bolted to the stock astro exhaust and the gas mileage was about the same. The headers and new exhaust helped power (and sound) but not fuel economy. Are you thinking that it might be staying in open loop?

Mr_Roboto · Post by **Mr_Roboto** » Sun May 13, 2007 10:06 am

There's the possibility it is, do you have a digital volt meter? You can check the coolant temp sensor with it, and the oxygen sensor as well.

http://www.megamanual.com/v22manual/mwire.htm#clt

If you scroll down, you'll see the table here with the ohm readings versus temperature. You can use this to check and see if your sensor is calibrated, get the van warm and see if the resistance is right. These can cause a vehicle to "run fat"

This is an oxygen sensor curve, it should dart around, if it doesn't the sensor is not becoming active. Past that, it will be "jumpy" but it should do it around .47 volts give or take.

canadavan · Post by **canadavan** » Sun May 13, 2007 11:17 am

That's great. I'll check this ASAP. Thanks for the info. Not sure if I can sneak away to do this on Mother's Day, but I'll post the results when I can...

Mr_Roboto · Post by **Mr_Roboto** » Sun May 13, 2007 6:35 pm

I understand, but I definitely don't think you should be getting 10mpg. It's unhealthy on the engine to run it rich too, so best to try and figure out what's up and get it locked down.

SafariRob · Post by **SafariRob** » Sun May 13, 2007 7:02 pm

Robo, thanks for posting that pic of the O2 sensor graph. I saved it for future use.
I see that the voltage output of the wide band is higher than the narrow band--wasn't aware of that.

Mr_Roboto · Post by **Mr_Roboto** » Sun May 13, 2007 7:30 pm

SafariRob wrote:Robo, thanks for posting that pic of the O2 sensor graph. I saved it for future use.
I see that the voltage output of the wide band is higher than the harrow band--wasn't aware of that.

Depending on the model of the wide band controller the voltage output can vary widely. My LC-1 will let you configure the outputs to various voltage ranges, I was emulating a narrow band for the stock ECM when I had it plugged in.

Kidhauler · Post by **Kidhauler** » Sun Jun 03, 2007 8:18 pm

Hey todd

Rocke was getting poor mileage and put in a heated O2 sensor as the headers alow too much heat to escape and the sensor runs too cold which as we all know is a common problem on even a stock van. For the $30-45 I would put in a heated sensor. You could also have a maf sensor not reading right which would make the computer compensate all the time and can give you bad mileage and also cause the high nox you talked about.

Even if your van was in open loop mode when you went to aircare you would probably still pass as the engine is still so new and clean inside.

SoCalli V8 · Post by **SoCalli V8** » Fri Jun 22, 2007 8:55 pm

Try fitting a 3 wire oxysensor, it has its own heating element which heats it up quicker, keeps it hot which inturn makes its output signal more accurate. Use one from a later model V8 truck.

canadavan · Post by **canadavan** » Sat Jun 23, 2007 8:08 am

Sorry I haven't updated this. I have purchased a 3 wire sensor. It is from a later model Astro. According to the parts store, GM used the same three wire sensor in the v6 and v8 trucks during much of the 90's.

I haven't got around to installing it yet because I want to get a three wire connector from the junkyard.

I've been running the stock v8 chip for about a month now. Fuel economy is up to about 14mpg. It still stumbles a bit and will occassionally stall when I put it in drive. I suspect that the lower idle speed is the main reason for my savings as most of my driving is city. I'd rather be using the custom chip for the smoother operation and better throttle response but I'm going to wait until I've installed the 3 wire.

Rockwerx · Post by **Rockwerx** » Sat Jul 07, 2007 8:54 am

Have you installed the new sensor yet? Did it fix your fuel economy problem? Let us know how it went for you.

Chevy & GMC Vans

Chevy & GMC Vans

Thoughts about my bad v8 fuel economy.

Thoughts about my bad v8 fuel economy.