1996 Gas Mileage

Maxium · Mar 16, 2012

I posted this in a couple other forums. I guess I'll post it here for a wider audience:

Ever since I did a PI intake/Cams swap, the power is good but my gas mileage has been down by a good amount. If I drive all freeway I can get around 20 mpg. All city I'll get around 15 mpg.I recently checked my mpg and with a 70% highway and 30% city I averaged 16.2 mpg driving conservatively. I've been hunting this problem down for a while and here are all the things that I have done/changed:

I've tried different tunes from Bama (same bad mpg, went back to stock)

Compression check: All near 180 psi +/- 5

Re checked timing (Pulled cover off and re-timed)

Checked fuel filler grommet for leaks

Checked for binding brakes

Checked vacuum at all locations I could think of (-21 in/hg)

Checked for vacuum leaks spraying carb cleaner from intake to bottom of intake manifold

Checked for exhaust leaks

Checked for clogged cats (using vacuum gauge and by infrared temp gun)

New Egr/Egr Solenoid

Checked vacuum pressure from solenoid to EGR

Checked EGR passage way by applying vacuum to EGR (engine stumbles and dies)

Checked voltage at DPFE

New Fuel pressure regulator

Fuel pump is only a couple years old (ford oem unit) along with the fuel filler grommet

Checked fuel pressure (40 psi with vacuum line off the regulator)

New Engine gaskets (valve cover, timing cover, etc)

New Oxygen senors (2 upstream only)

New 19lbs injectors with new o-rings and pintles

New Vacuum Lines

New Fuel filter

New plugs

New wires

New coil packs

New Iac

New PCV

New ACT temp sensor

New TPS

Datalogging with my SF3:

No DTC codes

LTFT for both sides are +/- 5% (although bank 1 is usually always higher by 4-5% than bank2)

STFT both banks +/- 2%

ECT within spec

ACT within spec

IAC duty cycle at idle is 40% and goes up to 98% at WOT

MAF ad counts is at 160 at idle and goes up to 810 at WOT

Spark timing at idle seems to be around 13 degrees

TPS seems to be in spec

LOAD at idle is around 15% at idle and goes up to 86% at WOT

My mods are on my profile page under the information tab.

I've logged misfires for each cylinder and I'll randomly (sometimes not) get only 1 or 2 misses on cylinders 4,5,6,7,8 (not enough to set a code) if i'm idling for a long time. But let's say I drive on the freeway for hundreds of miles, I will not get one misfire.

At this point, I'm a little lost as to what to do. I've thought that this might be normal mpg but seeing that other forum members get a lot better mileage with more mods than me, I think my car could be doing better. Perhaps I'm leaking fuel in a location that is not so apparent (I don't smell fuel anywhere) because if it was something related to a tune up wouldn't my fuel trims be off? Any leads are appreciated. Thanks

blkstanger · Mar 19, 2012

Are you running gears? Speedo off? Just an idea.

Maxium · Mar 19, 2012

Thanks for the reply. Yes, I'm running 3.73 gears, but I corrected it with a Speedcal. I also verified my odometer with a gps just in case.

wht93gtEd · Mar 19, 2012

my 98 is non-PI.
- vortech v1 s-trim @11lbs
- 60lb injectors
- SCT 90mm
- 255lph in tank & 255 lph in-line
- custom tune
- 3.55 rear

Just took a trip out to a car show which was all back road driving, some cruising. Maybe 50% steady 55mph and 50% rowing gears around turns and hills, with a few "jaunts" from stop lights.
Filled up when I got back home and for the 90mile round trip, averaged 20mpg. I will say I've gotten as low as 10mpg driving "spirited" for a weekend.

So with 3.73's at highway speed you're turning close to 3k? That probably has much more of an impact than you're giving credit for. IIRC in stock form I used to turn 1500 or so (in my '93...my '98 was never stock when I had it).

Are you saying it only dropped off after the intake/cams? Have you checked/monitored A/F ratios?

I also saw this post in another thread here...seems like you're right in-line where you should be

Mel96GT said:
I installed PI cams with the PI intake. I like the PI intake. However, the PI cam install was tedious but fairly fast with the Anderson tool. You only remove the valve covers and the cams since the special tool locks the chains so the cams can be removed from the gears. I would say the PI cams give you a few more horses up top but a slight low-end torque loss. I went from 24 mpg to 20 mpg on the highway with the PI cams but the engine can rev to 5500+ rpm's. All this is with the stock tune but I am not yet done with the mods. It definitely needs a dyno tune since it seems to run rich. This is a cheap mod if you can live with the mileage. I dyno'd 200 rwhp with PI cams, PI intake, Magnaflows and K&N FIPK.

Mel96GT said:
Is it worth it? I think you are better off with the stock cams since the PI cams are designed for PI heads and they suck gas with the NPI heads. I would just stick to the stock cams unless you have some 3.73-4.10 gears in back to take advantage of the higher-rpm capabilities. YMMV.

Maxium · Mar 19, 2012

Interesting. Well i average around 2500 rpm around 70 mph on the freeway. Yes, the mpg dropped after Pi Intake and cams. I haven't measured A/F ratio since I don't have a wideband, I've just looked at the fuel trims on my datalogger. I wonder if i got the PI heads it would increase my mpg?

blkstanger · Mar 19, 2012

My car is all stock. I get about 17mpg all city driving and average about 23mpg. I just went on an all freeway trip over the weekend and got 27mpg. That was the best tank ever for me. IMHO what you are getting isn't too far off for a car with mods done and gears.

Maxium · Mar 20, 2012

Well I just filled after an all freeway run. 19.6 mpg. I used to get around 24-25 on all freeway runs. After researching the quotes above, it seemed as though that user "Mel96GT" had his timing off a tooth on one bank. Other members have stated that they put in PI cams and intake with no hit (or even greater) to their fuel economy. My question is, if my timing or cam is introducing more fuel into the mixture, wouldn't the computer be able to compensate for that in the fuel trims? Or at least, shouldn't my fuel trims show the added fuel being put in?

blkstanger · Mar 21, 2012

Back when you were getting 24-25 mpg freeway had you installed the gears you are running.

bhuff30 · Mar 21, 2012

If the cam timing was off a tooth, the loss of gas mileage would be due to the incorrect valve timing making the engine very inefficient as opposed to a fuel/computer issue. The compression looks good, so it makes me think the cam timing is ok though.

When I installed PI cams, I didn't experience a loss of gas mileage. The cam grinds are very similar anyway, so you shouldn't notice any difference.

Maxium said:
Well I just filled after an all freeway run. 19.6 mpg. I used to get around 24-25 on all freeway runs. After researching the quotes above, it seemed as though that user "Mel96GT" had his timing off a tooth on one bank. Other members have stated that they put in PI cams and intake with no hit (or even greater) to their fuel economy. My question is, if my timing or cam is introducing more fuel into the mixture, wouldn't the computer be able to compensate for that in the fuel trims? Or at least, shouldn't my fuel trims show the added fuel being put in?

Maxium · Mar 22, 2012

bhuff30 said:
If the cam timing was off a tooth, the loss of gas mileage would be due to the incorrect valve timing making the engine very inefficient as opposed to a fuel/computer issue. The compression looks good, so it makes me think the cam timing is ok though.

When I installed PI cams, I didn't experience a loss of gas mileage. The cam grinds are very similar anyway, so you shouldn't notice any difference.

I was under the impression that I wouldn't take a hit to gas mileage as well. Although I'm not dying to get great gas mileage, the whole thing has me stumped and I would like to get to the bottom of it.. even if it's for learning's sake. Anybody have any data logging tips? Things that I should look for in a datalog that would give me a lead to my gas consumption?

bhuff30 · Mar 22, 2012

Definately look at the O2 sensors and make sure you are going into closed loop.

You should probably look at the indicated reading of the sensors to make sure they make sense. So ECT warming from ambient to around 190-200*, ACT around or just above ambient, reasonable MAF flow values (eg, at idle and WOT).

I assume you've checked for gas leaks? Specifically, at the donut between the filler neck and fuel tank, at fuel injector O-rings and in the FPR vacuum hose? Do you smell gas at anytime?

Maxium · Mar 22, 2012

bhuff30 said:
Definately look at the O2 sensors and make sure you are going into closed loop.

You should probably look at the indicated reading of the sensors to make sure they make sense. So ECT warming from ambient to around 190-200*, ACT around or just above ambient, reasonable MAF flow values (eg, at idle and WOT).

I assume you've checked for gas leaks? Specifically, at the donut between the filler neck and fuel tank, at fuel injector O-rings and in the FPR vacuum hose? Do you smell gas at anytime?

I've attached a recent datalog on a stock tune (I changed the extension to .txt but you can just change it to .csv to open it in Excel or some other spreadsheet program). It seemed like my numbers were ok. I also did a datalog of my O2's and the fronts seem like they switch (.1-.9) at a good rate. The rears do fluctuate more than I'd like though. As for checking for leaks. I don't smell any fuel. The filler grommet is fairly new and I checked for leaks there. No leaks that I can see or smell at the injector O-rings as they are new as well (oiled when installed). The fuel pressure regulator is new as well and the vacuum line is leak free and does not smell of gas.

1996 Gas Mileage

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