+1

Everytime I'm on the interstate I get concerned at how quickly the gauge drops from full to 3/4, but it slows slightly after that and pretty much stops moving for 50 miles within a needle width of 1/2. After that it does the opposite on the way down.

I too bent my sender arm slightly when I installed my FD pump, it actually reaches "F" but that 15-20 mile reserve capacity after "E" is definitely a thing of the past lol. Of course that wouldn't be so much of an issue if the FC didn't have the stupidest design for a low fuel light on earth and any of them still worked...oh well.

 

OK, still getting bad milage here.

I replaced the secondary rail with another for the change in FPR (S5 intake on S4 car), and am getting 32 psi w/ FPR vac line plugged in, 40 w/o. Still doing high 13's on milage when dividing miles driven by gas to fill!

I'm at a loss here- from my original thread it-


-New tires, and allignment
-New brake master & properly bled brakes
-Injectors were removed & cleaned, then re-installed
-Engine hasn't been in the car more than 1500 miles. It's a used S4 engine, but was rebuild 2 years back. Fires right up everytime, but do not know the compression figures.
-New O2 sensor, confirmed operation of closed loop with checker light
-Not pulling ANY trouble codes w/ code checker (was pulling an O2 sensor, but it has been replaced & no longer comes up)
-New thermostat & radiator installed with engine, and full shrouding. Temps are normal
-Rats nest removal, vac routing is identical to the 88
-Main cat is replaced with a straight pipe, 2 pre-cat casings still there, no cats inside though
-Properly timed, and idle properly set, as well as TPS, to FSM spec
-No exhaust leaks, no fuel leaks
-Good condition used spark plugs. I disassemble lots of these engines in the junk yard (cordless impact FTW), and always take the plugs if they're near new.
-New E8023 Airtex brand fuel pump and fuel filter (pre & post pump)
-Water temp sensor:Checked by FSM and is reading proper voltage range relative to temperature

Here are the few things I now think it can be, after fixing the FPR & confirming the water temp sensor & it's respected wiring is good.

-Clogged or too small diameter mufflers/piping. Vs my 2 FC's with all stock exhaust, & my other w/ RB system, it just sounds, funny. Maybe there's an excess in back pressure. I guess the way to check would be remove both mufflers & drive, & see if there's an increase in performance & mpgs?

-Gear drag. Car had a bad rear wheel bearing, I replaced that. However, the diff &/or trans makes a harmonic wummm wummm wummm noise going down the road. It coasts fine, but maybe when loaded it's sapping engine energy? I can't see it being bad enough to rob several MPGs though...

-Leaky brake booster- there's a hissing when I shut the car off, and the pedal is immediately hard to press to the floor, so there's definitely a vacuum leak from this.

Any other ideas?

 

I'd explore the vacuum leak first.

 

its actually easy to check the backpressure, just tee a boost gauge into the 6 port vacuum line.

on my 87 GXL, i was seeing around 0psi, up to about 4000rpm@wot then it starts to climb and i think it was about 3.5psi@7000rpm WOT in 3rd.

you need about 2.5-3psi to open the 6 ports.

the other thing i'd look at is the sensor on the bottom of the radiator, it does something with closed loop, so when its non functional the car runs fine, but it won't pass smog and mileage is down. the wires for the switch get pinched under the battery.

 

J9fd3s-

I'm running the 5/6 ports as well as vdi (s5 inake) from smog pump air & using rpm switches/solenoids to send the air. So sadly I can't easily check the backpressure

Switch is intact and working, as I've confirmed closed loop with a blinky LED inside the car

 

checked the air temp sensor (in the manifold not the afm) ?

 

This is a very good idea. Pin 2L (Green wire) ought to measure 1 to 2 volts w/engine fully warmed up (176 degrees). You should be able to measure the voltage at the sensor itself but be careful so as to not burn yourself.

 

Measure the dynamic chamber sensor voltage, it was within spec. Also pulled vac on the boost sensor as per FSM & it was within spec as well.

So still could be the leaky booster, drain from bad trans/diff, did not check the atmospheric pressure sensor behind pass kick panel.

Could PS be causing the drop? My other car has a manual rack. I just can't see it going that much lower when it was rated around 17 city from the get go, w/ PS.

 

Does your car have the 3000 rpm cold start working?

 

I tend to use the odometer on a long run to be sure I don't run out of gas. At 400 kms I look for a gas station, works for me

 

Aws is removed, it does rev up cold using the BAC to rev it. I've just capped off on the dynamic chamber where the aws would attach.

I use the odometer too, its needing fill up, typically around 12.4 gal at around 185 miles. Sorry its not metric! Haha

 

I should also mention all the emissions are removed, no egr, etc, but I did retain the bac & fast idle cam. The 88 which gets around 22-23 mixed is set up the exact same way.

 

what size tires are you running? are you sure its not just the odometer calculation?

 

Yea it's not. They're the 205/60/r15s' or whatever the door sticker says. Can't recall if it's 55 or 60

 

Its got to be something major to lose 10 mpg.

Any smoke on start up?
What do the plugs look like?
What about your OMP limp mode work around, is that effective?

 

Has your air flow been tampered with? I've seen people open them and "adjust" It. Massive change in a/f when this is screwed with.
And what size tires are you running? I lost about 5 MPG changing my tires from a 205 front, 245 rear - to a 245 front, 275 rear.

 

I get like 15mpg out of my 88 GTU. All stock (for now) except for streetport.

I was told I'd get like 20+ after my rebuild, but it's unchanged.

Nothing to contribute, just following your progress hoping to cure mine.

 

Still getting between 13 & 15 on every fill up. Floods everytime on a warm start up. AFM checked, wasn't tampered with. Thinking maybe there's a short somewhere in the wiring harness, sending a bad signal to the ECU- going to probe all the pins & check the readings the next free chance I get.

 

There are usually three causes of flooding when warm. Leaking injectors, bad clt temp sensor and low compression. Have you done a poor mans compression test already?

 

Yep, yep, and yep.

 

I would also check resistance from the ground pin on the ecu harness plug to the body of the o2 sensor, and between the o2 sensor harness plug in the engine bay and the pin on the ecu connector, if it's a universal sensor, make sure you aren't shorting the grounded shielding around the voltage wire where you spliced everything together, then last, try a different o2 sensor, even if it's new, you get bad parts sometimes.

 

get your injectors cleaned.

 

Injectors have been cleaned.

Did a poor mans comp test, even pulses around 100 on the front, & 85 on rear. Not GREAT but not power robbing by any means.

I honestly think there's a short in the wiring harness somewhere, I'm going to back probe all the pins @ the ECU as a start

 

Backprobed, & everything came out to be within spec! One of the CAS inputs was way high (should be below 1V), but upon a recheck, it was fine & in spec. weird.

It baffles me that I can't figure this thing out!

 

You know, I have a copy of the original fuel paper sticker that is stuck to the window when you buy the car. My 89 is supposed to go 15mpg city....