rickso84

My 91 died and was diagnosed with three bad and one open injector. Eight new Accel injectors were installed, stock pressure and the old injectors were original stock. Mileage used to average around 25 and almost 30 on the highway. Average mileage now with the new injectors is about 16, both on the display and calculated after a fillup!

What could have changed? Shop swears they installed the correct injectors. I checked the O2 sensors using the ECM (leave the ALDL jumper in, start engine, wait for SES to settle to 1Hz rate meaning closed loop, and check the light's on/off cycles are the same duration... on cycle is lean, off cycle is rich) and all appears fine... in closed loop and equal on/off light cycles.

One clue. The EGR was removed long ago and on the highway the SES would always come on after a while with an EGR code as to be expected. Now the SES doesn't come on at all.

StealthLT4

Was the ECM reset during the swap? I had this exact same problem when I got Bosch III's from Jon @ FIC, and it was because I didn't reset the ECM.

You need to disconnect the battery, THEN TURN ON THE HEADLIGHTS. The ECM will retain memory with something like 6 mV, which is definitely possible with all the electronics in the C4. After resetting the ECM, turn on the car and let it idle for like 15 minutes so the ECM can program itself for the new injectors. After doing that, I'm getting similar numbers to what I used to be getting, which makes me happy.

engle1147

With 3 bad injectors and one with a open coil = 4 bad injectors... you basically had a 4 cylinder car so it used less gas......welcome back to the land of the V8's......crappy mileage well maybe -- but man what power!

(4 cylinder='d 30 mpg) Double the # of cylinders you were using (8 cylinder = 16 mpg) and you get what your getting.

Good Luck!

cwyates4

Nope...if he is fairly stock, he should see around 25 on the highway and 16 in town. I get 12 in town and 22 on the highway, and look at my mods!

Do what stealth suggested, let the computer relearn.


thinking...


go look on the new injectors and get the numbers off 'em. Post 'em up and lets see what size they are...should be 22lbs.

BADDUCK

Not true....my near stock 1989 gets 29 on the highway with the original Multech injectors. He should get 25 to 30 MPG with 22 lb injectors.l

engle1147

More power to you guys! Years ago, I too used to get 25 to 30 then I changed my blocked/colapsed cats. If you figure out how to get the mileage back up there to 30mpg let me know so I can save some $ too!

MK 82

C4s don't get 30 MPG.

rickso84

Thanks guys.

I have "reset" the ECM a couple of times (disconnected battery for about a minute but didn't turn on headlights to totally drain). Car did run odd for a short while then seemed better but no change in mileage. I'll try REALLY resetting the ECM as suggested. Also will try to get #s off the injectors and post.

Forgot to mention it has a mild modded cam and a hypertec chip, coil, and 160 deg 'stat. Regardless, this car got 30MPG on a flat road at 55MPH. Now mileage is down almost 30%.

BADDUCK

Wrong....just because you don't doesn't others don't.

cwyates4

Get the numbers off the new injectors...maybe the shop put in some that were too big.

The only way to reset the ecm so that it relearns is to do what stealth said. disconnect battery and turn headlights on, let it sit overnight. Crank it up in the morning and let it idle for 15 minutes, then go for a spin.

Get those numbers first...

rickso84

Wow, it takes overnight to reset the ECM? I turned on the headlights for about 20-30 seconds... not enough, huh? I'll try overnight.

And the number is... are... 01D109B (one side) and 21LB /HR (other side).
Guessing they're 21LB injectors.

StealthLT4

I don't think overnight is necessary. Turning on the headlights should drain whatever residual charge that's being held immediately.

As for the nay-sayers about highway mpg, my last trip from Roanoke -> Annapolis averaged 31 mpg.

cwyates4

21lbs should be fine...might be a little small, but shouldn't cause the bad mpg. Reset that computer.

phat87

I'm with the 30 mpg guys! We just had our 87 on a road trip to Oregon and the highway mileage was 30 and the overall for 500 miles was 28.7 mpg.

Calderone

i guess you need to install Jon's injectors !

Eddie & the Cruisers

I was also told to turn on the radio to drain off the power the radio retains in capacitors for instant-on and also keeps the clock/station settings in memory. The clock will back flow voltage to the ECM. Only takes a few seconds to do it.

It doesn't take over night to drain the residual voltage off the computer, that's .

cwyates4

Eh, thats what Jon had me do after I put my new injectors in and was still having trouble...it was late already that night though, maybe he just didn't want me out that late at night.

Klondike

Just put Jon's Bosch III's in my LT4. Do the computer drain trick and when you reconnect and start, DON'T TOUCH THE GAS pedal at all till after the car warms up and goes into closed loop. This took about 18 minutes on my car while I anxiously watched the temp gauge. The computer takes a while to relearn the idle and the throttle positions while you drive. It took a little while for the mileage to creep back up in my car. At first, I was not getting the mileage I had before the swap. The performance is definitely better and the mileage improves with every drive. Yes an LT4 can get almost 30mpg on the highway. Last year I went to Colorado from Texas and got 28mpg average for the whole trip. Did much better in the mountains than I expected too.
P.S. Also check the regulator vacuum line to make sure you didn't crack it or leave it unplugged. That would make your injectors go to max pressure all the time.

Sam Lam

My 89 model with only 45k miles has never seen 30 MPG.....except when going downhill with a good tail wind! My original Multecs did not get good mileage either!

StealthLT4

I'm surprised Jon hasn't chipped in...