I have a 1992 with a 5.7 350. I am getting terrible fuel economy. Just did a complete tune up with E3 plugs, adjusted timing to factory setting and still I get 7 MPG in the city. I know these cars are not great on gas but 7 MPG ??? The car idles rough when it heats up, and from I read it probably needs new injectors. But would that cause such fuel consumption?

 

Yes, but even so im not running any emissions and i get an average of 17 mpg IF babied. No check engine light? Map sensor could cause it but not that bad i dont think. The rough idle could be used by injector not pulsating or being bad so its using up more gas.

 

There are a lot of reasons you might get bad mileage. Injectors are a good start. Clean your maf, new air filters, bad oxygen sensor. Scan your computer and see if you have set any codes. How hard are you driving it?

 

I agree, Question, are you towing a boat lol

 

its a 92 he doesnt have a MAF.

Top 3 things that come to my mind are injectors, O2, sensor, Coolant Temp sensor. I just put injectors in my 91 TA because the stock Multecs crapped out. go to www.fuelinjectorconnection.com and get a set of rebuilt injectors from Jon Banner. Tell him thirdgen.org sent you and you'll get a discount. Do the injectors and go from there. if it still acts up then put it on a scanner and watch the O2 outputs and see if the coolant temp sensor jives with reality.

 

If nothing else, replace the o2 sensor.

 

DING DING DING!

might as well spend the $8 and replace teh water temp sensor for ur ecm while ur at it

 

Right on the money!

 

 

THANK YOU everyone for your advice. First , I am not driving it hard or flooring it. Second the image above is right on the money , I live in brooklyn, today regular was $3.79 a gallon... so yeah I have to cry everytime I fuel up... I'll start with injectors, O2 sensor, fuel filter. I thought if temp sensor is bad it would not show temp on the cluster? Am I wrong?

 

the coolant temp sensor has nothing to do with the gauge in the dash, that is controlled by a different sender that is in the drivers side head.

 

then I'll definitelly replace it as well. thanks!!

 

yea, i did new injectors, o2 sensor, fuel filter, K&N air filter, high flow cat, flowmaster exhaust, and it def made a difference, in my 3.1 haha, i miss my 85 iroc sumdays, dont miss the gass milage tho!

 

Before you do or buy anything, I have one question for you? Did you unplug the one Tan wire plug on the passenger side of the engine near the blower motor to set the timing? If not, then thats the problem. Set the timing to 6*-8* degrees. No one never ask you this question.

 

I did do the timing after unpluging the wire. I think factory setting was 6*, and thats what I set it as.

 

we're chasing the same problem. So far i've replaced injectors, fixed my rough idle when hot. Replaced O2 because the old one cracked and just did the MAP. After I throw on a new CTS i'll let you know how much my mileage has gone up.

 

If you replace everything and you're still getting terrible mileage when the car is at temp get an ALDL adapter and download tuner pro to see what your computer sees... a past owner could have really messed up the tuning

 

At this point I replaced O2 sensor, coolant temp sensor and fuel filter. Didn't do injectors yet. I think I went up to 8MPG now LOL Still have the rough idle though. So will do injectors next. Another issue that I noticed, that at low RPMs, my votage is going up and down. I think this is probably the alternator?

 

Maybe the injectors just need a good cleaning.

 

if all that doesn't help maybe your cats

 

you should also check for proper operation of the egr valve

 

How do I check EGR valve?

 

once you have everything apart for the injectors... there are two nuts holding the egr, (round thing under the intake phlenum)
take it off and look to see if the passages under it are full of carbon.
And make sure the egr can open and close, hold it with both hands and press on the back of the phlenum with your thumbs and watch the valve at the bottom you should be able to see if its hanging open or you'll notice if you cant open it at all.
If you can't open it at all replace it, if its hanging open you can try and clean around the valve with carb cleaner, and clean out the passages with carb cleaner but if there is too much you'll probobly need to take the intake off to get at all of it

 

Changing injectors made the biggest difference by far. I was using a quarter tank every 20-25 miles of normal driving and 3 inked out bad. So you might check yours just to be safe.

 

Thanks ! As soon as weather gets a little more decent here on the east coast, I'll go for the injectors. I read a few posts, seems that everyone is happy with Bosch 3s from FIC.