poor gas mileage

[pmkls2]

Ok, I have had several problems with a surge and poor idle performance, and I have tried several things to correct the problems. I have replaced the o2 sensor, tps, plugs, wires, cap, rotor, fuel filter, temp sensor, removed AIV, etc....... Now my biggest concern is my gas mileage is plummeting like a lead balloon. When I bought the car a year ago, I could get about 375 miles on 12.5 gallons. Now, I am struggling to get 300 miles on 13.5 gallons. I have changed my muffler to a dual tip design with an upward bend and it turns my bumper black within a day or two. The car is a 1990 model with 127k miles and very well maintained. I am absolutely baffled by this thing and just want to get my gas mileage back since I live 41 miles from where I work. Any direction would be greatly appreciated.

[NISTECH]

check your airflow meter.

[pmkls2]

How do I check the meter to see if it is bad? I have changed my air filter to k&n and am aware of the problems they cause with the air flow meter, so I try to clean it frequently and sometimes it seems to make a difference if that helps any.

[pmkls2]

I also forgot to mention that after a cold start it will bellow black smoke if you rev it any and it sputters a little.

[NiTeRiDeR]

maybe your running rich... and ur timing might be off... check those two

[NISTECH]

check your airflow meter with a volt meter. one wire should be battery voltage ,one wire will be 0 volts, the other wire should be between 1.3 and 1.4 volts at idle warmed up. this wire is for the ecm if the voltage gets much higher then that and the throttle plate isnt open it will start running rich. also the 0 v wire you need to observe the voltage drop on that wire. your meter shoud have 2 places after the decimal. your voltage drop should be no higher then .02 volts if it is you need to attach a second ground to that wire and attach it to you intake where the other group of ground wires is attached. splice into the wire do not cut the existing wire the ecm still needs to see continuity on that wire or you'll get a code.

[pmkls2]

Ok, I will check that as soon as I can. Unfortunately as you are quite aware, as an auto technician myself I have time to fix everybodys car except my own.

[corn322]

hmm... I get 1.6v on the signal wire, would that be reason I get roughly 12 mpg?

[corn322]

or at least part of the problem... my dad came out and jiggled the connecter on the water temp sensor. something went "psst" and the idle jumped up and came back down. I think the temp sensor is more of the problem.

[NISTECH]

yep coolant temp sensors can also cause a rich condition. when you unplug your connector it will most likely be full of green crustation. clean it with battery terminal cleaner.

[pmkls2]

See, I was thinking the same thing about my car at one time and I actually changed my temp sensor, unfortunately it didn't fix my problem.I have replaced the o2 sensor twice, the last time it was with a Nissan sensor. I have checked or changed just about everything on this car, but this little gremlin running around in there is impossible to find. I even checked and cleaned all of the grounds as someone once advised. I have yet to check my airflow meter, but I am hoping to get the chance this weekend since I will be working saturday and will be here at the shop.

[pmkls2]

By the way, does my car have a CEL???? Because if it does the thing is completely inop, i.e. it does not flash when you first turn on the ignition nor has it come on when a problem has arisen that should turn it on.

[NISTECH]

your car may not have a cel if its a fed model. most 240's I deal with are california so they have to have it. so I am not 100% sure your does. you could always pull the cluster and shine a light from the back. if the bulb is not installed when you place the light where the engine light is suppose to be you will see it illuminated from the front. thats if there is not already a burned out bulb in its place.

[pmkls2]

Alright, I finally got around to checking my air flow meter today and this is what I found. On the one terminal there was about 13-14 volts. The other terminal had about 1.51-1.58 volts at idle warmed up. The last wire had about .02 volts. I went ahead anyways and ran a second ground for the 0v wire and attached it on th intake where the large ground is hooked up and now it reads 0.0v. On the way home the car drove completely different. There was no surge and no jerk with on-off-on throttle transitions. The voltage on the ecm feed wire is still at least 1.5 volst though, should I try changing the air flow meter and see if it drops? The car drives better, but that doesn't necessarily mean anything, as I wouldn't know if it is completely fixed.

[NISTECH]

moniter your gas milage for a couple tanks see if you have an improvement. but your air flow reading would suggest you still have a slightly higher fuel consumption rate. that reading should be a little lower. let it go though for a couple tanks and see if your ground has helped.

[pmkls2]

Okay, I was sort of thinking the same thing. So, far it ran a lot better this morning when I started it (no sputtering or black smoke), but it was pretty warm this morning for this time of year. Thanks for the help and I'll give you an update in a couple of days.