Maybe a little long!

Ok, so i checked my mileage the other day in my 87 2.8 liter. (MPFI) Go home, do the numbers, and it comes out to 10.52 mpg! Now, I may have a bit of a lead foot,(don't we all? ) but I don't think that's all the problem. I haven't changed anything major in the last while, (just fixin my t-top seals!), so I'm pretty sure I didn't screw anything up!

Here's a couple of things I've noticed that may (or may not) be related.

Sounds like a vacuum leak in open loop, I can't find it. Goes away (mostly) in closed loop.
Goes to closed loop just fine, at idle goes back to open loop.
In closed loop, ?injector pulse timing? (blink rate of SES light in diag. mode) seems erratic.
While driving, sucking vac. leak noise from engine bay while cruising (any speed), not while decel or accel.
Seems to run perfectly fine in closed loop, still sucks gas.

So, it seems to me that I may have a vacuum leak under certain conditions (maybe EGR?) If it's the EGR, can anyone help me with some bypassing/removing?

Oh yeah, one other thing. My exhaust smells a little gassy, mostly while accelerating from a stop. I also have one leaky injector (not too bad, just a little blackish on the plug).

I'm tempted to swap in a 350 and get better gas mileage! (whats up with that?) Help me keep my 20+ mpg!

 

measure ur vacuum if u suspect a vacuum leak.

easiest place to hook a vacuum reader is from the brake booster vacuum line, if u dont know where that is or dont have the tool,
turn the a/c on, and go on freeway and if the air from vent stops or weakens, you must have intake vacuum weather its related to ur problem or not. good luck

PS: if u smell exhaust, u might have an exhaust leak and that affects ur gas mileage too

 

im a lil confused on your description. you say your car goes back and forth between open and closed loop. your car will be in open loop w/ the o2 until it warms up and then stay in closed loop. it will run richer in open loop.

if you have a vacuum leak, that will definetly cause a problem w/ fuel mileage, find it and fix it. brake cleaner will help you find this, spray it around any vacuum line/intake gasket, or any other spot that you may get a vacuum leak. if you get a hesitation or jerking in the engine, theres your leak.

also if your SES light is blinking in diag mode, what codes are you getting? to read the codes ( if you dont know) youll get the same code 3 times in a row. for example, itll start w/ code 12, which means its running the diag scan. youll get 1 flash, a pause, then 2 flashes, then 1, then 2, then 1 then 2. then if there are more codes itll flash those codes. like xxx_xxxx for code 34. itll do it 3 times for each code until it goes back to code 12.

if you cant figure out the codes, go to an auto zone and theyll scan codes for free.


oh and try driving normally (no lead foot) for a while and see how much it goes up. im guessing you have more than that going on ,but see how much that may contribute to your issues.

 

Thanks for your suggestions, guys, and sorry my first post didnt explain any better what I've already done.

sogabe:
I don't have a vacuum tester, but I might have to pick one up. And yes, I do know where to test my vacuum. Experience with rebuilding and tuning a couple 350s goes a long way. I never knew the a/c thing to test vacuum, but sadly I have no a/c to try that! Also, I did have a couple bad exhaust leaks, but 5 bucks and 20 minutes at the muffler shop got those welded right up! Thanks for replying!

drdave88:
While the car is runnning, you can short ALDL pins A&B and the light will blink, but not for codes. Fast blinking means open loop, slow blinking means closed loop, and in closed loop the length the light stays on between blinks is I believe the injector pulse rate. Try it! It's pretty useful. And, I have no codes thrown whatsoever. I also tried carb cleaner around everywhere I could find a vac. hose connection. No luck there.

About the open/closed loop issue:
Heres an example

When I first start the car, it will be in open loop (duh) then when it warms up sufficiently, it will go to closed loop. This is without ever touching the gas pedal. After it is in closed loop, I can hit the the gas, and when the car goes back to idle, it goes back to open loop. Anytime after that that I'm idling, be it at a stop light, or in the driveway, the car goes to open loop.

Lightening the lead foot hasn't helped yet. 5 days after my last fill-up and I'm down to half a tank. Just guessing with my mileage, but thats right around 9 or 10 mpg.

Thanks for trying to help! I'm nearly convinced by now that my EGR is causing this problem, mostly because of this:
"While driving, sucking vac. leak noise from engine bay while cruising (any speed), not while decel or accel."
It would make me happy if someone helped me out in bypassing it. I checked it out a bit yesterday, but it was hard to tell how I would go about blocking it off.

 

Dont forget, most of us all now have winter gas. It brings your MPG down.

Colder air temps suck more gas, this bring your mpg down.

Tire air pressure, make sure they are inflated right, its cold and they "loose air pressure". This creates more friction on the road, bringing your mpg down.

Make sure the plugs are good, no vac leaks as you wonder, proper t-stat, blah blah blah.

Ive recently noticed the mpg on the GTP go from 23 in town to 19. And I'm driving the same or nicer.

 

Thanks Dale. good advice. I'll check all taht, and it may help some, but I don't think all that could cause 20+ to drop to 10 in under a month.

 

why would your ECM be jumping in and out of closed loop? it runs in open loop till the vehicle warms up enough and then goes into closed loop and starts monitoring the o2 sensor. i havent heard of jumping the A and B pins while its running and it monitoring the ECM going in and out of closed loop, it should always stay in closed loop once its closed. otherwise youre going to be going from proper A/F mixture to rich and back and forth.

 

Dave, isnt what controls open/close a combo of ECT, IAT, and O2? I'm no ubber ECM guy, so maybe I'm wrong.

If one of them is really off, or the engine is not reaching proper running temp, it wont switch modes.

 

So, if the open loop gas mileage is really bad, then I bet going back and forth from open to closed at every stop would not be very good for the mileage. And that could be my problem. Maybe I should talk to the ECM guys and see what sensors really control open/closed loop. I know too that the O2 and IAT for sure do, but i'm not sure of anything else. I have both a new O2 and IAT, both fixed other little engine problems.

That still doesn't address my odd vacuum sounding noises in open loop, though, I think I'm gonna buy a vacuum tester and see what's up there.

for helping!

P.S. Dave, try shorting your AB while the car is already running! Then just observe the blinky light for a bit. NEVER start it with the jumped pins though, that has a possibility of frying the ECM. Learn something new everyday, eh?

 

The ECM waits until the ECT reports a temp above 150* and that there's a signal above 450mv to start trying to enter Closed Loop. it remains in closed loop until the temp drops below 150 or the O2 stops responding to fuel metering changes.

O2's need to be hot in order to work, if the O2 cools down enough during idle, the ECM will not see a response from it and revert back to Open Loop until the O2 starts switching again. Same idea with the ECT signal, if it's not what's needed to be in closed loop, it reverts until it is. That's why some people have problems with low T-stat temps.

Alphius:

Your mileage IS really bad. The O2 sensor can only make SMALL changes to the fuel mixture, not ones like you're seeing. If I were you I would start checking out the load sensors and the ECT voltages since they have the most to do with fuel metering. Start with MAF, TPS, and ECT. Check out the voltages and see what you get. IIRC, there should be a table somewhere on here that shows all the sensor voltages. Just search it.

As for the vacuum leak sounds in open loop: The only place that I can think of that you might have a leak is at the AIR injection diverter valve. if the diaphram is bad, you'll have a leak when in open loop and the solenoid is commanded on, but as soon as you go closed loop and start diverting air, the solenoid shuts off and the diaphram no longer leaks because there is no vacuum getting to it.

The only way to check this is going to be spraying the diverter valve with carb cleaner RIGHT AFTER you start the car.

Good luck.

 

I was fairly close then

 

In the general area. I just added some more detailed info.

 

Thanks a lot, 2_point8_boy, thats just the kind of info I was looking for. I'm pretty sure that my TPS is good, and after doing some reading about the MAF, it seems that a change in mileage as bad as mine caused by the MAF would at least throw a code. I'll test the voltage on it, but I'm inclined to believe as you said that my AIR diaphragm is leaking. I'll try that too...

I bet my mileage problem and the open/closed loop problem are both related to my ECT. I've replaced most other sensors except the ECT, so I'll replace that very soon. I bet it's going bad. MAF is too expensive now, I might grab one from the j/y sometime if the ECT doesn't fix it.

This forum rocks!

 

i get 26mpg on highway and my engine runs like crap (possibly missfire)

 

2.8 mpfi

i also get very bad mileage on my firebird (14 miles per gallon) and have the vaccum leak /hissing sound...

im thinking my throttle body is leaking causing a vaccm leak


has anyone had this issue?

 

Yeah, my mileage is still doing bad. So far I've rebuilt the entire upper engine vacuum system with new rubber hose, because I broke a few plastic ones while removing my intake to fix my stripped ECT sensor hole. I've replaced my O2 sensor once again, and still I get right around 9-10 mpg.

Some people have put forward the theory that my car doesn't warm up enough in the time that I drive it, so I'm gonna flush the cooling system once again. I never have ever had a check engine light come on with this car, which really stumps me. Maybe my ECM is bad?

This problem is amazingly frustrating...

 

i've replaced my ecm with no luck also....

and for the cooling issue my car runs very cool but warms up eventually. probably stays around 170 almost always.

has anyone heard of a vaccum leak causeing terrible mileage and a small hesitation on part throttle?

 

alphus, how far are you driving it, how high hot does the coolnt get when driving it whatever distance? Where I'm at, I drive 8 miles to work and it is then just starting to reach op temps. very very doubtful your ecm is bad.

vac leak will case poor mpg and throttle hesitation(from the start), yes.

 

I mostly drive distances right about a mile, probly a total of 6-10 miles per day, so my coolant only just starts to move the guage (on the notoriously inaccurate gauges) before I shut it off. Even when I drive farther distances, while the car is moving, i get it to around 160. I have a manual fan switch, so when the car is sitting, it'll go up until I hit the switch, usually around 220 or so.

Yeah, even though I've been told vac. leak, I'm almost absolutely sure I don't have one, my car seems to run perfectly fine in all ways except this.
I figured the ECM was a long shot, I'm just yanking at straws here, but I think next I'll check for leaks around the tank area and fuel lines.

I swear my car doesn't run like it's double-rich all the time!

guys!




Just to update the record, here's what has been replaced in the last year:
Tires
Alternator
Water Pump
Heater Core + Heater hoses
New exhaust
O2 - Twice
TPS
IAC
ECT
Vac. Lines
Cleaned intake
Seafoamed

Probly forgetting something or other...

 

put a vac guage on it and find out. You can get a basic one for less then 10 bux's.

I'm almost sure your not reaching proper engine temps and that will be part of your problem.

 

I decided to flush my cooling system the other day, after letting the car warm up before driving for a week or so. I flushed the block and radiator seperate from each other, and this deep clean caused brown sludge to run out of my radiator. I think I've found my problems... I went to the gas station after hooking everything up, and the car warmed up faster than it has been lately. After getting gas, I figured my mileage out to 14.1 mpg.

I think the failure of my car to get up to operating temperatures, caused by a somewhat blocked radiator, contributed heavily to my poor mileage. I guess we'll see next time I fill up!

 

good find/fix even if that wasnt the problem.

 

this really has nothing to do with my gas mileage but i was changing my throttle body and noticed that there was major blockage in the lines leading to the TB...

what can cause this?

the blockage is so bad.. i forgot to tighten the hoses to the TB and it didnt even leak and i've been driving around for about a week now.

 

I wanted to add to this thread in hoping this may help some of you guys.I own a similar car with a v6.I wanted to mention a very important overlooked device that some dont realize its there.I am speaking of a fuel vapor solenoid or evap. control solenoid.It sits on top of the charcoal canister it has a vaccum line and a two wire connector.Theres two other large rubber lines ,but those are just for the fuel vapors.On mine the solenoid was stuck open,so I basically had a vaccum leak.The line from the engine to the top of the solenoid supplies engine vaccum to the solenoid and the ECM triggers it to open.You can check this by removing the line from the top(one that runs to the engine) and either blowing into it or use a vaccum pump to see if it holds vaccum.I just removed it and blew through it and it was clear it wasnt working.I dont know how far back in the years it runs ,but you might want to check it?.Funny thing is I never got a check engine light on?...and I cannot find a replacement part ,so its just disconnected.

 

The passages for coolant into the TB are pretty narrow and the flow through is not that much. Recipe for sludge. Its just the way it is. Mine is bypassed...

 

That's a good point to bring up, I have checked that, actually it was one of the first things I checked when I suspected a vacuum leak. Luckily, mine was still functional and working, but I see how it could be overlooked. Thanks!

 

I just tried testing that fuel vapor solenoid. I got all excited when I turned the key to "on" and air continued to flow through it, but then I remembered it didn't get power until the engine was running. Sure enough, it was working fine

Front end is getting checked out tuesday. I hope something there can be fixed and then realigned, thus upping the mileage a little.