crazymex83

I don't know what typical mileage for an '88 Formula 350 should be. I'm not doing very well on mileage right now and I'm guessing something is wrong, but I'm not sure what since the car runs well. When I pulled the car out of storage I drove from the Chicago area into central Illinois and did about 150miles on a quarter tank. Now I'm getting about 150-175miles to the tank. I counted my mileage from my last fill up to be about 150miles and I'm down to about 1/4 tank. I'm pretty sure the tank is only 15gal so give or take a bit that's only 12mpg.

I can usually start it on the first try but I have to crank it for about 1-2sec. I figured this is about normal considering the age and my other old car has a carb and may or may not catch if I don't prime it correctly. I also have a short stumble when it's cold and I'm at a stop.

I'm thinking there might be a vacuum leak but it idles fine. I also thought the IAC needs to be cleaned and clean up the throttle body too. I haven't done that yet and will buy some throttle body cleaner and get that done.
I used some lucas fuel injector cleaner and it helped some in the cold start area. The car seemed to run a little better after that too. I live 0.5miles from the highway, which I take everyday to work and noticed that if I just start the car and go without letting it warm up much then the idle seems to stay high until I get to a stop after getting to work. It's like the IAC is keeping the idle high to warm the car up. After I come to a stop, then the car is pushing and I have to hold the brakes until the idle drops to about 700rpm and all is normal. It also smells very rich at idle. I can't really have the car running in my garage, with the door open, or my clothes will smell like unburnt fuel very quickly.

I was thinking the evap valves may be bad. It smells strongly of fuel after coming home from work. I thought the gas cap was bad so I bought a replacement that didn't engage the threads and tried a few other nonvented caps but none fit so I asked for a gas cap for an '83 Monte Carlo and that one fit perfectly. (I know that the stock one is non vented and the new one is vented) I still smell fuel but not nearly as bad. I don't have a vacuum gauge or pump to test the evap valves. Is there another way to test them?

Am I missing something here?

nomind

What kind of mileage are you actually getting? Guessing by using the fuel gauge is not very accurate. I can run the same trip from one tank to the next, and the fuel gauge will be in a different spot, but I'll still calculate out the same mileage.

Keoman

iTrader: (3)

Whens the last time you changed plugs, wires, cap, rotor, etc...? Usually a good sign they are long past due. Not sure if Illinois does emissions but you would probably fail.

MBaggese

I'd start (as mentioned above) with looking a tune up items, and don't forget about an O2 sensor.

Pull #1 plug (easiest to do) and see if fuel fouled/black for confirmation.

I'd be careful about running a "rich running" car with a working cat converter.

A few simple tools for diag should get you sorted out.

And no-again as mentioned above-you can't figure milage based off the fuel gauge

jlacoste

Check all of your grounds. If they aren't all on all the way the comouter will stay in a closed loop and not read all the sensors. Remember there is a group of grounds on the fire wall. Also check you temp sensor. If its not working it could cause the same issue. The reason is the computer runs in closed loop until the engine temp gets into operating temperature. I had the same issue you are describing. The car seemed to run fine but was sucking gas like a hog. Mine was a ground issue that the previous owner caused by cutting wires.

ratfink6923

iTrader: (2)

Staying in OPEN loop. closed loop is what you want

jlacoste

Yes thats right. I had it backwards. It was early.

MrPackstin

iTrader: (9)

Make sure it is getting to operating temp and it has stock temperature thermostat. Need to see if it has any codes and try to get some live data from the various sensors.

vetteoz

iTrader: (13)

https://www.thirdgen.org/forums/tech...s-dumbies.html

crazymex83

well, I ran the car until it got close enough to empty as I dared and filled it up with 15.1gal of fuel and it went about 240miles. That comes to 15.89mpg and I do about 14miles of highway each day to and from work. It might be ok mileage for an older car, but I have an '83 Monte Carlo with no overdrive that will get 16mpg on the highway and is running 3000rpm at 65mph. I expect the firebird to have better numbers.
When I pulled the car out of storage I put on new plugs and new O2 sensor. I didn't mess with the rest of the ignition. I replaced the thermostat with a 180deg unit and put a new temp sensor, heater core valve, and radiator in when I found out that the old radiator had plenty of 'gravel' inside of it and my heater core lines were empty because the valve was clogged.
I hooked up my moates APU1 to the OBD port and my laptop and pulled some data, but not very much. It looks like the car is getting into closed loop because it would have the temp would be at 180, the BLM's were at 128, and idle would be at 750rpm. HOWEVER, the BLM would only be at 128 when I pressed the gas. At idle the BLM would be about 148. I didn't mess with it very much beyond that and am trying to figure out where I should begin.

Not that it really matters, but Illinois emissions only apply to vehicles 1995 and newer now, so the firebird is emissions exempt and will be modified accordingly in the future. My future plans involve a set of longtubes, maybe an x-tube into a larger flowmaster muffler and dual tailpipes. I've considered a new TPI intake, but since the heads and cam are not really HP makers I am considering their replacements as well.

crazymex83

I know this is an old thread, but I wanted to post an update.

So I've been experiencing the poor mileage again this year. I filled up the tank and ran it until almost empty and the results were similar with 15.5mpg and I baby the car to be consistent on the mileage results.

Anyway, it's still blowing a lot of fuel out the tailpipe and the smell is getting into the passenger compartment at times. I've taken time to check the exhaust for leaks and it looks pretty tight. It was easy to see in the colder weather and there would be an obvious tell tale sign of moisture in the air but there was nothing obvious. I'm starting to think that the drivers sides manifold is leaking as there is a low frequency booming sound when you stand on the drivers side with the engine idling and the hood open. There is no sound like that on the passenger side. Since the drivers side has the oxygen sensor any leaks would look like a lean condition and cause the ECM to richen up the mixture thereby causing excessive fuel consumption. I think these are the original manifolds but I'll take a look at the donut gasket on the manifold to y-pipe as well. Maybe this is a good reason to swap to headers. I saw a mechanics stethoscope for $3 at harbor freight. I'll pick that up since it's so cheap to see if it can help me out.

I did verify the fuel pressure is normal at about 36psi with vacuum and about 43psi without. I also found a few vacuum leaks near the fuel pressure regulator (fpr) and a couple of hoses that look very cracked and might be leaking. One hose was collapsed when I messed with it and as I did the idle speed kept changing. I've identified two sizes as 5/32" and 7/32" vacuum hoses. I can't tell what the others are, but if anyone knows specifically what they are please chime in. I plan to take samples with me to the store and buy replacements. I'll keep checking for vacuum leaks and see if there are gasket leaks anywhere. One idea is to use soapy water in a spray bottle or non-flammable carb cleaner. Any other suggestions?

I hope that the brunt of the issues are gaskets and vacuum hoses, but thanks in advance for your help.

no new tires

FP should be 38 PSI at idle and 45/46 PSI at full tilt.

crazymex83

Ok, so I've been tinkering here and there with the car and I fixed a few things recently. I found a collapsed hose near the fuel pressure regulator. As I messed with it with the engine running I could hear the idle speed changing.
I bought a vacuum pump from harbor freight and tested the rest of the vacuum lines. All of them held vacuum and none of them collapsed. I only bought 5/32" and 7/32" vacuum lines since that's what I pulled off the car. They were about $1 a foot and I bought 10ft of each at Advance Auto. I ended up only needing about 4" to fix that one area.

I also found a couple of exhaust leaks at the donuts off the exhaust manifolds. Both sides were leaking and since my winter plans include buying and installing a set of longtube headers I band-aided it by using an exhaust putty to seal up the donuts. It made a big difference when I was done. There used to be a stumble first thing in the morning shortly after starting it. There are a couple of stop signs near me so I felt the stumble each time. Now after sealing it up the stumble is gone.... well it was until another leak popped up at the manifold where it attaches to the cylinder head. I asked an exhaust shop to help me out and they inspected it for free and told me where the leak was coming. They didn't want to do the work because they were afraid of breaking old parts. I applied more putty and was able to seal that up as best I could. If you do a search for VER00160 you can see the stuff I used.

My mileage seemed to increase some after these fixes. It's still pulling mileage in the teens and I'm wondering if the catalytic converter is plugged but it seems to rev up quickly. There is a lot of low end torque and seems to run better. The exhaust still smells a bit. I wondered if it's the cat but it also smells like burning oil. I'm saving up to rebuild the bottom end but I think I'm chasing my tail or don't know what I'm doing. The mechanics around here aren't much help. I asked an older guy a couple of questions about what to look for and how to improve gas mileage and the 'best' answer I got was "you're not going to get good mileage in that thing." This is also the same shop that wouldn't work on the donut gaskets because they were afraid of breaking the studs and having to tow my car to another shop to drill them out.
Anyway, I'll have to start a build thread soon since I'm coming up with more questions.

8Mike9

Re: you cat...you can check for exhaust restrictions with your vacuum gauge.
Pull a line off the intake, or plenum, hook you gauge there..at idle you should have 18-20in/hg.

Now open the throttle to about 2000rpms...in a perfect world, you'll have the same reading...the lower the reading, the more restriction you have...specs usually allow for 2-3 inch drop, but even 2-3 inch drop is showing a restriction.

I see it mentioned above, did you replace the O2 sensor as well?

abray1

Mileage in the 75000 mile range you will need to check the converter. If you will install a heated O2 sensor that usually helps with mileage.

crazymex83

I will give it a shot with the vacuum test. I bought a vacuum pump, not a gauge so I'll see how that works.

The car has 164k on it as of yesterday so I'm certain the cat could use a replacement, if that were my intention. There are no emissions in central Illinois, nor are they required of a car this old in Illinois anyway. I'll have to see about getting creative and replacing the cat. I may just wait it out since the longtubes are near the top of the winter project list right next to new gaskets, camshaft, roller rockers, fuel injectors, etc.

no new tires

Long Tubes

Ghettobird52

Honestly when im driving all in town, i probably get 150miles to the tank as well. 14gal tank

150/14=10.7, Not that bad for stop n go with a v8.

all highway i could get 290-300miles, 75mph at 2100rpm

abray1

I dont remember how many gallons the tank holds but I do remember a stock decently tuned up 305TPI with 115000 miles would get 25.5 "interstate" miles "not in town" to the gallon on a full tank then filling it back up and taking how many gallons it takes to fill it back up and doing the math. I am pretty sure my stock 350 Iroc back in the day got 24 or close to 25mpg but that was in 1990 and I have slept since then. Mid 20s will put you close to optimal.

crazymex83

I'm pretty much running all highway so I'm thinking that 20+mpg shouldn't be such a stretch.
I attempted to pull some data the other night and what I was seeing confused me. I still smell a lot of unburnt fuel coming out of the tailpipes so I know it's running rich. Here's the confusing part, it looked like the computer was trying to take out fuel. The BLM and correction something or other said that instead of a normal 128, they were actually correcting down to about 90-110. Would you guys guess that the injectors are leaking? I would think that the cold start injector is leaking, but wouldn't that cause the car to run weird?
I fixed the exhaust leaks that were present and that took care of the hesitation issues I had, but didn't do much for the mileage issue. I'm still wanting to install a set of longtubes over the winter. This way I KNOW the exhaust is sealed up and not leaking, plus the performance benefits. (FYI, there are no emissions requirements in central illinois)
I didn't do a vacuum test yet so I can't say that there aren't more vacuum leaks. Would they cause a rich condition though?

Thanks in advance for any help!!

crazymex83

I should also mention that I noticed that there seems to be a pushing feeling when slowing down. It seems like the idle keeps trying to stay closer to 1000rpm when in drive and that when I come off the highway I feel the idle drop to about 700rpm at the first stoplight. Might the IAC need cleaning?

Ashton89iroc

I'd pull the injectors and get them flow tested just to make sure they aren't bad, Injectors would be my first bet. Next, I'd pick up a new Ignition module for the distributor and while you're at it put new plugs, cap and rotor in it. It seems like you're not getting a good spark if it isn't burning all the fuel. By the way, it is possible to check your injectors with an ohm meter, just check on here for the proper readings for a good injector vs. a bad one. I've had a very very similar problem with my '89 and after a whole lot of trial and error it turned out to be a combination of all the parts I listed.

crazymex83

I ended up changing out the injectors since I'm going through all of the trouble to pull them loose. It took me three days to do it. I only spent a couple of hours a night with it so maybe 8hrs total. That's including mistakes I made that required me loosening up some bolts.
I figured that the new injectors would have made a big difference but they didn't. The old injectors didn't look like I expected them to. It seemed like they were in pretty good condition. I installed a set of Bosch III's from Southbay and once I got past my reassembly mistakes the whole install went together pretty well. Again, I did not notice a seat of the pants increase in horsepower or anything. The exhaust smell is still there too.

I checked the spark plugs and none of them showed any signs of excessive fuel. They all showed proper burn patterns. The ones I pulled from the passenger side showed some carbon like an oil leak, but the drivers side looked pretty good. I later went and borrowed a timing light and started adjusting the timing. I pulled the oil pressure sender wire by mistake (I didn't know where the EST wire was located then) and set the timing to 6deg BTDC, per the underhood sticker. The car seemed to run very well and felt like it gained some torque that way.
A day or two later I found the correct EST wire location and set the timing again. While I was at it, the vacuum line to the EGR valve came loose from under the plenum. I was able to get that reattached, but noticed that the electrical connection to the EGR solenoid was disconnected. I reconnected it and started driving the car again. No difference in seat of the pants, but did notice a BIG difference in my wallet.

Apparently, I had been running around for the last couple of months with the EGR disconnected and getting 15mpg and my average speed had been about 60mph. After I reconnected the EGR solenoid my mileage went to 20mpg at the same speeds. I thought I was doing the math wrong every time I looked at the mileage but I was able to go 280miles and filled it back up with 14.05gal of 87 octane fuel.

Yes, the emissions equipment was able to give me a 33% increase in mileage. Your results may vary, but I am happy to leave it in place now.

I found that my exhaust leak is back and I'm losing coolant; possible head gasket issue. I've yet to trace this stuff down yet, but it looks like I'll be parking it soon and pulling the engine over the winter. I'll attempt to install a new cam since you can't just do that while the engine is still in place. I'll keep you guys posted!

Reid Fleming

iTrader: (2)

That is VERY interesting. I'm going to have to take a look at mine as I've been having the same crummy 14 mpg with mostly highway despite all new ignition components.