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90 Brougham 5.7, 05 Deville DTS
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Hey guys I was wondering what kind of gas milage you get out of your 5.0 Liter (Olds 307 4 bbl carb) powered Caddys?? I still have not calculated the milage I get from my 89 Brougham, as I have only drove it ~200 milaes, so I was wondering what should I expect? Thanks.
 

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1987 Cadillac Fleetwood Brougham
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183 Posts
89fatb said:
i have an 89 brougham with 115k on it and i get about 16 to 17 mpg, per tank
I have an '87 with the 307, 87K.

Best mileage: Cruising on Flat Terraine from RI to Cape Cod @ 55 MPH in cruise control =28 MPG

Worst mileage: Spending a week driving around Hartford/W. Hartford=11 mpg

Usual: 14-18 mixed driving, 20-22 highway @65-70 MPH in cruise control.
 

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1987 Fleetwood Brougham
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239 Posts
I think something is wrong with mine. Driving stop and go in flat Florida, I get 13 MPG on average. I think my Q-Jet is whack.
 

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1970 Sedan deVille hardtop
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Stop and go traffic, your right foot might be to blame. Try taking off more gently, everyone jumpng off the line like they do uses alot of gas.
 

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1987 Fleetwood Brougham
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BluEyes said:
Stop and go traffic, your right foot might be to blame. Try taking off more gently, everyone jumpng off the line like they do uses alot of gas.
My car doesn't jump off the line no matter what I do. I really think the Q-Jet is done. On the highway, doing a constant 70mph, I was only able to get near 17 MPG.
 

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94 Fleetwood Brougham
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Bad accelerator pump? That would make the Q-Jet a bit more lazy. Press on it separately quickly depressing it. It should stall the engine. I prefer some mods on it to increase the resons of the carb. I can post if you are interested. Involves carb dissassembly.

Clean the air tubes carefully with carb cleaner. Some internal mods here as well.

Ensure no vacuum leaks, carb is properly retorqued to 10 ft/lbs biannually. Replace PCV yearly, heat will make the springs in the PCV weak. There is 2 PCV's for Olds V8's, 1980 and older and 1981 and newer. Make sure you have the right one.

More internal mods are fairly easy with a pin vise and some home made special tools. My Q-Jet on my Olds 307 and later 350 Olds in my 85 Cutlass was a great running carb.

Careful mods based on much of what I read in Doug Roe's Q-Jet book applied to the not well documented E4MC/E4ME carb worked well.
 

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1987 Fleetwood Brougham
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I rebuilt the carb not to long ago and used like 2 cans of cleaner on it. It was downright clean inside. I just put in new breathers and PCVs, so they're clean. Maybe there's a vacuum leak, I'll spray some starter fluid around the engine to see if I can get it to pick up idle speed, but I really think this Q-Jet has had it. It's the original one from 1987 and my Mileage reads almost 98,000. The previous owner told me it was the original mileage, but should I believe a 70 year old drunk?
 

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1970 Sedan deVille hardtop
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Unless someone has mechanically damaged the carb, a carb rebuild should have restored your fuel mileage. The internal parts don't really wear out because there isn't that much that even moves. All of my carb'd vehicles have older carbs than yours and work fine.
You've got an electronically controlled carb, so assuming the rebuild was done right, I'd look at the electronics. Make sure the coolant temp sensor, manifold pressure sensor, throttle position sensor and O2 sensor are all connected and working properly. If one is sending the wrong signals, the computer might be directing the carb to meter more fuel than nessacary. Does your exhaust smell of gasoline?
Also might check for fuel leaks. It doesn't take much to affect the mileage.
 

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1987 Fleetwood Brougham
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BluEyes said:
Unless someone has mechanically damaged the carb, a carb rebuild should have restored your fuel mileage. The internal parts don't really wear out because there isn't that much that even moves. All of my carb'd vehicles have older carbs than yours and work fine.
You've got an electronically controlled carb, so assuming the rebuild was done right, I'd look at the electronics. Make sure the coolant temp sensor, manifold pressure sensor, throttle position sensor and O2 sensor are all connected and working properly. If one is sending the wrong signals, the computer might be directing the carb to meter more fuel than nessacary. Does your exhaust smell of gasoline?
Also might check for fuel leaks. It doesn't take much to affect the mileage.
The fuel used to be leaking from the pump, but I put a new one on. As far as I know, all the fuel lines are tight and dry. The car really doesn't have any pep, so I think the carb either needs a tune or there is something wrong with the way it's metering fuel. I can't really smell any gas in the exhaust though, just that good old cat converter stink.

It's weird, sometimes the car has plenty of get up and go, sometimes it's a dog. Last night I found myself having to stand on the gas pedal to get the car up to 60mph, but other times it hauls butt up to 70 no problem. Fruity electronics in cars! They should be illegal.
 

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1970 Sedan deVille hardtop
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That power problem sounds like the secondary lockout not disengaging when the engine warms up.
Does it ever sound like it is idling too fast? I'm wondering if something is wrong with your choke. The choke not opening fully could cause all the problems you describe, and if your cat is functioning correctly it would probably be burning up all the excess fuel the motor is seeing.
Since the power loss is internittent, I'd check that the wire for the electric choke (the one that hooks to the round part that sticks out on the passengers side of the carb) is getting +12V whenever the key is in the "run" position. If you've got a bad contact there, or an intermittent contact, the choke will not work right and cause all sorts of problems, including loss of power, poor fuel economy, and bad throttle response!

If you want to get rid of electronics, better find an older diesel, because spark ignition kinda needs electronics. :) Diesels only need electricity to start (well, except the newer common rail diesels).
 

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94 Fleetwood Brougham
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Do you have the heater hose strapped to the choke housing? You MUST have that to properly operate the choke, especially for the secondary lockout to work. Choke should be fully off within 4 minutes (MAX!) and allowing secondary operation. 3.5m is the ECM timer for worst case cold time.
 

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Don't be intimidated by the carb on that car. I know it's considered "electronic" or "computer-controlled" but the fact is that there is only one simple electronic part on it, the "mixture control solenoid", and it only works with the primary side of the carb. Once the secondaries enter the picture, there's no electronics involved. So if your issue is with WOT (Wide Open Throttle) only, chances are the problem is NOT with the carb (as long as the secondaries are opening obviously).
Now beyond the electronics, there ARE a bunch of specific settings that are unique to the computer controlled Q-Jet, and these CAN make a huge difference in how the engine runs. Unfortunately it may not be fun finding a mechanic that actually knows how to setup a CCC (Computer Command Control) car. I've seen and/or fixed so many of these poor cars bastardized by people that had no business being under the hood it would make the GM engineers who devised the system cry! A shame actually because once you know how the system works, it's actually one of the simplest yet accurate systems out there.
On my fleet cars, I found that at approx. 120K miles, the Olds 307 REALLY needed a carb rebuild and a timing chain/gears replacement. Those two items when done correctly made the car feel like new again. My normal MPG on our cars was approx. 20 in combined use.
Another thing you might want to check is that the engine is getting into "closed-loop" (computer controlled) operation. You can verify this quickly by feeling the air-injection hoses to see where the system is putting air to. In "open-loop", the air should be going to the air injection manifolds connected to the exhaust manifolds, in "closed-loop" the air should be going to the catalytic converter. In "open-loop", the fuel system will be running damn near full rich and can cause a problem if it continues like that too long.
 

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Isn't that the truth! I have some neighbors who believe that if there is a computer on there it MUST be removed, as it MIGHT give them a problem someday and they don't know how to fix it. "I don't need no [email protected] computers!" They seem to think their tinkering will make more power and give them better mpg than a tuned up TBI or E4ME/E4MC CCC system! And I have fixed their little screwups more times than I can count and they still won'tllisten to me....

Of anyone who has experience in 307's, Katshot likely takes the cake. How many have you managed?

Katshot said:
...I've seen and/or fixed so many of these poor cars bastardized by people that had no business being under the hood it would make the GM engineers who devised the system cry! A shame actually because once you know how the system works, it's actually one of the simplest yet accurate systems out there../quote]
 

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Try this, disconnect the vapor canister purge line. Plug off at the canister, but DO NOT plug the bowl vent at the carb. (big line on top "air horn" section of carb, 3/8" line)

See if the problem is corrected. Bowl vent pressure can wreak havoc on a carbonator, as it is a pressure sensitive device.

Doesn't sound like electronics to me. That carb is 90% mechanical/pressure. Only the primary side fuel is ECM controlled. You can connect a dwell meter on 6 cyl scale to the green lead on intake and at idle, it should be reading 50% scale.
 

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Yeah, you could say I've had a few ;). You bring up a good point, the purge valve "could" be bad. When it goes bad, it allows you to suck raw fuel in through the vac. supply line and gives the engine a super rich condition. Usually when this happens, the car idles real rough and generally produces a fair amount of black smoke at the exhaust. I usually test for this by pinching off the vac. line with a pair of needle-nose pliers. If the idle smooths as you pinch-off the line, the purge valve is bad.
Oh, and as for reading the M/C solenoid "dwell", it doesn't necessarily need to be at or near 30deg. (mid scale as you said). As long as it's between 10 & 50 and "varying", it should be fine. The important part is the "varying". That proves the system is making proper cross-counts. Getting the dwell into the mid-scale area is best because it gives the system the most correction room but if the system is operating okay, it's not necessary. Besides, unless you have the proper tools and experience, I wouldn't suggest trying to adjust the dwell. The attempt would most likely produce less than favorable results.
 

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Settle down there cowboys, I'm actually pretty mechanically savvy, so you don't have to dumb it down for me. I do appreciate it though, you know, just in case I had no idea :D

Alright, so I'm pretty sure my '87 doesn't have an electric choke. I can't recall hooking up any wires to the choke, but I could be wrong. Never seen a heater hose run to the thing either. I doubt the choke is working at all, and if it is, it's WAY out of adjustment.

Regarding the dwell, I've done the check before. I have all the Q-Jet special sockets and a dwell meter, and was able to get the dwell to hang out around 30 on the scale. It didn't vary in any sort of reliable pattern, but the needle did swing a bit once the engine hit closed loop and I popped the throttle to take it off of fast idle. Worked a lot better after I replaced the O2 sensor.

I honestly feel that the car is lean, not rich. I get arbitrary knocking and the engine is uncommonly hot feeling. It gets so hot under there that my crossmembers will actually burn my hands if I've been driving it all day. I don't throw any lean codes, but that's how it feels to me. The idle is rough, but not all the time. Most of the time it idles so smooth I think it's stalled.
 

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The dwell should only swing in closed loop. Open loop should be relatively stationary. The dwell change is due to the ECM changing cycle times on the M/C solenoid in an effort to nail the 14.7:1 A/F ratio. The back and forth transitioning is what is called "cross-counts" on later EFI cars. I use the term here because it's basically the same thing, the O2 bouncing from rich to lean.
So you think the engine's running lean, huh? Why? Have you checked the O2 sensor output? That should help determine whether it is or not.
The Olds engines have a tendancy to get carboned up and develope carbon knocks in worst case scenarios. Have you tried doing a top-engine cleaner? They also have a MAP calibration issue that can cause detonation at light throttle loads. I used to counter this issue by simply porting off some of the MAP vacuum via an aquarium valve I installed inline with the sensor. As for the heat on the crossmember, might it just be normal since the Olds engine uses a crossover pipe the runs right next to the frame crossmember? Just a thought. Can't remember ever noticing my cars doing that but I guess it would make sense.
 

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I assume it was lean due to the lack of power matched with the knocking. You say the MAP sensor could cause knocking at light throttle, huh? It only happens at light throttle, if I just tap the pedal and go back to what I was doing, the knock goes away. Tell me more about the aquarium valve trick.

I don't have an ALDL cable to monitor the computer, but I guess I could drop $40 on one to check it all out. I don't smell gas or smoke black, so I don't think richness is my problem.
 
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