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Only getting 22mpg HELP!

7150 Views 79 Replies 17 Participants Last post by  Bumpysbro
Hi guys,

After reading the sticky on gas mileage, I've started to replace a few things that could be the culprit to my terrible gas mileage.

Let me start with the specs of my car.

Its a d15b block, d15b7 head, si headgasket (8.55:1 compression)
Automatic transmission
Oem everything.
Tires @ 40psi
No tuning modifications at all.
New NGK Spark Plugs and Wires

Before I was getting an average of 22mpg. I just changed the fuel filter with an autozone replacement and now only got 190 miles out of the next tank of gas!! I verified there's no leaks after the replacement.

I pulled the o2 bung in the cat (obd2 cat on obd1 car, so bung closing the second O2 sensor hole) and it was BLACK as was the exhaust tip.

This leads me to a bad O2 sensor which I am going to change tonight.
I honestly have no clue how to check timing at the distributor, or valve lash so I'll be looking those up shortly after I finish this post. I highly believe it could be the distributor as I noticed recently it was only being held in with 1 bolt. I've since fixed that, so no more oil leaks at least.

From there if anyone has some ideas as to what could be causing such a major drop in economy I'll appreciate the support. I'll also be checking compression and would love to do a leak down test, but I say that term to all the tool rental places around here and they look at me as if I have 3 heads.

Thanks again in advance.


P.S. I have an intermitent sputter while the car is warming up, I would go as far as to say it sound like a misfire. NO cel lights

P.P.S I definitely think I have at least a distributor timing issue after reading a thread on how to set it.
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A quick and dirty way to check for exhaust leaks is for someone to block the tail pipe with the engine running while another person listens for leaks.
Valve seals won't kill fuel mileage.

Broken rings will kill mileage.
It's starting to make more sense now.

Black fuel smoke goes with real bad mileage.

If cam timing is a tooth out it will run like shit if it runs at all.
Like bone says.

Also, a lot to rich can soot up an O2 sensor and stop it working. They can be cleaned with heat from a slightly lean butane or oxy acetylene flame carefully applied to the correct area.
Oil burning leaves shiny black smooth deposits on the plug, like a coat of paint. Excess fuel leaves flat dull black deposits on the plug like it was dusted down with powder.

As it ran with the cam timing out one tooth, you may have bent valves.

Cam timing being that far out will kill VE and compression. That will impact on fuel metering and combustion efficiency.

Ignition timing being that far out will kill combustion efficiency.

Try it with the timing fixed. Once the soot burns off everything, it might be OK
With OEM everything as per your OP, how can you possibly change the belt half a tooth, unless you mean one tooth on the cam being half a tooth on the crank.
Poor fuel economy due to the engine is normally associated with rich mixture, fuel leaks, retarded ignition, miss fires or poor compression.

Due to chassis it is most often due to dragging brakes but can be low tyre pressures, external accessories like wings, or very large wheels and tyres.
It depends why your running rich.

Is your fuel pressure correct.

Is your MAP sensor OK

Are your injectors leaking.

Is it the same ECU that ran OK before.
Is your AIT sensor working correctly.

Is your oxygen sensor working correctly.
Just because something is new, that does not absolutely mean it works.
Put the old cap and rotor back on.
Are you sure the rotor is aligned properly and all the way down.

Are you sure the carbon rod in the middle of the cap is in place properly and it's spring is OK
Do you know if the distributor shaft bearings are OK.

If you remove the dissy does it spin freely and quietly.
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