MPG question with L28ET w/z31 ECU

[Milenko2121]

 

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What is your timing compared to stock? 18 seems off a bit. It will make a difference, but incremental...shouldn't be an earth-shattering change.

Fuel Pressure and Valve adjustment are related, if you have low vacuum due to tight valves it will change the injection but the O2 has a 10% correction available to get the mix back right. Your second gauge shows that's the case, but the times in open-loop you spend are definitely consuming more fuel than it should be...so it's worth getting right. 12" does seem low for highway cruise by about 3" compared to mine.

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Stock timing is 20, I was running 18 because I added psi and was worried, but I just bumped it back to 20, 2 degrees shouldn't be a 5+ mpg difference I'd think anyway.

 

I also have the injectors wired correct for the batch firing, and flipped inj 5 and inj 3 on the harness as well.

 

I'll do another valve adjustment today and see if I can pull more vacuum on the freeway.

 

 

And update on the codes.

I had the knock sensor wrong code, I forgot I fixed that awhile ago.

It's only throwing code 41 which is fuel temp sensor which from what I read, won't throw it in limp mode.

Edited July 19, 2015 by Milenko2121

[bradyzq]

Try free revving to say 3000rpm and measure timing advance. That will give you an idea if things are OK off idle.

 

Also, if you have an exhaust leak between the head and the O2 sensor, the ecu will add fuel to correct the perceived lean condition.

Edited July 19, 2015 by bradyzq

[Tony D]

Put a resistor to tie the fuel temperature low (the resistance is in the FSM)... It's a correction to fueling.

Hot fuel, more pulse width, cold fuel less pulsewidth.

 

JWT ties it in the harness mods with a resistor. I don't know why they don't actually USE the fuel temp sensor...not that hard to incorporate.

[SleeperZ]

Do you have an air dam?  Aero effects are going to be more noticeable at 70mph.  How are your tires wearing?  Excess camber and toe can affect mileage by a good amount too.

 

It's sounds like engine timing is close, verifying the advance at 3000 rpm sounds reasonable.  I also used the fuel temp sensor, but I doubt that affects the fueling very much.  I did not have a knock sensor, if I hooked it up, I immediately got the knock code.

 

Unless your air filter is plugged, that's about all I have.  I was on a road trip once where my mileage dropped below 20mpg, and noticed my air filter was very dirty.  Replacing it immediately restored the mileage.

[SleeperZ]

One more thought.  What is the condition of your injectors?  If you have one clogged it could throw off the whole engine while running closed loop as the other injectors will have to flow more to compensate for the plugged one.

[jeffer949]

My "one trip" statement is based on that was the only trip I took on that setup going on roads where I wasn't going 80-85 mph. The same setup went 12.8 at 110mph in the quarter mile so I'm pretty sure it was running well. 

 

Quick question. You state you have the hose off on your FPR. Does the z31 not use a vacuum/boost source to raise/lower the fuel pressure in accordance with pressure? 

[bradyzq]

Quick question. You state you have the hose off on your FPR. Does the z31 not use a vacuum/boost source to raise/lower the fuel pressure in accordance with pressure? 

 I _hope_ that is was off only for base fuel pressure verification.

[NewZed]

A few more things, that get overlooked but could explain why some get high mileage and some not so high.  

 

Altitude.  Higher tends to give better mileage, in my experience.  I lived at 5,000 feet for a year, and 300 feet for four, with the same car. But if you try to find supporting words on the internet you'll get way lost in numerous bizzare explanations of what happens at high altitude.  The most general explanation is that high altitude air is at lower pressure (fewer air molecules per unit of volume) which means less air enters the engine for each cycle.  So, basically, the engine is effectively smaller and creates less power.  Power comes from fuel.  Do a Google if you want to be entertained though.

 

Engine temperature.  Hotter tends to give better mileage.  Probably for a combination of reasons.  Again, the Google will entertain.  Thermodynamics and engine theory is hard.

 

Air temperature - summer vs. winter.  I've had my car long enough to see ~18 mpg in the winter, and 20-21 in the summer, consistently.  1978 engine, 1976 system, no EGR, essentially stock.  This might just be from the fuel maps in the stock NA ECU.  But 1987 is still fairly primitive.  Could be a combination of air and engine temperature, but my cooling system seems to do its job well.  Seems mostly air.

 

 

One other thing I wondered about, that I've seen overlooked in these swaps, is the wire cleaning step for the MAF.  They can get dirty.  Don't know much about it though.

[SleeperZ]

Good point about altitude.  The main thing is less air resistance at highway speeds.  The intake air is always metered and during cruising the oxygen sensor will maintain the right mixture, so the thinner air does not significantly affect combustion.

 

But my high 20s mpg at 70mph have always been at 5000+ feet.

[NewZed]

It's not combustion that's the issue, but fuel quantity available per cycle.  Not often thought of as "fuel" but oxygen is part of the equation. With good engine management, less oxygen used means less gasoline used. 

 

Of course, that's what turbos are for, so it may be moot.  Depends on boost level, I'd guess.

[Milenko2121]

Thanks for all the tips and help.

 

I will replace my air filter and see how that helps. Mine is a bit old.

My MAF is not wired for self cleaning. I cleaned it recently using MAF cleaner to see if it would help, but not much changed (voltage reading was similar).

No gasket leaks.

I'll see about getting the fuel temp sensor resister wired in, not sure how much this helps.

My injectors are a little over a year old, I bought them brand new from O'reilly. They're bosch branded. I've never flow tested them.

My spark plugs all look very similar as well.

Altitude varies, i'm in Vegas which says around 2100ft.

I have a stock front airdam.

[TimZ]

It's not combustion that's the issue, but fuel quantity available per cycle.  Not often thought of as "fuel" but oxygen is part of the equation. With good engine management, less oxygen used means less gasoline used. 

 

Of course, that's what turbos are for, so it may be moot.  Depends on boost level, I'd guess.

Probably the rest of this explanation is that because of the lower atmospheric pressure, you have less power available, and require a larger throttle opening to get the same power at cruise.  Larger throttle opening means lower manifold vacuum, which means lower pumping losses for the engine.

[Chickenman]

At low throttle openings and light load, an engine requires more Total ignition advance. Typical cruise numbers can easily be around 45 degrees BTDC.

 

How does the ECU compensate for light load factor and additional engine timing? Does it use a MAP sensor? MAP sensor may be faulty, or mismatched.

 

On a NA L28 ( or L24, L26 carburated cars ) with a vacuum advance pot, the vacuum advance adds additional ignition advance at engine loads. Having the vacuum advance disconnected can lower engine fuel mileage by 5 mpg easy.

 

More modern ECU's eliminate the mechanical advance and vacuum advance of the old dizzy's, but still calculate ignition advance by RPM ( mechanical advance ) and Load ( vacuum advance ) . They are dependent on the MAP sensor for that. A faulty MAP or incorrectly matched MAP will affect the LOAD values and thus Mialge.

 

Might be an idea to check out the MAP sensor carefully. Make sure it is accurate ( Test with DVM and Hand Vacuum Pump ) and properly matched to the ECU. By properly matched, I mean don't use a 3 or 4 BAR MAP when a 2 BAR would suffice. Resolution goes down on the higher BAR rated MAPS.