MPG question with L28ET w/z31 ECU

[Milenko2121]

I've been trying to chase down why I'm getting poor fuel economy (15 city / 19 hwy).

 

Just to get some background.

Stock junkyard L28ET with 220k on it (135 compression across the board, no leak down yet)

S14 transmission (0.759 5th) with a 3.545 r200

I cruise 70 around 2600rpm and my O2 reads around 14.5-15 AFR with around -12 vacuum

24.9 diameter tires (225/50r16) 16x8 0 offset

 

I'm hearing people get 25+ on stock EFI so I am a bit confused as to why I can't hit 20.

 

I'm planning on installing MS1 ECU that I have sitting around soon, but just wanted to understand a bit more as to why my MPG isn't the greatest.

[GeeZ]

Odometer issue? If it runs well, I wouldn't worry about it.

[jeffer949]

Ive gotten 29mpg on one trip when i was only driving 70mph. Same trans and diff as you. Much bigger turbo(he351cw) and bigger injectors with supporting mods. Im running MS1extra though. I did get like 5mpg once with all in town "spirited" driving. Had to hear the BOV every time i shifted. lol.  It could be a odometer that is off. You could have the wrong color speedo gear on the sender. 

Edited July 16, 2015 by jeffer949

[SleeperZ]

Ive gotten 29mpg on one trip when i was only driving 70mph. Same trans and diff as you. Much bigger turbo(he351cw) and bigger injectors with supporting mods. Im running MS1extra though. I did get like 5mpg once with all in town "spirited" driving. Had to hear the BOV every time i shifted. lol.  It could be a odometer that is off. You could have the wrong color speedo gear on the sender. 

 

Same here, if the ECU and engine are running well, this is appropriate fuel mileage.

[NewZed]

For every person who says they get 29 there's another guy who gets 20.  20 is the most common number I've seen.  That's combined city/highway though.

 

You're running an ECU designed for a different engine, one that should have EGR, and O2 sensor feedback.  Do you have an O2 sensor connected to the ECU or are you just using one for AFR readings?  Is EGR functional?  Are you getting any codes from the Z31 ECU?  Have you checked CHTS readings to be sure they're correct?  Are you using a stock FPR or adjustable or an FMU?  What is fuel pressure?  Did you put the system together or buy it that way?

 

There are lots of tuning tricks you can do to increase MPG.  A simple one is to reduce fuel pressure.  But that will have other effects also.

[SleeperZ]

No tricks, no codes, no emissions controls, I regularly got 29 mpg at 70 mph - Z31 ECU, stock L28ET in my S30.  If you get 20 mpg under similar conditions there is something not being accounted for.  That's 30% low.

[NewZed]

No tricks, no codes, no emissions controls,

I was asking the OP.

[SleeperZ]

I was asking the OP.

I understand, but it seems to me the idea is not to start modifying the system away from factory but to find out what is wrong, since the factory setup is capable of delivering the appropriate economy.

[NewZed]

He hasn't even established that his is set up right.  Each of my questions is relevant to determining if it is.  Doesn't matter if you or anyone else got 29 mpg.  Could be that yours is actuality set up incorrectly, down on power, but up on gas mileage.  Who knows.  All that's here so far is one Z31 ECU with 19 mpg, one with 29 mpg on one trip, and one "same here" (converted to "regularly").  There's nothing here to work with.  Let's get some facts.

[SleeperZ]

Believe me, mine was not setup incorrectly.  An incorrect set up will not return 50% better economy and normal driveability.  We are not entirely fact-free; fuel economy may be dependent on many variables, but it is not black magic.

 

However, I agree, more information is needed, especially ignition timing and tire size and inflation.  Those are critical, as well as a good clean air filters.

Edited July 17, 2015 by SleeperZ

[Milenko2121]

For every person who says they get 29 there's another guy who gets 20.  20 is the most common number I've seen.  That's combined city/highway though.

 

You're running an ECU designed for a different engine, one that should have EGR, and O2 sensor feedback.  Do you have an O2 sensor connected to the ECU or are you just using one for AFR readings?  Is EGR functional?  Are you getting any codes from the Z31 ECU?  Have you checked CHTS readings to be sure they're correct?  Are you using a stock FPR or adjustable or an FMU?  What is fuel pressure?  Did you put the system together or buy it that way?

 

There are lots of tuning tricks you can do to increase MPG.  A simple one is to reduce fuel pressure.  But that will have other effects also.

 

 

Engine was from an automatic so it has that cam.

No EGR

O2 is hooked up and functioning properly switches off at 3k rpm as well

ECU Isn't throwing codes other than knock due to it not being connected.

CHTS is working and functioning correctly.

TPS is adjusted correctly

Adjustable Aeromotive FPR set to 36 psi with hose disconnected

900rpm -19 vacuum idle.

Timing set to 18btdc

Cam timing is set to stock specs with a new chain(5k on it).

MAF tested good and is metering air properly.

Voltage sticks around 14.4v

 

I put the entire system together.

 

I posted tires before, 24.9 diameter inflated to 36psi all 4 corners(checked this yesterday).

 

My speedometer is accurate +-1mph pacing my buddies 2011 Audi S4 at 65mph, GPS, and getting pulled over going 49 in a 45.

Odometer I have not tested yet, but when I did my trip from Vegas to California it was around 330 miles, and my odometer was very close to this. I averaged 20mpg on the trip going 75@2800rpm for majority of the desert, and it was the same at 70@2600rpm.

 

Let me know if you need more info.

 

EDIT: fixed fuel pressure statement

Edited July 17, 2015 by Milenko2121

[NewZed]

It's a start.  

 

A few observations:

 

- 36 psi at 19 inches of vacuum is too high.  Maybe you were reporting two separate things.  Added a chart from the 1984 FSM.  36 is about right with the vacuum line to the FPR removed.

 

- there are six years of Z31.  Which year do you have?

 

- You can check your odometer via mile markers on the freeway at any speed.

 

- Extra resistance can make a difference on the CHTS circuit.   "Functioning correctly" is not the same as optimized for efficiency.  Always good to check the chart.

 

Just some small things that might add up.  There's also a test procedure for verifying that the O2 sensor is working right.  I don't know how bad the sensor has to get before you get a code, with the early ECU's.  Later ECU's like my 95 Pathfinder will tell you that the O2 sensor is bad.  Might not be so in early 80's.  But that's where you can lose a lot of efficiency, the ECU uses the O2 sensor to trim the fuel mixture as lean as possible.

 

Sleeper can help you with the fine details of getting it running best since he has one.  I only know generalities.

 

 

 

[Milenko2121]

Woops, I mean't to put -19 vacuum at idle on a separate line.

Fuel pressure is 36psi with the hose disconnected.

 

1987 Z31 Turbo California Spec ECU (I have two, no difference swapping between either) ECU connectors are also cleaned.

 

By functioning correctly, I meant that I had tested it according to the FSM, and it meets what FSM wants.

 

My O2 sensor is 3 months old bosch brand OEM replacement. It passes the FSM test and my Innovate LC-2 reads ~14.5-15 under 3k RPM during cruising and ~12-11 under WOT.

 

I'll see if I can get a better odometer test soon. Also, my method for determine MPG is I wait till near empty, fill up, write down odometer and how much gas was pumped. I use the best average over 4 fill ups (they're usually the same when I'm under similar driving)

 

Wouldn't my engine run rich even after warmed up if my CHTS was indeed still somehow bad?

 

Other than this MPG issue, the car runs perfect. Pulls hard (14.1@99mph with 10psi) and feels very smooth.

 

Thanks for your help so far

Edited July 17, 2015 by Milenko2121

[Milenko2121]

Just a question, could increasing my timing to 20 or 22 btdc effect my MPG? I think I may try this. I'm running 12psi intercooled right now which based on what others on here have, seems safe.

[NewZed]

The O2 sensor still seems like the place to focus.  Are you using the one for the L28ET, the VG30E, or the VG30ET?  The VG30ET ECU expects a different type than the other two.

 

Just asking.  I never really know what people mean when they say they "did the test and it passed".  

Edited July 17, 2015 by NewZed

[Milenko2121]

The O2 sensor still seems like the place to focus.  Are you using the one for the L28ET, the VG30E, or the VG30ET?  The VG30ET ECU expects a different type than the other two.

 

Just asking.  I never really know what people mean when they say they "did the test and it passed".  

 

What I meant by "It passes the FSM test" I had meant the Factory Service Manual that I downloaded from xenonz31.com for my exact model car the ECU came from. This is further supported by the fact it stays around 14.5 AFR while cruising under 3k RPM(z31 ecu switches over to open loop at 3k), and goes 11-12 while under wide open throttle.

 

The O2 sensor that I have installed in my car is the Bosch branded narrowband OEM replacement for a 1987 300zx Turbo installed at 12o'clock position. The Titania style O2 sensor. It uses a resistance rather than voltage to tell the ECU what the AFR should be.

Not only does it pass the Factory Service Manual test, it also is keeping my AFR's at 14.5-15 according to my Innovate LC-2 Wideband O2 Sensor with a 2 month old Bosch O2 sensor that it came with as a kit direct from Innovate themselves.

I installed O2 sensor 26" away from the turbo outlet(Innovate says 24" minimum), clocked it just before the 3 o'clock position as per Innovate's instructions, calibrated it properly according to Innovate's guide, and it has stayed consistent since I bought it. 

 

Let me know if you need more info, I tried to make it a little more clear so there's no confusion on my O2 sensors that I'm using.

 

EDIT:

I attached a copy of the test procedure from my FSM.

Edited July 18, 2015 by Milenko2121

[NewZed]

I'm just going through the basics.  Seems like you have them covered.  I'd separate the stuff that affects cruising mileage from things that don't.  Like "running 12psi intercooled" doesn't have much to do with cruising at 70 / 2600 RPM.  Then there are the basic tuneup items, like valve lash and ignition (clean plugs, no misfires, etc.).

 

That's about all I have.  Sleeper is the standard, maybe he's got something.

Edited July 18, 2015 by NewZed

[Tony D]

That sort of setup should net you higher mileage. My stock NA first series Ecu will return 22 on that same trip, in fact cross country. JeffP routinely got high 20's/Low 30's at 70mph cruising. He wasn't happy of it wasn't getting that. He drove that same route and up 395 to Reno all the time and got that kind of return.

 

You have touched all the bases, a key one being below the Ecu switchover point to open loop at 3K. I do not know if this is different between the 87 Turbo and 87 NA, but my Frontier was 3,500 and 80mph... The JWT conversions as I understand it use remapped N/A boxes.... This is only why I mention it.

 

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.

 

For a bit of contrast, my Blowthrough Triple Mikuni L28 on 83 Five Speed and 3.36 rear (correct diameter tires) returned 17 mpg in daily commuting duties through the canyon driven as you should drive through a canyon in a fat tired Z-Car. I could get 5mpg at a Track-Day or Auto-X, and generally it was returning 12-15 in town the way I drive. Long trips got maybe 18... It was admittedly rich safe. I would say the average you report of 20 would be OK for mixed driving... But for that LA Vegas Trip, "downhill" it should be better than that.

Edited July 19, 2015 by Tony D

[bradyzq]

Disclaimer: I've never played with a Z31 ecu. Having said that, if you threw a knock sensor code due to it not being connected, it is very possible you're running on a timing map for when the engine is knocking. This would of course kill fuel economy.

 

Can anyone confirm that these ecus do or don't work this way?

[NewZed]

bradyzq makes a good point.  The ECI control conditions are described in the usual place, including O2.