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Unread 02-27-2009, 12:20 PM   #1
mendelmax
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4.6 Stroker fuel consumption problems

Hi fellows.

As I have written some time ago, I have made a 4.6 stroker that is now fitted into 93 ZJ. It uses stock 21LBS, 39PSI injectors, but with adjustable fuel regulator, set at 45 PSI. It's got only 40 miles on it, so it's still breaking in.

The O2 sensor (both the old and brand new one- we changed it today) indicates perfect fuel trim, so does the emission test, and the overhead console reads 14 MPG (I know it's more since the fuel is under higher pressure, but assuming 10% pressure increase I can imagine only 10-15% EVIC mistake).
The problem is that the real fuel consumption varies somewhere around 5.5-4.5 MPG. Something is obviously wrong, but I have no idea what may be the source of the problem. The fuel gauge needle moves amazingly fast. There are no visible fuel leaks, nor sensible vapors. The engine runs silky smooth and there's no visible smoke. From time to time, the AFR get's a little richer, I mean it fluctuates a bit, but nothing big enough to cause so much fuel waste.

Does anyone have any suggestion what could be wrong and how could I check that?

Any idea, even the most unlike ones very appreciated

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Unread 02-27-2009, 12:32 PM   #2
StrokedWJ
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Are you just basing your AF readings off of the stock 02 sensor behind the cat or did you add another one? Have you had it on a wideband 02? What kind of specs did you build the engine with? SCR, DCR, quench?

You might have better luck asking this over on jeepstrokers.com. HTH!
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Unread 02-27-2009, 12:45 PM   #3
mendelmax
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It's OBD I, therefore it has only one, pre-cat 02 sensor. We were fine tuning it using this sensor with AFR gauge, but when we noticed something is wrong we went to emission test center and they checked it there. They said it's perfectly fine, though I'm not 100% sure how their analyzer works.
But, I guess, that so huge fuel volume difference would throw any o2 sensor out of it's band. I'm really confused.

The SCR is 9.81:1, DCR is about 8.55, quench 0.091". As I said, it doesn't ping, runs smooth, has great power. If I wouldn't see the gauge dropping I wouldn't believe anything is wrong.
I can only suspect too rich cold-starts, because we haven't done much driving at once, just short trips, where 02 sensor is not considered by ECU, but on the other hand, the AFR gauge then shows it's rich, but not that rich to get 5MPG...

Thanks a lot for the link, I forgot about that site. I should have post there at first place.
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Unread 02-27-2009, 12:52 PM   #4
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Are you determining the 5mpg # manually? ( as in fill, drive, fill, compute? )

I would have to agree it would have to be running excessively rich to get 5mpg....there would have to be visible fuel out of the tail pipe.

I'm wondering if there's something wrong with the fuel gauge....
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Unread 02-27-2009, 01:06 PM   #5
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So far it has been determined basing on gauge drop- I haven't had a chance to fill it up yet, but I will do it tomorrow, and then we will see precisely what the results are. I have already reduced injectors pressure a bit, so I hope this can reduce the problem a bit.

The fuel gauge worked perfectly fine before the swap, so I wouldn't consider it as a source of problem, but of course it will be checked tomorrow at fill-up.
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Unread 02-27-2009, 01:17 PM   #6
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Just dawned on me also....OBDI computers run open loop until they get a certain amount of data ( 20 miles of driving is in my head ) this might also explain the numbers you're seeing....
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Unread 02-27-2009, 02:55 PM   #7
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That could also be a good explanation, since we have unplugged battery twice, so it didn't had a chance to run over 20 miles at time. Still, mixture enrichment should be detected by AFR gauge, wouldn't it?

Does anyone know, how o2 sensor accuracy develops during engine run? I mean, when the readings are precise? Immediately after startup or maybe a bit later?
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Unread 02-27-2009, 03:46 PM   #8
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OBD II computers run in open loop, too. Both run in open loop during warm up and WOT.

Have you even run through an entire tankful since the rebuild? I wouldn't just fuel economy based on gauge drop, and I'd be hesitant to base it on a single tankful, too.
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Unread 02-27-2009, 04:48 PM   #9
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As I said, we will fill it first time tomorrow and count MPG precisely, but come on, no way engine could have that bad fuel economy, even during break in Even if the fuel gauge is waaaay off, it still drops much, MUCH faster than when 4.0 was in. Again, it's not a matter of wasting one tank of fuel- I don't care about it, since I know it will stabilize after some time. I'm only afraid whether it won't hurt the engine.
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Unread 02-27-2009, 06:09 PM   #10
Chrisfol82
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Is it a wideband 02 sensor your using or a narrow band? And i assume you have an AFR gauge in your dash that your refering to. If its a narrow band it wont really give you a very good idea of how much too rich you are, it will just tell you that your richer than stoich. If you have a wide band it will give you a more accurate actual air fuel ratio. So if you have a wide band and it looks like your only going a little rich it might really be going alot more rich but you cant tell with a narrow band sensor.
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Unread 03-01-2009, 01:48 PM   #11
mendelmax
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Ok, I'm a dumbest stroker builder ever

It appeared, that the locking screw on the fuel pressure regulator was not tightened enough, therefore engine got more and more pressure over time thanks to vibrations. This was fooling the computer alterations- it shortened times and got more and more pressure. The pressure when we checked again was 70 PSI...
That combined with running on closed loop made so much fuel consumption during first miles. Then when we checked the AFR at diagnostic station the engine was already in closed loop, not moving (no vibes, so no more pressure) and shortened the injection times.

That combines with a narrow band o2 (I didn't know about it) fooled us because we thought it runs only a bit too rich.

After setting everything up. driving 20 miles to let it learn, refuelling and driving another 60 miles to check the MPG it averaged 13 MPG. Not bad for engine breaking in with 4" of lift, 32" tires, snorkel, winch etc.

I feel sooo dumb, shame on me
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