What's your A/F on Overrun?, 'Nother D-Jet A/F Question Options

[Mark Henry]

Does A/F on overrun even matter?

Overrun and AFR means diddly squat. No load, it doesn't matter how lean it is.
I have complete fuel cut on my programmable FI, you can't get leaner than that, it's never hurt anything.

If it were lean I wouldn't worry about it, but its real rich in the 10.0 + range. So somehow it is dumping fuel in there. it may even be worse but my low reading limit on the Autometer AFR is set a 10.0

granted I have not put many miles on this engine and am still sorting things out but after following a truck down a long hill with it going rich, when the truck stopped at the bottom of the hill to make a turn and I had to stop behind it, the engine just quit... it did start right up again, but since I am looking for problems this might be one.

Engine is a 2056 w/ Raby 9590 cam and Ham's RS+ heads, SS HEs and a Triad muffler. ECU 037

Oh sorry, brain fart...guess I should read things better. (IMG:style_emoticons/default/smile.gif)
My first guess is it isn't dumping any more fuel in, just you cut off the air so it makes the charge rich. I'd say it doesn't have any fuel cut at all, I don't know if that is normal for D-jet or not.
I do know L-jet definitely has a fuel reduction, but not a total fuel cut.

[914_teener]

It depends on the ecu for fuel cut off.

[Mark Henry]

It depends on the ecu for fuel cut off.

But it would be reading a sensor, either the TPS or the MPS.
Totally guessing but I'd say likely TPS.

[barefoot]

IIRC D-jet is designed to cut off fuel on closed throttle until rev's drop to about 1200. then re-start fuel. Must be fuel leaks somewhere, or throttle position switch not fully to idle position

[914_teener]

It depends on the ecu for fuel cut off.

But it would be reading a sensor, either the TPS or the MPS.
Totally guessing but I'd say likely TPS.

IiRC it is TPS Mark.

[BeatNavy]

My understanding is that the TPS detects an idle situation and then mixture is controlled by ECU idle settings, not by MPS. Some ECU's cut fuel off above certain RPM on idle, although I can't remember which ECU models off hand. But I'm not going lean, I'm going rich. Mark's explanation about less air would make sense. But that would indicate a fairly normal operation, right? I was under the impression that most people see pretty lean conditions on overrun.

[Olympic 914]

Rob

What ECU are you running?

You said that you checked your TPS and I have mine set per these instructions.

http://www.pelicanparts.com/techarticles/9...alibrate_20.jpg

[BeatNavy]

Rob

What ECU are you running?

You said that you checked your TPS and I have mine set per these instructions.

http://www.pelicanparts.com/techarticles/9...alibrate_20.jpg

Pretty sure I'm running a 043 ECU. Yes, I follow those TPS calibration instructions as well. My ECU is set for idle so I usually run 13.5 to 14.5 a/f ratio. I will verify which ECU I have....

[914_teener]

My understanding is that the TPS detects an idle situation and then mixture is controlled by ECU idle settings, not by MPS. Some ECU's cut fuel off above certain RPM on idle, although I can't remember which ECU models off hand. But I'm not going lean, I'm going rich. Mark's explanation about less air would make sense. But that would indicate a fairly normal operation, right? I was under the impression that most people see pretty lean conditions on overrun.

I believe the overun or "with overun cut off circuit" are listed on Brad Anders site in his part description matrix.

I think it.s function is three fold. Driveability, emissions, and fuel economy.

I know I have it on my car. You can actually feel it when coasting to a stop when the tps hits the cut off. That is.....when it is adjusted right.

[DRPHIL914]

My understanding is that the TPS detects an idle situation and then mixture is controlled by ECU idle settings, not by MPS. Some ECU's cut fuel off above certain RPM on idle, although I can't remember which ECU models off hand. But I'm not going lean, I'm going rich. Mark's explanation about less air would make sense. But that would indicate a fairly normal operation, right? I was under the impression that most people see pretty lean conditions on overrun.

I believe the overun or "with overun cut off circuit" are listed on Brad Anders site in his part description matrix.

I think it.s function is three fold. Driveability, emissions, and fuel economy.

I know I have it on my car. You can actually feel it when coasting to a stop when the tps hits the cut off. That is.....when it is adjusted right.

so you are saying the correct setting is for it to "cut out"? which would then cause the AF reading to go up considerably- which is consistent with my previous experience- when idle circuit is set so that the ECU knob will adjust a/f on idle , it is set in correct position, when it is, I will see the A/F reading go full lean, like 22. - how I have it set now it does not do that - but ive never seen mine go rich like a 9 or 10. I would recheck your TPS board at idle and does it effect a/f and idle when adjusting the ECu knob ?

[914_teener]

My understanding is that the TPS detects an idle situation and then mixture is controlled by ECU idle settings, not by MPS. Some ECU's cut fuel off above certain RPM on idle, although I can't remember which ECU models off hand. But I'm not going lean, I'm going rich. Mark's explanation about less air would make sense. But that would indicate a fairly normal operation, right? I was under the impression that most people see pretty lean conditions on overrun.

I believe the overun or "with overun cut off circuit" are listed on Brad Anders site in his part description matrix.

I think it.s function is three fold. Driveability, emissions, and fuel economy.

I know I have it on my car. You can actually feel it when coasting to a stop when the tps hits the cut off. That is.....when it is adjusted right.

so you are saying the correct setting is for it to "cut out"? which would then cause the AF reading to go up considerably- which is consistent with my previous experience- when idle circuit is set so that the ECU knob will adjust a/f on idle , it is set in correct position, when it is, I will see the A/F reading go full lean, like 22. - how I have it set now it does not do that - but ive never seen mine go rich like a 9 or 10. I would recheck your TPS board at idle and does it effect a/f and idle when adjusting the ECu knob ?

I am saying that when the engine is under no load....why worry about AF. Because D jet adjusts mixture by vaccum signature from the MPS ....under decelerarion with the throttle plate in the closed position....the requirement of fuel is not meaningful....so why measure it? My answer would be that you are wasting fuel.

The ecu adjustment knob is to set the fuel mixture at idle only. Once the throttle plate is moved that no longer controls AF.

[BeatNavy]

Just closing the loop. I actually have an 044 ECU, which has NO overrun cutoff circuit. So...based on that fact and Mark's suggestion that it's probably less air, not more fuel, resulting in a higher A/R mixture -- and would seem like behavior. But the point of my OP is that I'm trying to determine if my mileage is lower than it should be and, if that is the case, whether this could be a cause. Right now I'm guessing the answer is no...

Thanks for everyone's input.

[Dave_Darling]

Very early cars would cut fuel on overrun, but that function was eliminated. The literature I have read is "due to the uneven cooling in air-cooled engines, it produced higher emissions". I believe they removed it in mid-71 model year?

I can't access Brad Anders' site right now, but I suspect he lists when the change happened.

--DD

[Mark Henry]

I can't see a system that has no fuel cut having lower emissions.

My bet is with fuel cut the car had more engine braking, eliminating fuel cut might actually have better MPG as the car coasts more if it's still burning a bit of fuel. Likely VW eliminated the fuel cut to increase the cars MPG figures. Without coasting you would be on the gas more when cruising, because of more erratic speed control, thus burning more fuel.

I see this in my bug (T4 conversion) that has full fuel cut, my engine braking is huge.
In fact I could see the Joe average driver not liking how much engine braking my car has. You can't lug my car, I have to keep the rpm up around 3K as the engine braking action makes it a PITA to cruise at lower RPM's.
You definitely wouldn't like driving it in constant stop and go every day.

[DRPHIL914]

Mark I agree, this is what I was seeing and why I readjusted my TPS so that it wasn't cutting out like that, and its much more smooth to drive this way, I have a good idle, no issue with the cut out, my mileage is excellent so I will probably leave it this way. I may not be able to adjust the idle with ECU knob but I don't need to , as the A/F and idle are perfect, having rebuilt then tweeked the MPS to get it where its at - .
BeatNavy doesn't have this circuit so its most likely very similar to the way I have mine set but I am still wondering about his issue having A/F showing increased richness in that situation. Please repost if you figure out the solution-

[Mark Henry]

I was thinking more on what Dave said, although the emissions part doesn't make sense to me, the 2nd bit about cooling does. The rich fuel charge would definitely have a cooling effect on the engine heads.

I think this rich condition on decell is perfectly normal and what the creators had in mind.

[mgphoto]

According to Porsche/VW the fuel cut off caused the cylinder walls to get hot, no fuel cooling, when the fuel returned there were atomization problems causing the hydrocarbon count to go up, not good for the environment and counter to what VW was trying to accomplish with FI.

Back to the OP, I think you may have a problem with leaky fuel injectors, overrun is the highest vacuum, that could cause fuel to be pulled from the injectors causing rich numbers on overrun, as mentioned this will not hurt your motor at all but your fuel economy will decrease.


Check the condition of your spark plugs, color will tell you a lot.

[Frank S]

Did you ever find out any more about this ?

I am also seeing rich AFR numbers on over run, same as yours 10-11

Mine is a 2056 with djet and seems to be similar to your build.

running a 043 MPS with initial settings at the 037 specs since I have a 037 ECU, but it was much too lean at those settings. Now I am in the neighborhood of 13.5 - 14.0 at a 3000 rpm cruise but going down hills on over run it goes to the 10.0 - 11.00 range.

I did set the throttle switch originally and reset it when I had the throttle body off for another issue.

haven't checked the mileage yet and only have about 400 miles on the rebuild engine.

Did you ever fix the problem with rich AFR on over run?
I'm running the same engine specs and had the same problem.
Car idles great but due to the needed MPS adjustments it idles rich 11,8 AFR, this is also resulting rich condition below 2600-2800 RPM 12,3 - 12,7 AFR all range above 2800 RPM is perfect.
I was able to improve the rich situation by adding and adjusting (Opening at 15,5 Hg) a Decel Valve. This improves drivability in low RPM range and also improves the rich overrun situation.

Cheers,
Frank

[BeatNavy]

I played around with my MPS, decal valve, tuning and a host of other things since I originally posted this. Car runs great, idles well, and is blast to drive. But I still see AFR numbers in the 12 range during decal, and my MPG numbers are not great. For the most part, I've given up worrying about it much, particularly since I have plans for a different motor before too long.

All good (IMG:style_emoticons/default/beerchug.gif)

[Frank S]

AFR of 12 will not hurt the engine and is reasonable I think. What MPG do you get?