Running rich in boost with stock EFI.

[pallnet]

I thought I would throw this out there to see if anyone out there has a different idea then what I think, I like to hear what others think.

 

The car:

1990 240SX with a bone stock L28ET and L28ET EFI installed.

 

The problem:

I've been running the car on stock EFI parts but I've collected the parts from different sources. I hooked up my LM1 and ran the car to find that the car is too rich (AFR 10's) when you ease into boost, it boggs but if you hammer into it then the car goes into boost like a beast and holds strong (I'm running stock boost on this one) Mean while when you're just driving around the car runs perfect (AFR 12.5-14.5).

 

What I've done:

All of the tests the FSM for the engine calls for. I've checked my grounds to make sure I don't have high resistance since this is a transplant/hybrid.

 

What I suspect:

The AFM spring has been tamperd with (the cover was not sealed when it arrived in the mail. It's a used unit I bought off someone)

 

What I'm gonna do:

Tweek the AFM spring a tooth at a time and run the car till AFR's are right accross the board.

 

I'm open to any other thoughts.

 

Thanks

pallnet

[boardkid280z]

That's what I've been doing this past week. I swapped my second turbo motor into my 78 coupe and wanted to put it in completely stock (I used to have intercooler, Megasquirt). So I swapped everything over exactly from the '83, except for the 3" downpipe and exhaust. So the car is a:

1978 280Z with a bone stock L28ET and L28ET EFI installed.

 

My AFM from the ZXT was also not sealed, so when I installed it, I drove it a little and turned the AFM spring a little to the rich side since I didn't know if the PO might have loosened it to make the car run leaner and I wanted to be safe. I didn't have my wideband in the car till last week, so I found out I was running pretty rich, and the original tension on the spring which I had marked was actually pretty close to where I needed to be.

 

After a few runs (and a quick scare past what looked like a motorcycle cop coming the opposite direction going 80+ on a highway service road - 45mph limit - who then put on his blinker, and I floored it and "got away from him"), I got the AFRs where I wanted them.

Flooring it in 2nd and 3rd gear, the AFRs stayed in the high 11s most of the RPM range. They start out around 3000RPM (full 7psi boost in stock guise) in the mid 12s then drop to the mid 11s by around 3500RPM and then by 4750RPM the AFR is back up in the mid 12s. So if you plotted it, it's kind of a flat slope with a long dip in the middle.

Not exactly what we're looking for, but I don't know how to get it any better. I don't want to run any leaner than mid 12s anytime I'm at WOT.

 

Another thing my car does, which has gotten worse since I leaned it out with the wideband: it bogs down a lot below the boost threshold until it's warmed up. You have to baby it, or just let it warm up. I don't know if it's too lean now or what.

Even after the car has warmed up it still seems really hesitant below 2500RPM.

 

I do need a new cap, rotor and wires, so maybe that will help some.

 

If you plan on sticking with the stock EFI for a little while at least, I think you'll find an improvement by adjusting the AFM spring. You said you're around 12.5 to 14.5 when you're just driving around. That's really pretty reach, any time you're not over 50% throttle I think above 14.5 AFR is just fine, and when you're just cruising 16:1 or higher is normal. My car was running 14.5-17AFR when I'm just driving around, and that'll be much better on gas mileage too.

 

What AFR during boost are you going to shoot for? I found that after the adjustments my car was still pretty lean in low boost conditions (0-5psi, 75% throttle) - 13-15AFR. Until I got on the throttle, it didn't add as much fuel as I would have expected.

[evildky]

a bad or open O2 sensor will cause the engine to run a bit rich, a bad chts will casue the engien to run stupid rich, and the afm allows you to tune a/f a bit

[boardkid280z]

Oh yea, I forgot, I am running without a stock O2 sensor. But I don't think the O2 reading affects the fuel curve at WOT, only at partial throttle/cruise. So I guess that may be part of the problem when he's easing into boost.

[WizardBlack]

I am not an expert on the factory EFI, but I'd say you're either going to be spot on while at full boost or spot on for drivability, but not both. Megasquirt.

[pallnet]

Ok as of this evening on the way home from work this is what I have from tweeking a bit here and there on the AFM. I have a cruising AFR of 14-16 and going into boost it will richen as it's supposed to, I can watch the climb of AFR richness as VAC drops and i get closer to boost. Right before I jump into boost the AFR is around 10-11. When I'm in boost it seems it's getting too rich in boost because by the time I hit 7psi I'm running AFR's in the 9's.

I did notice the samething you said, when my car is cold I get some flat lean spots here and there as I accelerate from dead stops unless I baby her till she warms up. Once she's warm she runs pretty good but I am going to mark where I'm at right now and give a little bit more of a tweek to see if I can lean out the boost a bit, this might smooth out that rich spot going into boost, it's like shifting the whole map up a little.

My O2 checked out fine when I installed it, I will check it again to be sure it's not a factor. If I continue to have issues with the stock turbo EFI then the stock EFI's days are numbered.

 

Here's a few pics of the car if anyone was wondering what it looked like.

 

 

 

 

Thanks

pallnet

[WizardBlack]

Definitely get the AFR's above 10.5:1. Below that becomes a concern for the engine. It can wash the oil off of the cylinder walls and damage them. The excess fuel can also make it's way into the oil and thin it out for the rest of the engine, too.

Likewise, you can start to experience spark blowout. It will feel like someone is randomly bumping the brake pedal while you are at full boost.

 

At 7 psi you can easily be up in the 12's. Just because it's boost doesn't mean you have to be at 11.0:1. I typically tune cars so they taper from normally aspirated AFR's (13.5:1) at 0 psi relative down to the richest you go (say, 11.5:1; it varies with the engine) at about half of full boost. Running 11.5:1 at 1 psi is just wasted fuel and power. It doesn't need that much fuel to avoid detonation and keep the chambers cool. Get it to taper smoothly and you'll have better spool and partial boost power.

[pallnet]

Sir

The goals sound just like what I was doing when I ran SDS on my Z, it's been a while but I found my old charts and maps that I made.

The biggest problem is that I'm running stock EFI and there is very little in adjustment that can be made without affecting another part. This evening I made a run to pickup my little girl right after work, it's a good 1/2 hour interstate drive each way. On the interstate I was cruising a good 14 all the way there and all the way back. I'm thinking I can tweek the clock spring a bit more (tighten) to lean it out a bit. In boost it's still 9's & 10's and kinda falling on it's face here and there. I really want to solve the turbo EFI issues I have but then again I'm thinking about just tossing the SDS on and spending my time tuning that instead.

Thanks

pallnet

[HowlerMonkey]

Playing with the spring tension won't do much good for richness while on boost since the air flow meter is pegged all the way open long before you start getting boost.

 

The stock tune is very rich at boost but obviously gets a little less rich at the higher rpms.

 

I believe the RPM at which the L28et "closes it's mind and starts ignoring sensors" is about 3500rpms from my monitoring on the highway and reading the turbo supplement of the factory service manual.

 

Kind of sucks knowing the the 280zx turbo was designed when the speed limit was 55mph since the stock (auto) configuration nets you rpms at the new speed limits that puts your car just slightly over the rpm threshold at which the ecu starts ignoring sensors..........unless you're running 3.36 with the auto or a manual tranny with 3.54 or lower gears.

[boardkid280z]

Sounds like you can do a lot of adjustment to the AFM spring towards lean. 14 is richer than stoich and, honestly, it's too rich for cruise, you're just wasting fuel. Idle should be 14.5 or so, but you should really aim for drivability and fuel consumption.

 

At cruise you should be at least 2 points leaner (16 instead of 14) and could easily be 3 points leaner (17).

In boost you should be at least 2 points leaner (11 instead of 9) could easily be 3 points leaner (12).

The only thing changing the spring affected for me was driveability before the car was warmed up. You said you noticed lean spots with a cold car, and that will likely get a little worse, but once the car warms up, you'll be fine. So just let your car idle a few minutes before you leave.

 

Your car is slower than it could be and you're wasting fuel by running it as rich as you are - lean it out. Remember 17:1 is fine for cruise as long as it doesn't affect driveability and 12.5:1 is great for boost unless you're having other issues.

[boardkid280z]

Playing with the spring tension won't do much good for richness while on boost since the air flow meter is pegged all the way open long before you start getting boost.

 

I'd be interested in seeing evidence of this. I don't think the AFM is pegged before you hit boost, and I think adjusting the spring tension changes the final position of the potentiometer in the AFM (changing the amount of fuel at full boost) - whether the ECU is still reading inputs from the AFM at full boost is another question. I thought it just stopped reading inputs from the O2 sensor after a certain RPM.

[HowlerMonkey]

I no longer have a flapper to confirm this but I seem to remember that the flapper even on non-turbo cars pegs full open most any time the throttle is all the way open...even at relatively low rpms.

 

One thing to consider when tightening the spring is that you are defeating one of the benefits of fuel injection which is the lack of a restrictive venturi in the carbs it replaces.

 

Anyone here can run put a meter on the terminals of the afm where it returns the resisted 5v reference back to the ecu and the sensor ground and then turn on the key and push the flapper completely open to read the full open voltage returned to check this.