DanielRicany

Hey guys. 1985 Corvette MAF voltage dropping at high RPMs causing a lean condition at high RPMs at full throttle.

What are the possible causes? Doesn't happen any other time.

bjankuski

Please explain what you are seeing? The MAF may drop in airflow if you have exceeded the airflow potential of the engine. Peak torque RPM is the highest VE that an engine sees (Which is the greatest cylinder filling). After peak torque the VE starts to drop and there is a point where the airflow actually starts to drop because the combination of engine parts is not designed to operate at the higher RPM and the air can no longer fill the cylinders in the short amount of time that the intake valve is open. So I would like to know what your scans are showing because this could be a normal situation.

DanielRicany

You do have a good point, however the Miniram that I have usually produces a nice flat torque curve. When I floor it, the torque "feels" consistent, doesn't drop off, (not an accurate way of measuring I know), however there is a huge drop of gms/sec, about 60 gms/sec from 3000 to 5500 RPMs. And then again about 230 gms/sec drop from 4500 to 5900.

I believe you saw my log in my other post.

How can I confirm that this is the case and not a malfunctioning MAF sensor? Vacuum gauge at WOT? An alternative method of measuring air flow?

Because it also seems that no matter how much more fuel I add to the higher RPMs, the AFR is not fattening up. The only way that it did fatten up was when I increased fuel pressure. It dropped from 14.3:1 at 5500 RPMs with 35 PSI no vacuum, to 12.5:1 at 5500 RPMs with 50 PSI no vacuum. When I made the fuel pressure change to 50 PSI, I also reverted to the stock WOT fueling vs coolant temp, and I kept the 20% enrichment at 4000-6400. The AFR at low RPMs at WOT is now about 10:1.

wydopnthrtl

Your double dip MAF count drops are not normal. If it was happening only on the top end then maybe its maxing out.
When I've maxed out a MAF my counts would just flat line and the A/F would get leaner on a linear slope. And it was a slowly rising slope too. IMO if a guy is maxing out the MAF w/only 500rpms to go.. don't sweat it. More than 1000rpm and I'd add a MAF signal modifier (mafia) and re-map the trans func at low rpms.

Easy thing to do is swap the MAF for a known good one.

Is it repeatable or is it sporadic?
If sporadic I'd clean and use dielectric grease on battery & chassie grounds, and all wires associated with the MAF & ECU. I'd also change the fuel filter.
If repeatable either the sensors are reading correctly or not.
If correct.. (unlikely) is there something in the intake (pre-MAF) causing an uneven flow through the tubing and MAF? Is there a bend or two right before the MAF? Personally I like to see at least 10" of straight before a MAF.
If not correct... data log each sensor to help narrow it down. If that's not possible you might try turning off inputs one by one to find out which one is trigging the bogus input.

My guess is either a dirty electrical connection or a bad MAF.

bjankuski

Good points,
I would like to see a longer data log in a higher gear, since the 1985 computer has such a slow update rate I am not sure if the drop in airflow is actually happening. I am wondering if the throttle has snapped shut during the middle of the scan where it still shows WOT on the TPS but the throttle is actually closed resulting in the low air flow reading.

DanielRicany

I doubt it. I picked this up on 3 different occasions.

DanielRicany

And I only made 3 WOT runs in that log.

tequilaboy

MAF signal vs rpm should resemble a hp curve and eventually peak and begin to fall.

Might want to verify your units/scaling. Are you referring to counts (a/d * 7)? Are you using the A20.ads (or adx) file with tunerpro or something else like WinALDL?

A log would certainly help to explain.

How can a maf signal with 255 gm/sec maximum drop 230 gm/sec between 4500 and 5900 rpm. Something doesn't add up with your data. If so, the load value would drop very low also, along with the injector pw. Are you seeing a similar drop.

DanielRicany

The injector pulse width follows the MAF gms/sec.

Here are two screens shots of my data log at WOT.

Refer to Column A for frame number.

Column C is Injector Pulse width

Column D is TPS

Column E is MAF gms/sec

Column F is Engine RPM

Thanks

tequilaboy

Your intake duct may be collapsing.

DanielRicany

That is a very good idea. And these results are not normal, correct?

And if my intake duct was collapsing, wouldn't my AFR stay steady all across the board?

tequilaboy

It depends...If the load falls far enough (< 40) you may drop out of PE. This could explain the lean AFR.

If it remains in PE, then it should stay on the rich side, despite the drop in flow and power.

If you have a long run of ducting without internal support, there is a good chance it is collapsing as a pressure difference develops as flow increases.

DanielRicany

Would I be able to confirm this with a vacuum gauge?

Also, I think the load would be under load variable in the data log. However, the highest my load variable ever went to was 18...

tequilaboy

Yes. If the duct is collapsing, vacuum should increase similar to closing the throttle.

DanielRicany

When I had my fuel pressure gauge taped to my window, the pressure dropped both with the vacuum hose connected to the regulator, and a second time around with the vacuum hose disconnected. It went from 43 to 49 and dropped to 47 the first time around as the RPMs increased. And with it disconnected, it went from 50 to 47 when I floored it.

My fuel pressure is set at 50 PSI. So, if it was collapsing, shouldn't I have seen more of a fuel pressure decrease with the hose connected compared to it disconnected? Especially with that much drop in gms/sec?

Thanks.

bjankuski

-Do you have an auto car or stick?
-You may have some unusual flow restriction at high RPM but you should feel that, it would be like you turned off the car when the intake duct collapses.
-After studying your data I still am skeptical that the data logs are long enough to give us a good clear picture, but based on what I see you are out of injector. I believe you are going lean because your injectors are maxed out. The data logs show a pulse width of 12.07 ms at 4475 RPM, by 6000 RPM you only have 10 ms of time to inject the fuel. (The 1985 car operates in double fire mode, which means it fires the injectors on each side of the engine every crankshaft revolution or twice per camshaft revolution) Since you are already at 12.07 ms at 4475 RPM I believe you will be above 10 ms by 6000 RPM. I actually believe you will start to run lean after 4500 RPM, when you factor in the injector response time of about 1 ms you exceed the amount of time for injection of the fuel at around 4600 RPM if you assume similar airflow per cylinder, that we saw at 4475 RPM. A mini ram intake typically flows near the same amount of air per cylinder from around 3000 RPM to around 5000 RPM, then it starts to slowly drop off. If I was looking at you fuel curve and it shows a near linear leaning of the AFR from 4500 RPM to 6000 RPM that would tell me the injectors are at 100% duty cycle or maxed out.

DanielRicany

The car is a 4+3.

It doesn't feel like I turned it off, it feels like it wants to keep going. It doesn't fall off even though it lacks fuel up there.

And also, using this calculator, at 10 ms duty cycle at 6000 RPMs it comes out to 50% duty cycle. At 15 ms at 6000 it comes out to 75% duty cycle. http://www.stealth316.com/2-calc-idc.htm

bjankuski

Daniel, that calculator does not know that you are running at double fire mode which means you only have 1/2 of the time available to do the injection. As I stated before with a scanned PW of 12.07 seconds at 4475 RPM you are already at 90% duty cycle, factor in the 1 ms response time and you are at 99% duty cycle. the scanner is reading how long the injectors are open at each pulse, since you have two pulses for every firing cycle you need to double all of the PW numbers to see how long your pulse width really is. At 4475 your actual pulse width was 24.14 ms.

If the car is not falling off in power at high RPM I will go back to my original statement that your scans are being influenced by you shifting gears and the slow scan time of the 1985 computer, that is why we need a third gear or fourth gear pull, to see what is really going on.

cumbercr

I'm sorry, but I have to say this again. In the first chart you were clearly decelerating. TPS went from wide open to closed and your RPMs dropped. Pulse width should have dropped. The fact that you don't see both events on the same frame is likely because of the slow data rate from the ECM. I would call this a normal condition.

DanielRicany

Through all the cells that are highlighted in blue, the TPS voltage is 4.78 volts for all of them. 0.53 on that data log is closed throttle. Anything in the "affected zones" was WOT.