Lean Misfires at 2700-2800 rpm

[TurboBob]

Originally Posted by DoctorDex

If it is the CAS itself, you'd think it would be directly related to rpm or timing. My car misses at slightly different rpm points when in neutral vs. while in gear.

Could the timing belt be "flapping" causing erroneous CAS signals?

Has anyone reported this on a 2G car?

Does anyone have a 2G engine in their 1G car that could wire in the crank sensor?

B

[DoctorDex]

Originally Posted by TurboBob

Could the timing belt be "flapping" causing erroneous CAS signals?

I'll pull my timing belt covers off to see if the belt flaps. I'm not sure if its possible to see, but I'm not sure how else to check it. Any suggestions?

I've noticed that my miss is most pronounced at ~ 2700 rpm and it'll miss constantly at this point, whether I punch it in neutral or if I'm at cruise. I've also noticed that @ if I'm accelerating slowly through the 2600-2900 rpm range, I get an occasional sustained 4-5 counts of knock until I let off the throttle. Maybe its some sort of resonant frequency of the slapping belt causing the CAS to misread? Dunno, this issue has me baffled.

[badass3000]

TurboBob-I dont know anything about belt flap. OEM belt/tentioner ect.

I have been stairing at some new logs and have gone cross-eyed, but may have stumbled onto something.


First a Q for Bob, I have SC3 wired to LC1 wired to Palm on MMcD. All works good.
I substituted BATT to log the WB#'s and it does, BUT on the palm there is another green graph that follows the WB#s only more detail (ie larger scale) that cannot be turned off. Is that somehow comming from the SC3 analog inputs? Can I use that somehow?

Q2, I have EGRT going to 3 bar boost gauge sensor, MMcD is set to 3bar, MMcD reads real low numbers. WTF?

okay, my new logs show a steady 2800rpm cruize but with a few spikes upto 3200rpm, without changing TPS, pressure, or AF#'s.
What could cause rpms to jump so much?
My MAPECU sees this fast increase in rpm and tries to dump more fuel in, boging the engine and backfiring.
WTF!

[TurboBob]

Q1, I don't know... what version of MMCd are you running? Turn off the H button and look on page 4 for the extra sensors. Could be you have the wideband turned on there. The Batt volt substitution was really intended for the people logging with Pocketlogger or other logging system that didn't support custom sensors.

Q2, dunno. You can set analog0 on the scanmaster to display straight countgs (0-255) and see what the EGRT input is doing. I suspect a bad connection......

I really think we are fighting the ECU's desire to do a lean decel or fuel cut. Also I bet the coil dwell is real low.... (assuming they control coil dwell based on load). With airflow scaling for large injectors you run into these kinds of effects at light loads.

I have a plan to fix this, gotta work on it a bit. It will become a software feature in the MAFT Pro........ but the data may be a help to you guys running other hardware....

When this happens, you can give it gas and it clears up.......right?
It also clears up right at 2800-3000 RPM, right? Really light load, like in vaccuum.... is there a moderate load that you don't feel it at? Its fine at WOT..... right?

Bob

[TurboBob]

The ignitor (power transistor, PTU) feeds ignition pulses back to the ECU. My understanding is that this is for misfire detection. This signal also feeds the tach.

You have aftermarket coils that have the reputation for jumpy or sticky tach effects. Perhaps this is affecting the MAP-ECU.

Is your MAP-ECU wired to the tach signal or the CAS signal......

We always wire the MAFT Pro's to the CAS signal......

2800-3200 is a big jump........

Bob

[DoctorDex]

Originally Posted by TurboBob

I really think we are fighting the ECU's desire to do a lean decel or fuel cut. Also I bet the coil dwell is real low.... (assuming they control coil dwell based on load). With airflow scaling for large injectors you run into these kinds of effects at light loads.

This theory seems very plausible, however, I can't explain how this problem would come on gradually over a number of months even though my injector size has been the same all along. I dunno, like I said before, maybe I have a couple of problems which are compounding each other.

Originally Posted by TurboBob

When this happens, you can give it gas and it clears up.......right?
It also clears up right at 2800-3000 RPM, right? Really light load, like in vaccuum.... is there a moderate load that you don't feel it at? Its fine at WOT..... right?
Bob

When you give it gas the majority of the problem clears up. However, I still get a very noticeable hesitation at 2700 rpm even if I'm at WOT. However, the rest of the problem rpm range (2600-3000rpm) feels better. If accelerating, in part throttle, this hesitation is noticeable on the logs (both front and rear banks go a bit leaner, O2 trim jumps and I occasionally get some knock). This hesitation can also be felt and it also creates a noticeable jump on my stock rpm gauge. With that being said, I wonder if by throwing more fuel/power at the problem all were doing is masking the problem and the miss is just less noticeable?

[badass3000]

Originally Posted by DoctorDex

...When you give it gas the majority of the problem clears up. However, I still get a very noticeable hesitation at 2700 rpm even if I'm at WOT. However, the rest of the problem rpm range (2600-3000rpm) feels better. If accelerating, in part throttle, this hesitation is noticeable on the logs (both front and rear banks go a bit leaner, O2 trim jumps and I occasionally get some knock). This hesitation can also be felt and it also creates a noticeable jump on my stock rpm gauge. With that being said, I wonder if by throwing more fuel/power at the problem all were doing is masking the problem and the miss is just less noticeable?

this RPM jump has to be the problem, im going to try using the CAS signal as my RPM input to MAPECU and see what that does.
Where do loggers get their RPM signal? B/C i havent logged this with my palm logger only the MAPECU logger.

[DoctorDex]

I also see this jump on my stock gauge when I punch the throttle. To see it on the stock gauge it depends on the rate my rpms are increasing. I see it especially in neutral, often in 1st and sometimes 2nd and 3rd. On the stock gauge it pauses for a second when it hits 2700 rpm, misses, and then jumps to try and catch up. The miss is audible and you can feel it lurch a bit too.

[badass3000]

TurboBob, what CAS wire are you refering to? I tried pin 69

[DoctorDex]

Noticed something peculiar today.

While cruising at 2700 rpm and the car is misfiring I'm seeing a increase in manifold pressure measured (with the digital gauge on the Blitz DSBC). Manifold pressure bounces quickly from ~ -15 mm Hg to 0 and then back to -15mmHg. At one point it even bounced from -8 mm Hg to 0.13 psi when there was a severe misfire. No change in pressure was evident on my mechanical gauge.

Anyone have any suggestions for possibilities?