BAD "miss" at 2200-2500 RPM

[portmod7]

HI guys,

I have an MS-II v3.0, 2.905 code. It is on a Gen-I small block Chevrolet 5.7L. I am running an MSD Street Fire distributor (http://www.street-fire.com/GM_EFI_distributor.html) with an 8-pin module and "Dual Connection" (PN 5526) external coil.

I have a BAD "miss-fire" starting at 2200 RPM that lasts up until around 2500 RPM. It's so bad that my ECU actually thinks that the RPM is 0 sometimes. It will clean up above and below that RPM range for the most part. It will "dry-rev" with the transmission in neutral just fine. It also stumbles and hesitates on acceleration at various ranges of RPM, but for right now I'm assuming that's more to do with the fueling calculations and acceleration tables. I've only had this project driving for a few hours now, I really haven't done any tuning.

The other issue I have, that may be related or may not be, is that when I hook up my timing light to check the timing, it lights very intermittently. The engine seems to be running smoothly, not missing, but for some reason my timing light isn't coming on regularly like it has on many engines in the past. I have meant to borrow a friend's light to see if it's just MY light that is messed up, but I haven't done that yet.

Startup at all temperatures is great(it's been pretty cold around here lately), idle is great, constant throttle above or below the 2200-2500 RPM range seems to be fine, but the 2200-2500RPM range is brutal.

Also, when you look at my datalog, you'll notice I don't have an O2 reading... I have not yet figured out how to get an O2 reading and not have it interfere with the fuel calculations. Right now, if I have the O2 influencing the fueling, the engine surges really badly (another bug I need to work out of the tuning). If I disable the O2 and run "open-loop" I believe it's called, the engine actually runs pretty well. It's on the rich side I think, but it will definitely get down the road that way.

Thanks in advance for any help!

[portmod7]

Ok. I've been doing some digging and in the assembly instructions (http://www.megamanual.com/ms2/V3assemble.htm), step 50a. viii, it states:

"If you later find you have an intermittent tach signal on the vehicle, but it doesn't work at all speeds or all temperatures in the car, check the Hall circuit modifications(http://www.megamanual.com/ms2/inputHEI.htm)".

On the Hall Input Modifications page it states that; "This (intermittent tach signal) may happen at lower rpms, or at higher temperatures, and seems especially prevalent for 7 or 8 pin HEI modules." I drove the truck again briefly after work, and while the engine was cold I definitely thought to myself "it's not so bad when the motor is still cold." Then I got home and started reading about this, it seems to fit my situation pretty well.

On the Hall Input Modifications page it has three different options. From what I understand, I currently already have the first option on my setup because I purchased a DIYAutoTune harness with my MS-II, and that harness already incorporates the twisted, shielded pair as described.

The second option seems like my next best bet because the start of the instructions for option #3 state: "If the MSnS-E mod (option #2) is not sufficient, you can add a transistor to the circuit (option #3)." So I am thinking about trying option #2, the MSnS-E modification.

Does anyone have any input on this, if I should bother trying it or not? It's the only thing I've found so far that really seems like it applies to me. I thought about trying to switch the input trigger in TS in case I got the VR pick-up wires switched by mistake, but I really don't think that's my problem.

Thanks!

[Matt Cramer]

We've had good luck on our Nova with the MSNS-E mod version.

[portmod7]

We've had good luck on our Nova with the MSNS-E mod version.

Thank you for the reply Matt. I will give it a try.

Reading the instructions, there seems to be some discrepancy on what size resistor should be used, do you have any input on that? I'm not even sure how I would know if 1k or 2k is too high, or how I would "experiment to find what works"?

Thanks!

[portmod7]

Well, the MSnS-E modification definitely made a difference. I can't say that my problems are gone, but "at temperature" (seems to be 175F+) I don't seem to have any ignition issues, it runs smooth.

The problem I have now is that from ~110F to ~175F there seems to be an issue in about the same RPM range as before, 2200-2500. As I run the engine speed up and it hits around 2200 RPM, if I hold the throttle constant the engine speed takes off on me. The datalog is showing as high as 8k RPM, which I know can't be right, but it's definitely running away, and then I'm pretty sure it hits the rev limiter that I have configured and comes back down. But if I continue to hold the throttle constant it keeps repeating that cycle until I take my foot off the throttle. It's also a "slow" rise in RPM, not like blipping the throttle and letting it dry rev, just a smooth moderate climb.

You'll be able to see the RPM spikes in the first few minutes of my datalog, until the engine gets up to good running temperature. Could this be the difference in R12 resistor values as mentioned on the MSnS-E modification page causing this?

Thank you!

[Matt Cramer]

What resistor value did you end up using?

[portmod7]

What resistor value did you end up using?

I went with a 1k OHM, 1/4W resistor, because I had it left over from building my ECU.

[portmod7]

I guess I'll just have to keep you guys posted. I took the truck for a little drive tonight and I didn't have any problems at all, even got the EGO correction, mixture control, and auto tuning to work well after trying to recreate the above mentioned problem. Hopefully whatever my problem was is gone, although I don't feel like I've done anything to fix it, so I don't know why it should be gone.

Thanks for the help everyone.