Well I have been on a week long - on and off hunt for a minor miss that actually was prompted from a fluctuating log from the DFI system.

The logs show an erratic RPM fluctuation that was also evident on my EASE logs, so I did not pay much attention - but it always did bother me that the flutter was there. Guess I dismissed it as software reporting errors. Now this "flutter" has been there since I modded the 350 in nearly every log I have ever made off the car. But now the DFI with its increased sampling capability has made it more annoying. A few of the turboGN guys that I speak with - saw one of the logs and said their DFI system is much smoother in the RPM recording. This was very alarming to me, meaning that something was not right.

This news commenced my search of the problem....which as you read this is still an open issue, but led to another finding.

The weather here in the last week in PA was downright HOT - not complaining - contrary to the bitter cold, 3 ft of snow this lapse in the weather prompted me to get a little crazy and remove the roof off the car. Upon doing this while in persute of my RPM flutter problem, I noticed what sounded like a very minor, almost undetectable popping under acceleration out of the exhaust - while making some logs to figure out the fluttering RPMs. At first I thought it was just my imagination, but the more I ran it, the more evident it became. I finally concluded that the it was a miss while the car was under acceleration and with my helmet on or the roof on the car, it was almost undetectable. This had to be cause of the fluttering RPM in the logs or mis-reporting of RPMs.

WHen I say mis-reporting for example, the rpms be on an upward trend, then out of the blue you will get a downward spike of ~400-600RPMs and then it will jump back up on trend with the previously correct RPM. THis repeats and contiues through the entire WOT logs. The result is a jagged graph as seen below.



While this looks horrible, I have seen other cars with the EASE software exhibit similar behavior, therefore it was not real alarming to me.

Well I figured that the cause of the problem had to be related to the pickup in the distributer or warn bushings in the distributer that was causing erratic reading. Rather than nickle and diming the distributer with replacing components I figured I would all-out replace it. Dropped the other distributer in and figure I would be good to go. Took it out and was ready for a nice and smooth log, free of the minor miss/popping. Well to my dismay - the miss and flutter was still there.

OK. Perhaps the MSD 6AL was messing with the DFI ECM which is located in near proximity and causing false reporting/ or simply manfuncting causing the popping. Next day I go out and remove the MSD and hook up the stock wiring to the distributer. Again to my dismay- the symptoms remain.

What the heck - plugs were new about 5 weeks ago and wires are Taylor 409s 10.4mm which look like jumper cables and I have yet to hear of a set failing.

Well I burn another evening after a long day at work, scrambling to change out the plugs( which I always carry and extra set in the toolbox) before the storm, that was brewing in the sky, opened up.
I got the plugs changed in record time - figuring about the time I fire up the car, the rain will be pouring in. I was lucky enough to get a test in before the rain started, but again the popping and fluttering was rearing its ugly head.

OK. I am now looking at one last thing before digging into the valve covers to test spring pressure, fearing one of them might be broke, and closing slow, letting out just enough to cause the popping. That last thing are the plug wires. God let it be.

We are now on the 4th night or is it the 5th....who knows. I run out the dealership after getting Dads OK to swipe his wires off the chevelle that could use a bad wire or two. I come home and make a swapout of the wires - and head out for what seems like its going to be another series of letdown tests. As I pulled out of the driveway on to the road the car seemed to be little more responsive, but I have been fooled before into thinking that the car is making more or better power when its actually the same or less. Out the road about 5 miles to my testing ground which consists of a straight-stretch about 1.5 miles long (hard to find in this part of PA ). On the road I never get on the car until in 3rd gear cause it just gets stupid and its a waste of tire and time. This time as I get on it in 3rd the car feels great and to my ears delight there is no more popping. Ahhh this can't be true...so I make another pass. I think we have a winner.

I made another pass along the road and stopped at one of two of the nice large turnoff the State of PA gratiously put on both sides of the straight-stretch for me to pull off and turn around - or to park in and review my logs. The win with the popping was bitter-sweet. I am now looking at very similiar logs that still exhibit the flutter of RPM, but the exhaust is free of the minor popping.

At this point I am pleased to have solved 1/2 the mystery - but am totally puzzled at what the other 1/2 is being caused by. The car pulls like a run-away freight train. And I am anxious to get back to the track and see what sorta gains I have uncovered, cause the miss was definately costing me some ET/MPH. The car feels terrific now, sorta one of those things you don't realize until its not there anymore - then its like WOW. Its hard to explain the noise it was making. Especially since the exhaust note at WOT is nearly defening.

So while my endevor is only 1/2 complete - the lesson learned once again even when you try to do everything right is the small things are ALWAYS there to bite you. One thing is not perfect, then its costing you power or your time trying to search it out. About the only thing you can do is put together a stategy to attack the problem with and take it one step at a time to eliminate possibilities. I see this all the time on forums where people come and ask for solutions, but these solutions that are given should be just used to formulate ideas to attack the problem with.

I was fortunate in this case, I had enough spare part to try each one of the items out in a systematic manner for free.

Now I need some more time to sort out the fluttering RPM issue in the reporting of the RPMs. I can't believe its mechanic, and think it might be RF interference causing some bad reporting of data. I had issues with the romulator in my car and I wrapped it with aluminum foil and the unit worked flawless. so perhaps the DFI is getting some interference....this too has to be causing some power loss, since the unit is jumping from cell to cell in the fueling stategies also.

Well thanks following this long azz post...and I will keep you updated on what I learn as events unravel

 

Jessie,

From what I understand it is only at WOT. Double check all your connections for security. That is one easy possibility, if any of the connections are not really good, vibrating at high rpm could cause a temporary open. If you think it is RF bleedover, try the same foil sheilding trick with the DFI wiring. If you are picking up RF interference, sheilding the harness should fix it. If it does, then you could sheild individual wires until you find the culprit and replace it with a sheilded wire or just leave the foil wrapped around it. You could also try wiggling the wires and connectors at idle while logging and try to induce the fault to check wire/connector continuity. It is real easy to have a wire breakdown inside the insulation yet be barely noticable from the outside.

Good Luck!



Doug

 

Doug.

Yes those are all great ideas. Thanks.

I have to think its interference. The DFI connections area screwed in and are very tough to get back apart. I will double check them thou - don't want to assume anything, that is what comes back to bite you.

I am going to try shielding the ECM tomorrow if weather permits. Seems those few days of decent weather were our summer and now it will be rain and then snow again. lol.

I have a new set of taylor wire that should have actually come in today, just did not have a chance to get down to pick them up. 3 yr old had his karate class tonight and I don't like missing that stuff.

I will let you guys know what I turn up. What all can give off RF interference in these cars. I am thinking this must be pretty strong in my car for some reason....since the DFI was effected also...perhaps its just vettes since they don't have a lot of metal compared to say other vehicles. Not even sure if the two have anything in common, but it sounds possible. At this point I am game for wearing a foil cap if it helps. Just not sure if it will pass tech.

 

Jesse,

Most of your RF is going to be generated by the ignition circuits, the high power ones. I have heard of the MSD box generating RF interference also. Basically any wires that are running parallel to each other are susceptible to mutual inductance. Another trick is to run them perpendicular to each other like you do with the plug wires if they have to cross. This will cause the induction currents to cancel as they will be out of phase to each other. It also comes to mind, what kind of wire is used for the the conductor in the plug wires you are using? Also check around your dizzy and coil wiring. Is the rpm reading being taken from the dizzy? Might try sheilding the computer wiring in the dizzy area with the foil.
As far as the simple things, that is always what it is when I make an assumption and don't check/doublecheck them. If the DFI wiring is screwed in, ensure the wires are wrapped onto the terminals so they tighten when you turn the screw. Also look for any stray wire strands, a single strand can cause fits as it bounces across terminals. Check for unusual angles at the connectors and any wires that are taut. These are prone to failure.



Doug

 

I am also trying to track a miss at around 5800 rpm's it is noticeable on the dyno and i can feel it when I'm racing down the 1/4 mile. I have also tryed replacing the distributer with no effect. I also replaced my msd 6al with the msd digital 6 and that made no differance. I also bypassed the msd system and ran the stock ignition still no change. This is driving me nuts the car still pulls very hard but i still feel the miss. I haven't tryed the plug wires yet since they were new last year and still look like new. I was also thinking maybe a injecter might be failing and causing the miss. I don't know what else to check. I am very interested in finding out why your car is doing something simmilar.

 

Thanks Doug.

Went to get my wires tonight and the guy ordered ones from that need assembled. Don't want or like them, so I had to order a set myself.


Will try isolating a few things this weekend if weather and other stuff permits.

TOY, my miss was a wire. You have to change yours out to eliminate them as the possible problem. Mine too show no signs of defectivity - yet they definately are tied to the miss problem cause its now gone.

Now if I could just isolate the reason for the bad RPM readings. Not sure if is playing a role in the RPMs flutter - but what about the tach filter. The DFI system goes right into the distributer so I think its out of the loop, but who knows.