Ronny

iTrader: (1)

I have had intermittant issues which I believed were caused by ignition failure. I am not sure now. Car ran great after I replaced ig mod with Delco, cap and rotor with pertronix brand. wires are newer not rubbing.

What i cant fiqure out is why the ECU is apaprently cutting fuel to injectors, sorry i cant post up analysis.

I captured break up at cruise at 3100 rpms 37 mph, tps steady at 27%, the tach goes crazy and the analysis shows rpm 6375 then 1000 then 5000. DC% is moving all over from 0 to 57 to 0 and constantly changing 0-59.

I then went WOT TPS 27 to 100, in analysis RPM goes from 3800 instantly to 6375 then instantly 4187 then 2500. in analysis DC hits 94 then instantly 29. AE goes frame to frame .12 to 1.12 back to .12 ms. So the fueling is changing as well. After I returned to cruise 3400 rpms at 59 MPH all was well.

As a test I slowly crept to 5200 rpms and held that at 28-33% DC 67 mph. Not in PE nor AE. All was well....

I do another WOT 3100 rpms 55mph to 4500 rpms 80 mph DC hits 50%. AE hits .24 msec No problems !

I do another WOT 2500 rpms 18mph to 4800 rpms 38 mph. DC goes to 65% AE .24 ms. No problems!

Anyone give me some direction as to where to look?

Would a fouled plug cause this as I fouled #2 a year ago due to oil(guide wear?). I doubt that is it.

5 years ago we cut the lead from dist to ECU to add Innovate RPM feed and aftermarket tach as well. 12V switched lead cut to feed the N20(wire nut).

The connectors from harness to dist are tight.

xch3no2

Sorry if dare state the obvious, but has time been creeping into your connections buddy?

Dual entendre intended

Ground glitches can behave this way.

Ronny

iTrader: (1)

Thank you. I should have know to suspect GROUNDS! 1984 vintage you know. We will go that route.

RBob

iTrader: (1)

The RPM is used to look up the VE. Could also be hitting the rev-limiter.

RBob.

Ronny

iTrader: (1)

I am aware of that. BLM over 3000 rpms is very close. Yesterday i was getting hits up there at 130.

Set in EBL.bin to 6000. with N20 enabled 5600.

It is clearly intermittant. I think tuned perf hit it on the head. We will check all the grounds.

I was thinking that it could be a failing TPS sensor. I discovered yesterday that it is not load sensitive as it was breaking up at cruise 45 mph. You could hear it and feel it. Same last night at cruise 70 mph in OD. I just wonder if TPS is failing intermittantly. Seems more apt to occur during first 5 min after startup. Tends to clear out at end of 20 min trip. last night on way home it dhappenned at 5 min in, then near my house after 20 min I got on throttle in 2nd gear and no issues. reved to 5500 cleanly.

No malf codes in WU. Would a faulty ground throw a flag? I bet not.

RBob

iTrader: (1)

My response was to why the injector PW was going to zero. Along with bouncing all over the place. With the rev-limiter set to 6K RPM, and the ECM seeing 6375+ RPM, it was shutting off the fuel from an over-rev condition.

RBob.

Ronny

iTrader: (1)

But the engine never approached 6375? Are you saying the signal from dist was read by ECU and ECU responds accordingly? Same on when it shows other eroneous RPMs? If that is the case how is it the dist can feed wrong ref pulses?

RBob

iTrader: (1)

The ECM thought that it did, as shown by the data.

Yes, that is what it is supposed to do. If the distributor reference pulse (DRP) is all wrong, the ECM doesn't have a way to know that.

Yes. Noise on the Ref Hi & Ref Lo signal pair to the ECM will do it.

Not a good thing to do. The aftermarket tach should come right off the coil, there is a tach feed from it. No way would I tap into the other ECM/distributor signal wires let alone the +12 feed to the coil/ICM.

RBob.

Ronny

iTrader: (1)

I may be WRONG on that.

This issue periodically occurred(tach swings) but never affected the engine operation. It would come and go. We may have tapped that coil wire as you state. I will double check that.

Do you think a ground failure is probable? What do you think the issue could be?

xch3no2

Also my next suggestion, RF from the wires or such, is a condition that can arise with time. Highlights two problems, the source and unprotected circuits.

EagleMark

You know better then that! If that is not bad by now it will be soon.

You've been chasing this RPM issue for awhile, seems some new parts helped... how old is the distributor? Was it a GM? I've seen all sorts of issues on old or aftermarket like loose base plate, reluctor wheel, corosion in pickup coil, pickup coil wire and plug issues just to name a few that would show up as your issue. I'd have had that distributor out, inspected or replaced by now with the issues you've been having. HTH!

Ronny

iTrader: (1)

I checked last night. The lead to aftermarket tach was taken fron the coil. Verified color of wire and also clearly says "tach" on dist cap for location.

Eagle: the distributor is old. Orginal. about 10 years back it was removed inspected and shimmed. since it is possible interfearance distributor is coming out and being rebuilt or relpaced. I suspect I will replace. Anyone have a suggestion for replace ? Delco, MSD, Davis, Summit, etc?

Also I believe the ground for ECU is on the manifold at the thermostat for the distributor and the ECU. That will be inspected and cleaned. I recall there is one more to front frame from engine. I looked at front of engine last night but did not see one. I recall there was one at frame drivers side rear that is visable. Any others you know of?

EagleMark

Stock GM AC Delco replacement! Wouldn't be to hard to go Small Cap Distributor while your at it? Other distributors of quality may be fine but stick with the OE module.

I don't even know what kind of car you have Ronny? But normally the EFI system has 2 grounds, one to back of head area and the other to thermastat housing.

If the system is all grounded back to block and battery negative is to block then other grounds from frame to body to motor are not an issue... unless your fuel pump is not grounded back to block.

I'd never rule out a ground but with what you've been going through it still sounds ignition related, even if a ground or loose connection there. You;ve chased half the problem away with parts... Have you looked at coil and button lately?

Ronny

iTrader: (1)

1984 Corvette. The coil is Accel in cap about 10 years aged 50K miles on it. Dont they fail all at once? The button is new with dist cap. Module is new Delco.

I understood a ground may be on on back of pass head but I could not see it. The grounds were removed 4 years back when I went to 7.4LTBI and Holley manifold. If ground at head, heads were done 8 years back and ground would have been cleaned. I store in dry garage seldom driven in rain. Same on prior owner. My mechanic is very thorough so he would have cleaned all. Nowthere is no room behind head to attach a ground. Like 1 inch clearance. do you need to see that from underside? haynes service manual does not list location of grounds. Seems all the electricals on engine ground to manifold by therm housing. Connections to dist are tight and clean when I swapped out parts.

Fuel pump has a dedicated ground and pump is one year old(TPI).

EagleMark

I just don't know the wiring on that car, but have taken many systems off cars and trucks and always remember 2 grounds for EFI system.

Coils can go bad, problems can be intermittent, your problem is higher RPM which could be stressing the coil, but it comes and goes so I don't think so? But I would not rule out anything at this point.

Distributors and ignition systems even in poor condition with faulty or intermittent parts will still run a stock motor all day and never notice, but once your pushing HP and RPM limits it becomes all to apparent. Rbob has chimed in so if there was a possibility of hardware or bin we would have known. The age of distributor even though serviced and new parts still,leaves a huge gap in a problem you've been chasing for a long time. You've got data and can feel it, so the issue is real. Really hard to find crap like this! Even going through the ignition system with an Osiliscope may not find it. By the time your done you could have dropped in a new distributor and saved time and money.

I hope I am sending you the right direction. Have found so many issues with aged and aftermarket distributors over the years it's not a guess.

Dominic Sorresso

Ron,

Have u checked wiring on fuel pump relays on fire wall? On my 84, I happened to look up while laying on my back under the car staring at bare wire. it sounds as if there is an intermittent short or ground affecting a signal to the ECM.
BTW, you don't still have tach filter capacitor on there do you?

Ronny

iTrader: (1)

Dom: I dont think it is a fueling issue since tach goes crazy during event. My FP is new and a year old. I think the distributor is sending erroneous info to the ECU and it reads as such in the WU logs. Each frame in WU shows a different RPM and then a corresponding msec of inj pulse.

OBTW is use to be load related. Only occuring at accelleration. Now it had morfed into a steady state misfire at cruise speeds. But intermittant regardless. And more likely to occur in first ten minutes of operation.

Tach filter was disconnected a long time ago. that was the stock GM filter. when I added the Accel coil 8 years ago a direct result was the stock tach similarly "broke up. I added an autometer and it was then A-OK.

I am leaning today to replace GM dist and go to a Mallory dist unit #6148201 with Mallory ext coil and Mallory basic 6 series ign box. It may better light the N20 charge as well. N20 not used this year at all. It is the original distibutor(125K miles) with exception of module, cap, and rotor being new.