86PACER

Search revealed no useful info. Found an old thread on the topic with no result/solution posted by the OP.

I've been chasing down a rough idle the last few days.

I've discovered that when I start the car with the EST connector unplugged, the idle is always smooth as glass, the timing mark stays dead on under a timing light, and all is well. Cold or hot.

But as soon as I reconnect the EST connector and restart the car (cold or hot), the idle becomes erratic every time, and wants to stall. The timing mark is jumping around when I look at it with the timing gun as this happens. The engine behavior seems to follow it.

What I've done:

Checked all of the wiring in the EST circuit for breaks, shorts, damage. Found nothing wrong. Wires to and from the dist. harness and ECM are good.

Pick up coil in dist. tested good.

Wiring inside distributor is good.

Timing is set to factory specs.

Ran with MAF plugged/unplugged. No change.

Ran with 02 plugged/unplugged. No change.

No Codes (only code 42 which is normal when you unplug EST)

TB and IAC is clean as a whistle. Reseting IAC to specs did nothing.

No vacuum leaks (or it would not idle perfect with EST unplugged only)

New Cap, rotor, plugs and wires were installed just before this happened. No other change. I put all my old stuff back on, and the problem persists.

The rest of the steps given in the shop manual seem to only apply when you have a no spark condition, which I don't have.

I wish I had spare parts laying around to swap in.

Has anyone fixed this issue on theirs before?

I'm going to be testing other components while I await for replies.

corvette1989bham

I am having a similar problem with my 86 that I am restoring, I havent started in depth trouble shooting yet, I am thinking a emmisions problem as in EGR / Throttle body vacuum (replaced all)/ or a hard distributor problem. If you figure it out let me know, I will do the same.

rons85

Normally the first suspect with this would be the module. The ECM will "switch" and take over timing control from the module at 400 rpm. The bypass wire unconnected prevents that from happening and leaves the module in control. It's that switching that can cause a bad module to mess up. That said...

I've recently gone several rounds with a similar problem after installing a new distributor (long story, but I was piecing together a "new" distributor from a pile of parts - all New parts..). Car would start and barely run, missing all over the place. Problem not evident or at least much better if the bypass wire was disconnected.

After several hours of swapping parts around, new and known good Used stuff, the actual problem Apparently, and I say that because I'm still not convinced, but it's Fixed and running OK, at least, was the pickup coil assembly. Not the actual Coil itself, but the clearance or interference between the stationary "teeth" and rotating "teeth" on the distributor shaft.

Never in 30+ years of playing with chevy motors have I had this happen, but I cured the problem This time by fiddling around with the location of the stationary teeth (loosen the retaining ring and tap the teeth lightly to move them slightly) to get the shaft/rotating teeth to clear without ANY contact.

After a few tries at That approach I finally (in desperation) took a file and shortened the tops of the stationary teeth a bit. It worked, no clearance issues and (presumably as a result) no problems when running. Beats me - I've never seen that be a problem before...

CFI-EFI

Here is a shot in the dark. Check your timing mark to see if it correctly aligned at TDC. The timing is significantly advanced from base timing when the EST is reconnected. If you are really setting the base timing at 14° or more (when you think it is 6°), due to a damper with the inertia ring that has slipped, you may be running so much advance with the EST active that it won't idle. You CAN go out and but a fancy piston stop, but you can check closely enough with a piece of coat hanger or other stiff wire. Maybe.

RACE ON!!!

86PACER

Thanks.

I've eliminated the Knock sensor and the ESC module as having to do anything with the erratic timing I'm seeing. I've already ruled out the rest of the mentioned possibilites in the Helms for a rough, unstable idle. At this point, I'm thinking either something in the dist., or a bad ECM.

I'll check the damper/TDC next and test the IAC valve itself. For the time being, I'm trying to track down and borrow a good used dist. or ECM locally, to swap in. That should tell the story.

CentralCoaster

Could a loose timing chain or worn distributor gear cause the timing to jump a little on its own, enough for the computer to keep chasing it and overcorrecting?

Does the timing appear to jump at all with the EST unplugged?

jimmers

My timing does the exact thing. With the est wire unplugged its dead on, but when its plugged in it just jumps all over the place. I was told awhile ago that this is normal. My engine runs great.

rons85

There is no way that timing jumping around "all over the place" is normal. Whoever handed out That bit of wisdom needs to be given another beer and led to a quiet place...

The ECM does not get any feedback to know what the timing actually is, so there is no way it would "chase" it. Now, if the RPM input (to the ECM) was varying then the timing Tables would be causing the commanded advance to change - which Could happen I suppose if that RPM signal, coming through the module from the pickup coil, was flakey. A false or erratic knock signal could be doing the same thing, presumably.

All that is Probably why the diagnostics point to the module.

For what it's worth, I had tried changing the EST and the ECM (I am in the midst of doing a TPI swap on another vehicle and have a Pile of 1985 spec. parts lying around. Sort of handy..) and disconnecting the knock sensor. Checked the wiring from the ESC connector and Distributor body harness connector back to the ECM harness plug. Etc. None of that was bad, and, like I said earlier, modifying the teeth on the pickup coil finally fixed it. I guess, either that or it was just dumb luck.

With the bypass line disconnected the ECM has no control over the timing advance. All you get is the 6° (crankshaft) or so advance that is built into the module, and will come in at 1000 rpm or so (I forget the exact numbers). Not much there - certainly not enough to run very efficiently. Not unless you had built a mechanical/vacuum advance Distributor to use with the TPI module....

jimmers

My buddy also has a 90 and his does the same thing. When the est wire is pluged in the timing jumps around. I don't know what to tell you.

86PACER

Timing mark stays spot on and rock steady with EST unplugged. If I unplug the EST with the car idling rough, the idle smooths out the instant I do so.

The rotor has the normal amount of side to side play it's always had before. I'll leave the timing chain for last. I don't want to pull a timing cover and lower the oil pan for nothing.

Ron, I'll check that out. I'm not leaving anything out.

kaiser

Here is the code for the later 8D mask. You might want to check what mask you are running for something similar:
http://forums.corvetteforum.com/showthread.php?t=556467

Tom400CFI

If the timing chain were the cause of the issue, then the timing would jump around w/the ESC wire unplugged too. The OP stated in his original post that is not happening.

"I've discovered that when I start the car with the EST connector unplugged,... the timing mark stays dead on under a timing light, and all is well. Cold or hot."

That means the chain and dist end play isn't the cause. I'm w/CFI-EFI here. It sounds to me like it COULD be over advanced w/the ESC wire connected. It's check that first b/c it's easy to check and free.

CFI-EFI

Forget the timing chain. As Tom400CFI said, regardless of their condition, the distributer shaft end play, and timing chain slop, doesn't change with the connecting and disconnecting of the EST. I will guarantee you have excessive distributor shaft end play.

RACE ON!!!

86PACER

Yes, once I though about this again, appeared to me that if it where the chain, the problem would be present all the time and not go away as soon as I unhooked the EST plug. I've managed to track down a dist from a local forum member. I'll update with my findings.

CentralCoaster

My point was if the ignition timing is shakey from slop in the crank/timingchain/camgear, this will affect idle, which I assume will feed some inputs to the ECM to alter the timing.

What criteria does the ECM use to control timing at idle?

mseven

what I've seen is, if the tune and the bl is real solid across the board and is right on 128/128 at idle, the timing will stay within 1-2* tops w/est connected. When the ecm is in controll (closed loop or open loop)it can move the timing up to approx. 4* to compensate (aside from knock retard which are seperate set of numbers) for unstable idle, BLM, AFR vs temp., idle rpm vs temp. etc.etc. Highway mode will bump it another 4*, cold start also has add ons to timing. IAC counts, and base pulse width at idle can change/slightly as the ecm is trying to calculate/maintain the blm, idle etc. Mechanically, having good bushings, a good gear and minimizing the vertical endplay helps stabalize timing fluctuation.

rons85

The only input to the ECM that would effect the timing at idle (closed loop = no cold start happening and assuming no Knock counts either, since there is no Load on the motor) is the actual reported idle speed. As long as that idle speed doesn't fluctuate to the extent that the rpm moves to a different spot in the main timing table (advance vs. rpm vs. load), and it certainly Shouldn't be doing that, then there will be no change in the commanded timing advance.

As an aside - the main timing table for my '85 (using TunerCat) doesn't even Start until a load (LV8 value) of 32 - it ain't doin nuthin at Idle...

The ECM does not (at least not directly) use timing advance to try and maintain the idle RPM or mixture. Now, if the RPM signal Was erratic enough that the ECM was moving the timing around then that RPM signal is the problem. Anything Mechanical (timing chain, distributor/gear were, etc) that might cause that much timing "flutter" would always be doing so - weather or not the bypass wire was connected. The only thing that is taken "out of the loop" by the bypass wire is the ECM/Module handshake. Take it from there.

Dave Grandon

Caught your article as first time subscriber. My 84's only got 24k and two weeks ago it began rough idling as well as an almost complete stall. It mattered not what spead I was going - further pressing the accelerator or attempt to gently pump it to life were unsuccessful. I am displaced from home where I have a Very competant tech at dealership, but find myself with a untried tech in Pa. They've replaced the igjectors, cap, rotor and ignition coil for about $1300 and they're still guessing. The tech needs to talk to the service mgr.? What's with that?
Am I being taken for a ride?
My tech advised the coil should simply have been tested with resistence meter- it works or it doesn't- I have yet to affirm that the nrew tech tested it!
Perhaps because it's older technology, they don't have experience with older models- could this be valid reason for "thei searching"?

anesthes

You're 85 is an oddball ECM. He's running $32B on a 165 ECM, which uses a combination of spark advance and IAC to steady the idle speed.

What he needs is a scanner. I agree a flakey ESC module could cause the problem, and if he has one on hard it should be fairly easy to rule out.

However, with a scanner he can see what his actual idle versus commanded is, iac steps, actual reported advance, and sensor input. He stated he has no codes, but honestly i've seen sensors report huge enough variances to cause operating problems but not pull a code.

Oh, and Ron you could get some knock retard at idle too! The code has no checks for load or vehicle speed. I have two setups that got knock at idle, one was false knock from the supercharger the second was false knock from the headers.

-- Joe

anesthes

On his '165 or your '870 ECM?

Generally speaking, the ECM has 'no idea' what the actual advance is if their is a mechanical problem. There is a constant for base advance that is built into the bin. It takes commanded advance, add's it together to get what it thinks is the total advance. On some masks ($32B, $6E on the 86-89 165 ecms, $8D on 90-91) advance is controller by the main load table, idle routines, and coolant sensor input.

For performance app's a lot of us zero out some of the other tables that will add or subtract from the advance, to make the car play more like an older mechanical (or vac) dizzy.

I'm not too familiar with the 1985 '870 ECM, but I know on the later MAF and MAP setups it uses commanded advance + iac to steady both the idle AND DECEL and anti stall AND idle adder in park/neutral based on
EST reference, coolant temp, MAF/MAP, and MAT.

-- Joe