GWW

Since I didn't get a response on my TPS question or the one I had on the cam affecting my idle. I've had to keep testing and searching for my idle issue.
My idle surges 150 rpms and the timing jumps 6-10 degrees. Disconnect the ESC wire and the timing is steady as is the idle. 25 rpm change no timing movement.
I don't think the timing jump and idle change should happen. The ECM should be controlling the idle with timing, IAC and fuel but this much of a jump and the surging idle is awful. The IAC is moving 10-15 steps to keep things idleing. I would like to know what the culprit might be. I have a GM service manual and I can't find anything other than how to set the timing. The ESC module is my first guess. Scan tool says there are no knock counts.
The car is an 89 l98 IROC nearly fully restored with no short cuts. More $ in it then it cost to by new but I can't fix this freakin idle after 2 years of trying. I'm ready to gut it and turn it into a drag car. Hell it idles like one.
GW

RFmaster

GWW

You may have EMI problem among other possibilities.
Make sure that you do not have the four wire harness from dizzy to ECM running in parallel with spark plug wires. What maybe happening is that DRP signal from dizzy gets cruded by a leaky spark plug wire. One way to observe this is with WinALDL - it will show DRP error. Another way is hookup oscilloscope and observe DRP signal quality.

Now there are several possibilities for lousy DRP - EST module and tired reluctor coil-magnet inside dizzy.
Make sure that reluctor harness (from pick up coil) is correctly plugged into EST module (p n). Weak magnet may also cause lousy DRP. Magnets may get weaker with age.
Check ground quality of the EST module.

//RF

GWW

Thank you much appreciated, I will check this. One thing to note after more testing. Set my base timing to 6, stock memcal installed. Set my Snap On timing light to 21 degrees, thats 15.87 d from the timing table + 6d base timing. The timing jump goes from 4d advanced to as much as 8d retarded, bad gas mileage, rough and surging idle, BLM out of line at 145. I will update later this evening
GWW

KMCCONKAY

have you checked fuel pressure its possible that your haunting idle could be from that. or the dist. Is it a tpi car with maf or speed density. those are known to cause problems

RFmaster

GWW

Aghhh, yep looks like you are loosing DRP signal from dizzy to ECM (Circuit 430, Pin B5, PPL/Wht wire). ESC retard is maximum 4 to 5 degrees depending on calibration. So if timing retard was caused by KS you would see just 17 degrees of total timing.
When you get DRP drop out in addition to timing retardation injector control pulse is also cut off. This is a safety feature since ECM must have dizzy reference signal indicating that engine is rotating. The only true way to see this is with oscilloscope - if you have one. A test light may give you an indication of injector pulse drop out. This may also explain surging idle: engine is being starved of fuel and ignition is being retarded.
Check for flaky wiring harness, pins inside ECM connector(B5), dizzy to harness connector (4 pin connector). I am just surprised that you are not seeing Code 42... Also, aftermarket EST modules are known to have bad DRP generation. Original GM module uses zero voltage crossing circuit to generate DRP pulse. Some of the lesser aftermarket EST modules skimp on this circuit and depending on reluctor coil voltage swing may produce inconsistent DRP signal.
If you have lappy with WinALDL you will see DRP flag frequently checked during surging.

I hope this helps...

//RF

GWW

Tonights update. I appreciate everyone stepping up. nothing more frustrating than posting and seeing the big 0 every night.
No codes guess the ECM just doesn't like to complain.
Checked for parallel wires running along the ESC or near any distributor wire.

Continuity tested the 4 from the ECM to the dizzy. Circuit 430-0 ohms from B5 to C.
C453 B3 to A 0 ohms
C423 D4 to D 0 ohms
C424 D5 to B 0 ohms
ECM to the ESC
C439 A6 to B 0 ohms
C485 B7 to C 0 ohms
C486 ground C1 to D 52 ohms hmmm...
C496 Detonation sensor didn't check, new sensor and ESC module in there. In my thoughts if the wire was open the ECM would never get a voltage signal signifying a knock count and then request a timing retard. I use Tuner Pro and it's showing no knock counts while idling, does when I rev it though so I think all is well there. However for giggles I will check it tomorrow-getting late.
Removed distributor cap checked connections inside-looked good visually.
Continuity checked the jumper harness from the coil to the dist-0 ohms both wires.

For the other question it is a MAF. Yes I checked fuel pressure tonight and flow volume and it's 43 psi vacuum line unplugged 39 plugged. Had it in a shop for 3 weeks, got it back last Friday and they couldn't figure it out either. Very reputable shop, put a ton of time in it, couldn't fix, didn't charge me anything. Think I'm gonna get them all some donuts. The blue hole of never ending investment keeps sucking up bucks.
Ordered a new MSD distributor tonight.

Keep the instructions or ideas coming, It helps!
G2W

GWW

[QUOTE=RFmaster;3455419]GWW

When you get DRP drop out in addition to timing retardation injector control pulse is also cut off. This is a safety feature since ECM must have dizzy reference signal indicating that engine is rotating. The only true way to see this is with oscilloscope - if you have one.

Don't have a oscilloscope. Looking at the schematic i see 2 reference pulse circuits C430 and C435. One says Hi and the other Lo. Can you explain why there are two? Hi voltage 5V low voltage .76V and what's the purpose of needing two? They appear to have something to do with the ALDL connector.

RFmaster

GWW

Thank you for checking harness.

C430 (ppl-wht) to B5 is a reference pulse from dizzy to ECM. This signal tells ECM when "points closed". When this line measured with DVM at idle (depending on DVM circuitry) it may read 1.5 Volts

C435 (gray) to A4 according to my documentation is EGR control line. I think you meant C453

C453 (blk/red) to B3 is a ground reference between dizzy and ECM. It should not read 0.76V ( which is by the way is forward diode voltage drop). This line should be at 0.0 Vdc when measured to ground! This maybe your problem.

C423 (white) to D4 is EST circuit which triggers the ignition module. The computer does not know what the actual timing is, what it does know is when it gets a reference signal. It uses this reference signal as a base point and advances or retards the spark from that point.

//RF

GWW

Thanks
Sorry. the .76V for circuit C453 just states that on my schematic. I didn't actually measure it.
I rechecked my ground circuit today with a better jumper and someone other than my teenager, bless his heart, helping me, it is 0 ohms like the others.

I've got a new distributor coming, should be here by the weekend. I'll touch back if the problem still exists. I also checked with an electric tech where I work and he will loan me a scope if I need it.

I have been working on this car for two years and never been really able to get a real nice all around idle. It's been in two shops and no one has fixed it. It's usually worse when it comes back. I've got hundreds of hours in it and it has worn me out. Chevy dealerships won't even touch the car. They say it's too old and their guys are only trained to fix new stuff.

It's nice just to have someone with more experience to bounce ideas off of. Thanks for the reference pulse lead. I'm still pursuing that as the problem or I've had an Electronic module failure. I hope I'm not chasing more ghosts.

I will stay in touch.
GWW

RFmaster

OK

Keep in touch - I'll re-read your measurements and visualize.
Word of advise, if I may - do not give up! These circuits are not that difficult to troubleshoot. Having proper tools and knowing what is acceptable - is the key.
In my view factory manuals are not as extensive as I have hoped. They are geared toward techs who do not really need to know finer circuit details. In my case, I did EFI retrofit into my carbed rig and having lab equipment allowed me to do some measurements to become somewhat familiar with these circuits. Furthermore, there is a lot of good troubleshooting info on third gen. Just got to be patient and take it step by step.

Question - when you disconnect EST by pass wire (tan-black wire) does your idle drops (to about 600 RPM) and is it at least stable (rpm wise) or is still bounces a bit as well??

//RF

GWW

It's tough to be patient but i'm still trying, bull headed as hell.

Yes to both of your questions. EST disconnected, idles like a champ and the timing stays dead steady at 6d.
GWW