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Timing issue with EST connected

Old 11-16-2018, 01:59 AM
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GregMartin
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Default Timing issue with EST connected

Hi all
I have been working through a couple of issues with my 84 Cross Fire and I now have a situation where the car will not drive with the EST connected. Please bare with me and I'll try and make this story short and full of facts.

Not long ago the car developed an erratic idle. The idle would start very hi when cold (probably 1400rpm) then settle around 800rpm when warm. However sometimes whilst sitting at the light the car would just idle down to zero and stop. This would also often happen when decelerating when coming of a freeway or after hard acceleration.

As with any old car I have ever bought I hatched a plan to replace plugs, leads, distributor cap, rotor plug, etc. When I looked into sourcing there parts I decided that I would replace the complete distributor so that I was starting from a good base with a new coil and timing module. The distributor I chose was a Davis Unified Ignition GM Computer Controlled Distributor DUI-12600RD. Anyway I installed it the other night, disconnected the EST and set the timing to 12 Deg advance. I plugged the EST back in and observed the change with the timing light; the timing advanced about 4 or 5 degrees at idle so everything looked good. I set off for a test drive and only got halfway around the block when the car started backfiring through the intake and missing badly. The car stalled repeatedly and didn't have enough power to let the clutch out at any revs. I disconnected the EST and the car returned to normal so I drove it back home. I then set the timing back to the factory setting of 6 Deg but as soon as I plugged the EST back in and restarted the car the problems started again.

The original distributor had been working apart from the rough idle: I didn't necessarily think that the new distributor would fix the idle but I thought it would be prudent to replace these parts in any case. The only code I have is 42 which should be expected.

Considering that the car runs quite well with the EST open circuit, there should be no issue with the coil or pickup coil and at least some of the modules functions are working correctly.
Has anyone here had this issue before? I found an old thread https://www.corvetteforum.com/forums/c4-tech-performance/1740187-erratic-timing-and-idle-only-with-est-connected.html where apparently the fix is to file the points off the pickup coil (I really don't want to do that).

I thought that a backfire in the intake indicated that the engine is retarded, Could the knock sensor be retarding the engine enough to cause this? I am really clutching at straws now.

Thanks in advance Greg




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Old 11-16-2018, 08:39 AM
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….. Your harmonic balancer's outer ring may have slipped due to degradation of the rubber that bonds it to the hub … you need to verify TDC or replace the balancer … beyond that , I'd take a good look at all electrical connections between the distributor and the ESC module …..
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Old 11-16-2018, 09:40 AM
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Originally Posted by C409 View Post
Ö.. Your harmonic balancer's outer ring may have slipped due to degradation of the rubber that bonds it to the hub Ö you need to verify TDC or replace the balancer Ö beyond that , I'd take a good look at all electrical connections between the distributor and the ESC module Ö..
Hi thanks for your reply.
The harmonic balancer has crossed my mind. Iíll try and confirm TDC tomorrow (itís night time here). Iím not sure how I will do that exactly because I donít think I will be able to get my dial indicator in a position to touch the top of the piston, maybe I can measure it with a screwdriver or something. The problem didnít happen with the old distributor and the rotor is pointing to where I think number one should be so itís probably ok, but it doesnít hurt to check.
im going to have a look at the wiring tomorrow as well. I resemtly had a set of headers fitted and when I got the car back there was an orange (possibly brown possibly pink) wire hanging down near the starter motor. The wire was not there before and there were bare strands like it had been pulled out of somewhere, but do you think I can find where? All the correct wires are on the starter motor and the knock sensor wire is in place. The car was running ok but this wire troubles me. This car was RHD converted in the nineties so the wiring is not always as per the FSM so thatís an added challenge.

The other thing I have noticed is that since I found that wire disconnected the car nearly stalls when I plug the 10K resistor into the ALDL connector to run winALDL. Itís almost like that wire provides power to the ECM and that it still kind of works without it but any electrical load drags it down.

Iím sorry to leak out additional information but it could be a red herring or otherwise unrelated or it could be important. One thinks for sure it does trouble me.

Last edited by GregMartin; 11-16-2018 at 09:44 AM.
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Old 11-17-2018, 04:07 AM
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ok I did a bit of work on the car today so here is an update:
I payed more attention to the backfires and to my surprise there were both intake and exhaust backfires and the timing was jumping all over the place.

The first thing I did was check that the harmonic balancer was really at TDC and it was but I think C409 was on the money because the behavior is like the timing is way way out.

The second thing I did was swap in the old module but there was no change.

So I pulled the new distributor out and put the old one back in. I swapped all the new stuff, module, coil, cap, rotor button onto the old distributor and the car seamed good. I went for a test drive and it went like stink (an Australian-ism for it was fast).

So hear is the interesting thing. When I put the new distributor in originally it looked like it located in a slightly different spot but I wasnít sure and in the end I decided that I was just second guessing myself. In hindsight I reckon that I my have been one tooth out on the spline. I was sure this wasnít the case because I could set the timing and the car ran with no advance. If I was one tooth out the Hall effect sensor would have been about 27 degrees out. In the days of point I guess the engine wouldnít have run but there may have been enough discharge to fire a sparkplug with an HEI distributor.

​​​​​​​anyway when I get a chance Iíll put the new distributor back in and make sure itís on the correct spine and see if the fixes the issue.
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Old 11-17-2018, 09:28 AM
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Iirc with hei you can't be one 1 tooth out. Just advance or retard until it comes to spec. 1 may be in the wrong position but it should have no bearing on how it runs. Very interesting. How does the timing sit now?
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Old 11-17-2018, 09:36 AM
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firing order ?
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Old 11-17-2018, 07:41 PM
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Originally Posted by 84 4+3 View Post
Iirc with hei you can't be one 1 tooth out. Just advance or retard until it comes to spec. 1 may be in the wrong position but it should have no bearing on how it runs. Very interesting. How does the timing sit now?
At the moment the timing is set at 8 Deg BTDC and the car runs perfectly (probably the best it has run since Iíve owned it). When I pick up the throttle the engine advances, no sign of backfires and on the road test it pulled quite hard. I donít think Iíll have time today to test the new distributor again so itís a job for next week.

So are you saying you can or you canít be a tooth out on an HEI distributor? With points you can see whatís going on and set the dwell angle so if you ran out of adjustment I guess youíd know there was a problem. I read an old post where someone had this problem and they ended up filling down part of the Hall effect pickup to fix it. Canít help but wonder whether It was just installed wrong.

Last edited by GregMartin; 11-17-2018 at 07:48 PM.
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Old 11-17-2018, 07:42 PM
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Originally Posted by corvettenorway View Post
firing order ?
yeah checked about 5 times haha
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Old 11-17-2018, 08:23 PM
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Originally Posted by GregMartin View Post

At the moment the timing is set at 8 Deg BTDC and the car runs perfectly (probably the best it has run since Iíve owned it). When I pick up the throttle the engine advances, no sign of backfires and on the road test it pulled quite hard. I donít think Iíll have time today to test the new distributor again so itís a job for next week.

So are you saying you can or you canít be a tooth out on an HEI distributor? With points you can see whatís going on and set the dwell angle so if you ran out of adjustment I guess youíd know there was a problem. I read an old post where someone had this problem and they ended up filling down part of the Hall effect pickup to fix it. Canít help but wonder whether It was just installed wrong.
the coil pickup on hei is locked inline with the cap tips. It can't tell the difference how you stab it in. If you advance it too far and move the plugs one over it'll act like its retarded too far again.
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Old 11-17-2018, 08:27 PM
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Actually the more I think about it the more I think Iím second guessing myself. Surly it shouldnít matter at what position the distributor is installed provided that one is set to fire around TDC. Now Iím thinking that the Hall effect pickup is either located at the wrong angle on the shaft or itís generating electrical noise which is mucking up the reference pulse on the ECM. Maybe the guy who filed the pickup was right after all.

Last edited by GregMartin; 11-17-2018 at 08:33 PM.
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Old 11-17-2018, 08:32 PM
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Originally Posted by 84 4+3 View Post
the coil pickup on hei is locked inline with the cap tips. It can't tell the difference how you stab it in. If you advance it too far and move the plugs one over it'll act like its retarded too far again.
Thanks again I just realized this see my last post (what an idiot haha). It brings me back to either electrical noise or the pickups being incorrectly located which I would think is unlikely.

Last edited by GregMartin; 11-17-2018 at 08:34 PM.
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Old 11-18-2018, 04:56 AM
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I had time to have a look at the DUI distributor today. It looks like the points of the pickup coil line up about 10 Deg before the rotor lines up with plug lead terminal. Iím not sure how that compares to the factory distributor because itís installed on the car and itís a bit difficult to see. Does anyone know if the orientation of the pickup coil is correct? The other thing I noticed was that the bar that the rotor button bolts to was bent. I know I didnít do it and wonder if that was effecting it.

Sorry to keep posting on my own thread but I figure more info equals better advice.
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Old 11-18-2018, 06:13 AM
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The orientation shouldn't make a huge difference. However if the rotor was out of alignment that possibly could due to the mount being bent. Interesting.
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Old 11-18-2018, 07:05 AM
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Originally Posted by 84 4+3 View Post
The orientation shouldn't make a huge difference. However if the rotor was out of alignment that possibly could due to the mount being bent. Interesting.
Interesting but frustrating.
Itís interesting that with that distributor the car ran very smoothly when the EST was disconnected. You would think that poor alignment of the rotor would be a problem at any amount of advance. Also Iím quite disappointed that the rotor mounting bar is bent on a new distributor. I did a bit of homework and the DUI seemed to be well regarded. I think this one may have been dropped.
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Old 11-18-2018, 09:36 PM
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Originally Posted by GregMartin View Post

Interesting but frustrating.
Itís interesting that with that distributor the car ran very smoothly when the EST was disconnected. You would think that poor alignment of the rotor would be a problem at any amount of advance. Also Iím quite disappointed that the rotor mounting bar is bent on a new distributor. I did a bit of homework and the DUI seemed to be well regarded. I think this one may have been dropped.
The factory Delco distributor module has a built in electronic advance curve that emulates the centrifugal advance curves in the older non-computer distributors. The curve operates if the computer fails or if the EST wire is disconnected. This electrical timing curve is in there to provide decent engine driveability until the customer can get the car to the dealership for repair.

I would suspect that the better aftermarket modules have something similar.
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Old 11-19-2018, 05:36 AM
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Originally Posted by 69427 View Post
The factory Delco distributor module has a built in electronic advance curve that emulates the centrifugal advance curves in the older non-computer distributors. The curve operates if the computer fails or if the EST wire is disconnected. This electrical timing curve is in there to provide decent engine driveability until the customer can get the car to the dealership for repair.

I would suspect that the better aftermarket modules have something similar.
Hi 69427 thanks for your replay.
I guess I wasnít questioning the fact that the car ran with the EST disconnected but what is wrong with this Davis distributor.

Currently the Delco distributor is back in the car but I have fitted all the Davis bits to it i.e. Dyna Mod module, DUI coil, DUI cap and DUI rotor button. The car is running quite nicely at the moment but considering that I bough a new distributor it would be nice to install it.

The only thing I can find wrong with the new distributor is the fact that the bar that the rotor button bolts to is bent. Which BTW is both amazing and disappointing.

Attached is a photo.


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Old 11-22-2018, 05:37 PM
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Quick update
I have spoken with the guys at Performance Distributors and they are going to sent out a new distributor shaft. Iíll post with results when it arrives.

Oh and happy Thanks Giving to all you Americans out there (I thatís what you say).
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