Timing Jumps in ESC Bypass Mode/Rough Idle

I own a 90 K1500 5.7L/700R4 Pickup with 120K miles. I am the original owner of this truck (so I know it very well). I have a Rough Idle that has been driving me nuts. I have read so many posts on the subject but have yet to get this resolved. At this point, I feel it has to be the Distributor and/or ECM.

The Problem:
Rough idle, almost surges at times (has never stalled). With a timing light connected and EST in Bypass mode, the timing seems to jump around (like 20 degrees or more advanced). It is hard to tell how much but it is clearly jumping around.

With EST in Bypass mode, can I assume the ECM is not going to be adjusting the timing? The timing chain is less than a year old so that it not causing this.

I ran WinALDL this morning (after a 30 minute commute) and the BLM is 108 so I know that the ECM is trying to take some fuel out due to the miss (or whatever else you want to call the timing jumping 20 degrees)

I can feel it when cold but it is worse when it gets warm. Sometimes, if the truck sits for a hour, it won't be as noticeable.

Ignition:
The stock coil was replaced with a MSD coil that I had in the shop. Reading this forum (and others), I decided to replace the Pickup coil (the stock one did not look so good). I had a spare ignition module but it also had no affect. The magnet on the shaft was loose so I replaced the shaft (with the updated version) including the gear drive. I could not detect any side to side motion of the shaft. I know that at some point I should have just bought a new dist (and I still may) but I had tunnel vision and wanted to find the cause of this problem. Plugs, wires, rotor and Cap are also new.

Emission Equipment Replaced:
EGR and Solenoid (Solenoid would not hold a vacuum)
Multiple 02 Sensors have been tested.
MAP Sensor
IAC
TBI: I rebuilt it including Fuel Pressure Regulator. Did not install new bushings. Refurbished injectors.

Vacuum Leaks: None found
ECM Codes: None stored
Compression: All 8 are 170 PSI +/- 7 PSI
Fuel Pressure: 14 PSI
Oil Pressure: 15 - 30 at idle (depending on temp). 30-45 PSI on highway.
Exhaust: Stock including original Cat. Backpressure at idle less than 1 PSI. A little over 1.5 at 2000 RPM.
Intake Gaskets were done last Nov when I did the timing chain and gears.

ECM Quad Driver Check:
I own a set of the factory manuals and I have probably read every page in Chapter 3 of "Fuel and Emissions Including Driveabilty" multiple times!

When I measure the resistance between the Quad Drivers and the case I get 11K ohms not 50K ohms that the book references. I have tried other meters with the correct scale. Does this mean the ECM is bad?

Known Issues:

I have a little blue smoke at startup. Most likely valve seals.
I installed a March Under Drive Pulley Set (Power and Amp version) years ago to try to save on gas.


Other than the rough idle, it runs fairly well above 1500 RPM. As soon as you touch the gas, the roughness it gone. Mileage for mostly city driving (maybe 25% highway) is between 12.5 to 14 PSI. Which is about what I have always gotten.

Now what??
What is causing the timing to move around in Bypass mode? Is this an ECM that has gone bad? Is this a worn dist housing? ECM? New Dist? Cat?

I need a little help and I hope someone can steer me in the right direction.

UPDATE STARTS HERE (Updated 10/24/2006)
After reading some posts about Misfires on the Vortec Engines due to the distributor vents being clogged, I decided to pull the distributor one last time before trying a replacement distributor. One vent was completely clogged and the other was mostly clogged (probably around 75%). I also noticed some corrosion on the terminals where the pickup coil connects to the ignition module. There was no play in the shaft and I tested the pickup coil with a meter and it looked good.

So I cleaned out the vents and cleaned up the terminals on the ignition module (using emory cloth), coated them with dielectric grease and dropped the distributor back in. I let the engine warm up and then set the base timing.

Well that was Saturday and it has been running better every day as the ECM relearns the fuel trims. The low end miss is gone and it is running a whole lot better. The idle is also much better. It will probably take a week or so for the ECM to adjust and I want to see what happens to the gas mileage. I am also due for a smog check which it should now have no problem passing the test.

I will report back once it passes smog and any change in gas mileage. Many thanks to all who helped me with this.

My guess is that the blocked vents caused the corrosion on the pickup coil terminals which gave a weak signal to the ECM.

//2000CAYukon

you probably have this figured out by now but what kind of ignition module did you put in? I have heard horror stories about aftermarket modules. But if it was oem then nevermind. I would lean to the ecm. They do go bad. I would try a junkyard one first before spending money on a new one. I don't even know what that would cost, maybe not that much.

This might not be anywhere near the same but I had a 90 Mustang GT that bent the dowl on the camshaft which caused a late cam timing.. Idled rough, still had all kinds of zing though.. Maybe you've got a bad chain, bent dowl something mechanical it sounds like more than electrical... You've already been through nearly everything electrical that would cause a rough idle..

No I have not figured this out yet although, I did work on it this weekend.

The ignition module is the original AC Delco Unit. At one point, I did replace it with a Borg Warner aftermarket piece but went back to the AC Delco unit. I have not ruled this out.

The timing chain and gears (double roller piece) is less than a year old. The old chain had some slack and I was sure the new chain would fix the problem.

This past weekend I decided to build a custom harness that removed the EST signal to the Ignition module. The goal was to isolate the ECM from the Ignition system. The timing issue did not happen when I installed this custom harness. Even after removing the custom harness the problem did not show up. The problem is very intermitten. Once it happens, it seems to happen until it cools off.

I cleaned up all of the connections to the dist and coil and lubed all the connectors with Dielectric grease. I drove it today but not as long as my regular commute. Most likely the problem will come back tomorrow.

I guess it is possible that the original Ignition Module and the Borg Warner piece where both bad. I also have not ruled out that the ECM could have a problem when it gets hot. The other funny thing is that is seems to be worse in the colder months than the warmer months.

I will be sure to update this post when it gets fixed. I would like to pinpoint the issue so any advice is welcomed.

//2000CAYukon

Try tapping the pcm see if the timing jumps around? Any knock sensor in the circuit?

Try tapping the pcm see if the timing jumps around? Any knock sensor in the circuit?

I have tried that in the past when the problem was occuring and it made no difference; although, I did not have a timing light on when I was doing this (I could still feel the miss).

The knock sensor is stock (only one on mine) and the last time I ran WinALDL, the knock count was 45 after driving for 40 minutes.

I may have to try tapping the pcm while I have a timing light on it.

//2000CAYukon

45 knock count seems high?

45 knock count seems high?

I thought so too but the count was not incrementing when I hooked up my laptop running WinALDL. Perhaps, I should hook up the PC first thing in the morning and watch the counts.

I have also unpluged the ESC Module (at least I think that is what it is called) when the problem happens and it makes no difference. This module is also original. Knock sensor itself has also never been replaced. Truck runs good once you touch the gas, so it does not feel like timing is being retarded under acceleration.

If it was not clear before, the problem is very intermitten but seems to happen more after it has been driven for 30 minutes or more.

I made a harness that bypasses the EST signal from the ECM to the ignition module. My goal was to isolate the ignition from the ECM; however, the problem decided not to show up and I will try this experiment again when I can and the problem is happening.

Thanks,
2000CAYukon

On the way home from work, I ran WinALDL for the whole trip. Knock count was 12 after about 15 miles of freeway driving. Getting onto the freeway caused the count to go from 1 to 4. I also noticed that when traffic slowed down to around 50 I would get an increase in knock count (1 or 2) when I accelerated back up to 65.

I don't think this is my issue. I still feel a slight miss (or at least I think I do) at lower RPM speeds. It is more noticable on the freeway when the RPMs are under 1800.

Even with the fact that I replaced the shaft, drive gear and pickup coil in the dist, I am leaning towards a bad dist.

If anyone wants to look at the WinALDL file, I can post a link to it.

//2000CAYukon

Pull the distributor. Check the shaft for radial and axial play. Radial play should be 0.001-0.0018" Axial play can be up to 0.045" before problems will occur. When you have it out, also look at the gap between the pickup coil poles and reluctor poles. It should be consistent, but with no mechanical interference.

With the EST in bypass mode, there should be NO advance ECM and NO variation/wandering of the base timing. If the distributor is sloppy, or the HEI switching module is not getting good signal or not relaying it reliably to the coil primary, you might see variation.

Pull the distributor. Check the shaft for radial and axial play. Radial play should be 0.001-0.0018" Axial play can be up to 0.045" before problems will occur. When you have it out, also look at the gap between the pickup coil poles and reluctor poles. It should be consistent, but with no mechanical interference.

With the EST in bypass mode, there should be NO advance ECM and NO variation/wandering of the base timing. If the distributor is sloppy, or the HEI switching module is not getting good signal or not relaying it reliably to the coil primary, you might see variation.

Blue Bowtie,
The shaft was replaced with a new AC Delco updated shaft. The reason I replaced it was the magnet on the end was loose. I did not check for radial play; however, it seemed to be fine with no mechanical interference between pickup coil and reluctor.

Right now I am trying to reproduce the timing jumping while in Bypass mode. If push comes to shove, I may swap in a new dist.

One interesting point is that when I checked the timing this morning after my commute. The timing did not jump while in bypass mode; however, out of bypass mode, the timing is all over the place (almost like the ECM is sending a bad signal to the ignition module).

One way or another, I am going to get this figured out.

Thanks,
2000CAYukon

Base timing with ECM in Bypass mode is still jumping around. Much easier to see it at night. Time to get a new distributor.

//2000CAYukon

Note that I updated the original post with the update and also posted here so that those who read this may benefit.

Here is the update:

After reading some posts about Misfires on the Vortec Engines due to the distributor vents being clogged, I decided to pull the distributor one last time before trying a replacement distributor. One vent was completely clogged and the other was mostly clogged (probably around 75%). I also noticed some corrosion on the terminals where the pickup coil connects to the ignition module. There was no play in the shaft and I tested the pickup coil with a meter and it looked good.

So I cleaned out the vents and cleaned up the terminals on the ignition module (using emory cloth), coated them with dielectric grease and dropped the distributor back in. I let the engine warm up and then set the base timing.

Well that was Saturday and it has been running better every day as the ECM relearns the fuel trims. The low end miss is gone and it is running a whole lot better. The idle is also much better. It will probably take a week or so for the ECM to adjust and I want to see what happens to the gas mileage. I am also due for a smog check which it should now have no problem passing the test.

I will report back once it passes smog and any changes in gas mileage. Many thanks to all who helped me with this.

My guess is that the blocked vents caused the corrosion on the pickup coil terminals which gave a weak signal to the ECM.

//2000CAYukon

I am having the same idle problem with my 90 gmc 350. Which vents are you refering to? Vents on the distributor or the block, when I pull the distributor? Please help. I already replaced plugs, wires, cap, rotor, and coil with MSD.

I am having the same idle problem with my 90 gmc 350. Which vents are you refering to? Vents on the distributor or the block, when I pull the distributor? Please help. I already replaced plugs, wires, cap, rotor, and coil with MSD.

First I all, I am sorry that I have not updated this post sooner. I was convienced that my original problem was ignition related especially since I could see the timing jumping around; however, the problem is still there and I know believe my issue is engine mechanical.

Using a laptop (using WinALDL) and a cable, I could see that the rough idle is caused by the ECM thinking that the truck is running rich so it leans out the fuel (BLM is 108 when the idle gets real rough). I went by my favorite machine shop this morning and he agreed with me that valve guides could be my issue. I plan to do a leak down test later to rule out the rings. According to the machine shop, the early TBI heads had poor quality valve guides. When they wear out, the valve rocks and does not seal well (especially at lower rpms) and unburned fuel makes it past the exhaust valve resulting in the ECM seeing the rich fuel.

To answer your question, I was referring to the vents at the bottom of the dist where the cap meets the dist.

At this point, a valve job with new guides is my next step; although, I may not be able to do this for a few weeks.

I will update this post when I make some progress on the issue.

//2000CAYukon

I'd change the 02 sensors before I did the valve job. I doubt the valve guides are the problem unless there is some missfireing through the TB or exhaust. Just my opinion though, I've heard of these sensors checking ok but actually being bad. I'll tell you something that happened to me once, installed a new GM EGR valve on a new GM crate motor, it ran great exept just off idle it would stumble and missfire, come to find out they gave me the wronge EGR valve it had a larger hole in the hose connector than the old one did. I went back and got the correct EGR and the problem went away. Keep us posted.

I'd change the 02 sensors before I did the valve job. I doubt the valve guides are the problem unless there is some missfireing through the TB or exhaust. Just my opinion though, I've heard of these sensors checking ok but actually being bad. I'll tell you something that happened to me once, installed a new GM EGR valve on a new GM crate motor, it ran great exept just off idle it would stumble and missfire, come to find out they gave me the wronge EGR valve it had a larger hole in the hose connector than the old one did. I went back and got the correct EGR and the problem went away. Keep us posted.

On the TBI engines, there is only 1 02 sensor and I have changed it twice now (always AC Delco parts). EGR and EGR solenoid have also been replaced (again AC Delco) and all vacuum hoses to it.

I have had the blue smoke startup issue for a while and at this point have convinced myself that the issue is mechanical. Ignition is completely new as well as:
- EGR and Solenoid
- 02
- ECM
- Plugs, wires, coil and dist

Valve job is now scheduled for head removal on 3/10 and installation on 3/17 so we shall see.

//2000CAYukon

Sorry, I misread your original post that it was replaced, don't know where I got it was just checked. I've rebuilt and installed a dozen or so TBI motors and the machine shop I use has never needed to replace a guide in any of the heads I've carried in, several valves though have been replaced. I'm kind of anxious to see how this turns out. good luck

I see the originar title has changed to "Fixed". What fixed it? I can't wait!

I see the originar title has changed to "Fixed". What fixed it? I can't wait!

At one point I thought it was fixed when I found the corrosion on the pickup coil connection to the ignition module. It ran a lot better after I cleaned the vents in the dist.

So at this point, the issue is not fixed. I am performing a leak down test this weekend to see what shape the rings and valves are in. After I have the leak down test results, I will decide on overhaul vs valve job (or maybe the engine is fine and the problem is something else).

Stay tuned!

//2000CAYukon

Well, I did not get a chance to perform the leak down test this past weekend. I did, however, replace the PCV valve and the gromets in both valve covers. Idle is a whole lot better and it no longer cycles up and down.

I plan to drive it this weeks to see how it goes. Right now, I am pretty happy with the way it is running.

Will post an update after driving it for a week.

//2000CAYukon