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