I just adjusted the timing on my 1995 5.7 Silverado. My truck was getting horrible gas mileage and I had already done everything else (i.E. Replaced every damn sensor). The timing was set at around 10 degrees so I set it at 0 (or about as close as I could get it to 0.) This was a pain as every time I tried to tighten that distributor bolt it adjusted the timing slightly. ANYWAYS. When I got the bolt sufficiently tight I checked the timing one last time and then turned the truck off and reconnected the black-tan wire under the firewall (ECT or something?) I drove it around for a bit and rechecked the timing when I got back. I left the black-tan wire plugged in and noticed that my timing was wayyy advanced (probably around 16 degrees) and fluctuating slightly as the RPM's went up and down while idling (This is one of the annoying things I have been trying to fix). So I unplugged the wire and started to re-do the timing adjustment. Well after I unplugged that wire the timing was back to 0 degrees.
So what the heck? When the wire is not plugged in the timing is at 0. When the wire is plugged in the timing reads around 16. Should I leave the wire unplugged? Or is it normal for the timing to jump back up when you plug the wire back in?
You disconnect the wire to reach base timing
When the wire is connected PCM is in control of the timing and it does advance it as needed
After base timing is set wire should be plugged back in
March, 28, 2011 AT 1:14 AM
So this is normal then? Or is something up with my PCM? Thanks for the reply!
March, 29, 2011 AT 12:48 AM
Yes this is a normal operation of the PCM
you should be OK
March, 29, 2011 AT 1:01 AM
I noticed that my transmission has been acting a little funky since I adjusted my timing to TDC. Is this even related? I'm worried that the timing has probably been off on this truck for several years and changing the timing back to factory settings may do more harm than good. Any thoughts?
March, 30, 2011 AT 11:15 PM
Check TP sensor, MAP sensor and or VSS sensor for proper operation. Repair or replace as necessary. If sensors are okay, check tire size and axle ratio. If tire size and axle ratio has been modified from stock applications, buffer (DRAC) must be recalibrated or replaced with correct calibration.
March, 31, 2011 AT 2:57 AM
TPS, MAP, IAC, Fuel Pressure Regulator, PCV, Engine Coolant Sensor, O2 sensor all replaced. I have been getting about 8-9 mpg and I have no idea why.
I have a new VSS that I haven't installed yet. I looked under the truck and noticed tranny fluid around the sensor. A month or so ago a mechanic told me that my "speedo gear" was leaking, whatever that means. I notice when I turn my vehicle on the speedometer starts at about 5-6 mph so that made me think I had a defective VSS.
The transmission just isn't shifting right. Like, it shifts clean but the truck just sounds like it is revving up too much to get up to speed. And if I hold the accelerator at say.25 mph before it shifts it takes a little while for it to change gears. This is especially bad when the engine is cold. Oh, and it starts really hard when cold and like a gem when the engine is warmed up.
March, 31, 2011 AT 3:09 AM
I also should mention that when I accelerate there is a lot of rattle almost directly under where I am sitting in the driver seat. This goes away after about 20 mph. The mechanic said something about how shocks on chevy pickups get compressed or something when you stop and then when you take off they are getting "stretched" or something so you are hearing those rattle. This didn't make much sense to me. I wondered if it was the exhaust so I checked for any play in the exhaust or missing hangers but nothing
June, 7, 2011 AT 7:34 PM
How many miles on the vehicle? Has the transmission been serviced? What color is the tranny fluid and does it smell burned?
The "speedo gear" the 'mechanic' was talking about is a gear inside the tranny that turns a cable that moves the speedometer. This is sealed the tranny body with an O-ring.
The shifting issue: Unplug the vacuum modulator and drive the vehicle. See if that clears up the problem. Try shifting manually. If either of these clear up the shifting problem, you have a bad modulator.
Your truck's tranny may be equipped with a vibration dampener. This dampener may have failed, causing the 'rattle'.
If the catalytic converter is equipped with a heat shield, these are notorious for causing rattles.
Hard start when cold sounds like a problem in the cold start circuit, like the engine isn't getting the extra shot of fuel it needs when cold.
Going back to the rattling problem, I'm going to presume that you've gone over everything with a socket, ratchet and wrench to ensure everything is tight, such as tranny crossmembers and driveshaft nuts and bolts.