95 GMC K1500 350... replaced distributor...timed it at 0 with computer unplugged... plugged it back in and timing changed by few degrees, engine surges and runs poorly. Checked plugwire route for interference(good). Is this a computer problem or maybe MAP sensor? New plugs, wires, cap, rotor, and distributor. Engine runs great before computer is plugged back in!!!

Spark plugs are set at 0.035 inch.
Ignition timing is set at 0 degrees BTDC. The computer then controls the timing from there. There will be a wire that you unplug before you set the timing, then once you have verified that the timing is at 0 degrees, then reconnect the wire and you're done.

Remove spark plug cylinder #1 place cylinder end of compression time. Crankshaft pulley timing mark at 0 degree
remove distributor cap then find on the cap where is the number 1 cylinder
while placing distributor in place place rotor to face cylinder 1 wire
replace cap and wires
start engine and move dist. to the best rpm then with timing light adjust timing
hope it help

Not sure if the 95 is the same as the 93. On the '93 there is a timing advance wire you have to unplug (brown wire near the distributor on the firewall with a plug connector.) Start it up with the wire unplugged and set timing to 0 degrees BTDC. Turn off engine. Plug wire back in and computer takes over. There should be a sticker on the radiator shroud that gives timing and spark gap settings.

The timing (base), is 0 degrees before tdc. Keep in mind the timing connector must be unplugged. usually a brown wire with a black tracer, has a single connector, located under glove box, or by distributor. Thanks...

The engine timing markers down on the harmonic balancer/crankshaft pulley should be aligned with the timing indicator on the engine block to "0" degrees when the engine is in the compression stroke for the #1 cylinder, and this is TDC or "Top Dead Center". When something blows out of the spark plug hole that only means that the piston is coming up on TDC, but the crankshaft will still have to be turned until the timing markers are at "0" degrees. You should be sure that you do know how to properly align the timing marks to "0" degrees, and the timing marks must be set at "0" degrees before the distributor can successfully be installed.

Be certain that the timing marks are set at "0" degrees and on the compression stroke for the #1 cylinder, or the timing will be 180 degrees off when you install the distributor. This has to be precise and guessing will not get the distributor properly clocked in so that the ignition timing can be correctly set.

Once you are certain that the timing is set to "0" degrees "TDC", then mark the distributor housing where the #1 position on the distributor cap will be when the cap is installed on the distributor, so that the distributor housing shows a reference mark where the #1 position is, then the distributor should be installed so that the ignition rotor is pointing directly at the #1 position on the distributor housing, or just before the #1 position mark according to the rotation of the ignition rotor. The distributor should be able to turn and point the ignition rotor on either side of the #1 position mark to provide plenty of timing advance or ****** so that the ignition timing can properly be adjusted to specifications after the engine has been started.

Be sure that you follow the proper timing procedure for that vehicle and remove any timing spout connectors that are described in the set timing procedures

Also, be certain that you are actually using the #1 cylinder to set the engine to TDC and to actually time the engine with a timing light. Here is a firing order diagram that should also help to assist you. Let me know if you require any further assistance.

compression check ?sounds like head gasket

The v-8's timing is supposed to be 0 degrees plus or minus 2 degrees and is only adjustable if you have a scanner. A timing light does not work on them.
These engines can be "timed" in the way you are used too.
The distibutor housing holds the cam sensor, The PCM calculates the timing by measuring the crank signal and the cam signal.

What you are seting by turning the distibutor housing is the correlation between the 2 sensors. On the V-6 model, the housing holddown bracket is fixed to the housing so if you return the rotor to the proper position when reinstalling the housing, the engine should be at the correct cam sensor position setting.they did not fix the holddown bracket to the V-8 models, it would have saved a lot of headaches.
To set the correlation between the 2, you will need a scan tool that can read camshaft position offset. With the engine RPM over 1200 adjust the offset too 0 degrees.
TEST PROCEDURE
The ignition timing cannot be adjusted. The distributor may need adjusting to prevent crossfire. To insure proper alignment of the distributor, perform the following:

With the ignition OFF, install a scan tool to the DLC.
Start the engine and bring to normal operating temperature. IMPORTANT: Cam ****** Offset reading will not be accurate below 1000 RPM.
Increase engine speed to 1000 RPM.
Monitor the Cam ****** Offset.
If the Cam ****** indicates a value of 0°+/-2°, the distributor is properly adjusted.
If the Cam ****** does not indicate 0°+/-2°, the distributor must be adjusted.

ADJUSTING PROCEDURE
With the engine OFF, slightly loosen the distributor hold down bolt. IMPORTANT: Cam ****** Offset reading will not be accurate below 1000 RPM.
Start the engine and raise engine speed to 1000 RPM.
Using a scan tool monitor Cam ****** Offset.
Rotate the distributor as follows:
4.1. To compensate for a negative reading, rotate the distributor in the counterclockwise direction.
4.2. To compensate for a positive reading, rotate the distributor in the clockwise direction.

Repeat step 4 until 0°+/-2°, is obtained.
Turn the ignition OFF.
Tighten the distributor hold-down bolt to 3 Nm (25 lb. ft.).
Start the engine, raise engine speed to 1000 RPM and recheck Camshaft ****** Offset.
Thank you for using fixya and good luck.

0 degrees BTDC would be the spec , there is a brown w/tan tracer wire near the distributor , you have to disconnect the connector before setting timing

after connecting your wire try resetting your distributor or pull timing chain cover and ckeck timing marks on gears

did you check to make sure your distributor is timed right whith the engine?