96 5.7 ltr timing off

[ls2002]

My truck idles fine but when driving and trying to accellerate or under a load it feels like the eng is bogging down as if the eng is getting too much fuel or missing. Took it to the mechanic and he said the timing is -45 degrees and MAF out put is 0.0. Mechanic thinks the gear on the distributer has slipped. When I went to the salvage yard to get another distributer talking to the guy there he thinks it would be more likely the timing chain jumped. I went to autozone tonight checked compression on the #1 and #3 both read 90psi.
Question how much does it take for the timing to read -45degrees?
Can either one of these be able to let the timing get that far out and still run smooth at idle.
Could either a bad crank or cam sensor show timing off by -45 degrees.

[ls2002]

wanted to add that the mechanic said there was no miss fire codes being read and the MAF sensor was reading 0.0 he said it should read .4 to 4.2 volts but has no effect on the timing.

[jdmccright]

90 psi is way low. Are the other cylinders higher? They should be in the 170-180psi range.

You may have a leak in the head gasket in between those cylinders, but a jumped timing chain may also cause this as well if present in more than just those two.

The waaay off timing could be a bad sensor, but a truck that far off is usually not runnable. I'd recheck the ignition wiring & make sure they go to the right cylinders and make sure they are reading the right cylinder as #1 from the distributor....it can happen.

Good luck!

[tblake]

Did you only check compression on 2 cylinders? The only accurate way to check compression is with all 8 plugs out, battery charger on and throttle blocked open.

As JDM stated, 90 is too low, my truck had around 160 psi after 8 puffs of air on all 8 and it runs fine.

[j cAT]

Originally Posted by ls2002

wanted to add that the mechanic said there was no miss fire codes being read and the MAF sensor was reading 0.0 he said it should read .4 to 4.2 volts but has no effect on the timing.

I maf should be at normal idle 1.2 volts. measure the intake vacuum if normal range of 15-21 inches replace the maf.

the maf has 3 wires ground , output , and voltage input of 5 volts dc.

[j cAT]

distributor gear is possible. this should cause a miss fire. poor compression on some cylinders will cause poor power . also with checking engine vacuum you can see at idle then increase rpm to observe how the vacuum changes. this is a quick way to narrow down how well the engine is operating.

at idle the vacuum should be 15-21 inches and steady. If when gradually increasing RPM the vacuum start pulsing rapidly the valves are bad ...

other testing is possible using the guage . most repair manualls have details on this .

[777stickman]

The only true way to check timing on your 5.7 vortec is with a scan tool that can enable "cam retard or cam offset" mode. This is called base timing. When in this mode the timing should read 0 +/- 2 degs at 1500 rpm's.

I agree that at -45 it most likely would not run at idle. Either he had it on the wrong plug wire or maybe he didn't reset his timing lite?

Normal VCM controlled timing at idle is around 24-30 degs advanced. This is out of the base timing as described above.

Also you need to figure out the MAF & get that fixed. It plays a big role in how the motor runs going down the road.

[ls2002]

Thought I would post an update, I bought a compression test kit and have rechecked the 2 cyl they are checking 180 plus and holding. I will be testing all 8 cyl and will post back.
Can someone give me a quick overview of how to check the eng vacuum. Most important where do you hook up the vac gauge, also what I should look for as readings.

On one of the reply posts it was stated maybe he had his timing light hookup to the wrong wire. There was no timing light he was using a scan tool (professional) that was showing a -45 degrees. the scan tool was only plugged into the obdII plug.

Also is there anyone that can recommend a good scan tool and program for the pc. Have done a lot of looking but would like to hear from people that use a pc base obdII reader.

Thanks

[ls2002]

When checking the MAF the 3 wires can someone tell me which is ground and which is power and is the power wire 12 volts
with the eng running can i use a multi meter and check the out put volt by tapping into the wire at the MAF

Just finished the compression check on all 8 cyl readings are as follow:

1. 190 2. 190
3. 185 4. 185
5. 185 6. 185
7. 185 8. 185

the plugs has a little carbon suet, which i think is from running so rich and/or the timing being so far off
also the plugs are double platnium.

[j cAT]

Originally Posted by ls2002

When checking the MAF the 3 wires can someone tell me which is ground and which is power and is the power wire 12 volts
with the eng running can i use a multi meter and check the out put volt by tapping into the wire at the MAF

Just finished the compression check on all 8 cyl readings are as follow:

1. 190 2. 190
3. 185 4. 185
5. 185 6. 185
7. 185 8. 185

the plugs has a little carbon suet, which i think is from running so rich and/or the timing being so far off
also the plugs are double platnium.

I thought you said the map sensor. my mistake.. the Maf wires are like stickman described 12volts pink, ground is black white, the output is yellow to the PCM..If you have 12volts and a good ground but have no output voltage the MAF sensor wires are broken/open...the greater the air flow thru the MAF the higher the output voltage.. when engine is running if no voltage the engine will run very poorly..



any hose connected to the intake manifold will be vacuum. pull a small hose off and feel the suction..connect vacuum gauge for a reading.

I can time an engine using this method very closely. I have done this many times..using a timing light on some engine this is possible..

I believe your 96 5.7 would require a tool for an accurate setting..but you should be able to get it close..If you never messed with the timing I would suspect the distributor/and or gear as defective;and or worn. remove cap check rotor for too much play wobble etc arcing.

[777stickman]

This is out of my '98 C/K shop manual. Your '96 maybe different.

Pink wire is 12v with key in the run position thru fuse #21 (ENG 1) in the underhood fuse block.
Blk/Wht wire is ground.
Yellow wire is signal to VCM. With key in run and engine off you should have 5v nominal. Anything less than 4v or more than 6v is not good.