Odd Timing issue 1999 5.7L Vortec - SOLVED

[West 1]

Working on a friends 1999 Suburban 5.7L Vortec engine. A little history, this rig has 340,000 miles on it, he won't sell it because of all his family history they have in this specific truck. Trust me I have tried to get him to sell. It is not a financial issue, he has a new Diesel truck and his wife has a new car. They just love this old. truck and want to keep it running.

The current engine has less than 40,000 miles on it. I believe it was a Jasper rebuild. It has had random tune issues going on for 3 years now, but maybe only 5,000 miles. In that time it has new wires, plugs, distributor, Cap, New Spider, lower manifold removed and Fel Pro Perma Dry Plus intake gaskets used on assembly. I have done a smoke test looking for intake leaks and found nothing. Fuel pressure is 60 psi key on, running seems to be 52-55, accelerate the engine and it jumps to 60 psi. If I remember correctly the new spider uses slightly lower fuel pressure than the OEM unit. The new spider does have individual fuel injectors. The spider connector has been checked and cleaned. Compression tested good all 8.

My friend asked me to look at it because it was running poorly, had check engine lights for misfires and he did not trust it to drive. It had 2300 misfires in #5 recorded so I focused on that issue. It was also low on coolant and I could see a small leak at the front of the intake at the head so I pulled the intake and installed new gaskets and seals. I could see the original lower intake gasket had been leaking. All seemed to go well on the new install.

Issues today: At idle it will sometimes have misfires. #4 does it the most but almost all other cylinders will sometimes misfire. Most of the time my scan tool shows zero misfires on all cylinders, when it does misfire it will record 2-4 misfires on a cylinder and then clear up.
Before replacing the lower intake manifold gaskets #5 cylinder showed 2300 misfires now you never see more than 2 and it clears. This random low count misfire may be normal in all of these trucks but I do not know. Under throttle it pulls hard and runs well. Gas mileage is good. No check engine lights.

With this said, why this note? Sometimes the engine starts slowly, starter spins it fast but it fails to start on the first try. Maybe 20 seconds of cranking gets it started. I have seen it hard start while the fuel pressure reads 60 PSI. I have read that the GM ECM looks for a proper crank sensor signal and a proper cam sensor signal and tells the engine to start if it sees both are good?

Thinking this might be the cause I checked the Cam retard. I know I set it to almost dead zero 2 days ago. Check it this morning and the cam showed it was at 13.5??? It was below 1, maybe .975 when I set it last. Set at 1200 RPM, checked when the distributor was loose, checked again after it was tight. All good. Now it reads 13.5. How can that happen. I reset the engine to 1.2 at 1200 RPM this morning. Tighten it down and double check it was still at 1.2. Go to set the Crank sensor to make sure it is in spec, after it instructs me to Rev the engine foot on the floor, foot on brake, AC off and rev it till it hits the rev limiter the scan tool says it can't set the crank sensor because the engine is out of spec? I go back and check the timing at the cam and it shows it is at 6.4. What the hell it was at 1.2 30 minutes earlier.

I think maybe it is the cam sensor failing? New crank sensor and cam sensor installed and it tested almost identical after install. Will not stay in time???

More history, my friend had a shop work on it after I last worked on it, so the spider was installed and other work done by a third party. More hands in play here.

I took apart the upper and lower manifold and double checked the spider connections and routing. All good.

Thinking about why the timing is jumping has me a little baffled. Distributor new, engine at low miles so I am not thinking timing chain worn out or distributor to cam gear wear.

I seem to remember the first time I installed a distributor in this engine the timing changed slightly as I tightened the distributor down??? This may be a clue this time because in the past 3 or 4 days I have reset the timing several times now and each time it stayed exactly where I set it as I tightened the distributor down??? I did try with 2 hands to move the distributor after it was tightened and nothing moved so it is not sitting in there loose.

I wonder if the new distributor he bought at Summit has play in the shaft allowing the gear to walk up and down on the cam gear changing my timing settings? I have done as many as 15 restarts after setting timing and it does not change at all. Let it rev to 5,000 RPM just once and the timing will now be off by more than 5*. Have any of you bumped into this with the 96 to 99 5.7L engines?

When I removed the manifold, I marked the distributor, and the hold down bracket for the distributor so I could re install without being far off. I am wondering if the hold down bracket was upside down when I marked it and not properly holding the distributor in place??? Just a thought as I scratch my head on this one. As mentioned a shop had it apart last time and I put it back in the same position it came out in.

I appreciate your thoughts.

Edit: I went out and checked the hold down on the distributor. It is properly installed and tight. Dang, that would have been too easy to repair. I am thinking the distributor has to have excess play letting the shaft float up and down on the cam gear changing my timing? I would think it would jump back to proper time if I revved it again but it seems to stay where it moves to after I rev it?

 

[West 1]

For anyone reading this note: The 99 Suburban is repaired. Thinking about the fact that it won't hold timing I pulled the distributor. Looking hard at the gear on the bottom of the distributor I noticed no wear. That is good, so it is meshing with the camshaft gear and causing no damage.

Next I checked the end play on this gear. This was a new Distributor from Summit and probably only has 4,000 miles on it so far. This distributor shaft had .080 of end play. I believe it should be closer to .010. I pulled the gear off this shaft and made a thicker shim, from the factory the shim on it was .060 and had .080 of total end play. .140 total. The new shim installed as a second shim is .070 thick for a total shim thickness now of .130 and this tightened the shaft up to .010 total end play. It formerly moved up and down a lot, after the second shim was added it barely moves at all but measures to have a tight .010 end play now.

On install I set the distributor tight enough that it takes a firm hand to move it at all. First stab the Cam Retard was 6.5* at 1200 rpm, I was off as these need to be +2 to -2 to work properly.

To speed up this process I decided to hook up my scan tool, engine running at 1200 RPM and adjust it running. Got it down to .125*, very happy with this. Proceeded to tighten the hold down bolt with it running. With each movement of this bolt while tightening I witnessed slight change in my timing setting. This is normal and I have seen this on many engines while tightening the distributor and I am very happy to see it on this truck now, previously I saw no difference as I tightened the hold down bolt before fixing the shaft end play.

With the distributor tight now the timing retard is showing 1.315*. Well within the allowed +2 to -2 so I am happy and left it as is.

Prior to the distributor end play being tightened the timing would change when I revved up the engine and never went back to the proper setting, sometimes it bounced as far as 13.5* off from the initial setting.. Now I have Revved it many times, driven it about 70 miles in the hills with lots of accelerations needed. The timing retord has not changed at all. It is now steady. Finally fixed. So distributor shaft end play can really mess up one of these Vortec Engines. One more thing to look for. Hope this helps the next poor soul going down this road. BTW: it still has very minor random misfires, no certain cylinder, but about once every couple minutes one cylinder or another will show 1 or 2 misfires and then clear up. IT is never one cylinder having all the misfires and they are not often.

On the test drive initially the after CAT O2 sensors were bouncing like the before CAT O2 sensors. The more I drove it the after cat O2 sensors started reading steady between 600 and 750 showing the CATs are starting to work again? Wow, did not expect that. Thought for sure they were dead and gone after it was driven so much with tune issues but maybe they will have more life? It actually passed the CAT scan so it will pass smog testing now.

Mark