accelaration problem ..exhaust

[laurie88]

Guys

The distr was never touched befoe only changed cap and rotor after sympton was noticed pcm said the timing may hav been slightly off anyway and steadily went out of spec as the timing chain worn...........not sure if I buy this

The cam retard is now set to 7 degree following pcm update and boat runs a dream

[91xl]

96-98 gm vortec 4.3l and 5.7l in trucks have the same issue with cam retard changing and causing power issues worse on high mileage trucks so i would suspect some chain stretch could of moved out of spec

[91xl]

also noticed laurie88 has an excalibur not gt 40 which is gm not ford the excalibur 330 and 343 are based of the gm vortec engine

[DanielC]

I totally agree with trying to learn as much as we can about our engines.
I am a bit of a gearhead, and went through the automotive program at a local community college, back when cars had carburetors, and distributors, with points.
I recently went back, to the same community college, first, to get access to the shop, and get an engine rebuilt, and kind of refresh my knowledge, and then to take a class in fuel delivery and emission systems. It was about a 35 year time difference between the time I first took auto classes, and the most recent time.
Many things have changed, many things stayed the same. A chevy 350 is still a chevy 350, only now, it is a 5.7. In the 1970's, almost every multi-cylinder engine had a distributer, and it totally controlled the ignition timing. Now, many engines do not have distributers, they have coil packs, or even Coil on plug configurations.

Back to the chevy 350/5.7. It still has a distributer, but it no longer has much to to with the ignition timing. The engine's computer now has taken over that function. The old 350 distributer had some weights in it, and the faster it went, the more it advanced the ignition timing. It also has a vacuum diaphragm, and the higher the manifold vacuum, the more it advanced. When the distributer was made, they could run the distributer on a machine, without the engine, and they could perfectly align, or index the timing of the points opening to the position of the rotor under the cap to the eight towers on the top of the cap. Once that was done, a notch on the distributer cap kept everything in "good enough" alignment. Before you could rotate the distributer body far enough to crossfire, the ignition timing would be so far off the engine would not run anyway.
Fast forward to a modern chevy 5.7. The distributer is still there, but no longer controls the ignition timing. The engine's computer now does. It now has two functions. It is a high voltage switch, that connects the rotor under the cap to the proper terminal, and it is a convenient place to put a cam position sensor, because it is driven by the camshaft, directly.
Because the engine will start, and run with the distributer indexed slightly wrong, and it no longer controls the ignition timing, it is very easy to rotate it far enough to cause a crossfire. So, when working on the 5.7, instead of the relationship of ignition timing to rotor location being built in when the distributer was made, you have to set it, by the cam retard spec.

Laurie88,
Yes, the timing chain can wear. As it wears out, the camshaft slowly retards behind the crankshaft position. Because the distributer is driven off the end of the camshaft, as the camshaft retards, it takes the distributer rotor with it.
It could be very possible that your engine was assembled with parts that had it close to one end of the allowable tolerance for the cam retard specification. As these engines were put into service, and had hours built up on them, PCM may have started noticing problems, and issued a new cam retard specification.

Lets all hope this thread helps someone in the future with similar problems.

[Andy3535]

I really think you should all read the first PCM update that i uploaded!!!
This is a problem that only some boat owners will have,my boat started to play up at the end of the season. Many people would not notice the misfire as it is at low speeds and and at low revs.(there are no bangs or pops the engine is just missing slightly)
Streched timing chain!!!!!Laurie's boat has done 400 hours my boat also a 206 built 2007 fitted with the vortec excalibur engine has run 1344 hours,it also runs just on LPG gas:

Andy

[91xl]

i know how the ignition works the point i was trying to send is that if it is far enough to arc to the wrong post it will do it on all cylinders and therfore will not run. yes the ecm controls spark but it is like a fuel map it has a set table of adjustment. electricity takes the path of least resistance. i went to a 2 year gm tech school and i am a 3 time recertified ase master tech. the crab style cap this engine uses is also known for burning through the pastic were the leads are run through it. not arguing just applying my 2 cents and maybe help a fellow nautique owner!! i also found that on the trucks it may be in spec with no light on but can be moved to the plus side for some power... found this out while doing many intake gasket jobs on 5.7l vortec trucks