Hello, I have an issue with my 1997 Chevy Suburban with 5.7 Vortec and dual fuel conversion, 215k kilometers: it developed a consistent rough idle since a few days that started when driving in very rainy weather, but it does only do this when running on LPG, not on gasoline. On gasoline it runs very smooth. Hot or cold makes no difference. Any idea where I could start with the troubleshooting? I also hear a very distinctive tick once every revolution when it is idling rough, sounds a bit like a noisy valve. But when on gasoline there is no noise. Any thoughts on where to start? Thanks!
Greetings Fredje and welcome to the GMT400 forum!
By coincidence I've been down in a deep Vortec distributor rabbit hole trying to get
a better understanding of precisely how the performance of these units can deteriorate
and we end up with intermittent misfires with no Check Engine Light, P0300-P0308 codes
with a light, or a CEL with a P1345 code.
You have a very interesting problem description in that you have a somewhat rare dual-fuel
setup, and the problem presents when running on LPG but not gasoline. Disciplined troubleshooting would have
us focus on how the use of one fuel 'stresses' the ignition system differently than the other fuel.
Which, at first glance, sounds either impossible or highly improbable, since the distributor is located on the back
of the engine working with electrical arc distribution, and isn't inside the combustion chamber where the different fuels
are actually burning?
****
According to the following youtube video, one major difference between running on gasoline vs LPG is that
there is much more timing (spark advance) involved when the engine is running on LPG. This additional spark
advance leads directly to an appreciably larger gap that much be traversed between the spinning rotor firing tip
versus the stationary spark plug wire terminal inside the distributor cap. Please observe closely as the owner
of a clear distributor cap-equipped
dual-fuel Vortec motor gives us a good view of what's going on:
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I find this really interesting, and it actually makes sense that they would aggressively turn up the spark advance
on the LPG, mimicking a higher compression engine on the fly by doing so. (Here in the states Honda has been
selling a Honda Civic GX, featuring a single-fuel CNG engine that's running a 12.5:1 compression ratio.
HIGH COMPRESSION Civic GX link)
As for the misfire showing up after very heavy rainy weather, all distributor caps find this weather extra challenging,
and quite possibly internal aging combined with the extra humidity has uncovered an internal weakness allowing
misfire under stress? New cap & rotor or not, the symptoms warrant a 'close' inspection of the inside of the
distributor cap and rotor. (Sometimes the sparks blow through the rotor to the inviting ground presented by
the distributor shaft it's bolted to.)
Lastly, the 'once per revolution noise'? First of all, we have to keep in mind that the distributor actually spins at
1/2 of the engine rpm. (360° of spark distribution occurs over 720° of crank rotation.)
So if you can prove it's once every engine revolution we would either have to declare that there's excess play
in the distributor bushings allowing interference between spinning rotor firing tip and the stationary spark plug
wire terminals twice per rotation, *or* expand the search for the noise to other parts of the engine where a
'once per revolution' noise could be generated.
But let's say that what sounds to the ear is 'once per revolution' is emanating from the dizzy itself. As in once
per dizzy revolution? Assuming you have an old inductive clamp-on timing light lying about, you can try to
move the clamp around the 8 spark plug wires and see if you can't get the noise & light flash to sync up,
thereby showing you which cylinder is associated with this noise? For example, unbeknownst to you the
PO (Previous Owner) broke one of the distributor cap mounting screws off, the cap is now at a slight angle,
and the spinning rotor tip is hitting the #4 terminal as it goes by? (Note: This works best on a slow idle, not at
speed. But it
does work. Also works to help identify a single noisy lifter. [with a little math to account for the
offset in intake or exhaust valve opening vs. TDC...but I digress.] ) (
OLD inductive timing light link)
****
This is starting to turn out to be a term paper, so I'll wrap this up here. Thanks to your detailed problem
description & unique dual-fuel setup, if I were you I would look carefully at the condition of your distributor
cap & rotor, and replace them at any sign of wear/weakness/failure. Make sure that your distributor doesn't
have excess slop/play -- they are known to develop bushing wear when a lot of miles have accumulated.
And if the spark plugs are old soldiers, then the amount of Kilovolts requred to jump the worn gaps goes up,
further stressing an already stressed situation. BTW, you don't need fancy plugs, a set of non-exotic metal
NGK plugs with the V-groove have worked well for me over the years.
So give all of the above some thought, look for the worn out parts and replace them, and by all means if you
have any further questions -or- fix the problem, please come back here and share the answer for the 3 other
dual-fuel owners out there.
Happy Hunting!
Cheers --