Hi all,  I spent a couple of hours the other day gathering clues to my rough running problems.  First the facts, then my theories.  I’m looking for any input.

Description of the problem:  The problem is really more of a surging rather than rough running.  At less than about 2200 RPM the engine will begin to surge.   It gets worse the more I lower the RPM.  Each time I make a change to the throttle setting I adjust the manual mixture knob to find the leanest point where the engine does not surge.  I have Tracy’s standard mixture monitor.

Each cycle of the surging takes a little over 1 sec.  RPM surges buy 300-400 RPM.  Manifold pressure fluctuates by 2-3 inches, and the mixture monitor fluctuates the full range of its scale.

At this point I can manually increase the mixture up to the first red bar.  This is over rich and the engine runs a little rough and the RPM drops a couple of hundred.  It will smoke a little more, but at least the surging stops.  If I lean down into the amber area the surging starts again.  The surging gets very bad if I lean down into the middle of the scale.  This is what happens around 2000 RPM and 15in MAP.

If I further close the throttle, the surging is similar but to a great scale.  In order to stop the surging I have to increase the mixture beyond the top of the scale.  Here it will be running rough from an overly rich condition but still surging a little too.   Down around 1000 RPM it is still surging but so rich it will barely run.  Beyond that point, either the richness kills the engine, or the surging kills the engine.

Assorted clues:  Several additional clues may shed some light on the situation.  Mostly centering around the Fuel Pump.  I have noticed that if the engine is running rough from being too rich (wont run leaner because of the surging) and I turn the engine off (by shutting off the fuel pump) the will run perfectly smoothly for several seconds before it stops.  In other words, as the fuel pressure drops below 10 psi the mixture is lean enough for the engine to run well and it purrs for a couple of seconds without surging until the fuel pressure reaches zero.

In an attempt to track this down more I ran a couple of experiments.  I normally run 45 psi of fuel pressure.  I have the TWM adjustable with manifold compensation.  First I turned down the fuel pressure to 40 ( as low as mine goes) and possibly saw a slight improvement.  Then I turned up the fuel pressure to 55psi and the surging definitely got worse.  The cycle time was faster, it was harder to get it to go away by enriching and the problem continued up to about 2800 RPM.

So it wants a lower fuel pressure.  But why?  More experiments:
1. Did my best to match the mixture between the two rotors, but with the turbo I only have a single common EGT.  I noticed a slight improvement.
2. Unplugged the turbo air inlet (engine now running n.a.) – no change.
3. Disconnected the MAP line from the fuel pressure regulator – no change.
4. Had my EGR passages welded closed (in case they were leaking) – no change.
5. Tried assorted arrangements of where to sample MAP  -  no change.
6. Run engine on either the primary injectors (stock turbo in the engine block) or the 52 lb secondaries in the Throttle body – no change.
7. Run on leading or trailing coils. – no change.
8. adjust timing with light – no change.

Other findings.  I had previously that turning off my alternator caused my engine to stop.  I turns out that turning off the alternator just makes the mixture leaner.  The effect is smaller if running on only one set of coils, and exaggerated if there is a high amp load.  But this situation is always fixed simply by enriching the mixture (about 110 deg turn of the knob).  The buss voltage

Summary:  So it seems that installation is particularly sensitive to changes in fuel pressure.  My next  step will probably be to buy a lower pressure fuel regulator.  The thought is that maybe running 35 PSI will stabilize everything (i.e. maybe with only having to hold 35 psi the pump will no be as affected by changes in fuel flow or system voltage.  I will have my injectors checked and cleaned although I don’t imagine that will help.

Any other thoughts?  

Thanks ahead of time for any input I may get.

Dave Leonard