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Engine surges at mid RPMs

kaboodle

New member
My 1990 PMC Chevy 305 is running great...EXCEPT that between about 1800 and 2500 rpm it surges, almost like I am pumping the throttle. When it starts to ssurge, I push up the thottle and it settles in nicely at about 2800 rmp.
What's up?
One theory, and please comment. With a lot of past fuel problems, I installed a second fuel filter this past winter. Is it possible that at these mid ranges the fuel pump is not putting out enought pressure, but at the higher rpms (above the mystical 2800) it regains control?

I would suspect that there is crud in the tanks as well, but that does not address the mid-range issue very well.
 
Are these EST ignition systems w/ knock sensors by chance?

IMO, 1.8K to 2.5K rpm is right in the range where detonation may be occurring.
The hull is most likely not planing, and you are likely above hull speed.
This is generally not a good gasoline engine RPM to operate at for any duration, IMO.


If a knock sensor is picking up on this, it may be cutting back on spark lead....., thus causing the surge that you are noticing.

If not EST, then it is likely a fuel delivery problem in some form.
 
Rick and Justintime
Thanks for your input. I have just read the thread to Larry302 Ignigiton Issues (about 5 items above this one) , and I want to ask a follow up for your thoughts. I did not mention in my description that I have had some voltage issues. (Please remember I am a converted sailor, and my knowledge of engines is scary.)
I was not getting 12+ voltage, and thus had my alternator rebuilt. Did not do the trick, so I started to poke around. By doing just that, wiggleing wires and opening a multi wire plug behind the voltage regulator (which I cannot get to without taking off the radiator tank), I pretty much fixed the problem. But, then on the next cruise, the voltage dropped again until I wiggled wires again. CLEARLY I need to fix that, but, to the point, could this be the cause of my surging as well. From my interpretation of the Larry 302 issue, it sure could be. Should I start by replacing these wires that have been sitting atop the engire for 20 years.
Thanks .
Bill
 
Bill, I hate to see any circuits circumvented from the OEM, but it may be a place to start.

As suggested to Larry, you may want to pull your Hull harness/Engine harness interface connector/connection apart, and look for any corrosion that may be causing a voltage drop.
If you were to circumvent any of this, I'd do so in a way that will be obvious to any other person who may end up working on this.
Or see if the wire alone can be replaced, but kept within the harness.... if possible.

Just a thought!

Also, in my previous post, I asked what ignition system is being used.
EST or a more standard system likely used in 1990.


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................ I would say first things first get the charging system 100% then you can diagnose the surging issue if still there.
Agreed! Good call! First things first!

However, be careful with the trickle chargers. These are known as battery selling agents.
IMO, best to use a smart charger that will cut back proportionately, and go into "float" or "maintenance" mode....... especially if you want to leave it connected for any length of time.

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