slight surging at idle

jvandyke

jvandyke

True Classic
car will surge a bit at idle, not a big deal but curious as to what it could be, seems to go away when good and warm but after a shut down and restart, the surge will be back. Idle tends to fluctuate slightly too, sometimes at 1200, sometimes right close to 1000, on cold start lower than that....no bid deal, just curious.
I did notice some arching from plug wires when watching engine run at night, rerouted wires.
 
Will guess year.

If it is an injected engine a cold manifold that has a leak at the flange will allow more air into the intake manifold. This drops the pressure and the MAP sensor increases fuel as it cannot tell the difference between a manifold leak and moving throttle plate. When the engine is very hot the manifold expands enough to seal the leak.

Now if it is a carb engine the opposite occurs, vacuum drops,which inturn decreases fuel supply and the idle will wander. Over 2000 rpm the leak is so small compared to the air requirements of the engine and now off of idle jets that surging is not noticed.


TonyK.

Grimsby Ontario Canada.
 
Replace the spark plug wires.. they are done wore out. There should be no arcing regardless of how close they are routed if the plug wires are any good.

New set of plugs while you're at this..

Bernice

jvandyke said:
I did notice some arching from plug wires when watching engine run at night, rerouted wires.
Click to expand...
 
Again Jeff... all good advice... BUT...

... most surging is caused by a very small vacuum leak.

Heat and cold causes materials to contract and expand and airs gets sucked in and then it starts to die and then there is more fuel than air and the thing just cycles and cycles over and over.

Do fix the wiring and plugs but surging will still probably occur.

Find a vacuum leak using an UNLIT propane torch and slowly pass the gas over and around all suspected areas of vacuum lines, intake manifold, and all FI componants.

WD40 can be used also but is a bit messy... But clean up with a rag makes things all look shiny and new...

Let us know what ya find.
 
vacuum leak seems viable, will hunt.
The tube that connects the air intake hose, right before the throttle straight down to....? something... is suspect just because it looked rough, split at the end. I'll start with that.
 
replaced the hose between intake and block, showed oil seepage, no difference in idle. Pulled fuel pump to clean and assess it's increase in noise, seemed fine but it's quieter, side effect is idle has smoothed out quite a bit. Could it have been a flux in fuel pressure due to partially clogged pump causing erratic idle? Or I could have inadvertently fixed a vacuum leak but I didn't remove much. I'll take it anyway.
 
Welp... ya really didn't...

... PROVE the problem... so you'll never really know.

I've done the same thing and the REAL problem being... the issue comes back when its either embarrassing or inconvenient!

I don't think messing with the fuel lines in this case woulda caused or cured anything... but the vacuum line coulda sealed at least for the moment!

Anyway... now you'll have to WONDER and WAIT and SEE of if it ever comes back. You may wanna try a different tack next time while the trouble is evident.
 
Idle is definitely different after mucking about with fuel pump, that's not why I went in after the pump, it was loud so I assumed it was clogging. Just a happy side effect I guess. Probably bumped something or, well, I don't know. Oh, ya know I did dump in some Lucas Fuel Injector cleaner, that may have done something too I suppose.......oh well, whatever.
 
!

So this car is FUEL INJECTED? If so, a failing fuel pump will do some really interesting things to the idle.....including causing it to stop completely. As the bearings wear, the pump/motor will bind and slow down. You should hear the change in the pump speed though!

If the injectors are dirty, and the fuel isn't atomized, it's the same as the engine running lean, which will cause a surge. Clean injectors will atomize the gas, and richen the mixture.

Plugged fuel filter will drop the overall fuel pressure, causing lean condition as well, but that seems unlikely. Listen for your pump changing speeds, and use injector cleaner every few months, and watch for intake leaks.
 
eflexter said:
So this car is FUEL INJECTED? If so, a failing fuel pump will do some really interesting things to the idle.....including causing it to stop completely. As the bearings wear, the pump/motor will bind and slow down. You should hear the change in the pump speed though!

If the injectors are dirty, and the fuel isn't atomized, it's the same as the engine running lean, which will cause a surge. Clean injectors will atomize the gas, and richen the mixture.

Plugged fuel filter will drop the overall fuel pressure, causing lean condition as well, but that seems unlikely. Listen for your pump changing speeds, and use injector cleaner every few months, and watch for intake leaks.
Click to expand...

This is good advice if you find no vaccum leaks and the issue points back to fuel.

I had a bad injector,clogging FP and the car was missing on #3 but just prior it did the surge thing. I sent the injectors out to be cleaned/tested, replaced the main fuel filter (meaning the filter after the pump) and installed a pre-pump filter.

Made a huge difference.
 
I took off the pre pump filter after I redid the tank, maybe I should be running one even though the tank is like new.
 
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