Oscillating Idle- Surges-settles-surges-settles

I have an 93 Elantra 5-speed. The engine ALWAYS oscillates rpms,
surging, then settling back down then surging again. It's especially
noticeable at idle where it cycles with regularity of a few seconds
between regular idle speed and what sounds like double that, say between
700 and 1500 rpm.

I'm not techno-savvy, but I'm wondering if someone might have a best-guess
troubleshoot diagnoses.

Suggestions much appreciated.


Bob
Ashley
******************************

 

This might be worth a try, it sure worked great on my fuel injected '86
Fiero, so I think this might pertain to any multi port fuel injected
vehicle:
Remove the "snorkel" assembly that feeds air into the fuel injection
manifold. What you should see now is the throttle plate. It will be closed
because nobody has their foor on the gas. It will open up as the throttle
is depressed. You can also open it manually. If you look on the back of
the throttle plate you should see a build up of black carbon. Take a can of
brake clean (or other suitable cleaner), a box of q-tips, and go to work
cleanning up that whole area, especially the back of the throttle plate.
After cleanning and reassembly it may take the engine a while to burn off
the cleanning stuff and return to normal but, at least in my case, the
surging idle was gone. It is still hard for me to believe a little carbon
could have that much effect on the idle quality, but it did.

Dan

..

 

Thanks for weighing in, Dan. I've got your prescription in my repair
folder. I'll report back if it succeeds or not.

Scouting around, I notice that the idle speed controller (ISC) can also be
associated with this problem of surging-settling idle oscillations in
early 90s Elantras?

Thanks,

***
rib

Bob
Ashley
******************************

 

You'll want to check anything that has to do with idling. This condition
occurs because the ECM wants to lower the engine RPM, but can't restrict
the air flow any further. It then begins shutting off the injectors in
bursts.

Check your throttle body for excessive carboning as the other poster
suggested.

Make sure the idle switch is closed. On your throttle body, there's a
little switch with one wire that grounds when the throttle is closed.
Also check to be sure the idle switch isn't holding the throttle plate
open. It can do that if it's far enough out of adjustment.

Check to make sure the throttle cable isn't pulled so that there's no
slack and is holding the throttle open. Adjust it to have a little slack
when the throttle is closed if necessary.

Check your idle motor in the bottom of the throttle body. It'll have six
terminals in two rows of three. Unplug the connector and check the
resistance on the motor from the center pin to each outer pin in each row.
Each should be about 30 ohms (+/- 10%). Readings way off indicate a
shorted or open idle motor. Check your throttle body for signs of coolant
leakage. Coolant can leak into the idle motor and damage it. A short
circuit (zero ohms) will damage the ECM. If you have coolant leakage,
you'll need to replace the gasket between the two halves of the throttle
body.

Check the idle adjustment screw. You'll need to ground a wire to set base
idle. It's been a long time since I've done one of these, so I don't
recall exactly where the wire is. The service manual on www.hmaservice
should be helpful. Then, with that wire grounded, adjust the base idle
screw (should be under a rubber cover coming inside a hole in the top of
the throttle body). You may need to back the screw out to keep the car
running with the wire grounded. When at operating temp, adjust the screw
so the car runs at the RPM specified on the underhood sticker.

After you've corrected anything as necessary above, go back and adjust
your idle switch so that it opens/closes as the throttle is moved slightly
open/released. Also adjust the throttle position sensor to be about 500mV
at idle.

 

Thanks hyundaitech for this comprehensive advice. I won't be doing this
work myself, but I'll print your prescription and hand it over to the
person who will be doing it. Even so, I'm still learning something from
your input. Many thanks!

Bob

Bob
Ashley
******************************

 

as johhny 5 would say, morrrrre inpuuut =)

 

This did the trick. Problem solved. Thanks! Thanks to Dan K, too. He also
suggested the clean-up fix.

Bob

 

93 Elantra 5sp. Had the surging/pulsing problem with the idle described
below. That got solved by a thorough clean of throttle body.

Now today we were toodling around doing short trip errands. Warm dry day.
First few stop/start trips were fine. Then the car would not idle period,
just die at every stop light/sign. Why would it run fine for half to a
full hour, then decide to start fainting like this?

Ideas/suggestions welcomed.

Thanks

Bob