Idle Surge FAQ

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This VFAQ used to be located at http://www.myzero.com/engine/surge.html

This VFAQ is based on Terry's Troubleshooting Tips for Idle Surge, but this FAQ written for the layman who may not know all the lingo. In my experience, there are 4 main causes for the idle surge:

Dirty Throttle Body ^*[$0]
Leaking (or missing) Base Idle Set Screw (BISS)^* [$0.17 - $15]
Malfunctioning EGR Valve^* [$0 - $10]
Malfunctioning ISC (Idle Speed Control) [$200+]

As you can guess, the number in brackets [] is the estimated cost of repairing each of these items yourself, and the asterisks (*) indicate that these items have failed on my personal car ('91). The listing is also listed in order of difficulty to fix or access.

In summary, the car sucks all air in through the air filter (technically the MAS [Mass Air Sensor]) where the amount of air is measured by the computer. It does it's computations and figures out how much fuel to mix with the air. If either the second or third things in the list fail, the engine starts sucking some air in other than through the MAS and so the computer's calculations get screwed up, then the mixture isn't correct, then the engine tries to correct it causing the idle surge. If the first or last item in the list fails, then it's just unable to keep the idle steady through a malfunctioning part (nothing to do with the air).

I'm splitting up how to check and repair each of the items in the list. If initially troubleshooting the idle surge, go in the order of the listing. Use these target pointers to jump to the section you want: Throttle Body, BISS, EGR, and ISC. If you do all of these things and it doesn't fix the problem, check out Terry's page (at the top). He has mentioned other possibilities that I don't cover here, so maybe it's one of those.

Throttle Body Cleaning

The Throttle Body Plate gets dirty over time, and will sometimes stick open a slight bit at idle allowing more air than normal. Follow the FAQ I have on cleaning the TB (Throttle Body) to eliminate this possibility.

BISS (Base Idle Set Screw)

The BISS has a small rubber O-ring at the top of it. Over time this O-ring deteriorates, and the screw slowly starts backing itself out. If you don't catch it, it can even fall out totally. If the screw is gone, or the O-ring is old, the BISS allows air to get by into the Throttle Body causing the surge. This procedure will show how to replace the O-ring:

First find where the BISS is located (see picture). [note: in the picture you'll see the RTV sealant mentioned below]
There should be a small rubber cap, under that a head of a screw
If you don't have a cap, don't worry (see below). If you don't have a screw, then you'll need to eventually get one (see how to get one below)
Back the screw out with a screwdriver and remove it.
Take a pair of pliers and remove the rubber O-ring around the top of the screw
Replace the O-ring with a new one (you can get one at any home store, the size needed is #5 O-ring 3/8x1/4x1/16)
Put the screw back in, and screw in until it stops. Then back it out about 2 turns.
Start the car and check idle speed. Adjust the BISS to get the idle ~700 RPM.
If this didn't fix it, then go on to the EGR Valve step

After I replaced my O-ring last time, I set the screw in the correct place then filled the hole to the top with high-Temp RTV sealant. So now even when the O-ring deteriorates, it won't be able to back itself out through the sealant. Hopefully, fixing this problem forever.

If you have lost your BISS screw (Mits. Part #MD614948), Tallahassee Mitsubishi sells a replacement kit (BISS, O-ring, and rubber cap) for ~$15. Tell John (parts manager) that you're a member of Club DSM and he'll give it to you for a discount. This is who I buy all my factory parts from, and have had nothing but excellent service from them...but as the saying goes, you're mileage may vary. You can search the CLUB DSM archives for other/cheaper sources for finding the BISS.

If you did lose you're BISS, you can temporarily fix it by putting one end of a short piece of vacuum hose in the BISS hole, and simply plugging the other end of the hose with something (a screw) to block any air from getting sucked in.

EGR (Exhaust Gas Recirculation) Valve

If you're EGR Value goes bad, it will allow air to get by causing the surge. The only way to test that is to temporarily block it off, replacing the gasket with a piece of coke can cut out with some scissors. If this turns out to be the problem, you'll need to put a more permanent gasket in there since the coke can fix will only last a couple of months before the exhaust gases blow through that thin piece of tin. I already have a FAQ on blocking off the EGR Valve so just follow it.

ISC (Idle Speed Control) Unit

This is a procedure to see if the ISC is bad or not. If it is bad, you'll need to replace it. I've never had to replace the ISC, so I don't have a FAQ or procedure on how to do that...Sorry.

Remove the ISC connector.
On the ISC side of the connector, you will see 6 pins (see picture). A top row of 3 and a bottom row of 3.
With the car shut off, take a ohmmeter and check the resistance between these pins: 1&2, 2&3, 4&5, 5&6.
This checks the 4 different coils in the ISC that can go bad. If any of the values are GREATER than ~30 ohms (like in the kohm range), then you have a busted ISC and have to replace it. L
If they check out OK, then it isn't the ISC
Be sure to reattach the connector when done! I don't know how many times I forgot about that.

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