Idle surging/revving

[aipri]

I did some quick searches and didn't find this particular issue, although I may not know exactly what to search for. Anyway last week the car began revving very high during idle. As soon as you start the car it will rev up as if you're giving it gas in park or neutral (it's an auto), the rpm doesn't seem to go up and down its just a steady high rev and loud. Once you put it into drive or reverse the rpms will seem to drop and it quiets down, however I did notice when driving it's accelerating slightly at around 25-30mph even when not giving it gas. I didn't drive it too much like this for obvious reasons. Almost feels like the throttle is stuck but it I looked at it I didn't notice any obvious issue with it. The car also had an aftermarket cruise control put in, sometime in the 90s probably, it hasn't worked in some time though and I wasn't too concerned with getting it fixed. Thoughts? Thanks!

[David T]

Verify that the throttle is opening and closing all the way (with the motor off). Examine the decel springs. Check for any air leaks in the induction system.

[Rich]

It's not an uncommon problem for a car this old. Can't point to an exact thread for it.

This is potentially a safety issue, especially in an automatic, so make this a priority if you drive it regularly. Your report about it acting like it's on the gas at 25-30mph is a good clue.

Start by making sure your throttle is actually closed at idle.

With it idling high in neutral push the arm that's on last part of the throttle linkage forward to be sure it's on its stop and is engaging the black idle microswitch. If the idle drops to normal then we'll work upstream from there to see what's holding the throttle open.

[dmcnc]

With it idling high in neutral push the arm that's on last part of the throttle linkage forward to be sure it's on its stop and is engaging the black idle microswitch. If the idle drops to normal then we'll work upstream from there to see what's holding the throttle open.
^exactly the first thing I would check. I had a failing microswitch that led to your same symptoms. Make sure the screw on the arm is engaging the microswitch (you should hear the click) when the throttle is released (closed position). After that I'd check the connection on the back for continuity with the switch engaged and not engaged.

[DMC-81]

Good advice offered so far. I'll add the suggestion to make sure the cruise control system is completely unplugged/disconnected while you are troubleshooting to eliminate that as a cause.

[Jonathan]

What RPM does it sit at while idling high and in park?

Another possibility (in addition to the good advice you got here already about the throttle spool and cruise control system) is that the electrical connector attaching to the thermistor is either loose or just not making good contact. The thermistor is one of the sensors underneath the intake (accessed by "going into the valley.") It's function is to check for engine coolant temperature and report that back to the idle ECU. If that signal is absent, it defaults to idling high and it is usually a fairly specific RPM, like 2,000 exactly.

I've had this kind of issue before and it is no fun feeling like the car wants to give itself gas even though your foot isn't on the pedal. Getting into the valley is a pain, but there is a temporary work around you can do that involves making a jumper at the back of the ECU to simulate the signal. Report back on what you found on some of these other suggestions first though before going this route.

[Spittybug]

Check your throttle cable. Age/rust/rot can make them frayed and sticky at the point where the cable exits the shield. The shield's end cap can also pop out of the threaded fitting.

[AugustneverEnds]

there is a temporary work around you can do that involves making a jumper at the back of the ECU to simulate the signal

Sorry to hijack but could you point me to where this is explained? thanks

[Jonathan]

Sorry to hijack but could you point me to where this is explained? thanks

No worries.

Here's a thread that discusses which wires to jumper and some other background on what is going on with that idle ECU.

http://dmctalk.org/showthread.php?10205-100-in-the-valley-and-car-started-to-rev-high-What-part-is-over-heating

I've attached a couple pictures of the jumpers I made up. Basically, you need to continue on the proper connection for two of those four spots (as you'll have the white plastic connector pulled away from the ECU) and then those other two spots are what you need to loop a wire back into. You're skipping the thermistor under the intake with this jumper wire. I think it works better in the long run with some sort of resistor inline. Mine kept me going for the balance of the summer and then I went into the valley and fixed it properly that winter.

5872858729

[Bitsyncmaster]

I've tested the ECU and anything under 5 Kohms works the same. So your jumper is fine to bypass and open circuit fault.

[aipri]

Thanks for all of the suggestions! I'll start trying to troubleshoot. Unfortunately another issue my car has is that the tach is frozen, so I can't tell what RPM it is at, however I think it's higher than 2000 RPM, it sounds pretty high. FWIW the car has always been garage kept, even before I got it, and only has 10k miles, so there isn't much in the way of corrosion. Looks like I'll be giving myself a crash course on the throttle and linkage.

[Rich]

Thanks for all of the suggestions! I'll start trying to troubleshoot. Unfortunately another issue my car has is that the tach is frozen, so I can't tell what RPM it is at, however I think it's higher than 2000 RPM, it sounds pretty high. FWIW the car has always been garage kept, even before I got it, and only has 10k miles, so there isn't much in the way of corrosion. Looks like I'll be giving myself a crash course on the throttle and linkage.

You don't need the tach for the initial troubleshooting.

First step is to see if the throttle linkage is fully closed (against the hard stop and the idle microswitch). You can check this with the engine on or off. Push against the top of the upper throttle arm closest to the intake manifold to see if it's bottomed out (closed) or not. If it moves even a mm or two then the throttle isn't normally closed and you need to figure out why.

[aipri]

With the holidays over I finally had a chance to take a closer look. The throttle appears to be closing and it looks like screw on the microswitch is making contact properly and doesn't appear to have any play in it when closed. I took the air box off and cleaned the throttle body with some carb cleaner as well as the microswitch. With the air box off and after cleaning it seemed to be revving a little lower but still seems higher than usual. Originally when starting and in park it was idling so high that it sounded like the throttle was like half open. I took a couple pictures as well in case anything looks out of place? I haven't done anything with the cruise control, which wasn't working anyway either. What do you think should be the next step? I also saw some talk about the Idle speed regulator or Idle ECU being an issue that causes the rpms to spike.

5896758968

[Rich]

The throttle appears to be closing and it looks like screw on the microswitch is making contact properly and doesn't appear to have any play in it when closed. I took the air box off and cleaned the throttle body with some carb cleaner as well as the microswitch. With the air box off and after cleaning it seemed to be revving a little lower but still seems higher than usual.

Before moving away from the linkage let's review the two things in bold above just because it's where you get the most bang for your buck on this kind of problem.

For both of these tests start by pushing the accelerator then slowly releasing it to see where the linkage ends up. Helps to have a friend do this while you watch/listen in back with the engine off. Without pushing on the linkage or throttle arm to begin with.

About the microswitch be sure that not only the screw hits the switch lever but the switch also makes a click at the end of the throttle travel. If you do not hear the switch close then push the linkage by hand to see if it goes the rest of the way and clicks the switch - which indicates a linkage/cable issue.

Also check - before pushing the throttle arm closed - that the idle arm stop screw is fully against its stop. Use very thin paper like a stiff dollar bill as a feeler. Any gap after means the throttle isn't in idle position. If you do find a gap don't be tempted to fiddle with the idle stop screw. The problem is upstream if there is one.

With these final checks that the throttle closes by itself every time and the microswitch arm clicks you can move to more complicated tests. This test doesn't rule out a bad switch, only bad actuation of the switch.

[aipri]

Before moving away from the linkage let's review the two things in bold above just because it's where you get the most bang for your buck on this kind of problem.

For both of these tests start by pushing the accelerator then slowly releasing it to see where the linkage ends up. Helps to have a friend do this while you watch/listen in back with the engine off. Without pushing on the linkage or throttle arm to begin with.

About the microswitch be sure that not only the screw hits the switch lever but the switch also makes a click at the end of the throttle travel. If you do not hear the switch close then push the linkage by hand to see if it goes the rest of the way and clicks the switch - which indicates a linkage/cable issue.

Also check - before pushing the throttle arm closed - that the idle arm stop screw is fully against its stop. Use very thin paper like a stiff dollar bill as a feeler. Any gap after means the throttle isn't in idle position. If you do find a gap don't be tempted to fiddle with the idle stop screw. The problem is upstream if there is one.

With these final checks that the throttle closes by itself every time and the microswitch arm clicks you can move to more complicated tests. This test doesn't rule out a bad switch, only bad actuation of the switch.

Thanks Rich, I did verify that the switch was clicking as well, but it's obviously the original so I'm sure it's possible it could be bad. The revs drop a bit when putting the car into Drive or Reverse, but are much higher in Park and Neutral, not sure if that reveals anything else. I will try the tests you suggested as well.

[David T]

Even if the micro switch clicks it doesn't mean it is good. The only way to really know is to disconnect the wires, hook up a continuity meter to it and see when it clicks that you get continuity. Even that is not a complete test. A complete test would be to get to the plug going into the idle ecu and make sure the switch is connected all the way into the ecu.

[CFI]

My vehicle was experiencing the same problems. Last night I cut the two thermistor wires from the idle ECU and spliced them together (a technique mentioned in another thread). So far so good. The engine idle is lower and does not surge when placed in park or neutral.

[CFI]

Well the problem is back but I’ve found (hopefully?) the answer: the throttle is sticking and not activating the idle micro switch.

I had tested this several times with the engine not running and it always check out fine. I could hear the click from the switch every time I let my foot off the gas pedal. Well today it acted up (2,500 RPM in park, no lower than about 1,700 in Drive) and I pulled over, popped the engine cover, and pressed on the idle micro switch. Immediately the idle dropped way down. Took my finger off, idle surged back up.

So, I’m going to try just lubing the cable, spool, spring, etc before I make any adjustments. Any recommendations for lube?

[David T]

Service Bulletin SC-01-1/82 has a lubrication procedure for the throttle cable. The throttle spool should also be disassembled, cleaned and lubricated. The quadrant link (102555 section 1-4-1) ends wear and when they do you get enough play so that the throttle arm is not positively returned all the way to idle and hit the micro switch. Either replace the link or lengthen it so that it pushes the throttle arm all the way down at idle.

[Bitsyncmaster]

Grease the spring on the cable spool and see if that quick work fixes the problem. It did on mine.

[Elvis]

How funny - I've been telling this for many years but nobody gave a sh..

I asked people to help to search for the root cause but all I got was to be blamed not to help :-(


My guess is that the bearing is worn and the reacts on the vacuum on the throttle plates
or
the spring has lost force.

solution is to bend the spring at the throttle plates a bit and give it a bit more kick.
That worked for my car and two others I had here.

All the other here will tell you something different - therefore - this is my solution,
it works perfectly for >10 years now, I never had to use the new butterfly unit I
bought in Houston back then.



Well the problem is back but I’ve found (hopefully?) the answer:
the throttle is sticking and not activating the idle micro switch.

I had tested this several times with the engine not running and it always check out fine. I could hear the click from the switch every time I let my foot off the gas pedal. Well today it acted up (2,500 RPM in park, no lower than about 1,700 in Drive) and I pulled over, popped the engine cover, and pressed on the idle micro switch. Immediately the idle dropped way down. Took my finger off, idle surged back up.

So, I’m going to try just lubing the cable, spool, spring, etc before I make any adjustments. Any recommendations for lube?

[CFI]

solution is to bend the spring at the throttle plates a bit and give it a bit more kick.
That worked for my car and two others I had here.


The springs are all greased up. I’ll bend the throttle plate spring next. Thanks.

[David T]

The throttle system has 3 springs. Two are in the throttle spool and one is on the throttle body. (There are also 2 springs, one each on each decel valve but that has noting to do with getting the throttle arm all the way back to idle). Even if you tighten up the throttle spool springs, and/or the throttle body spring, it is not always enough to get the throttle arm to trip the idle micro if you have a lot of play (looseness) in the quadrant link ends. My quick fix is to lengthen the quadrant link but it does mean more lost motion in the throttle pedal. Remember the quadrant link has a left-hand AND a right-hand thread for precise adjustment and it also has the little clips like the door struts to prevent the ends from popping off.

[CFI]

My quick fix is to lengthen the quadrant link but it does mean more lost motion in the throttle pedal. Remember the quadrant link has a left-hand AND a right-hand thread for precise adjustment and it also has the little clips like the door struts to prevent the ends from popping off.

Wouldn’t you want to shorten the quadrant link? Seems like lengthening it would push the bottom of that rocker arm and create a wider gap between the idle micro switch and the adjustment screw that trips it.

Between greasing everything up and making slight adjustment on the idle micro switch adjustment screw everything seems to be working normally. Thanks everyone for the advice.