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Diagnose my fast idle problem!

It starts up readily. It runs a little rough at idle, as though a cylinder occasionally misses, for a minute. Idle speed starts at 600. I don’t need to put my foot on the gas to keep it running. Over the next 10 minutes the idle speed slowly rises until it tops out at 1300. I’ve taken long drives (50 miles) and measured the idle at about that twice last week.

The idle adjust screw and fast idle adjust screws are all the way out. There’s slack in the throttle cable at the carb end. The throttle and carb valves move freely.

I tried to find a vacuum leak both by putting my hands on every vacuum hose I could reach and spraying propane on them. I got no reaction.

I installed a new fuel pump 7 years ago. The plugs look okay. They’re 15 years old but only driven 22K, the same for the distributor cap, rotor, plug wires, PCV, air filter, fuel filter; I replaced the oil and oil filter 9 years, 3K miles ago.

The idle has risen gradually from 800 12 years ago to 1170 in 2015 to 1300 this year. Spec is 700.

The Haynes mentions the dashpot, a device I’ve never dealt with. I’ll fiddle with it next.

Unless I get a better idea, I’ll replace all the vacuum hoses, plugs, etc - wave a dead chicken over it.

You-all have any ideas?

'87 toyota pickup dlx, 4-cylinder, 5-speed manual, 22R engine, red

One more thing: it doesn’t have too much power when it’s idling at 1300. If I lift my foot off the brake after a stop, it doesn’t jackrabbit off. It’ll move without giving it gas, but accelerates slowly. I would think that a car meant to idle at 700 would deliver a lot more power at 1300.

Any check engine light?

On an '87? There’s no way to scan it either. I suspect that if I could, I’d know the problem without asking.

LOL, I liked that one. Thanks for the chuckle.

The temp sensor for the choke may be bad.

Additionally, if the float is sunk the needle valve that it closes to manage fuel feed to the float bowl will remain open all the time, allowing too much fuel into the engine. This can cause all of your symptoms. I could be wrong, but I believe these carbs used stamped metal floats, and these can corrode over time and leak. The float could even be only partially filled with fuel, creating havoc with its floating characteristics. The fuel feed could even vary depending on how much fuel has leaked into the float at any given time.

I seem to recall a discussion about carb rebuild kits for this vehicle. How’d you make out with that? Will you be rebuilding it? Or is my memory becoming unreliable?

WoW. I’d replace the oil/filter on an annual basis at least.


Do you mean the metal coil that rotates as it heats up and moves the choke?

Is there a way to diagnose this? Does one see evidence of this in the ‘fuel window’ on the front of the carburetor? I haven’t found a way to look into that; I need some kind of mirror.

Your memory’s okay, I just wanted to do everything else first. I was hoping for a vacuum leak. I’m still hoping that all of them leak a little, too little to detect any single leak but enough to add up to this problem; the hoses are all 30 years old.

A re-build kit costs $35; I can buy a new carb for $114, which seems like a better deal, if it’s really as good as the $430 carb from AutoZone.


Unfortunately, there’s no way I know of to diagnose this other than to pull the top off of the carburetor. If someone here knows of one, I too would be interested in hearing it. [quote=“RandomTroll, post:6, topic:99848”]

A re-build kit costs $35; I can buy a new carb for $114, which seems like a better deal, if it’s really as good as the $430 carb from AutoZone.
I’m not familiar with the $430 Autozone carb, but I personally have had problems with things I’ve bought at Autozone. I’ve gotten parts that don’t fit right, a left-side headlight module for my '91 Camry that didn’t match the right side… or fit quite right, and a friend recently got a bad “knockoff” MAF sensor. AZ seems in my experience to carry a lot of junk.

I remember when Jesse James was promoting Autozone in the tv commercials.

Can I assume they don’t currently have any celebrity endorsements . . . ?

I much prefer napa auto parts over Autozone, anyways

Did you use propane to check around the intake manifold gasket?
Try pinching off or capping each vacuum hose, including the brake booster.

1 Like

This truck uses a carb, right? If you take a piece of cardboard when it is at warm idle and completely block off all the airflow into the carb’s throat, does it stall straight-away? It should. If it doesn’t, try to listen for “rushing” sounds indicating where the air is getting in. First suspect for an older truck with a well used carb is the throttle valve bushings. With use they can wear and the space created provides a path for air directly into the intake manifold.

If you buy a $114 replacement carb, be sure to keep the existing one at least until you are certain the $114 version meets all your expectations. Best to keep the re-building the existing carb option alive .


Okay, a carb from whatever you think is a reputable shop. I’ve never had a problem with the stuff I’ve bought at AutoZone, but I’m an undemanding customer.


That’s a good idea. I should have thought of it.

That’s a good idea too. Tomorrow.

Of course. Of all the crap I have around, a carb would be silly to discard.

I have no suggestions. I’ve always rebuilt the existing one. I guess if I was buying one I’d try to go directly to Holley or Carter.
Perhaps someone here has purchased carbs and has a suggestion.

Does the carb have an idle cut-off solenoid? This part failed on my 1979 Toyota truck. It’s screwed into the side of the carb and has one wire going to it. If you apply and release 12V you should hear it click.

check the throttle position sensor,if it is malfunctioning you can get one on the net for about $25

I see a ‘solenoid valve’ that connects to the ‘fast idle cam’ which connects to the ‘choke opener’

I don’t see this in the diagrams. I see a ‘throttle positioner’ , which is a diaphragm.

Do these things come in a re-build kit? I don’t see them as separate parts.

A piece of cardboard made no difference, probably because the top isn’t flat so I can’t make a good seal. A cloth makes it stall in seconds. I don’t hear any sounds of air rushing or hissing or anything like that.

I tried the checking-for-leaks trick with carburetor cleaner (gets further than propane, is more stuff), and I pinched every vacuum hose that connects to the carb; none of that made a difference or showed a sign of a leak. Because I can’t run the engine with the air cleaner removed I can’t reach many hoses.

If I pull out the acceleration pump I can slow the idle down, even stall. I can’t get access to the auxiliary acceleration pump: it’s under the air cleaner. I guess I’ll pinch its hose with a clamp when the engine’s off

Take off the air cleaner and you’ll have a much better view of the carb and all its plumbing and external parts such as AAP housing, idle cut-off solenoid if present, etc., etc. Also the sight glass at the front of the carb’s main body, to see if the float is at its correct level.

One of the usual suspects is the AAP diaphragm, broken and leaking fuel into the engine downstream of the carb. If you remove the AAP’s vacuum hose and smell gas in it, then it is. It’s easy to remove the AAP housing (dome-shaped, one vac hose to it, three screws hold it to side of the main body) to inspect the diaphragm. Be careful to catch the spring in there and remember where it goes.

It’s good that you can stall it out by 100% covering the carb’s airflow inlet. That means there’s nothing hugely wrong. The warm idle rpm depends entirely on how much air goes into the carb. You’ve proved you can make the idle rpm 0 by blocking all airflow. You should then be able to then slow it down by partially blocking the airflow into the carb. Might be worth a try anyway. If you can slow the idle rpm down to where you want it to be by physically blocking the airflow, you’ll know you are on the right track.

When you got fast idle and wandering idle with a carb, the most likely reason is there’s too much air getting through the carb. The throttle valve plate position is supposed to be what determines the airflow through the carb and therefore the engine idle speed. Perhaps the throttle valve isn’t staying in one place for some reason. Either opening slightly due to engine vibrations or such, or shifting laterally in the throat. For it to work reliably of course the throttle valve has to move perfectly freely. If it or its linkage tends to stick in certain positions, even slightly, that could cause this symptom.

I’ve noticed. But I can’t run the engine with the air cleaner off.

So another has written. As near as I can tell from the diagram in Chilton, the AAP is under the air cleaner on the engine side, inaccessible when the engine is running. Can I just unplug its hose, stick a golf tee into it, then start? The AAP page at places it on the front of the engine. Which is it? It’s a California car.

Doesn’t that assume that gas isn’t sneaking in by some other means, such as the AAP?

I don’t see either of the valves moving at all. Perhaps I miss it. Since I’m not putting my foot on the gas, the throttle valve shouldn’t move, right? And since the car responds properly to throttle, it must work, right? Perhaps it’s resting in the wrong position.

The choke valve rotates freely by hand. Moving it by hand doesn’t seem to make a difference. It’s ‘sprung’, that is, if I move it, I feel the force of the choke’s thermostat coil moving against it, holding it in its resting place. The choke ‘broke’ 15 years ago. All that happened was that the arm that fits into the loop at the end of the thermostat coil had popped out, probably when I started it on that -17° day. To figure that out I had to drill out the rivets that hold the cover on (earlier models had screws; the $114 replacement carb has screws). Until I fixed it I was running too rich, and failed emissions test in 2005 until I fixed it. But I just assumed the correct position was slack, no tension on the coil. I checked it again this time. Unfortunately taking the cover off means I can’t see how it was, but the arm must have been in the coil because it didn’t move freely. Maybe voltage isn’t getting to the coil. Would just opening the choke valve by hand test this?

I had a sticky throttle problem on my carb’ed truck , just last summer. The symptom was I couldn’t get it to warm idle at the correct rpm. At least not repeatedly. It would be spot on one time, then the next time I drove, it would be idling too high or too low. It wasn’t a big deviation, but it was annoying. For example if I wanted it to idle at 700 rpm, I’d set it there, then it would idle at 900 the next day instead. Or 600 the next day. Adjusting the idle speed adjustment screws didn’t do the job b/c I’d get it correct one day, then the next day it was wrong again. The problem turned out to be a combination of things. The throttle plate bushings needed a spot of lube, and the throttle linkages were sticking a little, especially as they were affected by the engine on/off throttle position solenoid (the anti-dieseling solenoid). A little wd40 and working the mechanisms by hand did the job on those. In my case the warm idle problem was annoying, but the fast idle was the bigger problem. It was varying all over the place. That setting is very sensitive to where the fast idle screw sets on the fast idle cam, and cleaning & lubing & fine-adjusting that mechanism is what fixed it.

Too much gas sneaking can indeed cause idle speed wandering. But that usually causes other problems even more severe, like the engine being impossible to start when hot. I had a problem like that too, where the power valve was leaking. But that problem was really obvious as it caused very noticeable drivability and fails to start problems.