diagnosing high idle

[dirklc]

Hello - My son and I are sorting through our 1984 T4WD (MT SR5 w/AC). We're trying to figure out a high idle issue and would appreciate any advice as we try to diagnose the most likely culprit.

I bought the Tercel years ago and trailered it home, where it's been sitting in storage awaiting some attention. When I purchased it, the PO had removed the gas tank trying to figure out a fuel issue (PO said he drove it regularly until he couldn't get it fired up one day and couldn't figure out the fuel issue). I was able to rig a gas can to the fuel pump to get it started and drive onto the trailer.

Fast forward to this summer. We're trying to get it back on the road to serve as my son's daily driver. My son and I have fixed the fueling issue (fuel pump was leaking and fuel pickup lines in the fuel tank were broken). Installed new radiator and thermostat, checked the fluids, and fired it up.

It starts and warms up but the throttle runs open at about 4k RPM. If you bump the throttle, it will accelerate but doesn't drop below 4k RPM. It appears that the PO had installed another carburetor (paint on the carb, probably from a pull & pay, and what appears to be the original in the truck). PO did say that it ran fine on the current setup before the fuel issue.

I'm guessing it's a vacuum issue or something with the throttle positioner? I see five blocked off vacuum ports on and around the carb. See these photos:
https://photos.app.goo.gl/Jb3uFFY666MS3VYR7
https://photos.app.goo.gl/6yXwEZwCzJwyQikU6

Could one of these ports be the culprit? If I look at what I assume is the original carb, I can see that these ports are missing so I wonder if this carb came off a similar Tercel or Toyota. We didn't find any obvious vacuum leaks but sure seems like a vacuum issue. Maybe one of the vacuum lines is missing or not run properly? We're trying to work through the diagram in the FSM but we're not really familiar with this setup and the non-original carb is throwing us for a loop.

Also, I noticed this apparatus below the EGR vacuum modulator. Not sure what it is but it looks a little suspect (the linkage above it is very loose and the top of this valve looks cracked):
https://photos.app.goo.gl/5VtHWrye2N5Lkw146

Thank you in advance for any advice!

Best,
Dirk (and Luke)

[dirklc]

And I tried to embed images linked to my Google Photo account but they didn't load. I just edited to urls in my message. Maybe the images are too large or something? Anyway, let me know if I missed something with the image function and I can update my original post.

[Mark]

Wow, my tercel (3-speed auto) doesn't even do 4000 rpm at highway speed. I don't think a vacuum leak could account for that kind of rpm.

[NWMO]

Good Morning dirklc,

I'm with Mark, there is no vacuum problem that will cause those RPM (I believe the upper limit on high idle is spec'd over 3,000 though). The best starting point is finding the cable between the pedal and the carb, it is surely holding the "butterfly" open and fueling. I think uploads are pretty good now (can go up to 6MB or better I believe) and just attach them to you message. You can then embed after uploaded as well. With the newer phones, file sizes are growing and may create a problem. If so, simply edit the file or I often find it easier to email the photo to myself and it then asks what file size to use and I can downsize it that way.

Chris

[dlb]

I agree with Mark and Chris, that's too high of RPM to just be a vacuum leak. The butterfly valve in the carb will be open while the engine is cold but it should gradually close as it warms up so confirm that. Also confirm there is slack in the throttle cable at idle.

In this pic

t1.jpg

the big hose circled in red must be plugged when you are testing the car with the air filter housing removed;
the hose that goes to the port circled in yellow must be plugged;
the hose circled in green goes to the throttle positioner and even though the diaphragms are capped there, I *think* it can still leak through the ruptured diaphragms -- just suck on the hose that goes to the TP and if you can suck air through it, it's bad and the hose going to it need to be capped.

I can't make out what that is in the second pic but I think it's just a capped port, but it looks like it might be a stock? Not sure.

The thing in your third pic is the AC idle kick up. It increases the idle speed when the AC is on. Doesn't matter if the linkage is loose, just suck on the hose going to the diaphragm and make sure you cannot suck air through it. If you can, cap the hose that goes to it.

[dirklc]

Thank you for the suggestions. Over lunch, I disconnected the throttle cable, plugged up that open vacuum port, and it's running at a much more reasonable RPM. I still want to work through the vacuum lines to make sure we don't have a leak before we start any tuning but we definitely need to adjust the throttle cable. Again, I really appreciate everyone's advice.

[dirklc]

I'm still trying to chase down a high idle. Starts (cold) and idles around 2k. As it warms up, the idle increases (doesn't drop). When warm, it's 'idling' around 3k. Idle speed doesn't drop if I bump the gas or if I completely disconnect the throttle cable.

A bit more background (building on my earlier post this summer). The car had a replacement carb (with a bunch of ports plugged) installed and the original carb in the truck. I had the original carb rebuilt and we installed it, trying to get everything back to the original state (thinking maybe the issue was with the throttle plate in that non-original carb or something?). So, the car now has the original carb that was recently rebuilt and I'm still struggling with this high idle.

What could be the culprit? I don't have a lot of experience with carbureted cars (most of my weekend wrenching on my T100's). I would like to take a thoughtful approach to diagnosing the issue - don't want to create more headaches by just throwing parts at it or tearing stuff apart haphazardly. I have tried to diagnose any vacuum leaks but not finding anything (and could a vacuum leak really drive the idle up that high?). I completely disconnected the throttle cable so don't think that's the issue. Dumb question, but anything other than the carb that could be the culprit? Something related to timing/distributor?

As always, I really appreciate the advice! My son and I are having fun getting the car back on the road but we're scratching our heads on this one.

[dlb]

So both carbs have had the same high idle when warm problem, is that correct?

[dirklc]

So both carbs have had the same high idle when warm problem, is that correct?

Correct - same high idle when warm with either carb.

When we installed the rebuilt carb, we labelled and reinstalled the vacuum lines as they were set on the carb we removed.

Again, we really appreciate any advice.

[dirklc]

One additional observation. After we swapped carbs, I believe that we had it idling just above 2k. Went on to tackle other projects and figured we either had (1) a vacuum leak somewhere or (2) needed to adjust idle.

Fast forward a few weeks. We're back in the project and back at 3k+ idle. So, I don't know if the car just wasn't fully warmed up when we were seeing 2k idle or if the issue is intermittent.

What's odd to us is that the idle increases as the car warms up - starts at 2k and idle keeps increasing as it warms up.

[NWMO]

dirklc,

When we installed the rebuilt carb, we labelled and reinstalled the vacuum lines as they were set on the carb we removed.

With the age of these cars, I wouldn't trust the vac hoses were correct to start with. The procedure in the FSM isn't bad, I feel like mine is working nearly correct for the first time since I've owned the car (5 years). When I first start it in cooler/cold temps, it will idle around 1,000 rpm briefly, but then increases to my high idle setting around 3k (very near spec). If I let it idle long enough to warm up, it will idle down on it's own or more often I will get in to go and after stabbing the throttle it will idle down.

Chris

[dlb]

+1 for Chris' comment. Every t4 I have bought has had the vac hoses connected incorrectly.

A few more thoughts just came to me too:

1) Have you checked the AAP diaphragms on both carbs? They are prone to rupturing and lead to gas being sucked through the vac hose to the intake manifold. That could cause a high idle.
2) With the idle being high on both carbs, I thought of what outside the carb could cause a high idle, and high fuel pressure is about the only thing I can think of. Do you have a way to check that?

[Mark]

I don't know how high the idle would go if the idle speed adjustment screw was turned all the way in, but it sounds like something is holding the throttle open. I'd suspect a mechanical issue with the throttle plate/linkages instead of a vacuum leak with the rpm that high. Sort of like the fast idle cam is always stuck on.

[Mark]

Have you looked down the carb to see if the throttle plate is being held open somewhat? Spray the whole assembly of linkages to make sure nothing is seized-up?

[Petros]

it seems likely that it is not carb related, so you need to check all of the non carburetor items that can cause a high idle like a vac leak. it is rare, but entirely possible, that both carbs were set up with the incorrect linkage adjustment, or vac line misrouting, or even a big vac line leak.

I once was ready to pull the head off a badly running tercel4wd (low compression in adjoining cyl #3 and #4, usually an indication of ruptured head gasket between the cylinders), when I discovered the big brake booster line was leaking at the vac assist at the brakes, easy fix.

down load a copy of the Factory Service Manual (FSM) from elswhere on this site and go though the diagnostic section on the fuel suystem and the emissions system sections. they are pretty good guide that gives you a list of symptoms (including high idle), and refers you different sections of the manual that gives you a diagnostic test (usually something simple, like pulling a vac line and block it off to observe the engine reaction). Go though each system and section systematically and usually that will narrow down the cause (it also gives you a basic education of what each of the lines and systems do in the car). it is not that time consuming, you can likely do all the tests in less than 30 to 40 min using simple procedures, usually not testing equipment is necessary.

Do not cut corners, and do not randomly replace parts and make multiple changes at once. it is a waste of time, you may be causing new problems and you end up chasing your tail, tying to find the old problem and creating new once on top of it. follow the testing procedures in the FSM and it usually results in you finding the problem and getting it fixed. finding the issue is always more time consuming than fixing it, it may just be a bad or misrouted vac line, easy fix.

Always diagnose the problem BEFORE you try to fix it. otherwise you waste time and you get frustrated by replacing good parts and you still have the problem.

good luck.