Surging RPM

[tercle]

My new car is a 1985 4x4 automatic wagon with Federal emission control.
It is finally on the road after some post-purchase repair and modification.

I can't get it to idle smooth.
It does idle steady at first when cold, but then starts surging.
You can feel it surge at low speed city driving, and will sometimes die when I roll to a stoplight if I don't shift to neutral and tap the gas.

A new PCV valve (Microgard) amplified the affect.
I put a green Japanese 71010 PCV valve (Toyota?) from another Tercel and it got much better - but it's still there at idle and low speed.

I have swapped in a known-smooth carburetor and there was no change.
Swapped in a different distributor and there was no change.

The fuel filter is new.
PCV grommet new.
All vacuum hoses check out.

[TURTCEL]

Sounds like a vacuum leak to me. I had the exact same issue on my "85 and most of the issues were due to vacuum leaks.

Just spraying carb cleaner around the vacuum lines and components is not enough, you must use the plastic hose that comes with the carb cleaner too so you can spray at exact points. I used to just spray it out of the can but a huge spray pattern can suck into multiple leaks making it hard to find the exact area of problem.

I am still having a little runnability issue myself and am having a little trouble fixing the issue but yours sounds like a vaccum leak for sure. And it won't necessarily be a hose but could be a component as well. I would spray around the throttle shaft to see if there is a change in RPM's. I had a leaky shaft and just by spraying carb cleaner on that area woud kill the car. Made it idle like crap too.

Good luck.

Peace

[ARCHINSTL]

There were a number of threads on this back in 2005-6. I had a similar problem, save mine was not at idle - it was cycling between 2200 to 1800 to 2200, ad infinitum.
I was able to eliminate it by disabling the vacuum switch "A." See the Club's longest thread (65 posts) - I believe some of the idle issue is covered later on: https://tercel4wd.com/forums/viewtopic.php? ... it=surging
Tom M.

[Tercel__jon]

I had the same problem with the surging. I put a glass fuel filter off a tractor on mine so I could see if there was trash and I saw tons of bubbles! Replaced the rubber lines back near the tank and no more bubbles, surging, or dieing.

[dlb]

tercle, PCVs are easy to test. you just blow through them from each side. air should pass easily from the engine side to the carb side, and it should pass with difficulty the other direction. that was an interesting guess for a bad idle—i wouldn't have guessed that but after reading about them more just now, i guess a stuck open one could act as a vac leak at idle since it should only be slightly open then. fun facts.

that link tom posted is a good place to look. it's huge but there's lots of good info in there. basically, it sounds like you have a vacuum leak somewhere in the hot running portion of the emission system, so you just need to test each hot system and find which one(s) are malfunctioning, then test each component of that system to figure out which ones are the problem. all the tests are in the FSM and all are quite easy to do.

i'm going to take a guess and suggest it's the EGR. if the EGR valve does not seal properly, it will allow exhaust gases to recirculate into the intake at all times, including idle, virtually no throttle, and full throttle, rather than just at partial throttle. you may not notice the leak at full throttle because the fuel mixture is richer then and can offset the leak, but it would be very noticeable at those other times.

[tercle]

I should have mentioned how random the surging is.
Not a cyclical "hunting" that I have had carburetors do before.
And when the rpm comes way down or the engine dies, it feels like it's starving for fuel.
Adding a bit of gas pedal always keeps it going.

by Tercel__jon » Mon Jan 14, 2013 12:32 am
I had the same problem with the surging. I put a glass fuel filter off a tractor on mine so I could see if there was trash and I saw tons of bubbles! Replaced the rubber lines back near the tank and no more bubbles, surging, or dieing.

I was wishing the Corolla fuel filter was see-through and I fully expected the symptom to go away or change when I swapped in another carburetor from a running 86.

This weekend, I am going to buy some new fuel hose and run lines directly from a mini gas tank to the fuel pump.
I think If I submerge the fuel supply line and the return vent line below the surface of the gasoline in the container, it would work.

I will bring a fire extinguisher.

The way it dies, it feels like fuel supply problem.
And the second carb never died like that on the Old Reliable automatic FWD wagon.

I did pull the vacuum line from the EGR with no change, but haven't removed it and blocked it off.

Today I plugged the vacuum line going to Vacuum Switch A to no avail.

[tercle]

I looked down the carb and discovered there is liquid gasoline trickling/spilling in the primary barrel.
Having a basic understanding of carburetors, I suspect this may be the cause of the strange idling and running.

The other (original) carburetor was surging the same way, although I have to swap them again to confirm the over-rich condition.

Can a vacuum leak cause this?

Can a failing fuel pump put out too much pressure?

The float level is a little high.
The image on FU-3 of the manual shows the Correct Level range.
My level is at the high end of this range.

[dlb]

i would check your AAP diaphragm. check the FSM for the test, it's very easy and only takes a second. the AAP is used to create a richer mixture at cold temps but if the diaphragm is broken, it will leak excess fuel all the time. usually it will manifest with a rough cold idle and cold driveability issues but these things can present differently sometimes. they're easy to replace, too. i just pulled one off an old carb for my current project.

[TURTCEL]

Another thing to check that is eay is the accelerartor pump. While looking down the barrel of the carb manually turn the throttle with the car off. You should see a squirt of fuel come out of the little brass tube at the top of the primary. It will only drip if you very slowly turn the throttle. If you do not see fuel coming out of this tube inthat fashion thenthe accelerator pump could be bad too.

Having a bad acelerator pump cancause hesitation but usually won't be intermitent.

Peace

[tercle]

AAP diaphragm looks good, although it's hard to test cold because the random racing, slowing and dying is more severe when cold.
I also put a couple pounds vacuum on it with a vacuum gun and it held it.

It has to be something outside of the carburetor, because this carb came off a car that was idling fine.

I swapped it to try to get rid of these symptoms, but the symptoms didn't change.

I was thinking there was an outside chance it was the AAP when you described it, but then there would still need to be an outside component that would blow it on both carbs.

And the accelerator pump seems to work.
Like I said, this is happening on two different carburetors.
This one was running fine the day before on a different car.

[4wdchico]

Try plugging the purge line that runs from the evap canister to the carb.

[dlb]

AAP diaphragm looks good, although it's hard to test cold because the random racing, slowing and dying is more severe when cold.
I also put a couple pounds vacuum on it with a vacuum gun and it held it.

i would still try the FSM tests. i think they just involve starting the engine when cold, pinching the hose, shutting the car off, and then un-pinching the hose while shining a light down the barrel to watch for fuel spray. then you do it again when the engine is warm and it shouldn't spray. i've had bad AAPs that appeared fine when i did similar checks like you did but when i looked further, they were actually bad and i was surprised.

the AAP is activated by manifold vacuum via the TVSV during the first two heat stages of the TVSV. might be a good idea to remove you TVSV and test it too. i find most of them wear out with age and don't make it to the third and last heat stage, which means they always supply vacuum to the AAP even at full operating temps (along with not doing a variety of other things). this would also cause your AAP to always operate. a properly working TVSV is surprisingly crucial in the proper operation of all the vacuum-controlled devices so check yours out. it's easy to test, just dip its end in water warmed to certain temps and verify that air passes through the correct passages. see page EC-22 for the deets.

Can a failing fuel pump put out too much pressure?

i'm not sure about this. the tests in the FSM don't seem like they would give a clear indication of a failing fuel pump but you might try it anyway.

The float level is a little high.
The image on FU-3 of the manual shows the Correct Level range.
My level is at the high end of this range.

as long as the level is still between the two points of the sight glass, i think it's fine. above or below those points is when there's an issue.

[tercle]

I did the factory tests as you described.

Is EVAP cannister the Charcoal cannister?
Does a Federal car have an EVAP?

[tercle]

Can I just cap the TVSV and everything vacuum-related temporarily?

[dlb]

I did the factory tests as you described.

oops, sorry. i misunderstood.

Is EVAP cannister the Charcoal cannister?
Does a Federal car have an EVAP?

yes and yes.

Can I just cap the TVSV and everything vacuum-related temporarily?

you could but the thing would run like hell so you wouldn't be able to tell if the problem was gone since you'd have about 8 more problems going on. a few guys here experimented with bypassing the TVSV but all reported it as unsuccessful.