Erratic Idle - If it Idles at All

[tabercro]

Hi All,

About two years ago I bought a 1985 T4 with 167k. Last year I replaced the timing belt and front oil seals. After that I adjusted the timing to 10 degrees BTDC. I haven't put a lot of miles on it, but it was the best it has run so far. Very smooth and quiet idle. No problems. Late last fall I put the car in the garage for the winter.

When I pulled it out this spring, it was harder than I expected to start. Took a fair amount of cranking. It idled high once it started (around 2000 rpm) which has been normal for the about the first minute of operation since I've had it. I drove it around the neighborhood for a quick joyride and once it was warmed up it would stall at every stop sign. It would restart very easily. I got back to the house and checked under the hood and didn't see anything odd. Later I took it for a ride and it seemed to be running much better. I thought maybe it needed to run a bit and I stopped at the gas station and topped up the tank (about 3 gallons) with fresh fuel. I drove a few more miles and then it started idling very erratically often idling so low that the engine would knock a bit. I drove it a couple of miles home with no improvement.

I've driven it almost 100 miles since then with varying results. While driving and not shifting it's fine. Normal power. Normal response. However, at full temperature it's either stalling repeatedly, idling so poorly that it's painful to listen to, or idling almost normally (at about 850-900 rpm). When I say almost normally I mean that it's a little higher than it should be and it's fairly quiet. However, it's never the way it was. It's always skipping every so slightly.

I didn't do any work over the winter other than add an inclinometer, which I don't think should have had any effect on how the engine runs. Although if any of the wiring in the dash could have something to do with this, please let me know. I'm still having some issues with that wiring, which will be for another post in the near future (insert eyeroll emoji).

I'm fairly stumped by what's going on. Any input will be appreicated. Thanks.

[dlb]

I would thoroughly look for vacuum leaks, check the AAP (if it's ruptured it will allow the port on the intake manifold to suck fuel in whenever manifold vacuum is high, which it is at idle), and double check the timing.

Once it's warm, does it run fine as long as you keep it moving? Any bucking while driving?

[Neu]

I'd check the simple things like fuel filter and make sure the carb is getting fuel, check the carb, check vac leaks, pull the air filter off and see if it runs better. Maybe check the distro rotor real quick.

[tabercro]

Thanks for the responses.

I've pulled the air filter and it makes no difference.

dlb, the problems seem to start as it warms up. Although once warm, the problems seem erratic, it idles fine (and high) upon start. Once it warms up it ususually stalls every time i stop. Then I drive a bit more and I'll come to a light and it will idle okay (never perfectly). No bucking or skipping or loss of power while driving.

I realize there are some basics that I need to look over. I'm just perplexed by the inconsistent nature of the problems. And that they came out of nowhere after getting things running smoothly.

[Nordical25]

It is good to have patience with these cars because they all are pretty soon 40 years old. It means that gaskets start to fail due to high age and other parts may corrode etc. So vacuum line check etc. are good ideas as dlb & Neu suggested.

The car has two idles as you mentioned, high idle at the beginning and low idle when engine has heated enough. At the beginning idle is like choke on at older vehicles. So basically air to fuel ratio is not ok on low idle when RPM fluctuates with warm engine. This can be caused by many issues.

What fuel did you tank 3 gallons? I mean how high is the ethanol content? And has ethanol content in fuel changed in your state recently? Older cars run usually a lot better with 5% ethanol fuel because they were never designed to run with ethanol fuel in the first place. And higher ethanol content means less energy in fuel since ethanol has lower energy content compared to fuel (petrol). It means that basically one would need bigger jets to carb with higher ethanol content to compensate lower energy content of ethanol fuel. Especially if you ever try E15 or higher ethanol content.

[Mark]

I'd suspect a ruptured erg vacuum modulator diaphragm. The EGR system is only activated when the engine is warm. On my wagon this happened several years ago. It would idle fine when it was cold, but after the engine warmed up, it would often stall at red lights and idle low. I'd often have to keep my foot a bit on the gas pedal to keep it from stalling. For now, I'd try pulling and plugging the hose to the modulator and see if the problem goes away. I'm not a fan of disabling the EGR system permanently. Even if you don't care about emissions, when it's disabled the combustion temperatures are significantly higher which can cause overheating issues and possible component damage.

[tabercro]

Thanks for the responses.

Today when I started my Tercel it was idling for long enough that I could check a few things myself. After it warmed up, I drove it around a bit and RPMs would drop way down at stops, but it didn't stall.

Nordical25, I'm not sure specifically what the ethanol content was where I fueled up but it should not be more than 10%. As far as I know, it's 10% or less at the pump in Maine. I'm not sure of anywhere that guarantees no ethanol added. It would have been the same content of anything I put in it and ran last year.

Mark, I tried plugging the port on the EGR modulator. From your response, I wasn't sure if I should block that off, or leave it open and actually plug the hose leading into it. Plugging that port once warmed up made not difference. If I pull the vacuum line going to modulator, RPMs go up and it runs a bit smoother. When I pull it off I hear a "pop", which I'm assuming is the gate closing. When I either plug the vacuum line with my thumb or plug it back into the modulator RPMs drop considerably and it runs a bit rougher. Is this more or less normal operation for this part of the EGR system?

I also checked timing today, which shouldn't have changed since last fall. And it was right where I set it at 10 degrees BTDC. However, it continues to run really rough.

I'm going to keep plugging away at it. Thanks for any help.

[Mark]

Here's the instructions from the service manual for checking the egr vacuum modulator:

egr.jpg

The reason I suspected the egr system is that (off the top of my head and I could be wrong) it is the only vacuum system that is activated only when the engine is warm (unless you have a feedback carb like mine with an oxygen sensor and a primitive computer). If I had a vacuum leak (which is usually what causes a rough idle) only when the engine is warm, the egr system is what I'd suspect. It's more common to have a rough idle when the engine is cold and it goes away when the engine warms up. Your issue seems like the rare reverse.

[Mark]

I'd also check the vacuum lines shown in red to make sure they are connected and that they aren't deteriorated:

vacuum.jpg

That is vacuum coming from the manifold (basically from when the engine is at idle) that is activated when the engine is warm. So any leaks there would cause a rough idle when the engine is warm. The choke opener is also on that circuit. I'd suck on the hose going into it to make sure the linkage moves and the diaphragm isn't ruptured. With vehicles this old, it would be rare to have all the diaphragms intact in all the emissions control components. The diaphragms are just rubberized fabric. The rubber cracks and flakes off over time causing leaks.

[Nordical25]

E10 petrol could be fine especially if other Tercels run fine with E10 including your Tercel previously. I just started to wonder this since your issues started after refueling. However, it sounds to other issue as Mark suggests above me.

We have 2 different fuel types in Finland, 98 E5 high octane and 95 E10 low octane fuel. I use always 98 E 5 in my three older cars and 95 E10 in my 2018 daily driver. Finding E5 fuel could improve your idle. However, it depends. It may be that your carb has already been adjusted to run well with E10 fuel. You can try to see any difference if you find E5 fuel from Maine.

In my country car makers had to make a statement which models can use E10 fuel. Finnish Toyota importer suggested that all 1998 and afterwards built Toyotas are good to go with E10:
"
All petrol models manufactured for European market from January 1998 can use E10 petrol with an exception of the following models:

Avensis 2.0 VVT-i (1AZ-FSE) manufactured between July 2000 and October 2008
Avensis 2.4 VVT-I (2AZ-FSE) manufactured between June 2003 and October 2008
"
source: https://www.aut.fi/en/road_transport_in ... enger_cars

[Mark]

The regular gas here in Canada is 87 octane with 10% ethanol. My tercels run fine with it. I assume it's the same in the US. In hotter places, there may be more of a problem with the fuel vaporizing easier in the lines from the ethanol, but we've been through a few heat waves here over the years (30+ degrees Celsius) and I haven't noticed any problems.

[Nordical25]

I am glad to hear that Tercel 4wd works with E10 because one day we could also have only E10 fuel. How long you have had E10 gasoline and has your carb been adjusted during E10-period?

Gasoline has 46MJ/Kg energy density meanwhile ethanol has 26,8MJ/Kg energy density.
It means that E5 fuel has energy density of 5%*26,8MJ/Kg + 95%*46MJ/Kg = 45,04MJ/Kg
Meanwhile E10 fuel has 10%*26,8MJ/Kg + 90%*46MJ/Kg = 44,08 MJ/Kg
source for energy contents: https://energyeducation.ca/encyclopedia/Energy_density

It means that E5 fuel has about 2% less energy than 100% gasoline and E10 fuel has 4% less energy than 100% gasoline. This means higher consumption of new type ethanol fuel because engine needs same amount of energy with both fuels for constant operation. Which could in theory mean carb adjust for new fuel type, especially for E10.

[Mark]

We've had it here for years, but I don't remember exactly when it started. The govt. info says it has slightly less heat energy, but that is partially made up for by its easier combustion. I never made any adjustments to the carb. To try and compensate for it by changing the air-fuel mix (which is factory sealed in the Tercel carb) accurately would require some kind of wide-band O2 sensor that you could monitor as you drive in varying conditions and throttle positions, acceleration rates, etc. Basically, I don't think it's realistic to dial in the carb accurately for such a small change without lots of monitoring/diagnostic equipt. I think considering the efficiency limits of a carb, most people's Tercels run much more than 4% inefficient anyway, based on driving style, altitude or the condition of all the carb/emission components anyway.

[Nordical25]

You have very good proof there with sealed carb that you haven't adjusted idle. Ethanols easier combustion is also a very good point. Then I should be ready to move into E10 when they start selling 98 E10 here. Octane recommendation according to manual for European Tercels is at least 97 so I will stick into 98 gasoline anyway. I have noticed that local 95 octane can cause some dieseling. And I noticed that already back in 2000's (00's) when they did not blend any ethanol to gasoline.

[xirdneh]

When idle is high try pushing throttle cable cam down. If idle drops i would suspect the brass butterfly is hanging up due to wear on forward side (usually) This is pretty common. I add a spring to pull it the rest of the way down.