Sudden Super High Idle Speed!!

[canoeontheriver]

I have had my 1983 Tercel 4 WD Wagon since The last part of 2014. I'm the guy who courted the one and only previous owner of this car for a year and she finally consented to sell "Pete". Well I have been driving him to work everyday and mostly with out any major issues. Tonight on my way home I pulled off the highway onto my road and noticed the idle was suddenly way high, I mean really revving. I got home, pulled the breather, checked for vacuum hoseds being off, idle adjustment screws were checked, throttle cable, everything looks normal, and yet I can't get the idle down. It changes just a little with the idle screw, and a little with the choke adjustment, but still is about to blow up with the high idle. HELP

[Petros]

check for vacuum leaks, use a spray of carb cleaner by puffing it around various vac operated components, gaskets, vac lines, etc. when you get a change in speed, you found the location of the leak.

Note, some brands speed the idle up, some slow it down (depends on the propellant used), so test it down the carb to verify if the one you use will speed up the idle, or slow it down.

If no vac leak found, check all of the linkage that could cause it hang up. If for example pushing the throttle closed by hand brings it back to normal, it means some friction or binding somewhere in the linkage or on the carb itself.

good luck.

[canoeontheriver]

Ok, I will give some more time to it, but when its revving at such high RPMs its a little nerve racking. "Captain, she's gonna blow!" Thanks for the input!

[Petros]

high idle will not harm the engine, I have way over reved mine, no issues.

the only thing that will blow the engine is run without oil, or allowing it to overheat badly. a few engines have bundled up when they used the factory 5 deg BTDC spark setting, it makes the exhaust run hot and will damage the exhaust valves. Sometimes an exhaust valve will break off and do a lot of damage to the engine.

It is it running lean because of a vac leak, that too could make the exhaust valves run hot. but it is not the rpm that will harm it.

[Jonymoto]

Petros, I've read in other posts that the FSM is wrong in saying that the timing is 5 deg BTDC. I believe that was for leaded petrol (sorry gas) and 10 deg is now for unleaded. Just a thought.

[Petros]

not quite: there had not been leaded fuel available for a decade before, leaded fuel has higher octane rating so it would allow more advance.

the 5 deg timing was purely for emissions, it yields lower combustion chamber pressures and temperatures, so it produces lower NOx emissions. however the lower temps means the fuel air mixture is still burning when the exhaust valve opens so the exhaust gases are hotter, damaging the valve, also reduces emissions.

the best efficiency (best economy and most hp) occurs with the most spark advance without ping or preignition, which is damaging to the engine and wasteful of fuel. ping is that crackly sound you get on hot days when you lug the engine.

I have to wonder how much overall emissions are actually reduced since more fuel per mile is burned with the less advanced timing. So more combustion products are produced over the same distance. I suspect it comes from measuring parts per million, vs. Parts per mile of emissions.

[Jonymoto]

I said that because overhere, unleaded petrol wasn't available until around 1987. That's why my 1983 Tercel isn't fitted with a catalytic converter. When unleaded came in, it was recommended that the timing be changed to 2 deg BTDC. I found that caused the engine to ping a lot under heavy acceleration or when going uphill. Until I read the posts here, I was running at 7-8 deg BTDC.

[Petros]

I think we got unleaded fuel in about 1973 or '74. I know '71 and earlier all depended on leaded fuel. you had to switch to unleaded premium fuel if you drove and earlier car. At some point leaded fuel was completely off the market in the USA, perhaps 1980 or so.

[irowiki]

Remember, from the factory the car only needed to last 3 years or 36,000 miles give or take, so they didn't really care about the long term when it came meeting emissions.

[canoeontheriver]

I replaced the timing belt a year and a half ago. I set the timing like I always have with No. 1 plug at top dead center and it seems to run a little bit rich, I get 28-30mpg so I have not tried to reset it. I downloaded the vacuum diagram and if the rain will stop I'll get busy trying to find my leak. Thanks for your help.

[marlinh]

There could be something very small stuck in the secondary throttle plate. I found a tiny piece of silicone holding one open. That carb could not be adjusted down to normal idle.

[canoeontheriver]

OK, are you ready? I checked every vacuum line, and could go on and on about it but I won't. The linkage at the foot feed was bent. I straightened it out and woo hoo no more idle problems. Took the opportunity to adjust everything carb related and Pete runs like a well oiled sewing machine. Thanks for all the input! CASE CLOSED.....MYSTERY SAUL VED

[helipilot77]

Good news! Glad you got it figured out.