slightly high/low idle once warm

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dlb
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slightly high/low idle once warm

Post by dlb » Mon May 16, 2016 2:00 pm

i think i posted about this a while back but i can't find the thread. i'm STILL trying to find the cause and wanted to check in with the gurus about it.

backstory: my ae95 corolla has developed some idle issues in the last 6 months or so. once it's warmed up, it tends to idle a little high -- stock idle is 800 rpm but mine will be anywhere from 1100-1400. less frequently, it will idle too low, around 600 rpm.

the first thing i thought was vacuum leak so i went though the brake booster, PCV, EGR, charcoal can, and power steering systems but the issue remained. i sprayed propane around all that stuff and the intake manifold but no rise in rpm. the next thing i thought was marginal electronic component so one at a time, i've checked the o2 sensor, various coolant temp sensors, vacuum switch sensor, and alternator, and swapped the ECU, distributor, and entire throttle body with spares i have kicking around. still no dice.

i haven't been able to check the fuel pressure yet because my fuel pressure tester kit doesn't have an adapter that T's into the corolla's fuel system. that seems like the next thing i should check but i don't know if or where i can get the adapter i'm looking for. could i cut an existing fuel hose, T into it, and then use a fitting with clamps to splice the hose back together afterward? that seems like a very bad idea to me, creating a potential fuel leak point. i guess a better idea would be to do that but then remove and keep that hose for future diagnostics, and replace it with a good one.

does anyone have any thoughts on the fuel pressure testing, or any other things i should investigate re: my idle issues?

merci.

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Re: slightly high/low idle once warm

Post by Petros » Mon May 16, 2016 2:12 pm

This is what I would check:

did you clean the throttle body assembly with throttle body cleaner? sometimes that causes the throttle to stick.

also, lubricate the throttle cable and all related parts.

Most modern EFI cars have a device called an "idle air control valve", it is a devices that allows air around the throttle to even out throttle response just off idle, sometimes these get gummed up like the throttle plate and need to also get cleaned like the throttle body.

I am not sure fuel pressure would cause high idle, usually if the fuel pressure low or fluctuating, it will idle okay, but the car will miss or get weaker at highsr rpm.
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Re: slightly high/low idle once warm

Post by keith » Mon May 16, 2016 3:44 pm

For an EFI vehicle, I'd look for a vacuum leak, especially around the intake manifold gasket.

If it is a pre OBDII vehicle, there usually is an idle position switch (IPS) ganged to the throttle position sensor (TPS). The IPS may not be making when you take your foot off the gas. One common reason for this is that someone mistook the throttle stop screw for an idle adjustment and adjusted it out to the point that the IPS cannot make. Then they adjusted out some more for a good cold idle but that makes the hot idle too high. A dirty throttle body can also contribute to the if the throttle plate can't completely close.

If the IPS doesn't make, the PCM (computer) thinks your foot is on the gas so it makes no attempt to send signals to the idle air control (IAC) valve. Many mechanics assume the IAC is bad when it is not.

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Re: slightly high/low idle once warm

Post by dlb » Mon May 16, 2016 4:28 pm

Petros wrote:This is what I would check:

did you clean the throttle body assembly with throttle body cleaner? sometimes that causes the throttle to stick.

also, lubricate the throttle cable and all related parts.

Most modern EFI cars have a device called an "idle air control valve", it is a devices that allows air around the throttle to even out throttle response just off idle, sometimes these get gummed up like the throttle plate and need to also get cleaned like the throttle body.

I am not sure fuel pressure would cause high idle, usually if the fuel pressure low or fluctuating, it will idle okay, but the car will miss or get weaker at highsr rpm.
i swapped throttle bodies with a known good one but nothing changed. the throttle body contains an auxiliary air valve (same thing as an IACV) which i also tested just for good measure, no problems there.

i wouldn't have guessed to look at fuel pressure either but the FSM listed it as a possible cause.
keith wrote:For an EFI vehicle, I'd look for a vacuum leak, especially around the intake manifold gasket.

If it is a pre OBDII vehicle, there usually is an idle position switch (IPS) ganged to the throttle position sensor (TPS). The IPS may not be making when you take your foot off the gas. One common reason for this is that someone mistook the throttle stop screw for an idle adjustment and adjusted it out to the point that the IPS cannot make. Then they adjusted out some more for a good cold idle but that makes the hot idle too high. A dirty throttle body can also contribute to the if the throttle plate can't completely close.

If the IPS doesn't make, the PCM (computer) thinks your foot is on the gas so it makes no attempt to send signals to the idle air control (IAC) valve. Many mechanics assume the IAC is bad when it is not.
already looked for vacuum leaks around the manifold. will keep looking though.

regarding an incorrectly adjusted IPS (i've never heard of that, not sure if the ae95 has one), i should note that i've owned this car for several years and it idled fine up until roughly 6 months ago. at no point did i adjust the idle screw or IPS so i don't think it would be out of spec. also, i swapped the entire throttle body with a known good one already and the problem remained.

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Re: slightly high/low idle once warm

Post by Petros » Tue May 17, 2016 12:02 am

the throttle position sensor (TPS) could be worn or bad. We had a ford one ton that had this, it seemed to run fine, but it idled fast. It had a worn out place on it, which I learned Ford is notorious for wearing out. Only cost about $35 to replace it (the dealer wanted $90 to connected it up to their machine to see if it shows anything). It did not send a code because it was still within normal range, it was just sending a signal so the ECM thought the throttle was open at idle. changing it also improve the fuel economy.

you can test it with a volt-olm meter, you put the probes on the correct contacts and swing the throttle through the full range of movement, the resistance showing on the meter should increase smoothly from idle to full throttle. If it jumps just off idle, than it is worn out right there at idle. This is where it would wear out first because that gets the most throttle movement on the contacts.
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Re: slightly high/low idle once warm

Post by dlb » Tue May 17, 2016 9:47 am

the TPS is located on the throttle body, which i replaced with a known good one, and the symptoms didn't change.

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Re: slightly high/low idle once warm

Post by Petros » Tue May 17, 2016 11:23 pm

I was afraid you would say that...you are on your own now.

I am out of ideas for you. fuel pressure might be your only hope, though again I do not see how fluctuating fuel pressure would make it idle fast. Might test the fuel pressure regulator first, usually a device on the fuel supply rail.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
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Re: slightly high/low idle once warm

Post by dlb » Sun May 22, 2016 10:28 pm

looks like high fuel pressure is my problem.

i finally got brave and cut the rubber fuel hose between the fuel filter and the fuel rail, and T'd in the pressure tester. i followed the steps in the FSM and found where the pressure should be 33-40 psi, it is 45, and where it should be 23-30 psi, it is 36.

i have a friend with a parts corolla i can grab the FPR from so i'll be paying him a visit soon. fingers crossed that this finally solves my high idle issue.

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Re: slightly high/low idle once warm

Post by Petros » Mon May 23, 2016 3:23 pm

usually an easy test is to swap out a FPR from a wrecking yard, or borrowed off another car. these are usually very reliable, so getting a used one at a wrecking yard is a good bet. these are standard items and should be common to a lot of different models, cross check compatibility before you go to the wrecking yard.

Of course if new price is decent just getting one from the local autoparts store or Rockauto, or similar is not a bad idea either. Though sometimes parts like this are way unreasonably over priced, that is when I hit the wrecking yard.

BTW, we have owned EFI Hondas, Mazdas, Toyotas, and Nissans we have driven to over 200K miles, some well over 300k, and never replaced a fuel pressure regulator.
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Re: slightly high/low idle once warm

Post by dlb » Mon May 23, 2016 5:38 pm

if i had had a FPR on hand i would have swapped it in but unfortunately it's one of the things i did not grab from the parts corolla i had. hindsight is 20/20. oh well. i have a friend nearby with a parts car that i can get one from soon.

i'm just as surprised as you that this seems to be the source of the problem. i'll be thrilled if this fixes it though since it looks very easy to replace.

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Re: slightly high/low idle once warm

Post by Petros » Mon May 23, 2016 8:36 pm

it is possible over-pressure could cause it, it is just that I do not think I have ever seen a failed FPR. Though I know it can happen, it is just very rare.

Usually if you trouble with the fuel pressure it is because it is too low, either a weak pump (going out) or plugged filter or screen in the fuel tank. that would never cause a high idle, but will cause it cut out sometimes, and run weak and funky other times.
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Re: slightly high/low idle once warm

Post by dlb » Tue May 24, 2016 5:06 pm

the problem persists.

i borrowed another FPR from a friend, installed it, and the pressures are all the same. so either my brand new FP gauge is out of calibration or i have some cause of high fuel pressure other than the FPR. for now, i'm going to assume the gauge is working correctly and go from there.

the pressure is high both when the FPR is opened and closed so i don't think it's a plugged fuel line. i will look for anything obvious though. i've never heard of a fuel pump putting out too much pressure, it seems unlikely but is it a possibility? i have a spare fuel pump but don't want to drop the tank if i can avoid it.

*edit* i just connected the FP gauge to my spare gas tank's send hose, then connected the pump wires directly to my car's battery. the gauge showed that pump putting out 47 psi. this is essentially the same as the static (ie engine off) FP test in the FSM. the only difference is i don't have a fuel filter in this circuit but in a static test where the pressure builds and then holds, i don't think it would make a difference. so i now think that my gauge is faulty. geeeeez louise. i might ask a mechanic friend if i can borrow his FP tester, just to confirm this.

meanwhile, my quest for a normal idle goes on.

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Re: slightly high/low idle once warm

Post by Mark » Tue May 24, 2016 8:59 pm

I'll be one of those anal pricks on the internet (like the people who correct grammar) and say that a pump doesn't create pressure, it creates flow. The resistance to flow in the system creates the pressure. The only way your pump could be responsible for the high pressure is if it puts out too much flow. Unless it's the wrong pump for your car (too large of a pump) or it's rotating too fast (high voltage?), I wouldn't think the pump would be the problem. Does your regulator re-route the excess flow back to the tank? Could this line be obstructed? Could the gas tank be pressurized? I agree that it's most likely a buggered pressure gauge. Or... could one or more of the fuel injectors be leaking by? I don't know if there's a test for them in the FSM. I shouldn't post things on the internet after a few IPAs. I tend to ramble.

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Re: slightly high/low idle once warm

Post by Mark » Tue May 24, 2016 9:14 pm

Not knowing much about fuel injection controls, I did a search and this page might be a bit simplistic, but there are a couple of quick checks like a loose/leaking intake duct to a coked up throttle body:https://www.2carpros.com/articles/engine-idles-too-high

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Re: slightly high/low idle once warm

Post by Petros » Tue May 24, 2016 9:53 pm

all EFI fuel pumps put out more pressure than the system can handle, the system depends on the fuel pressure regulator to keep the fuel system within specification. too high a fuel pressure is just as bad a tool low (though it is far less common). If the FPR is good, than check for pinched or damaged return line, plugged up charcoal canister, improperly working fuel cap (the cap is part of the system and prevents over pressure of the tank).

And of course, correct spark timing, no vacuum leaks, and smoothly operating throttle.

You might also check all of your sensors: air and coolant temperature, air flow, O2, etc. they might be within normal spec so they will not throw a code, but if the air temp sensor is reading too low it will make the mixture more rich and make it idle too high. same with coolant temp, and O2 sensor. The FSM usually has simple tests you can do with a volt-olm meter on the sensors, make sure you check them all both cold and hot.

Some autoparts stores have sensor test machines, you bring them in and they can bench test them for you.
'87 Tercel 4wd SR5 (current engine swap project)
'84 Tercel 4wd (daily driver, with on going mods)
'92 Mazda MPV 4wd (wife's daily driver)
'85 Tercel 4wd DLX auto(daughter's daily driver)
'01 Honda Civic (other daughter's daily driver)

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