I need help with this, I have a 1993 FJ80, that I have replaced almost everything, the last problem was the throttle cable was sticking so the RPM would always be around 1000-1200 when in park or neutral unless you pull back on the accelerator with your foot, so I replaced the throttle cable, readjusted the TPS to spec by the FSM using feeler gauges and a ohm meter, I replaced the cap, rotor, plugs, wires, every vacuum line, including all under the intake manifold, rubber intake hose, the evap canister, fuel filter, both 02 sensors, fuel pressure regulator, TPS, cleaned and inspected all the injectors, cleaned the throttle body, the IAC, every hose I can find, I replace, disabled the egr, it runs great and pulls strong without vibration at anything other than idle now after replacing the cable, when you come to a stop, it slowly falls from 1000 to around 550 where it idles rough and vibrates really bad, as soon as you blip the throttle in park or drive it stops and runs great, until you come to a stop, when it's cold it revs to about 1800 for a minute or so then falls to idle. it runs great when that happens, any ideas where to go with this? I disconnected the battery after i adjusted the TPS thinking it needed to relearn the idle because the cable was sticking and it was fighting that, but that didn't seem to change anything.
1993 Fj80 Rough Idle, runs great other than that (1 Viewer)
- Thread starter brandonatx13
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LS1FJ40
SILVER Star
After you disconnected the battery how many times have you driven it? For how long? When I uninstalled my EGR I had to drive mine (92 3FE 80) about 5 times for 15 minutes or so each before the idle "settled in" at around 650ish where it should be. Now it runs perfectly and idles better than ever.
- Thread starter
- #3
I drove it 3 times for around 10-15 minutes, it still stumbles, shakes and idles rough when you come to a stop, like it's hunting for the right idle, it's fine for a second when it's around 700 but as it falls to 550 it gets worse, still think the computer is adjusting after i replaced the throttle cable and adjusted the tps?
LS1FJ40
SILVER Star
Something to consider: after years of operation, battery disconnects, other variables and and wear factors, the Idle Air Controller (IAC) can be operating at one end of its range of travel. To correct this, the valve must be "centered". If you've had the throttle plate sticking slightly open due to the recently replaced throttle cable, the IAC system has been trying to slow the idle speed down and is probably out of whack.
Not sure if the LC PCM has this capability or not, pretty sure it does since it's OBD1 and this should be universal , but here it is:
To center the IAC, you'll need to cycle the ignition switch from "off" to "run" at least 14 times within 20 seconds. Do not start it, just go from "off" to "run". This will cause the stepper motor in the IAC to center the air valve. Wait about five minutes, ignition off, then start the truck. Most likely the idle speed will then be so low it won't even run, so you'll need to drive it right away, with plenty of idle and stop-and-go situations. Around the neighborhood, with plenty of stop signs, usually works fine. Be prepared to drive the thing for 20-30 minutes before the idle stabilizes at the correct speed. If the idle is still too low, there's a good chance the air passages are clogged with soft carbon and oil vapors, not unheard of, but by centering the IAC, you've eliminated one variable from the equation. Come to think about it, you may want to remove and inspect the operating part of the valve and then do the centering procedure.
Another cause of rough or low idle is an EGR valve that's not completely closed at idle. This is also very common, and the fix is to remove it and clean the valve and seating area. If you've ever been in a highway speed situation for a long time, abruptly gone to a stop-and-go situation and experienced a low and rough idle when you've previously had no problems, this is the first likely culprit, and holds true for pretty much any engine with an EGR that's vacuum controlled.
I know you're wondering "who's this guy, come out of the blue with all this drivel", as well you should. But I've had a couple of these things myself, and spent a few years at a Lexus dealer as a Lexus and ASE Master Tech. The last ten of close to 30 years in the business was at my own shop, which I sold five years ago. I'm sort of semi-retired now, and it's a slow time of year at work, so I'm on the internet. Hmmm....
Not sure if the LC PCM has this capability or not, pretty sure it does since it's OBD1 and this should be universal , but here it is:
To center the IAC, you'll need to cycle the ignition switch from "off" to "run" at least 14 times within 20 seconds. Do not start it, just go from "off" to "run". This will cause the stepper motor in the IAC to center the air valve. Wait about five minutes, ignition off, then start the truck. Most likely the idle speed will then be so low it won't even run, so you'll need to drive it right away, with plenty of idle and stop-and-go situations. Around the neighborhood, with plenty of stop signs, usually works fine. Be prepared to drive the thing for 20-30 minutes before the idle stabilizes at the correct speed. If the idle is still too low, there's a good chance the air passages are clogged with soft carbon and oil vapors, not unheard of, but by centering the IAC, you've eliminated one variable from the equation. Come to think about it, you may want to remove and inspect the operating part of the valve and then do the centering procedure.
Another cause of rough or low idle is an EGR valve that's not completely closed at idle. This is also very common, and the fix is to remove it and clean the valve and seating area. If you've ever been in a highway speed situation for a long time, abruptly gone to a stop-and-go situation and experienced a low and rough idle when you've previously had no problems, this is the first likely culprit, and holds true for pretty much any engine with an EGR that's vacuum controlled.
I know you're wondering "who's this guy, come out of the blue with all this drivel", as well you should. But I've had a couple of these things myself, and spent a few years at a Lexus dealer as a Lexus and ASE Master Tech. The last ten of close to 30 years in the business was at my own shop, which I sold five years ago. I'm sort of semi-retired now, and it's a slow time of year at work, so I'm on the internet. Hmmm....
- Thread starter
- #6
I tried that and it didn't help, still the same low, rough idle, i did notice when i disconnect the fuel pressure vacuum line, it idles a lot better, i noticed that when it's a idle there is a strong vacuum on the fuel pressure regulator and it doesn't change when you rev the motor....I also replaced the fuel pressure regulator with a new denso one...any ideas now??
Did you do the IAC dance and make sure your EGR was fully closed?
Did you put a gauge on the vac line to the fuel pressure regulator? It should be manifold vacuum, so at idle it'll be high, dipping as load goes up, which is hard to see just revving an unladen engine. When you disconnect the line, your pressure regulator senses low (zero) vacuum and sends full fuel pressure to the injectors, so the idle speed should go up with it disconnected from its vacuum source.
Speaking from experience, I would go back and be double-sure of your TPS adjustment, then double-check that the EGR closes completely in high vacuum situations (idle and coasting) before you go any further or spend any more money.
Did you put a gauge on the vac line to the fuel pressure regulator? It should be manifold vacuum, so at idle it'll be high, dipping as load goes up, which is hard to see just revving an unladen engine. When you disconnect the line, your pressure regulator senses low (zero) vacuum and sends full fuel pressure to the injectors, so the idle speed should go up with it disconnected from its vacuum source.
Speaking from experience, I would go back and be double-sure of your TPS adjustment, then double-check that the EGR closes completely in high vacuum situations (idle and coasting) before you go any further or spend any more money.
- Thread starter
- #8
Ok i am still having problems getting this to idle right but i made several observations maybe someone can make sense of is:
1: When driving in gear when you come to a stop it will idle fine at 650 for 10 seconds then the rpms fall to 600, then 550, then 500 where is idles really rough and shakes bad, as soon as you blip the throttle or just slightly touch the accelerator so it raises the rpm's it's smooth and nice
2) I adjusted the tps to spec by the fsm with the shims, it accelerates much better and smoother and has more power, but the idle is worse, if you slightly turn the tps clockwise like a mm or so and hear the stepper motor on the IAC click it idles fine.....Why it's out of spec? but hesitates slightly when you step on the gas and power surges after 3,000 rpm strong
3) I took off the IAC and tested it to the FSM, when i bump the s1,s2,s3,s4,s1 then s4,s3,s2,s1,s4 you can feel it vibrate, and it barely moves, very hard to even notice the movement, it's doesn't open and close much at all, is this how it is suppose to work, tiny adjustments or is it faulty? The FSM shows in the pic the plunger moving quite a bit..
4) unplugging the fuel pressure regulator helps it idle a little better?
5) EGR has been bypassed and blocked off a long time ago
Is all this just a bad iac? or something else i am missing? Thanks for all of your help so far!!
1: When driving in gear when you come to a stop it will idle fine at 650 for 10 seconds then the rpms fall to 600, then 550, then 500 where is idles really rough and shakes bad, as soon as you blip the throttle or just slightly touch the accelerator so it raises the rpm's it's smooth and nice
2) I adjusted the tps to spec by the fsm with the shims, it accelerates much better and smoother and has more power, but the idle is worse, if you slightly turn the tps clockwise like a mm or so and hear the stepper motor on the IAC click it idles fine.....Why it's out of spec? but hesitates slightly when you step on the gas and power surges after 3,000 rpm strong
3) I took off the IAC and tested it to the FSM, when i bump the s1,s2,s3,s4,s1 then s4,s3,s2,s1,s4 you can feel it vibrate, and it barely moves, very hard to even notice the movement, it's doesn't open and close much at all, is this how it is suppose to work, tiny adjustments or is it faulty? The FSM shows in the pic the plunger moving quite a bit..
4) unplugging the fuel pressure regulator helps it idle a little better?
5) EGR has been bypassed and blocked off a long time ago
Is all this just a bad iac? or something else i am missing? Thanks for all of your help so far!!
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Methinks the IAC should show more movement..... and it's not unknown for them to fail. With it removed, is it possible to look into the passages in the throttle body (I haven't looked in a while, don't remember) and if so, does it look like the inside of a coal mine? You may need to remove the throttle body for a good cleaning also.
- Thread starter
- #10
I cleaned the throttle body and intake a month ago when i replaced all the vacuum lines, anyone else bench test their IAC and see movement? It really feels like the motor is just choking for air at idle, like the iac isn't moving....
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I tested my IAC today, having trouble idling, and my bench test results were just like yours. When I applied power, you could feel it bump, but not much noticeable movement. Im thinking it should move through its whole range of motion. My opinion is its bad. fwiw
ppc
M Go Blue
If you feel the movement when grounding each of the four wires the IAC more than likely is OK. It is a stepper motor that only moves in pulses. You can also check for proper resistance. This device controls an air bypass in the throttle body to regulate engine RPM. It doesn't have to move very much to change the airflow. In the test you are only making it move 4 or 5 times. In operation the ECM may send 50 signals to effectively change the RPM by 200.
It is far more likely that the air bypass is plugged to some degree.
It is far more likely that the air bypass is plugged to some degree.
I have a '94 FZJ80 with what I thought is a rough idle problem, but after reading through this thread I looked up spec idle RPM, which it seems like you guys know, is 650. At operating temp, mine idles @ 400rpm, so would this be considered rough idle or low idle, and are the causes the same?
I would definitely call that "low". If it's also smooth, you may just need to do that "14 times in 20 or less seconds" dance.
I might also suggest verifying that idle speed on a separate tachometer, seems like I remember the vehicle's tach is not terribly accurate.
I might also suggest verifying that idle speed on a separate tachometer, seems like I remember the vehicle's tach is not terribly accurate.
- Thread starter
- #15
I still been having lots of trouble trying to figure out the rough idle problem, I did all of you'll suggestions and some, cleaned the idle air passages, cleaned the throttle body again, checked the IAC and centered it by turning the key 15 times without starting it, I put the paper clip in the e1 & te1, verified check engine flashing, no codes, ac off, neutral, adjusted the timing to 3 degrees btdc. Runs great at anything other than at idle, here is a video of what it does, when the rpm's start falling it shakes violently, when you just blip the gas it's all good again until it starts falling. maybe someone can make sense of this.....
I'm thinking a SMALL vacuum leak somewhere that is slowly building and dropping RPM. Intake hose, PCV Valve grommet, vacuum driven accessories such as brake booster, any other servos? Just fishin.....
- Thread starter
- #17
When the RPM's start dropping and it idles really rough and shakes violently, i can unplug the TPS and it idles right back to normal very smoothly, could this be something simple like a fuel pump relay, or fuel pump resister? Because when the TPS is unplugged, it doesn't complete the circuit for idle, the tps is brand new and adjusted to spec, this is a long shot but the whole problem starts when the TPS circuit closes and the ecu is in the idle program this is when the rough shaking idle starts, like it's starved for fuel or air....i did change the IAC with another one and it does the same thing, we know the IAC is working from the 1,800 rpm cold start and the a/c kicks up the rpms. What do you'll think?
Did it do it with the OLD TPS? Otherwise, could it be another bad TPS?
- Thread starter
- #19
It acted the same with the old tps, but when i unplug either TPS, it idles perfect and smooths out, I checked with a multimeter and when IDL & E2 have no continuity is when when it idles good
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When you unplug the TPS, you're making the PCM think the throttle is open, so it enriches the mixture, which smooths your idle, therefor making me think you have a lean condition at root fault. I think your surge upward in rpm's is the IAC reacting to the very low idle. So, that being said, I think you have, as already mentioned, a small vacuum leak, maybe at the PCV, engine side of the brake booster ck. valve, any small hose on the throttle body or intake manifold that's open to manifold vacuum, or, and here's where I see a lot of problems crop up, at the EGR valve itself. I realize you've had the valve off, and replaced it, but if it's leaking exhaust into the intake, it will give the exact same symptoms as an intake vacuum leak to atmosphere.
It's a pain to go back over items you've already covered, but it surely sounds like you've already hit all the high spots, no change in your symptoms, so if it were me, I'd start going back to the likely suspects.
My .02, but I can surely say from personal experience that missing something first (or second, or third) time around is no thing unusual.
It's a pain to go back over items you've already covered, but it surely sounds like you've already hit all the high spots, no change in your symptoms, so if it were me, I'd start going back to the likely suspects.
My .02, but I can surely say from personal experience that missing something first (or second, or third) time around is no thing unusual.
