Cold start problem - low idle.
10-08-09, 11:28 AM
#1
Cold start problem - low idle.
Whats up guys, this problem just started yesterday. When I start the car, idle stays low, pretty much at 650rpm - which is the normal speed for when the engine is warm. It of course makes for a rather rough/vibrating idle until the car warms up. When it warms up, the car is perfectly normal, idle doesn't drop below 650, everything is nice and smooth.
So what could it be? I have a few theories.
1) ECT Sensor - it could be broken and telling the computer that the engine is warm, and therefor the computer isn't sending enough fuel. But I kind of rule that out, because that would create problems on acceleration while the engine hasn't reached operating temperature. Plus, on a cold start the exhaust has a heavy gas smell, so I'm more inclined to think that it isn't getting enough air.
2) Cold start injector - I really don't know how it works and whats its function. Can somebody shed some light onto that?
3) Idle Air Valve - thats my most likely suspicion. Could it be that its not opening and the engine isn't getting enough air? But in that case, wouldn't I still have a problem once the engine is warm?
So what could it be? I have a few theories.
1) ECT Sensor - it could be broken and telling the computer that the engine is warm, and therefor the computer isn't sending enough fuel. But I kind of rule that out, because that would create problems on acceleration while the engine hasn't reached operating temperature. Plus, on a cold start the exhaust has a heavy gas smell, so I'm more inclined to think that it isn't getting enough air.
2) Cold start injector - I really don't know how it works and whats its function. Can somebody shed some light onto that?
3) Idle Air Valve - thats my most likely suspicion. Could it be that its not opening and the engine isn't getting enough air? But in that case, wouldn't I still have a problem once the engine is warm?
10-08-09, 12:39 PM
#2
Hey there Och, I will help you to try to come up with some answers instead of bring in this one in. Sorry for asking, but what year do you have? I ask because it will help me diagnose a little easier, as a 89'-92' 1UZ uses the Cold start injector, and the 93'+ doesn't. That being said, this is more for the pre-92'. 94' and later, just ignore the cold injector part. Does the vehicle drive fine even during a cold start or is it a little sluggish? Once the vehicle is warmed up, start it back up and note what it does. Is it a cold engine running thing (start up, idle, driving, etc.) or is it just during idle? I think as far as the theories go, your there, however I would like to add that the ECU controls all those things, so if you have an issue that can always a likely candidate. 1st rule of diagnosis, don't rule anything out. Here is a normal cold start sequence. As you turn the key, the computer takes a snap shot of the sensors to get an estimate of what the operating conditions are and should automatically start up in cold start, which is an electronic high idle. This is achieved through the ECU controlling the idle air control valve, fuel, and sensors. During cold start the engine idles fast (1000+rpm) because it is running rich in order to warm up sooner to achieve operating temperature quicker, it is basically running off of pre-programmed memory. The 8 fuel injectors, I do believe, do not add the extra volume, but the cold start injector does. Once the engine reaches operating temp it shuts the cold start injector off in order to resume warm idle, which is around 650rpm. Once the ecu idles the engine down, it is then in closed loop, which means it is reading from the temp sensor, O2 sensors, and all the other various ones that monitor all the things like fuel strategy, accessory loads, etc, etc. That being said.......
1) Well this could be true, but typically if the sensor breaks it normally fails to it's default, which is -30 or something. How new is the sensor? Try unplugging the sensor to see if you get a different result. Measure the resistance of the sensor, Ohm it.
2) This is what I am leaning towards, however I am not sure how to test it. However you can either clean it with some solvent or you could source one from another junked LS4. If it's a 93 or later, you don't have one, and you will need to start looking for another solution.
3) Obviously controls the idle, and like you said it works, just not during the cold start. Normally air is never a problem as naturally aspirated motors suck, literally suck air to stay running. If you don't think if getting enough air, take the hose from the MAF tubing to the IAC (idle air control) valve off, and put your thumb over it quickly, like your plugging the IAC valve up, it will suck you thumb in, and the motor should stop as it is not able to suck any air in to burn. Try taking the IAC off and cleaning it with some break cleaner, it's simply it's just a few clamps and 2 bolts.
I hope I helped a little bit..... Let me know if I need to clear anything up.
1) Well this could be true, but typically if the sensor breaks it normally fails to it's default, which is -30 or something. How new is the sensor? Try unplugging the sensor to see if you get a different result. Measure the resistance of the sensor, Ohm it.
2) This is what I am leaning towards, however I am not sure how to test it. However you can either clean it with some solvent or you could source one from another junked LS4. If it's a 93 or later, you don't have one, and you will need to start looking for another solution.
3) Obviously controls the idle, and like you said it works, just not during the cold start. Normally air is never a problem as naturally aspirated motors suck, literally suck air to stay running. If you don't think if getting enough air, take the hose from the MAF tubing to the IAC (idle air control) valve off, and put your thumb over it quickly, like your plugging the IAC valve up, it will suck you thumb in, and the motor should stop as it is not able to suck any air in to burn. Try taking the IAC off and cleaning it with some break cleaner, it's simply it's just a few clamps and 2 bolts.
I hope I helped a little bit..... Let me know if I need to clear anything up.
Last edited by 3UZFTE; 10-08-09 at 12:45 PM. Reason: edit.
10-08-09, 02:55 PM
#3
Those are pretty good tips. My car is a 94, so I guess then cold start injector is ruled out.
The card drives just fine when its cold or warm, its just that cold idle is low. If I tap the gas to raise RPMs to around 1400, like it should be on a cold start and hold the pedal there, it keeps running just fine, and drives fine as well.
The first thing I'll do is test the IAC the way you described, I don't know why I didn't think of it myself. Thanks!
The card drives just fine when its cold or warm, its just that cold idle is low. If I tap the gas to raise RPMs to around 1400, like it should be on a cold start and hold the pedal there, it keeps running just fine, and drives fine as well.
The first thing I'll do is test the IAC the way you described, I don't know why I didn't think of it myself. Thanks!
10-09-09, 04:00 PM
#4
Those are pretty good tips. My car is a 94, so I guess then cold start injector is ruled out.
The card drives just fine when its cold or warm, its just that cold idle is low. If I tap the gas to raise RPMs to around 1400, like it should be on a cold start and hold the pedal there, it keeps running just fine, and drives fine as well.
The first thing I'll do is test the IAC the way you described, I don't know why I didn't think of it myself. Thanks!
The card drives just fine when its cold or warm, its just that cold idle is low. If I tap the gas to raise RPMs to around 1400, like it should be on a cold start and hold the pedal there, it keeps running just fine, and drives fine as well.
The first thing I'll do is test the IAC the way you described, I don't know why I didn't think of it myself. Thanks!
Colin
Last edited by Amskeptic; 10-09-09 at 04:02 PM. Reason: wrong acronym
10-09-09, 08:02 PM
#5
Im thinking IAC also. Drown it in brake cleaner like he said. There's also a great CRC product i've used for this kinda stuff called Carburettor Cleaner. It doesnt evaporate as quick as brake cleaner does.
I remember you mentioning you're running a straight vac hose in place of your removed P/S idle up valve. Pointless IMO, but you can use it to see if the IAC is actually moving (squash it to block it and see if the IAC adjusts itself to keep the idle the same). Infact that hose could be the only reason the engine is running if the IAC is stuck completely closed.
I remember you mentioning you're running a straight vac hose in place of your removed P/S idle up valve. Pointless IMO, but you can use it to see if the IAC is actually moving (squash it to block it and see if the IAC adjusts itself to keep the idle the same). Infact that hose could be the only reason the engine is running if the IAC is stuck completely closed.
01-26-11, 12:55 PM
#7
Driver School Candidate
the missing cold start injector
Hey there Och, I will help you to try to come up with some answers instead of bring in this one in. Sorry for asking, but what year do you have? I ask because it will help me diagnose a little easier, as a 89'-92' 1UZ uses the Cold start injector, and the 93'+ doesn't. That being said, this is more for the pre-92'. 94' and later, just ignore the cold injector part. Does the vehicle drive fine even during a cold start or is it a little sluggish? Once the vehicle is warmed up, start it back up and note what it does. Is it a cold engine running thing (start up, idle, driving, etc.) or is it just during idle? I think as far as the theories go, your there, however I would like to add that the ECU controls all those things, so if you have an issue that can always a likely candidate. 1st rule of diagnosis, don't rule anything out. Here is a normal cold start sequence. As you turn the key, the computer takes a snap shot of the sensors to get an estimate of what the operating conditions are and should automatically start up in cold start, which is an electronic high idle. This is achieved through the ECU controlling the idle air control valve, fuel, and sensors. During cold start the engine idles fast (1000+rpm) because it is running rich in order to warm up sooner to achieve operating temperature quicker, it is basically running off of pre-programmed memory. The 8 fuel injectors, I do believe, do not add the extra volume, but the cold start injector does. Once the engine reaches operating temp it shuts the cold start injector off in order to resume warm idle, which is around 650rpm. Once the ecu idles the engine down, it is then in closed loop, which means it is reading from the temp sensor, O2 sensors, and all the other various ones that monitor all the things like fuel strategy, accessory loads, etc, etc. That being said.......
1) Well this could be true, but typically if the sensor breaks it normally fails to it's default, which is -30 or something. How new is the sensor? Try unplugging the sensor to see if you get a different result. Measure the resistance of the sensor, Ohm it.
2) This is what I am leaning towards, however I am not sure how to test it. However you can either clean it with some solvent or you could source one from another junked LS4. If it's a 93 or later, you don't have one, and you will need to start looking for another solution.
3) Obviously controls the idle, and like you said it works, just not during the cold start. Normally air is never a problem as naturally aspirated motors suck, literally suck air to stay running. If you don't think if getting enough air, take the hose from the MAF tubing to the IAC (idle air control) valve off, and put your thumb over it quickly, like your plugging the IAC valve up, it will suck you thumb in, and the motor should stop as it is not able to suck any air in to burn. Try taking the IAC off and cleaning it with some break cleaner, it's simply it's just a few clamps and 2 bolts.
I hope I helped a little bit..... Let me know if I need to clear anything up.
1) Well this could be true, but typically if the sensor breaks it normally fails to it's default, which is -30 or something. How new is the sensor? Try unplugging the sensor to see if you get a different result. Measure the resistance of the sensor, Ohm it.
2) This is what I am leaning towards, however I am not sure how to test it. However you can either clean it with some solvent or you could source one from another junked LS4. If it's a 93 or later, you don't have one, and you will need to start looking for another solution.
3) Obviously controls the idle, and like you said it works, just not during the cold start. Normally air is never a problem as naturally aspirated motors suck, literally suck air to stay running. If you don't think if getting enough air, take the hose from the MAF tubing to the IAC (idle air control) valve off, and put your thumb over it quickly, like your plugging the IAC valve up, it will suck you thumb in, and the motor should stop as it is not able to suck any air in to burn. Try taking the IAC off and cleaning it with some break cleaner, it's simply it's just a few clamps and 2 bolts.
I hope I helped a little bit..... Let me know if I need to clear anything up.
Oh yeah, the original engine was mistreated. Right head gasket was blown and they evidently kept running this fine car, melting on the bottom 2 pistons on the right bank. Sorry, but I had to cry on someone's shoulders!
The following users liked this post:
rcunamay (03-25-20)
Thread
Thread Starter
Forum
Replies
Last Post
