Full Version: engine running oddly..HELP!
okay...so my LT1 is running weird. I noticed a while ago that if i stopped quicky that the rpms would kinf of drop down a little bit like the engine wanted to stall out but quicky came back to normal idle range. it did this only when i would stop quicky and not every time but a good amount of time but now, when the car idles it does the same thing. it will idle around 1k-1100 rpms then every once in a while it will quickly drop to about 700 and while it does this the oil pressure drops from around 40psi to about half way between the line between 40 and 0. so the needle drops about a 1/2 inch i guess. then it goes right back to normal. i had everything in the front of the engine replaced when i bought it (water pump, optispark, timing, chain and waterpump) so i dnt see why it would be either the timing chain or the opti. I hope that this isnt going to be a major problem but id like to catch this issue before it causes something worse than it already is. it will do this about once or twice every 2 minutes or so. ANY information will be helpful..thanks guys
This seems like it could be a TPS or even an IAC...both can affect idle quality. Here's a test: When starting a cold engine, does the idle come right up to around 1400 or even as high as 1700, then gradually drop down to normal idle speed as the engine warms up? Unless the engine goes to a high idle on a cold startup, then the IAC is defective. The IAC is a precision stepping motor and it can freeze up or work erratically as it fails. The TPS can cause "blank" spots in acceleration or even give a false throttle blade position to the PCM. If that happens, then the PCM will calculate the incorrect fuel air ratio for the engine and that can affect the idle and idle speed.
QUOTE(Me Too @ Jun 8 2010, 10:48 AM) 
This seems like it could be a TPS or even an IAC...both can affect idle quality. Here's a test: When starting a cold engine, does the idle come right up to around 1400 or even as high as 1700, then gradually drop down to normal idle speed as the engine warms up? Unless the engine goes to a high idle on a cold startup, then the IAC is defective. The IAC is a precision stepping motor and it can freeze up or work erratically as it fails. The TPS can cause "blank" spots in acceleration or even give a false throttle blade position to the PCM. If that happens, then the PCM will calculate the incorrect fuel air ratio for the engine and that can affect the idle and idle speed.
i havent really payed much attention to the idle when i start the car because its always different lol sometimes i will be at a stop light and it will sit at 1500rpms for some reason. other times it will be at about 1k. AND WHAT I DONT GET AT ALL is that even if i am rolling 3mph, the idle will rev at 1500 in NEUTRAL until i completely stop then it drops to normal. Its like the car has a mind of its own or something, i dont see any connection what so ever with the speed of the car and the idle speed when it is in neutral. i dont get that at all and have been wondering about it because my car is loud and i know it pisses off the neighbors when i come home at 4am and im doing 5mph in neutral but the engine is revving at 1500 LOL
sounds like TPS to me.
Hmmm...I'd check out both the TPS and the IAC. Problem is, the IAC is not cheap and the only way to check it is to replace it. The TPS is relatively inexpensive, a good one will cost you about $25. Be careful, though...the cheapies, available at some discount auto supply stores are not very good and give some odd idles. The best ones are sold by GM, so maybe there is a discount available from a GM place on the Internet.
To make matters worse, it could be something else. No codes from the PCM makes this a difficult problem to figure out. Bare in mind that the PCM should be throwing a DTC, and if it isn't, it could also be defective.
Here's a test you should try. Disconnet the negative terminal from the battery and leave it off for about 5 minutes. That should wipe out the Long Term Memory (Block Learns) and force the PCM to operate the engine on the Intergrator or Short Term Memory. If the engine runs better, then deteriorates after about 20 miles, it's a good bet that the PCM is also defective. In fact, one of the clues that the PCM has failed is that it doesn't throw DTCs when something has failed and it won't do a diagnostic on itself.
To make matters worse, it could be something else. No codes from the PCM makes this a difficult problem to figure out. Bare in mind that the PCM should be throwing a DTC, and if it isn't, it could also be defective.
Here's a test you should try. Disconnet the negative terminal from the battery and leave it off for about 5 minutes. That should wipe out the Long Term Memory (Block Learns) and force the PCM to operate the engine on the Intergrator or Short Term Memory. If the engine runs better, then deteriorates after about 20 miles, it's a good bet that the PCM is also defective. In fact, one of the clues that the PCM has failed is that it doesn't throw DTCs when something has failed and it won't do a diagnostic on itself.
QUOTE(Me Too @ Jun 9 2010, 12:14 PM)
Hmmm...I'd check out both the TPS and the IAC. Problem is, the IAC is not cheap and the only way to check it is to replace it. The TPS is relatively inexpensive, a good one will cost you about $25. Be careful, though...the cheapies, available at some discount auto supply stores are not very good and give some odd idles. The best ones are sold by GM, so maybe there is a discount available from a GM place on the Internet.
To make matters worse, it could be something else. No codes from the PCM makes this a difficult problem to figure out. Bare in mind that the PCM should be throwing a DTC, and if it isn't, it could also be defective.
Here's a test you should try. Disconnet the negative terminal from the battery and leave it off for about 5 minutes. That should wipe out the Long Term Memory (Block Learns) and force the PCM to operate the engine on the Intergrator or Short Term Memory. If the engine runs better, then deteriorates after about 20 miles, it's a good bet that the PCM is also defective. In fact, one of the clues that the PCM has failed is that it doesn't throw DTCs when something has failed and it won't do a diagnostic on itself.
To make matters worse, it could be something else. No codes from the PCM makes this a difficult problem to figure out. Bare in mind that the PCM should be throwing a DTC, and if it isn't, it could also be defective.
Here's a test you should try. Disconnet the negative terminal from the battery and leave it off for about 5 minutes. That should wipe out the Long Term Memory (Block Learns) and force the PCM to operate the engine on the Intergrator or Short Term Memory. If the engine runs better, then deteriorates after about 20 miles, it's a good bet that the PCM is also defective. In fact, one of the clues that the PCM has failed is that it doesn't throw DTCs when something has failed and it won't do a diagnostic on itself.
well actually my check engine light is on so it is throwing a code but it has been on since i got the car and when i had the opti and everything replaced they said the code was the 02 sensor since i have no cats or 02 sensors. they cleared it but it came back on. And for the past two days the car hasnt done the weird idle thing but maybe once..it comes and goes..oh and i had a mexican in a dodge neon (non SRT) try and race me for money..so i said yeah while we were driving anddddd pretty much blew his doors off and listened to him revv his rice can as i turned off. It was good enough that i blew him away than to take his money haha
You should get some O2 Syms. They will duplicate the O2 sensors (your engine takes 3), and turn off the code. However, without O2 sensors, your engine will run in "open loop" mode and never go into "closed loop". Basically, it will run off the Integrator and never go into the Block Learns. It will run well when the throttle is mashed past 70% or WOT! Or, is that WFO!? Without the O2 sensor(s), the engine will not have very good partial throttle response, gas mileage and emissions will suffer.
So, if you think the engine is running well at this time, it will run much better if you reinstall the O2 sensors. If necessary, bungs (if they aren't already welded in), will have to be welded onto the header collectors, then a third one will be needed back where the CAT should be. Most use a single Sym where the CAT was, and one real O2 sensor on each collector. The CAT sensor pretty much monitors the CAT output, but the sensors at the headers tell the PCM whether or not the engine is running rich or lean, and without them, the PCM will stay in open loop mode.
So, if you think the engine is running well at this time, it will run much better if you reinstall the O2 sensors. If necessary, bungs (if they aren't already welded in), will have to be welded onto the header collectors, then a third one will be needed back where the CAT should be. Most use a single Sym where the CAT was, and one real O2 sensor on each collector. The CAT sensor pretty much monitors the CAT output, but the sensors at the headers tell the PCM whether or not the engine is running rich or lean, and without them, the PCM will stay in open loop mode.
QUOTE(Me Too @ Jun 14 2010, 11:37 AM)
You should get some O2 Syms. They will duplicate the O2 sensors (your engine takes 3), and turn off the code. However, without O2 sensors, your engine will run in "open loop" mode and never go into "closed loop". Basically, it will run off the Integrator and never go into the Block Learns. It will run well when the throttle is mashed past 70% or WOT! Or, is that WFO!? Without the O2 sensor(s), the engine will not have very good partial throttle response, gas mileage and emissions will suffer.
So, if you think the engine is running well at this time, it will run much better if you reinstall the O2 sensors. If necessary, bungs (if they aren't already welded in), will have to be welded onto the header collectors, then a third one will be needed back where the CAT should be. Most use a single Sym where the CAT was, and one real O2 sensor on each collector. The CAT sensor pretty much monitors the CAT output, but the sensors at the headers tell the PCM whether or not the engine is running rich or lean, and without them, the PCM will stay in open loop mode.
So, if you think the engine is running well at this time, it will run much better if you reinstall the O2 sensors. If necessary, bungs (if they aren't already welded in), will have to be welded onto the header collectors, then a third one will be needed back where the CAT should be. Most use a single Sym where the CAT was, and one real O2 sensor on each collector. The CAT sensor pretty much monitors the CAT output, but the sensors at the headers tell the PCM whether or not the engine is running rich or lean, and without them, the PCM will stay in open loop mode.
so are you saying if i get 02 sensors my car will be faster???????? and yes my gas mileage is very bad like 11 mpg LOL
QUOTE(91formula @ Jun 15 2010, 12:46 AM)
so are you saying if i get 02 sensors my car will be faster???????? and yes my gas mileage is very bad like 11 mpg LOL
At WOT, it could be faster, by a lot or maybe by a little--depends on the engine. Here's the deal, at 70-80% throttle, the engine goes into open loop mode and during that time, it runs the last known good fuel curve. That "last known good fuel curve" is set during closed loop operation. So, what happens if the engine never operates in closed loop? If the engine stays in open loop, the PCM never fine tunes the engine. So, there is a strong possibility that it will run better during WOT if there are O2 sensors, becuz it will establish the proper fuel curves during that time.
The lack of O2 sensors will affect how the engine runs and it will affect gas mileage to a significant amt. 11 mpg is way too low.
If the engine has other problems, the lack of O2 sensors forces it into open loop operation and that could cover up a bunch of difficulties. You will have to add the O2 sensors and find out what happens, then fix as needed. For instance, if the engine has a 58 mm TB (too large for this engine), running in open loop mode, means that the TB size is basically ignored and it runs premapped airflow curves for the TPS output and with those premapped air flow curves, the PCM runs basic injector dwell rates--it doesn't care what the air/fuel ratio is. With the O2's installed, the PCM will try and fine tune to the throttle body TPS output, which means the engine will at first run lean, then the PCM will try and tune the lean output by using the O2 sensor values.
QUOTE(Me Too @ Jun 15 2010, 11:04 AM)
At WOT, it could be faster, by a lot or maybe by a little--depends on the engine. Here's the deal, at 70-80% throttle, the engine goes into open loop mode and during that time, it runs the last known good fuel curve. That "last known good fuel curve" is set during closed loop operation. So, what happens if the engine never operates in closed loop? If the engine stays in open loop, the PCM never fine tunes the engine. So, there is a strong possibility that it will run better during WOT if there are O2 sensors, becuz it will establish the proper fuel curves during that time.
The lack of O2 sensors will affect how the engine runs and it will affect gas mileage to a significant amt. 11 mpg is way too low.
If the engine has other problems, the lack of O2 sensors forces it into open loop operation and that could cover up a bunch of difficulties. You will have to add the O2 sensors and find out what happens, then fix as needed. For instance, if the engine has a 58 mm TB (too large for this engine), running in open loop mode, means that the TB size is basically ignored and it runs premapped airflow curves for the TPS output and with those premapped air flow curves, the PCM runs basic injector dwell rates--it doesn't care what the air/fuel ratio is. With the O2's installed, the PCM will try and fine tune to the throttle body TPS output, which means the engine will at first run lean, then the PCM will try and tune the lean output by using the O2 sensor values.
The lack of O2 sensors will affect how the engine runs and it will affect gas mileage to a significant amt. 11 mpg is way too low.
If the engine has other problems, the lack of O2 sensors forces it into open loop operation and that could cover up a bunch of difficulties. You will have to add the O2 sensors and find out what happens, then fix as needed. For instance, if the engine has a 58 mm TB (too large for this engine), running in open loop mode, means that the TB size is basically ignored and it runs premapped airflow curves for the TPS output and with those premapped air flow curves, the PCM runs basic injector dwell rates--it doesn't care what the air/fuel ratio is. With the O2's installed, the PCM will try and fine tune to the throttle body TPS output, which means the engine will at first run lean, then the PCM will try and tune the lean output by using the O2 sensor values.
okay i gotchaaaa. ill see what i can do about getting some o2 sensors then. and let me state that that 11mpg was calculated on the HIGHWAY all the way to orlando from jacksonville. took a whole tank of gas pretty much and its only like 150 miles LOL
QUOTE(91formula @ Jun 16 2010, 06:53 AM)
okay i gotchaaaa. ill see what i can do about getting some o2 sensors then. and let me state that that 11mpg was calculated on the HIGHWAY all the way to orlando from jacksonville. took a whole tank of gas pretty much and its only like 150 miles LOL
OHMYGAWD! That's awful mileage. There is no way that a properly running engine should do that. Good luck hope you can track it down.
QUOTE(Me Too @ Jun 16 2010, 11:39 AM)
OHMYGAWD! That's awful mileage. There is no way that a properly running engine should do that. Good luck hope you can track it down.
i know its rediculous. i hate it so much but my other formula is the same way it was getting only 10mpg for some reason. seems that every vehicle i have had so far winds up having some kind of issue that just strait KILLS the fuel mileage lol
QUOTE(91formula @ Jun 8 2010, 04:06 AM)
okay...so my LT1 is running weird. I noticed a while ago that if i stopped quicky that the rpms would kinf of drop down a little bit like the engine wanted to stall out but quicky came back to normal idle range. it did this only when i would stop quicky and not every time but a good amount of time but now, when the car idles it does the same thing. it will idle around 1k-1100 rpms then every once in a while it will quickly drop to about 700 and while it does this the oil pressure drops from around 40psi to about half way between the line between 40 and 0. so the needle drops about a 1/2 inch i guess. then it goes right back to normal. i had everything in the front of the engine replaced when i bought it (water pump, optispark, timing, chain and waterpump) so i dnt see why it would be either the timing chain or the opti. I hope that this isnt going to be a major problem but id like to catch this issue before it causes something worse than it already is. it will do this about once or twice every 2 minutes or so. ANY information will be helpful..thanks guys
I was havin the same problem with my formula....idle would get really high then gradually come down on a cold crank, then when driving and stopping the rpms would drop down below 500 and my engine would stall out. i changed the iac valve and it idles perfect now at 900 rpms.
ha now my formula is on the sidelines for a jacked up clutch
So, I was about to post a topic with similar symptoms when I read this one so I'm gonna piggy back it. I just put a mild cam in and had it tuned at CAM. running about 300hp at the wheels, 313 ft lbs. Everything was fine until about 2 weeks ago. I cranked the car, and I noticed a high rev, almost 2000. then it came back down, but my oil pressure is guesstimated about 60 psi. Whether the car is moving or sitting still, the oil pressure stays that high. As I am driving, when the car changes gears (A4), it seems to almost stal out, then jumps to high rpm and I have even squeaked the tires when it does that. After the car warms up, the oil pressure drops to about 40 psi then fluctuates as I am driving. If I am accelerating, it is about 40, but when I am at an idle, it drops to about 30...
Is that the TPS or is my PCM failing? Why is my oil presure so high?
Is that the TPS or is my PCM failing? Why is my oil presure so high?
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