what happens when these fail
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Joined: Dec 2005
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From: Indianapolis
If your MAP and IAT are dead it's going to throw off your fueling, your pcm will fall back to using your O2's and MAF for fueling. I believe the MAP is on the back of your intake and the IAT is normally in the lid. I'd say the MAP is more important but is a pain in the *** to get to.
If your MAP and IAT are dead it's going to throw off your fueling, your pcm will fall back to using your O2's and MAF for fueling. I believe the MAP is on the back of your intake and the IAT is normally in the lid. I'd say the MAP is more important but is a pain in the *** to get to.
I've had a very wierd thing has been happening. First start of the day, cold engine. RPM's jump up and down for like 2 minutes before it finds it's proper idle. Then, its 100% normal until I shut the engine off. Then if I start it up again say after its been off for like 5,10,30 minutes, doesn;'t really matter...it hunts again for about 1-2 minutes until it settles down again. Then its 100% perfect again until the car is shut off and restarted.
So after its warmed up, its perfect. But even after its warm, on every restart it acts crazy again.
No codes, no ses light. Its been doing it for like a year I just thought I'd ask about those two sensors since I've checked everything else possible. I can live with it but its annoying. I can also just drive off if its still hunting and everything is fine as long as I'm on the throttle driving normally. But if I come to a stoplight or stop sign within that 2 minute period where it would normally still be hunting. It keeps jumping around while my foots on the break at the stop light. Then it settles down and its good to go until its turned off.
Do you know if a speed density tuned engine does that on initial start-up?
Last edited by LS6427; Jan 18, 2009 at 08:43 PM.
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Joined: Dec 2005
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From: Indianapolis
No a SD tuned car will not hunt on a cold start-up. I ran mine on a SD tune for a month or so and really couldn't tell a difference except the car felt snappier. Did you check your battery connections kinda sounds like your fuel trims and idle trims are getting blanked every time you power down. You would never throw a code as the PCM is continuously being reset. Also might want to take a look at your IAC motor. Just a thought.
No a SD tuned car will not hunt on a cold start-up. I ran mine on a SD tune for a month or so and really couldn't tell a difference except the car felt snappier. Did you check your battery connections kinda sounds like your fuel trims and idle trims are getting blanked every time you power down. You would never throw a code as the PCM is continuously being reset. Also might want to take a look at your IAC motor. Just a thought.
When you say "power down", what do you mean, just turning the car off? It just sits like any other car until I start it up again. I don't ever lose power altogether if thats what you mean.
Can the IAC just act crazy in the beginning and then settle and work perfectly?
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From: Indianapolis
By power down I meant turning the ignition off and opening the door. I take the IAC back thinking about it more makes it less likely but I have something you can try. Unplug the MAF and see what she does then. Your fueling trims don't kick in till you reach ECT of 92. It effectively puts your car in a kind of SD mode.
By power down I meant turning the ignition off and opening the door. I take the IAC back thinking about it more makes it less likely but I have something you can try. Unplug the MAF and see what she does then. Your fueling trims don't kick in till you reach ECT of 92. It effectively puts your car in a kind of SD mode.
Thats why I wondered if SD tunes do that, because I was told long ago that my PCM has defaulted to a SD tune, since I can run fine without a MAF.
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TECH Regular
Joined: Dec 2005
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From: Indianapolis
I'd start with the maf try cleaning it or seeing if you know someone that will let you swap with them real quick to see if it idles clean or does the same thing. Could be a voltage drop in the wiring or the sensor inside is dirty or shot.
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Verdad Gallardo A couple years ago when my MAF could be unplugged I did that, I went and bought a new MAF. It made no changes. I think my PCM is firmly in a SD tune mode.
1998 WS6 A4
my original 1998 PCM
2002 LPE LS6 blocked 427 bottom end
I added ported LS6 heads and ported LS6 intake
42 lb injectors
stock ported TB
Grotyohann 1 7/8" LT's (QTP, identical manufacturer)
No cats
Car/engine ran perfect from 2002 till about 1 year ago when this hunting thing gradually began.
Nothing special other than that really. No wierd mods. Has a **** ton of miles, closing in on 100,000, runs like brand new. Code free. Just has that wierd rpm hunting problem at start up and on every restart up for 2-3 minutes. Other than that it runs picture perfect.
Since it runs perfect after start-up I know the IAC must be working. And I would expect everything in the entire system is working since it runs perfect after that initial rpm hunting. I don't know however if an IAC can act up for a few minutes after start. If it can then maybe its got some kind of problem.
This is after a 45 minute drive and about 30 miles. I turned it off and then immediately started it back up.
**Never mind the "check guages" light, thats been on for 7 years after I put the 200mph speedo in because its actually a Camaro cluster they used.***
my original 1998 PCM
2002 LPE LS6 blocked 427 bottom end
I added ported LS6 heads and ported LS6 intake
42 lb injectors
stock ported TB
Grotyohann 1 7/8" LT's (QTP, identical manufacturer)
No cats
Car/engine ran perfect from 2002 till about 1 year ago when this hunting thing gradually began.
Nothing special other than that really. No wierd mods. Has a **** ton of miles, closing in on 100,000, runs like brand new. Code free. Just has that wierd rpm hunting problem at start up and on every restart up for 2-3 minutes. Other than that it runs picture perfect.
Since it runs perfect after start-up I know the IAC must be working. And I would expect everything in the entire system is working since it runs perfect after that initial rpm hunting. I don't know however if an IAC can act up for a few minutes after start. If it can then maybe its got some kind of problem.
This is after a 45 minute drive and about 30 miles. I turned it off and then immediately started it back up.
**Never mind the "check guages" light, thats been on for 7 years after I put the 200mph speedo in because its actually a Camaro cluster they used.***
427 bottom end? You said it only does it when the engine is cold. It could be improperly tuned for warm-ups causing cam surge. I'm going to ask a friend to see what he thinks.
It simply does that on every start, no matter what. But then its perfect after it hunts for 2 minutes or so.
Maybe since I have almost 100,000 miles something is simply getting worn out???
Can something like a cam get worn out or any other part, and do that?
BUT.....why would it run 100% perfect after the hunting stops if something was worn out. This is wierd man!!!
Can something like a cam get worn out or any other part, and do that?
BUT.....why would it run 100% perfect after the hunting stops if something was worn out. This is wierd man!!!
Also, if I do start it up, and it starts its hunting thing, if I give it a touch of gas pedal and hold it, it will idle smooth at whatever rpm I hold it at. Say if I hold it up at 1,200 rpm, it'll be smooth, no jumping around. If I let it go after about 1-2 minutes up at 1,200 rpm, it won't hunt and it'll settle into its idle pretty quick.
I just changed the plugs last week too, no change.
I just changed the plugs last week too, no change.
TECH Regular
Joined: Dec 2005
Posts: 437
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From: Indianapolis
After your car is started and there is no errors thrown you will not be in a SD mode. When you start it up and the coolant temp is below a certain temp you will be in open loop, once the temp is met your car will switch over to closed loop. In closed loop your MAP, IAT, and O2's come more into play then your MAF until WOT. So it sounds to me that once you go into closed loop (ie speed density) you are fine. Eliminating the MAF will force the car out of an open loop start and put it in a closed loop right off the bat, but if you get no errors sent you will stay in that cycle. If you pull the MAF plug and it runs fine that means something isn't right with your open loop portion. Is there someone around you that has HP Tuners that can get a scan file for you? Seeing that would answer everything and it's free to do for anyone with HP Tuners.
After your car is started and there is no errors thrown you will not be in a SD mode. When you start it up and the coolant temp is below a certain temp you will be in open loop, once the temp is met your car will switch over to closed loop. In closed loop your MAP, IAT, and O2's come more into play then your MAF until WOT. So it sounds to me that once you go into closed loop (ie speed density) you are fine. Eliminating the MAF will force the car out of an open loop start and put it in a closed loop right off the bat, but if you get no errors sent you will stay in that cycle. If you pull the MAF plug and it runs fine that means something isn't right with your open loop portion. Is there someone around you that has HP Tuners that can get a scan file for you? Seeing that would answer everything and it's free to do for anyone with HP Tuners.
What specifically should I ask him to look for? It'll be warmed up when I get there, so do I need to let it cool off for a couple hours then start it for the scan?
So.....I just went out there, engine is 100% cold. I unplugged the TPS and the MAF, started it up and it didn't really hunt much. Just went right up to 11oo rpm and stayed there. Throttle worked fine, revved up and don't normally without the TPS and MAF.
So is it in SD tune right off the bat?
Tomorrow I'll start it and go driving and leave both the TPS and MAF unplugged, see what happens after warm up.
TECH Regular
Joined: Dec 2005
Posts: 437
Likes: 9
From: Indianapolis
Yes with the MAF unplugged you are on a SD fall back but leave your TPS plugged in and leave the MAF off for now. Have him look at what the timing is doing when it's hunting and when it is not. Also if there is a specific coolant temp as to when the hunting stops, make sure your O2's are switching quickly. If it stops hunting when your fuel trims kick in you have a problem with your open loop which pretty much the only sensor being used is your MAF. Of course check to see if you have any codes as well, some have to trip 2 or more times to actually show up. After you get done you may want to clear out your PCM and let it relearn the fueling ect.
Yes with the MAF unplugged you are on a SD fall back but leave your TPS plugged in and leave the MAF off for now. Have him look at what the timing is doing when it's hunting and when it is not. Also if there is a specific coolant temp as to when the hunting stops, make sure your O2's are switching quickly. If it stops hunting when your fuel trims kick in you have a problem with your open loop which pretty much the only sensor being used is your MAF. Of course check to see if you have any codes as well, some have to trip 2 or more times to actually show up. After you get done you may want to clear out your PCM and let it relearn the fueling ect.
So, the MAF is what takes care of things at intitial start up, do I have that right?
I could pick up another MAF and just throw it on and see if that fixes it.
But I have Z06 MAF, will a stock OEM MAF not coincide with whatever tune I have for this set-up?
I could pick up another MAF and just throw it on and see if that fixes it.
But I have Z06 MAF, will a stock OEM MAF not coincide with whatever tune I have for this set-up?