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Old 02-28-2004, 03:41 PM   #1
ym42
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Default Vacuum Leak?

I described this condition before, but did not get many asnwers. The problem is that when I let go the gas pedal, or when I only open the trottle a little bit, the engibe seems to choke. Also, when idling, opening the trottle a little bit will result in initial increase in RPM followed by huge misfires and drop of RPM almost to the stall, and than repeating the same sequence. Under load, there is no problem, when cold and mixture rich, no problem. This happens only when the engine is warm. When I drive and let the pedal go the engine sounds like a motorbike or a turbo charged HONDA in a similar situation, you know the sound... long seris of misfires under no load...


I was thinking this would most likely be caused by the vacuum leak somewhere. I looked at the condition of the rubber part between the TBS assembly and intake, and it is very bad. I fixd it a little, but prefer to get a new one....

Where else the air would leak? EGR? CCPS? Where can I get that thin rubber gasket? Any other ideas will be appreciated.

I already checked ignition, tried to disable EGR - does not help.
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Old 02-28-2004, 06:51 PM   #2
Kevin94SL
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2006 ION-3 Sedan
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Have similar symptoms with my '94 SL1. What are you driving (Year, model??). Would like to figure this out myself as it is anooying me greatly (although, I'm not panicked anymore when the car actually does stall.. it's just like... meh!)
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Old 02-28-2004, 08:26 PM   #3
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If you think it's vacuum, have you put a vacum gauge on it? I don't see any vacuum readings posted.
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Old 02-28-2004, 08:48 PM   #4
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Probably NOT a vacuum leak, as a vacuum leak would affect idle speed as well when the throttle was closed (idle speed would be too high) It sounds like the initial air/fuel mixture as you accelerate from idle is too lean. Possible causes (in order of likelihood)

1. Bad ECTS
2. Clogged fuel injectors or fuel filter
3. Bad TPS
...
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Old 02-28-2004, 09:57 PM   #5
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Wolfman:

It sounds like the initial air/fuel mixture as you accelerate from idle is too lean/

IT DOES NOT HAPPEN WHEN I ACCELERATE! It happens when I close the trottle, when I take my foot off the pedal. The faster I do it, more sympthoms (shudder ) I get.

When in N i press the pedal down VERY slowly, and after reaching about 1800RPM the engine starts to choke and lose RPM, as if the fuel is being cut off, similar to what the limiter is doing, but a little slower.

This DOES NOT HAPPEN ON COLD ENGINE!
This does not happen under heavy load, but while maintaining constant slow speed the car starts to shudder because of the misfires and might cause strange shifting behavious as the tranny thinks there is not enough power from the engine goes from higher? to lower? gear ( eg from 3rd to 2nd and back, as on higher RPM output is normal and no misfires ) THIS IS VERY ANNOYING and I can not maintain slow constant speeds without this shudder! (<45mph )




ECTS is NEW. I used to get rich mixture code, and after putting new ECTS there it went away, get better mileage.

Fuel injector or filter would pose a problem under heavy load - by my car accelerates so well it slips the wheels ( dry weather, straigh road ) when shifting from 1nd to 2nd ( Auto, WOT ). It shifts very hard when going WOT, and I do that a lot, as unfortunately I miss a DOHC engine that I do not have.

TPS - would always be a problem, not only on a warmed up engine....


Any other ideas?

PS: can I get that TBS gasket from the Saturn?
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Old 02-28-2004, 09:59 PM   #6
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OH, forgot to say what car I have, I presume with all my problems everyone would remember

I have white SL1, everything power, SOHC, 130000 miles, Auto.
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Old 02-29-2004, 02:24 AM   #7
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Well, this will be difficult. It's one of those "if I had the car here" type problems. Misfires can be caused by lean fuel conditions on a warmed up engine. You say the problem occurs ONLY when the engine is warmed up and ONLY when the throttle is suddenly closed OR when you are operating at slow speeds in relatively high gears, but disappears at higher speeds/rpm/or throttle settings. (correct me if I am wrong again) This suggests to me a problem in the fuel curve. (You are running too LEAN during those events, and that is what is causing the misfire) As it only occurs when the engine is warmed up (PCM is using CLOSED loop operation to set fuel mixtures) and you are not reporting any SES light activity, it suggests to me that the PCM is getting faulty sensor data, but not so faulty as to trip the SES light. I would think that if the O2 sensor was detecting a lean condition (in relation to the other sensor data it was getting) it would set an SES light for the "lean" condition when the event occurs. This suggests that the PCM is seeing data from other sensors that suppors the data it is getting from the O2 sensor as being correct, and hence no code. This suggests to me that the data from either the TPS or MAP sensors is incorrect, or (less likely) the IAT sensor. It is either that or the fuel flow from the injectors at closed throttle operation is way LOW under closed or partial throttle operation, but since you say your acceleration is fine, it suggests that fuel flow is correct under WOT, when sensor data is ignored.
...
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Old 02-29-2004, 02:06 PM   #8
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Yes, you understood everything right . I will try to replace MAP sensor soon - not a big deal. I was thinking about that already. I already know it is not the O2 sensor - because I swapped it with another Saturn, and nothing changed. I suspected lean mixture too, and was thinking this is due to the leak...

The bottom line: you think it is not the vacuum lines, like cracked EGR/CCPS lines or that gasket between the TB unit and intake?
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Old 02-29-2004, 08:22 PM   #9
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Those, if damaged, would generally affect idle speed as well. Although they are positioned for flow with partial throttle operation, if they were leaking then, the sensor data would be correct, and I would think the O2 sensor data would read lean enough to set the SES light. You can easily confirm/deny this by unsnapping the connector at the top of the throttle body, plugging the ports underneath (pencil erasers work well) and driving the car. If the problem goes away, you have found the problem.
...
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Old 02-29-2004, 11:32 PM   #10
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Today swapped MAP sensors with another SL, no difference. As an experiment disonnected O2 sensor, and somehow nothing changed, car still ran, no SES light! Engine was hot and I drove for a couple of minutes. Strange... But I did get a rich mixture code before I replaced ECTS couple of weeks ago... No more though... At this moment working on the coil packs, preparing to exchange them and the wires... I suspect may be it is ignition?? Next thing will be disconnecting vacuum lines and may be swapping TBI at the end...
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