Does a leaky intake cause lean or rich running in a NA car?
05-27-2008, 04:05 PM
#1
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Does a leaky intake cause lean or rich running in a NA car?
I read in the shop manual (p. 24:39 ; DME Diagnosis) that air leaks in the intake system will cause a rich running condition. This seems counter-intuitive to me. I'd think that unmetered air (air coming in after the MAF) would cause a lean condition. For a turbo, I'd expect leaks=rich, but for an NA, leaks=lean. Am I missing something?
I ask because I recently discovered that my car had only ONE (of four) screws attaching the throttle body to the intake runners.
I ask because I recently discovered that my car had only ONE (of four) screws attaching the throttle body to the intake runners.
Last edited by TheOtherEric; 05-27-2008 at 05:44 PM.
05-27-2008, 05:01 PM
#3
05-27-2008, 05:20 PM
#4
Race Car
Yes, it will make your car run rich, not lean.
The MAF doesn't read that air coming in since it was leaked in after the MAF. The ECU has a reading for how much air went into that combustion cycle.
Once the combustion takes place, the ECU takes a reading on the CAT O2 sensors and sees that there is more oxygen than needed. The ECU then compensates by adjusting the gas/air ratio by making the mixture richer to try to bring the CAT's O2 sensor readings back to spec and in line with what should have been the CAT's O2 sensor readings given a particular MAF reading.
You'll burn more fuel than needed, gas economy and performance would be decreased as the car runs rich...
The MAF doesn't read that air coming in since it was leaked in after the MAF. The ECU has a reading for how much air went into that combustion cycle.
Once the combustion takes place, the ECU takes a reading on the CAT O2 sensors and sees that there is more oxygen than needed. The ECU then compensates by adjusting the gas/air ratio by making the mixture richer to try to bring the CAT's O2 sensor readings back to spec and in line with what should have been the CAT's O2 sensor readings given a particular MAF reading.
You'll burn more fuel than needed, gas economy and performance would be decreased as the car runs rich...
05-27-2008, 05:53 PM
#5
Rennlist Member
Thread Starter
Yes, it will make your car run rich, not lean.
The MAF doesn't read that air coming in since it was leaked in after the MAF. The ECU has a reading for how much air went into that combustion cycle.
Once the combustion takes place, the ECU takes a reading on the CAT O2 sensors and sees that there is more oxygen than needed. The ECU then compensates by adjusting the gas/air ratio by making the mixture richer to try to bring the CAT's O2 sensor readings back to spec and in line with what should have been the CAT's O2 sensor readings given a particular MAF reading.
You'll burn more fuel than needed, gas economy and performance would be decreased as the car runs rich...
The MAF doesn't read that air coming in since it was leaked in after the MAF. The ECU has a reading for how much air went into that combustion cycle.
Once the combustion takes place, the ECU takes a reading on the CAT O2 sensors and sees that there is more oxygen than needed. The ECU then compensates by adjusting the gas/air ratio by making the mixture richer to try to bring the CAT's O2 sensor readings back to spec and in line with what should have been the CAT's O2 sensor readings given a particular MAF reading.
You'll burn more fuel than needed, gas economy and performance would be decreased as the car runs rich...
This answers two big questions I've posted about lately: 1) why was my car running so rich on the dyno, and 2) why wasn't I making top-end power on the dyno. I think I lost power at the top end because it was running soooo rich (10.1:1) and it was running so rich there because that's where the leak will be the worst; at high rpms and gobs of air flow.
Funny thing is that I did notice getting worse fuel economy at the track last year. Happened pretty suddenly, too.
05-27-2008, 06:21 PM
#6
Race Car
Aha! Brilliant. I figured I must be missing something.
This answers two big questions I've posted about lately: 1) why was my car running so rich on the dyno, and 2) why wasn't I making top-end power on the dyno. I think I lost power at the top end because it was running soooo rich (10.1:1) and it was running so rich there because that's where the leak will be the worst; at high rpms and gobs of air flow.
Funny thing is that I did notice getting worse fuel economy at the track last year. Happened pretty suddenly, too.
This answers two big questions I've posted about lately: 1) why was my car running so rich on the dyno, and 2) why wasn't I making top-end power on the dyno. I think I lost power at the top end because it was running soooo rich (10.1:1) and it was running so rich there because that's where the leak will be the worst; at high rpms and gobs of air flow.
Funny thing is that I did notice getting worse fuel economy at the track last year. Happened pretty suddenly, too.
Now, the real question is...where did the other 3 screws went? and how did they get loose in the first place? I'm checking mine later just to be safe...
05-27-2008, 06:33 PM
#7
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03-12-2019, 09:50 PM
#8
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Yes, it will make your car run rich, not lean.
The MAF doesn't read that air coming in since it was leaked in after the MAF. The ECU has a reading for how much air went into that combustion cycle.
Once the combustion takes place, the ECU takes a reading on the CAT O2 sensors and sees that there is more oxygen than needed. The ECU then compensates by adjusting the gas/air ratio by making the mixture richer to try to bring the CAT's O2 sensor readings back to spec and in line with what should have been the CAT's O2 sensor readings given a particular MAF reading.
You'll burn more fuel than needed, gas economy and performance would be decreased as the car runs rich...
The MAF doesn't read that air coming in since it was leaked in after the MAF. The ECU has a reading for how much air went into that combustion cycle.
Once the combustion takes place, the ECU takes a reading on the CAT O2 sensors and sees that there is more oxygen than needed. The ECU then compensates by adjusting the gas/air ratio by making the mixture richer to try to bring the CAT's O2 sensor readings back to spec and in line with what should have been the CAT's O2 sensor readings given a particular MAF reading.
You'll burn more fuel than needed, gas economy and performance would be decreased as the car runs rich...
03-12-2019, 11:41 PM
#9
Rennlist Member
Thread Starter
Ha ha. Just saw the thread title and thought "ooh this sounds like an interesting read." Turns out it's a thread I started 11 years ago. Ha!
Timely too cuz I just found another intake leak during a routine checkup ( intake pressure test) and I was about to ask about it. Derp.
Timely too cuz I just found another intake leak during a routine checkup ( intake pressure test) and I was about to ask about it. Derp.
03-13-2019, 02:53 PM
#10
Generally speaking, too much, or unmetered air, create a lean condition. However, the compensating factors of the ECU and O2 sensors will try to rectify the lean condition, right?
I would imagine a lazy or malfunctioning O2 sensor could cause a real problem, but wait, wouldnt the ECU detect the lazy or malfunctioningaO2 sensor and throw a CEL?
Based on the explanations above, why would the ECU and O2 sensors allow a car to run anything other than perfect, or why would there even be a rich condition?
Could that be a typo in the workshop manual?
Also, if holing a piston (from consisently running lean) is not considered detrimental, then by all means, run that bad boy lean!
Internal Combustion Engine, 101
I would imagine a lazy or malfunctioning O2 sensor could cause a real problem, but wait, wouldnt the ECU detect the lazy or malfunctioningaO2 sensor and throw a CEL?
Based on the explanations above, why would the ECU and O2 sensors allow a car to run anything other than perfect, or why would there even be a rich condition?
Could that be a typo in the workshop manual?
Also, if holing a piston (from consisently running lean) is not considered detrimental, then by all means, run that bad boy lean!
Internal Combustion Engine, 101
Last edited by nine9six; 03-13-2019 at 09:26 PM.
