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How much effect of a engine misfire effect your gas milage?
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indy4192
User
Sep 16, 2009, 6:59 AM
Post #1 of 23
(28106 views)
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How much effect of a engine misfire effect your gas milage?
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I was wondering would a engine misfire problem effect your overall gas mileage and if so to what extent?
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DanD
Veteran
/ Moderator
Sep 16, 2009, 9:03 AM
Post #2 of 23
(28101 views)
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Re: How much effect of a engine misfire effect your gas milage?
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There are so many different scenarios that it’s not all that easy of a question to answer; but I’ll take the easy way out and use this one example.
You have a four cylinder engine and one cylinder is misfiring; that means you’ve lost 25% of your engine's ability too produce power.
You still want to run the car a 50 miles an hour; so you push the throttle a little (a lot) further down; this feeds the other three cylinders more fuel so that they’ll produce more power.
In the mean time the misfiring cylinder is still getting fuel (non OBDII system); as much as the power producing cylinders; that it cannot burn and it’s going out the tail pipe.
So you guess how much extra fuel the thing is going to waist and don’t guestimate low.
50% or more wouldn't surprize me.
Dan.
Canadian "EH"
(This post was edited by DanD on Sep 16, 2009, 9:04 AM)
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indy4192
User
Sep 16, 2009, 9:18 AM
Post #3 of 23
(28096 views)
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Re: How much effect of a engine misfire effect your gas milage?
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Well mine is a 6 cylinder and my gas milage has gotten better. I had new plugs and wires installed and a new cam shaft sensor. I am presently getting about 19-20 mpg according to my last fill up. New off the showroom floor in 1998 the car was suppose to get that kind of milage. So that is why I am so baffled, I would think my gas milage would drop to maybe around 14-15 mpg but is has gotten better and I still have the misfire code and you can tell when at idle the car is firing just right. But it starts everyday, runs without issue and sounds fine. I am just baffled by this.
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DanD
Veteran
/ Moderator
Sep 16, 2009, 10:00 AM
Post #4 of 23
(28094 views)
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Re: How much effect of a engine misfire effect your gas milage?
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Like I said a lot of different scenarios; a 98 (OBDII system) will have misfire detection capability. When the computer senses a misfire it will shut down fuel to that cylinder in an attempt to save the catalytic converter.
As for you believing that the car is getting better fuel mileage; I’m not sure; but can you imagine how much better it might be if you find the misfire issue?
Dan
Canadian "EH"
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indy4192
User
Sep 16, 2009, 11:25 AM
Post #5 of 23
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Re: How much effect of a engine misfire effect your gas milage?
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Wow this is news to me. So you are saying that my car is able to sense the misfire in that No. 1 cylinder and basically stop using it till it is repaired? I was not aware that the car can do that. What damage does that do to the engine?
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speed
User
Sep 16, 2009, 3:10 PM
Post #6 of 23
(28078 views)
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Re: How much effect of a engine misfire effect your gas milage?
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correct me if im wrong guys but it could hurt the piston or cylinder wall. becuase that extra space that is missing from no fuel has to be filled up somehow. and that is by more air intake. and if its compressing air too much it could result in blow by(piston rings go bad) and that would mean a loss of out put and most likely power too. as for gas milage like he said the reason you get better milage is becuase that cylinder isnt using fuel so you now have less fuel comsumption.
GM ASEP 26 SCC Milford ASE certified in Brakes and Electrical on Thursday April 5th 2012
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indy4192
User
Sep 16, 2009, 3:34 PM
Post #7 of 23
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Re: How much effect of a engine misfire effect your gas milage?
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But I haven't really seen any loss of power, it takes very little presure on the pedel to get the car going. Also even though you would be using less gas because of the loss of the one cylinder, wouldn't you in fact use more because you would have to excerllerate more due to the loss of the one cylinder and thus consume more gas? What little you would "save" by not having that one would be easily loss by the other 5 having to make up the loss of power?
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Tom Greenleaf
Ultimate Carjunky
/ Moderator
Sep 16, 2009, 4:54 PM
Post #8 of 23
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Re: How much effect of a engine misfire effect your gas milage?
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Among other issues what would concern me is uneven engine temps. So many now use dissimilar metals that it a big concern IMO. Better MPGs - I seriosly doubt that,
T
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DanD
Veteran
/ Moderator
Sep 17, 2009, 2:30 AM
Post #9 of 23
(28055 views)
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Re: How much effect of a engine misfire effect your gas milage?
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Yes the computer can shut down individual cylinders that are misfiring, by shutting down the injector pulse, too that cylinder. The manufacturer doesn’t want raw fuel to enter the catalytic converter; causing it to overheat and burn out. The computer does not shut the fuel down and then forget about it; it will randomly turn the fuel back on to the cylinder. If for whatever reason the misfire is no longer present; fuel is allowed to continue and the cylinder will run again.
Anytime that you see the amber check engine light flashing; that’s the computer telling you, it is sensing a misfire. If the misfire is there long & bad enough; the check engine light will come on solid and set a code. Now too put a time frame on this; it can all happen within a few milliseconds; you may not even feel that it’s happening.
As for engine damage; nothing immediate; but left unattended there will be excessive internal ware to most rotating components; due to the unbalanced condition.
Speed
There won’t be any damage to the piston or cylinder walls; that cylinder turns into an air compressor. There’s no fuel, so there’s no combustion; the heat from combustion is what promotes ware.
The total volume of what enters the cylinder doesn’t change with the lack of fuel. If the compression of that cylinder is 150 psi it will still only be 150 psi without the fuel. Again without fuel there’s no combustion; combustion pressures reach into the thousands of pounds per square inch.
Look at electric air compressors we use in the garage for our air tools. They run for years on end with very little maintenance.
Indy
The misfire in your engine must be very light that the computer can sense it but not your butt. LOL
The rest that you posted in your last post, about the fuel and power distribution, is right on the money. That’s why I have a hard time with you saying that your mileage is up?
Maybe the vehicle before you had some work done to it (spark plugs) the thing was running very poorly; now you just have a random intermittent misfire.
Like I said above; the manufacturer didn’t design or program this function into the vehicle to save you fuel; it’s to not harm the converter. They don’t want too warrantee one, while the vehicle is still under the government’s imposed extended emissions warrantee.
The manufacturers don’t give a RAT”S A$$ about saving you anything; it’s their wallet they’re worried about.
If they could have those features shut down, after the warrantee runs out and not get caught doing this; they’d do it in a heartbeat. They’d love too SELL you a converter or another new vehicle.
Dan.
Canadian "EH"
(This post was edited by DanD on Sep 17, 2009, 2:36 AM)
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indy4192
User
Sep 17, 2009, 6:57 AM
Post #10 of 23
(28046 views)
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Re: How much effect of a engine misfire effect your gas milage?
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The engine temp is good, I did have the radiator flushed completely out this past saturday, but even before then it was fine. As for gas milage, I filled up last night and with both some city and semi highway driving it figured out to about 28 mpg, which given the age of the car (1998), the milage on the car (128,000) and this misfire I won't complain. I bought this car at auction for $450 and have had now plugs and wires installed and a cam shaft sensor. The misfire was there before the new plugs so I don't think it's a plug issue. As for "feeling" the misfire, if you are sitting in the car you can tell the engine is just off, but you give it some gas and it is really hard to tell. It always starts right, there has been no hesitation or anything. Hope to get the No. 1 cylinder looked this weekend.
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Sidom
Veteran
/ Moderator
Sep 17, 2009, 11:49 AM
Post #11 of 23
(28045 views)
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Re: How much effect of a engine misfire effect your gas milage?
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I would have to go with Dan on the mpg, more than likely all the previous work you had done corrected and issue you had before so even though you are getting a misfire the mpg has increase....
There are a lot of things that can set a misfire, a couple that could be your case would be either a vacuum leak or bad crank sensor.
A vacuum leak is most noticeable at idle, that is where it will have it's greatest effect. Once you get off idle the air flow is high enough that a vacuum leak isn't going to affect the air/fuel ratio so driving around the mpg won't be affected as much. This can be observed in a data stream. High fuel trims @ idle that return to normal at higher rpms means a vacuum leak, false air or something is getting around the MAF & diluting the air/fuel mix.
Alot of times when you get a misfire you will also get a corresponding lean code. For a few reasons. One a vacuum leak.. For obvious reasons but even with ign misfires or other ones, even though you have more HCs & CO due to the misfire (so you would think you would have a rich code) you also have more O2 due to the incomplete combustion & O2 sensors only measures oxygen.
A crank sensor for this problem would be rare but I have seen cases of a misfire with no running problems. The PCM uses the crank sensor for a lot of reasons & one is to detect misfires by monitoring the engine speed & performance. If one cylinder is dropping out it will show up on the pattern. If the sensor is faulty and has a bad pattern the PCM can mistake it for a misfire.
Check fuel is good but it is so subjective. Unless there is a very noticeable drop that is one area I don't like to go. There are dozens of things that can affect mpg and none have to do with the motor. Simple things like correct tire pressure and correct alignment can have a big difference on mpg. Having a good week, making all the lights, not getting caught in traffic. The list goes on
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indy4192
User
Sep 17, 2009, 11:55 AM
Post #12 of 23
(28043 views)
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Re: How much effect of a engine misfire effect your gas milage?
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So what I get from everyone is that I should have my mechanic look for a vacuum leak. Since the code read out the first cylinder, does each cylinder have it's own vacuum or are there just 1 centeralized place to look? Also, since Dan let me know that the car will shut off fuel to that first cylinder, what damage am I doing to the engine till I can afford to get this fixed?
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Sidom
Veteran
/ Moderator
Sep 17, 2009, 12:33 PM
Post #13 of 23
(28042 views)
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Re: How much effect of a engine misfire effect your gas milage?
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This is just information on your problem and systems, so when your tech talks to you about the problem he has found, stuff will make more sense to you. I would probably hold back on telling him what type of diagnostic routine you what him to run, just tell him the problem and he should be able to find it on his own. Now don't take this the wrong way but honestly the tech is a professional and I'm sure he is well aware of all the systems & strategies we covered. If he isn't and does need you to provide the info we have covered here, I would strongly suggest finding another shop & tech......
What Dan said about the injectors is correct. The PCM will disable an injector if the misfire is severe enough, also (not to confuse things even more ) on newer systems in an overheat condition the PCM will alternate disabling injectors as a cooling strategy.
It's not good to driving around with any known problem but this type of problem is a slow type of wear process. Meaning just ignoring it & driving it any way over a long period of type is going to cause some type of internal damage that more than likely wouldn't have happened if the problem had been addressed in a reasonable amount of time...
In a nutshell..... Do you need to stop driving the car & have it towed to the shop?...... No
Can you hop in it and drive across the country with out a worry?..... Definitely not.
To answer your question, the cylinders create the vacuum. If there is a leak close to one cylinder, if its a gasket, injector o ring, vacuum port close to that cylinder, then that is the one affected. If it's in a central area then all cylinders are affected and thats when you get the lean bank 1, lean bank 2 codes.....
Vacuum leak is just a term used. It is just a difference in pressure. Technically it doesn't "leak" vacuum but rather it draws air in from a high pressure area (outside) to a low pressure area (inside the intake manifold). It's unmetered air, not going thru the MAF, so the comp doesn't know where it's coming from but can only see the results in the O2 reading and try to compensate for it but riching the mixture.....
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indy4192
User
Sep 17, 2009, 12:45 PM
Post #14 of 23
(28039 views)
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Re: How much effect of a engine misfire effect your gas milage?
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First off thanks. I am the worlds biggest worrier when it comes to things I don't know or understand and when it comes to cars, well I don't know nothing, and I understand very little. I have good faith in my mechanic, he has worked with me in trying to solve this issue. With having no background knowledge on this car since I bought it at a Goodwill autction, I have nothing to go on. Add on top of that a lack of financial stability, I can only have things fixed 1 time a month. I know "fixing" the problem will probably be easy, but what I am finding out is that finding out what is causing the problem is the real task. I do feel better that since I am only driving the car back and forth to work (about 150 total a week) and maybe a short trip shopping on Saturday, that I am not in danager of killing myself. The engine light being on will still bug me, but having the great info everyone here has posted really helps.
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Sidom
Veteran
/ Moderator
Sep 17, 2009, 1:10 PM
Post #15 of 23
(28035 views)
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Re: How much effect of a engine misfire effect your gas milage?
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Well it sounds like you've got a good tech for the job. Also sounds like you take care of your car which is good. I see the reverse on a daily basis, people that don't worry about the SES light or other problems and just drive it until it dies and then just can't understand why it's so expensive.....lol
It's good to get a background on whats going on.......miscommunication or lack of understanding is sometimes one of the biggest problems.
from what you've said this does really sound like a minor problem
(This post was edited by Sidom on Sep 17, 2009, 1:10 PM)
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indy4192
User
Sep 17, 2009, 1:44 PM
Post #16 of 23
(28033 views)
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Re: How much effect of a engine misfire effect your gas milage?
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How does the car stop the fuel injector from operating and injecting gas into the cylinder that is misfiring?
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DanD
Veteran
/ Moderator
Sep 17, 2009, 1:52 PM
Post #17 of 23
(28032 views)
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Re: How much effect of a engine misfire effect your gas milage?
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The computer just doesn't turn it on.
Dan
Canadian "EH"
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indy4192
User
Sep 17, 2009, 1:55 PM
Post #18 of 23
(28027 views)
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Re: How much effect of a engine misfire effect your gas milage?
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Dan I also want to thank you, your assistance has been great. My knowledge of how a car works can fit on the head of a pin. I was unaware and other places I have asked this question never told me the computer has the ability to shut off the flow of gas to a specific cylinder. You advise is greatly appreciated.
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indy4192
User
Sep 18, 2009, 2:37 PM
Post #19 of 23
(28010 views)
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Re: How much effect of a engine misfire effect your gas milage?
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This may sound stupid, but I have the car to a couple differnt mechanics dealing with the misfire issue and not a one said anything about the PCM being able to assist the engine and misfire by stoping fuel to that cylinder to avoid unburnt gas getting the the CC. IS this PCM something new or were they just trying to scare me into getting them to do the work? All of them (except for the mechanic who changed out the plugs and such who didn't say it would damage the CC) said that ANY driving would destroy my engine by destroying the CC. Could they have not know that the PCM can do what Dan said it can do?
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Sidom
Veteran
/ Moderator
Sep 18, 2009, 6:45 PM
Post #20 of 23
(27999 views)
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Re: How much effect of a engine misfire effect your gas milage?
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Just so there isn't any confusion in certain cases a PCM will disable an injector but this doesn't mean that for every misfire the injector gets disabled. With a misfire where there is a rich condition there is a very good chance the cat will be damaged (over time). If the misfire is severe enough to cause damage, the check engine light will flash indicating the condition. For example,if you had a ruptured pressure regulator diaphram. This will be dumping fuel into the engine and could hurt the cat but disabling an injector would do nothing to minimize the damage.
It's very possible with your condition they didn't feel this strategy was happening and personally when talking to a customer about a problem, I don't bring up every possible scenario that could happen especially if I feel it's not relevant to the problem. I've found that all it does is confuse the customer even more if they aren't familiar with the system & problem I'm explaining, an information overload if you will.
This information that has been discussed isn't any hidden secrets for the chosen few. It is something that is covered in any update class that is offered covering the new systems. In the automotive field techs are constantly going to classes that range from a few evenings to a week long with hands on. Plus system descriptions are also available in professional data bases.
I'm not able to see the car to make an assessment and can just go off what you've posted. It doesn't sound critical. You said it's basically running good but just has the light on. If that is the case it does need to be addressed and if it's causing a rich condition, then over time it can cause damage if ignored. Now if it was chugging and dying while pouring black smoke out the back and shaking so bad it was loosening your fillings then yea you would need to stop driving it then & there.
It sounded like you had a pretty good relationship with the original tech. As long as you haven't had any major problems with him it would probably be best to stick with him. As far as the techs go, in a nutshell what they said is true, driving on a misfire can damage the engine and cat.
Unfortunately this is a sue happy time we live in now. When people are winning millions for getting burned due to holding scalding hot coffee between their legs, everyone in any business has to really dot I's & cross T's. Sounds like they didn't know you and were just covering themselves. You would probably bust a gut laughing at some of the stories some techs could tell about total common sense stuff customers were trying to hang on shops just because "no one told them"
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DanD
Veteran
/ Moderator
Sep 19, 2009, 2:12 AM
Post #21 of 23
(27991 views)
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Re: How much effect of a engine misfire effect your gas milage?
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Please don’t be offended by this, because you shouldn’t; but maybe the people that you have talked too, haven’t wanted to explain this; because they thought it would be over your head.
They didn’t want to open the door to a thousand questions that took them (a technician) years to understand themselves?
The other thing is; a lot of automotive technicians (or any type of repair tech for that matter) are very adept at working with their hands and figuring out problems. We on the most part (but changing) are not the most enhanced at verbal or people skills. DO YOU WANT IT FIXED OR NOT attitude; is what a lot of techs think is explaining the issue to their customer consists of?
We are also usually on the defence because people don’t trust us until they get to know us.
Here’s something to think about; people can talk about a repair tech’s mistakes or what seems like mistakes to the uninformed because the car; being mechanical, can always be fixed. Now I’m not comparing myself or the trade to the medical field or Doctors; but they burry their mistakes.
Dan.
Canadian "EH"
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shawn99corolla
Novice
Sep 6, 2014, 3:39 PM
Post #22 of 23
(17937 views)
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Re: How much effect of a engine misfire effect your gas milage?
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Issue with 99 Corolla CE with 147,000 miles....I have a misfiring P0303 Code - Cylinder #3 Misfire. Here is what has been done in effort to remedy the misfire. A major tune up, new spark plugs, new fuel injectors, new spark plug wiring, The car runs fine once warmed up but in the morning the engine vibrates until warm and also shivers a little while the car is stopped at a stop light. I have dropped it off at the mechanics a couple of times now to no avail. Once the code is reset it will not display the CEL for couple of days but the symptoms still remains. The CEL comes back on within 2-3 days with same code. Really frustrating! Well at least this time CEL in not flashing like before it comes on and stays on. Any ideas what can still be causing the misfire. I really appreciate your help. Thank you in advance.
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Hammer Time
Ultimate Carjunky
/ Moderator
Sep 6, 2014, 3:47 PM
Post #23 of 23
(17936 views)
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Re: How much effect of a engine misfire effect your gas milage?
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Please read the FORUM RULES before posting. This thread is 5 years old.
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We offer help in answering questions, clarifying things or giving advice but we are not a substitute for an on-site inspection by a professional.
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