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Question on Engine Code

#1 User is offline   ratracer49 Icon

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Posted 23 April 2018 - 08:33 AM

This isn't really racing related (unless you count that it's for my tow vehicle) but I have a question on an engine code. I've seen on here in the past some good help from people on daily drives so I thought I'd ask. I have a 2001 Silverado with the 6.0 gas engine. I am getting a P0300 Random Misfire Code. The check engine light seams to trigger mainly when I'm coasting from speed or maintaining speed. I don't notice any actual misfire, at least not enough of one I can discern. My question is, can this code be caused by an exhaust leak? I know I have a leak at the exhaust manifold gasket and was wondering if once I rectify that my misfire code might go away. I guess I'm trying to be lazy and hoping to not have to go through looking for vacuum leaks, checking coils, and checking injectors to track it down.
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#2 User is offline   Verwayne Icon

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Posted 23 April 2018 - 04:08 PM

I asked a friend about that code and he told me:

"If you did a search on that code it could be triggered by numerous issues. Doubt there is a way to find out unless he takes it to have scanned."

I'm not sure who is and around your area, but Auto Zone will generally do a free scan if they aren't waist deep in customers.
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#3 User is offline   DRTmotorsports Icon

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Posted 23 April 2018 - 08:21 PM

View Postratracer49, on 23 April 2018 - 08:33 AM, said:

This isn't really racing related (unless you count that it's for my tow vehicle) but I have a question on an engine code. I've seen on here in the past some good help from people on daily drives so I thought I'd ask. I have a 2001 Silverado with the 6.0 gas engine. I am getting a P0300 Random Misfire Code. The check engine light seams to trigger mainly when I'm coasting from speed or maintaining speed. I don't notice any actual misfire, at least not enough of one I can discern. My question is, can this code be caused by an exhaust leak? I know I have a leak at the exhaust manifold gasket and was wondering if once I rectify that my misfire code might go away. I guess I'm trying to be lazy and hoping to not have to go through looking for vacuum leaks, checking coils, and checking injectors to track it down.


An autozone scan will not reveal anything other than what you already know. A P0300. They may give you a list of things it could be, but the bottom line is that code can be triggered by many things. Could be anything from old plugs, bad wires, O2 sensors, even an ECM. It is a pretty large list of things that can trigger that code. If this was my truck, I would start by looking at spark plugs and wires. If they are old, and just do not look good, change em. If they are newer, and look to be in good shape, it may be more cost effective to take it to a trusted shop and let them do some live scanning. They can actually observe most of the sensors, and what they are doing while the vehicle is running to help pin point the issue. Sorry, but this code is probly the worst one to have, just too many variables. Good luck.
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#4 User is offline   mod911 Icon

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Posted 24 April 2018 - 07:57 AM

Check the intake gaskets for leaks,they leak under high vacuum conditions such as cruise and decel. Does the light ever FLASH? Be glad its a 6 litre since they are not a DOD motor. A partly clogged injector will give similar results. Look at mode 6 data or watch misfire COUNTER rack up misfires cyl by cyl live. A scanner is of little help mostly,people ask for their codes to be checked and when I say pxxx they say what is that. They already knew since their bud or auto zonnies get them the code. They must be expecting maajic instructions to fix it for free....AS I tell people when they ask me THE question......DO you do diagnostics? I just YES, we do.....NEXT come THE question.....Do you charge for it? I say yes and they say just for checking codes,isn't that customer service? I say whats your name,put in my data base and up comes NOTHING....I say but you are not a customer......well I would be maybe if you give me something free that I already have....I then say, why do you expect the SMARTEST guy in the room to work for free? I pay monthly for information about 4 hundred PLUS the cost of a Snappy top line diagnostics platform + updates and other expensive quickly outdated equipment....

Search for clues is the best advice....coils are rare fix,injectors too unless someone take a probe or power wire to them....clean throttle bore first....spray water with some wiper fluid in it and see if it make a worse misfire there is a good place to look....
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#5 User is offline   ratracer49 Icon

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Posted 24 April 2018 - 08:18 AM

Yes unfortunately this is a very vague code. I have one of the MaxiScan units (don't recall the model) which is how I got the P0300 code. It was the only engine code nothing more specific yet. One thing I probably should have added is that I just replaced the plugs. I had new ones waiting to put them in and once I got the code I replaced them so I could look at the old ones and see what I could find. The plugs were definitely due to be replaced but none of them looked off in color. I cleared the code and it came back again. Like I said it runs fine but obviously something is triggering the code.

Being it's such a vague code is it possible that the exhaust leak I know I have at the manifold gasket could cause this code? I don't want to waste a lot of time and money fixing things that don't need to be so I figure bringing it to a shop will be the way to go if its not the leak. However I also don't want to pay for someone to just tell me I have an exhaust leak that I know I have. So my next plan of action will be replace the gasket, and probably some studs and bust a few knuckles. Then see if that clears it up. If not off to the shop.

Thank you both for your time and response.
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#6 User is offline   ratracer49 Icon

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Posted 24 April 2018 - 08:34 AM

View Postmod911, on 24 April 2018 - 08:57 AM, said:

Check the intake gaskets for leaks,they leak under high vacuum conditions such as cruise and decel. Does the light ever FLASH? Be glad its a 6 litre since they are not a DOD motor. A partly clogged injector will give similar results. Look at mode 6 data or watch misfire COUNTER rack up misfires cyl by cyl live. A scanner is of little help mostly,people ask for their codes to be checked and when I say pxxx they say what is that. They already knew since their bud or auto zonnies get them the code. They must be expecting maajic instructions to fix it for free....AS I tell people when they ask me THE question......DO you do diagnostics? I just YES, we do.....NEXT come THE question.....Do you charge for it? I say yes and they say just for checking codes,isn't that customer service? I say whats your name,put in my data base and up comes NOTHING....I say but you are not a customer......well I would be maybe if you give me something free that I already have....I then say, why do you expect the SMARTEST guy in the room to work for free? I pay monthly for information about 4 hundred PLUS the cost of a Snappy top line diagnostics platform + updates and other expensive quickly outdated equipment....

Search for clues is the best advice....coils are rare fix,injectors too unless someone take a probe or power wire to them....clean throttle bore first....spray water with some wiper fluid in it and see if it make a worse misfire there is a good place to look....



I'm a bit slow on the keyboard and got side tracked so my previous reply was before I saw yours. Thanks for the tips it gives me a couple more tools to self diagnose.

In case you haven't noticed the smartest guy in the room is always expected to work for free. I have never understood though why people feel that someone should provide a service for free. Just because you don't hand them a physical product doesn't mean there is no work or cost of equipment involved.
You have to learn the rules of the game. And then you have to play better than anyone else.

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#7 User is offline   mod911 Icon

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Posted 25 April 2018 - 07:28 AM

Its not likely that the exhaust leak is to blame...soak them in oil for as long as you can and maybe a reverse drill will screw them out...random misfires are fun. just because the plugs look good doesn't mean anything. They can get chemically coated and other things that cannot be seen but measured on a scope. Check burn time of the plugs if you can,its old but look anyway...
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#8 User is offline   ratracer49 Icon

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Posted 13 July 2018 - 09:56 AM

So this project has gotten beyond frustrating. I spent some time narrowing things down and still have not fixed the problem. Just as routine maintenance the fuel filter, wires and plugs have been replaced. I wasn't expecting these to solve the problem but they were past due needing done and wanted to eliminate the variables. I was suspect that it could be a vacuum leak and started going through lines, gaskets, and seals while checking and cleaning sensors. PS the plunger on an air idle control valve can really be fun to find when you let it run off the end of the screw testing it. During this process I replaced the MAP sensor due to a torn seal (probably from me removing it) and it was pretty crusted up anyway. Probably could have got by with just a cleaning but the torn seal dictated the replacement. At this point a test drive was looking like good for no check engine light until right at the end. At least this time it resulted in a new code; "Engine rpm higher then expected" (don't recall the number or exact text). So my first thought is to focus more on the vacuum leak thinking its getting extra air. So I start going over it again and then come across this.

Attached File  IMG_20180703_190949_opt.jpg (42.38K)
Number of downloads: 2

Apparently the cruse control cable is occasionally hanging up and not always rotating not allowing the throttle to return all the way closed. So hopes up that I was onto something I fixed this and fingers crossed. Well it did fix the "Engine rpm higher then expected" code but still not the random miss fire.

Along the way I feel I have corrected some problems (I believe there were multiple contributing) and no longer have a check engine light that stays on. However I do get a flashing check engine light that happens around 65-75 mph and at times when pulling a trailer.

So admitting defeat it's time to bring it to a shop an have someone with better equipment and experience fix it. This is where the real frustration comes in. I get a call that after their diagnostics I have a leaking EGR tube and a sticking vacuum purge valve. Just happy to get it over with I have them do the repairs. So I pick up the truck and no check engine light on the way home (all highway 60 mph and under). However today the real test on the expressway low and behold the same flashing check engine light between 65-75 mph. So the repair shop will be getting a call today. I want to believe that the repairs they did were necessary and don't doubt there are multiple things going on with an older truck like this. I'm hesitant to have them look at it further if it's going to end up costing me more money and them not knowing it they've really fixed the problem. I don't have a problem paying for legitimate work done. However just taking guesses at what part to replace until the problem gets fixed is what I was trying to avoid and quite honestly could do myself.
You have to learn the rules of the game. And then you have to play better than anyone else.

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#9 User is offline   ratracer49 Icon

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Posted 13 July 2018 - 05:27 PM

New bit of info. I can trigger the check engine light in any gear if I hold 2500 rpm. No code higher or lower. Could it be a crank position sensor problem?
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#10 User is offline   mod911 Icon

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Posted 14 July 2018 - 11:20 AM

Possible.Put in a new one and do a relearn on it FIRST,check for rust jacking of the sensor,. Did you have some one smoke check your intake gaskets? A lean injector may do it since the on time is very short and it will be lean and go away with longer injector pulse.If either the inj or gaskets cause a lean mix it is a problem.. When your light is FLASHING it means your cat is melting down. That is the ONLY reason it flashes. Check your short term fuel trim numbers at 2500 and see what that tells you....check individual injector on times. Watch your 0-2 sensors when light is flashing to see if one bank is lean or both. A bad maf sensor may cause lean running at if it is off or getting un reported air to the PCM...I chase this shit daily and sometimes it is fun....A customer last week said he had cam sensor codes at A-Z and bought all the sensors,cams and crank and had his neighbor kid install them and a day or 2 later it quit and he wanted me to tow it in and check it out for a no start....I told him as I was laughing that he need a new motor now.........he looked puzzled so I asked if it cranked over funny now......he says ya it cranks different now.....good job there I said because his SENSORS were CORRECT,his chains were stretched out and ready to fail and his new sensors said the same code again so he figured the job was botched or sensors were no good again after 10 miles.....lol....it was an aluminum dual overhead Caddy motor....90cents a pound scrap metal now...
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#11 User is offline   mod911 Icon

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Posted 14 July 2018 - 11:23 AM

View Postmod911, on 14 July 2018 - 12:20 PM, said:

Possible.Put in a new one and do a relearn on it FIRST,check for rust jacking of the sensor,. Did you have some one smoke check your intake gaskets? A lean injector may do it since the on time is very short and it will be lean and go away with longer injector pulse.If either the inj or gaskets cause a lean mix it is a problem.. When your light is FLASHING it means your cat is melting down. That is the ONLY reason it flashes. Check your short term fuel trim numbers at 2500 and see what that tells you....check individual injector on times. Watch your 0-2 sensors when light is flashing to see if one bank is lean or both. A bad maf sensor may cause lean running at if it is off or getting un reported air to the PCM...I chase this shit daily and sometimes it is fun....A customer last week said he had cam sensor codes at A-Z and bought all the sensors,cams and crank and had his neighbor kid install them and a day or 2 later it quit and he wanted me to tow it in and check it out for a no start....I told him as I was laughing that he need a new motor now.........he looked puzzled so I asked if it cranked over funny now......he says ya it cranks different now.....good job there I said because his SENSORS were CORRECT,his chains were stretched out and ready to fail and his new sensors said the same code again so he figured the job was botched or sensors were no good again after 10 miles.....lol....it was an aluminum dual overhead Caddy motor....90cents a pound scrap metal now...

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#12 User is offline   mod911 Icon

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Posted 14 July 2018 - 11:28 AM

Do the relearn first. It is a common prob my info says...can be caused by oily connector,rust and crud jacking it away from tone ring or damaged reluctor ring notches,your crank is pulled up into the block on decel because of the high vacuum pulling pistons up more causing more gap in the sensor to tone ring causing a fallout of the signal....kinda like a GM speed sensor in a wheel that engages at 3 mph when a jacked up sensor looses its signal intensity...I can watch that and catch that easily tho
Diagnostics are fun huh?
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#13 User is offline   ratracer49 Icon

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Posted 16 July 2018 - 02:27 PM

Your definition of fun and mine must be two completely different things. lol The only response from the shop I brought it to is they would have to see it again to try and diagnose the problem. Which is to be expected however I'm not sure my confidence level in them being that other them them doing some diagnostics and putting some new parts on it the truck is exactly the same as when I brought it in. I'm not doubting the work they did on it was completely unjustified and wouldn't complain if they did a couple extra repairs that needed done but might not be necessary to fix the problem if they actually fixed the problem. I see no change from when I brought it in to now. Maybe they never replicated the problem after the repairs and thought it was fixed, I don't know. I can drive it all day without making the code come back if I just avoid maintaining 2500 rpm and this was something that I didn't think to try before I brought it in. I assumed it was speed related (65-75 mph area) and towing related. Which is false now that I tested my new theory. Doesn't matter if I'm towing or what speed the light only flashes at that magic 2500 rpm spot.

Yes they did a smoke check on it. At least they said they were going to so have to believe it was done.

I'll take a look and see what I can do with the info you provided. Also I'll get in and take a look at the crank position sensor see what condition it's in and see if excess oil or corrosion might be an issue. I can't do the relearn myself so after I see what it looks like I will see about getting the relearn done. Self diagnostics is not what I'm setup for. I'm working with a Maxidiag 703 (which I'm not even fully versed on taking full advantage of I'm sure) and a home shop. And it's not a high-tech shop by any means. I make a lot of my "specialty tools" the same way I fab most of my own racing components for the car. If I had to buy all that stuff my racing schedule would be even less then it is now. Luckily this year I've had very little in repairs need to the car "knock on wood" so it's helped keep me going and get a few more nights in. Which is also part of the frustration with spending money on the truck with no improvement as is comes out of the "play" budget.

The basic stuff I can track down and do myself. However the more complex stuff I leave to the experts and bring it in. However the repairs I can't do are far enough between that I have no real relationship with a shop so finding someone trustworthy is the trick. If I lived closer you'd have already seen my truck and you could be getting paid for this free advice you've been giving. I do appreciate it very much.
You have to learn the rules of the game. And then you have to play better than anyone else.

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