Persistent misfire on a GM 4.3 motor during damp weather.

Hi Paul and everyone, I hope you all had a great Thanksgiving!
My name is Doug and I am a mobile mechanic over here in Northern California and have a couple of questions.

For a while now I've been a bit confused about what the difference is between short-term and long-term fuel trim, even after taking the Advanced Engine Performance class at the college here. So I reviewed Paul’s section where he discuss it in detail in class and I think I understand it much better now.

I am wondering though why the long-term trim only gets updated after moving the throttle position either up or down. Seems to stay constant until the RPM's change. Does anyone know why?

And also on another point, I have a friend who has a 2000 GMC 4.3 with a spider injected TBI motor that runs pretty decent in dry weather but whenever it's raining, especially during damp weather it has a very bad misfire. I've been dealing with this for a while now trying to get it figured out and I think I'm starting to narrow it down. I kind of thought that it might have something to do with an intake leak until Paul mentioned that if it is misfiring and showing high fuel trim numbers at higher RPMs, it's NOT an intake leak. And of course after thinking about that made sense to me.
I'm getting positive numbers of 25 to 30 short term trims on both banks, not just one, at RPMs above 2000 under heavy load. Would you say this has to do with the spider injection that might need to be updated like I've heard so many others have done? I was thinking maybe, like was mentioned in class, that it could have a bad oxygen sensor, but it has the same numbers on both Banks 1 and 2 which kind of rules that out, but of course not necessarily and I have not specifically tested them yet either. I have tested compression on this motor and has good compression of 115 or above with no more than 5% variation between cylinders. I have also replaced the coolant temperature sensor that was having a problem at one time. Thought I had it fixed then but alas not so. I did have to replace the fuel pump a while ago and the other day I was thinking that what if the connection down up under there is having a problem with moisture.? Maybe I need to get up under there and check that out also what do you think? One persistent code that keeps coming up his P0304. Although there is nothing wrong with the wires or cap or plugs. Other times it's a random misfire. And of course as you all know it could run really crappy for a while without setting any codes.

Thanks in advance for any and all help and suggestions!

Doug Foote
BigFoote Mobile Automotive
(707) 391-0577

Working where I am in a cold wet country, damp weather to me makes me look for ignition secondary arcing or water flooding a module

Bigfoote13 wrote: For a while now I've been a bit confused about what the difference is between short-term and long-term fuel trim, even after taking the Advanced Engine Performance class at the college here. So I reviewed Paul’s section where he discuss it in detail in class and I think I understand it much better now.

I am wondering though why the long-term trim only gets updated after moving the throttle position either up or down. Seems to stay constant until the RPM's change. Does anyone know why?

The PCM/ECM has a fuel map table, think of it as Engine Load (vertical scale) vs Rpm (horizontal scale). Depending on what the RPM is will justify what the computer has in memory for the long term trim. At idle you will see the long term trim sit still unless its correcting for a consistent low or high short term. Let say you raise the RPM to 2000rpm in park, now you reached the portion of that fuel map table that is based on 2000rpm so you may see the long term change based on what is in the computer's memory for that rpm.

You want to look at both when dealing with a misfire because even though we automatically look at Short Term FT, that might be at 0 while Long Term is at +15. Reason LTFT would be at +15 is because the STFT was at +15 for a specific amount of time. +15 is a problem at idle. Some say +/- 10% is OK.

And also on another point, I have a friend who has a 2000 GMC 4.3 with a spider injected TBI motor that runs pretty decent in dry weather but whenever it's raining, especially during damp weather it has a very bad misfire. I've been dealing with this for a while now trying to get it figured out and I think I'm starting to narrow it down. I kind of thought that it might have something to do with an intake leak until Paul mentioned that if it is misfiring and showing high fuel trim numbers at higher RPMs, it's NOT an intake leak. And of course after thinking about that made sense to me.

Usually idle will give you the most positive percentage for fuel trims if you have a vacuum leak and as you raise the rpm fuel trims will drop, sometimes will drop to 0 though you may still have slightly higher than 0 correction.

I'm getting positive numbers of 25 to 30 short term trims on both banks, not just one, at RPMs above 2000 under heavy load. Would you say this has to do with the spider injection that might need to be updated like I've heard so many others have done? I was thinking maybe, like was mentioned in class, that it could have a bad oxygen sensor, but it has the same numbers on both Banks 1 and 2 which kind of rules that out, but of course not necessarily and I have not specifically tested them yet either. I have tested compression on this motor and has good compression of 115 or above with no more than 5% variation between cylinders. I have also replaced the coolant temperature sensor that was having a problem at one time. Thought I had it fixed then but alas not so. I did have to replace the fuel pump a while ago and the other day I was thinking that what if the connection down up under there is having a problem with moisture.? Maybe I need to get up under there and check that out also what do you think? One persistent code that keeps coming up his P0304. Although there is nothing wrong with the wires or cap or plugs. Other times it's a random misfire. And of course as you all know it could run really crappy for a while without setting any codes.

Those spiders I think need 50psi minimum for the poppet valve to open so I am guessing chances are fuel pressure would be good for idle at least. I would still rule out fuel pressure with a fuel pressure gauge as GM is nice and gives us a schrader valve to work with.

- 115 PSI seems a little low to me for compression.
- Plugs, Wires and Cap are good but are they new? Basically how are we saying they are good?
- The spider can cause problems if there is not enough flow through the injectors. Could do an injector balance test
- I have had to replace MAFs on these in the past. I forget what the values should be for that motor.
- Upstream O2 sensors you could try to unplug them both and see if the problem persists. Engine should stay in OPEN loop.

seen it before. check the distributor. theres a tsb out for them theres a screen in there that gets clogged. can find the tsb on all data bbbindustries etc.

cheryl hartkorn wrote: seen it before. check the distributor. theres a tsb out for them theres a screen in there that gets clogged. can find the tsb on all data bbbindustries etc.

I'll look for that TSB. A screen in the distributor? Oh uah, you mean the of course the spider injection distributor, not the spark distributor Got it I'll look for that.....

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graywave wrote:

Bigfoote13 wrote: For a while now I've been a bit confused about what the difference is between short-term and long-term fuel trim, even after taking the Advanced Engine Performance class at the college here. So I reviewed Paul’s section where he discuss it in detail in class and I think I understand it much better now.

I am wondering though why the long-term trim only gets updated after moving the throttle position either up or down. Seems to stay constant until the RPM's change. Does anyone know why?

The PCM/ECM has a fuel map table, think of it as Engine Load (vertical scale) vs Rpm (horizontal scale). Depending on what the RPM is will justify what the computer has in memory for the long term trim. At idle you will see the long term trim sit still unless its correcting for a consistent low or high short term. Let say you raise the RPM to 2000rpm in park, now you reached the portion of that fuel map table that is based on 2000rpm so you may see the long term change based on what is in the computer's memory for that rpm.

You want to look at both when dealing with a misfire because even though we automatically look at Short Term FT, that might be at 0 while Long Term is at +15. Reason LTFT would be at +15 is because the STFT was at +15 for a specific amount of time. +15 is a problem at idle. Some say +/- 10% is OK.

And also on another point, I have a friend who has a 2000 GMC 4.3 with a spider injected TBI motor that runs pretty decent in dry weather but whenever it's raining, especially during damp weather it has a very bad misfire. I've been dealing with this for a while now trying to get it figured out and I think I'm starting to narrow it down. I kind of thought that it might have something to do with an intake leak until Paul mentioned that if it is misfiring and showing high fuel trim numbers at higher RPMs, it's NOT an intake leak. And of course after thinking about that made sense to me.

Usually idle will give you the most positive percentage for fuel trims if you have a vacuum leak and as you raise the rpm fuel trims will drop, sometimes will drop to 0 though you may still have slightly higher than 0 correction.

I'm getting positive numbers of 25 to 30 short term trims on both banks, not just one, at RPMs above 2000 under heavy load. Would you say this has to do with the spider injection that might need to be updated like I've heard so many others have done? I was thinking maybe, like was mentioned in class, that it could have a bad oxygen sensor, but it has the same numbers on both Banks 1 and 2 which kind of rules that out, but of course not necessarily and I have not specifically tested them yet either. I have tested compression on this motor and has good compression of 115 or above with no more than 5% variation between cylinders. I have also replaced the coolant temperature sensor that was having a problem at one time. Thought I had it fixed then but alas not so. I did have to replace the fuel pump a while ago and the other day I was thinking that what if the connection down up under there is having a problem with moisture.? Maybe I need to get up under there and check that out also what do you think? One persistent code that keeps coming up his P0304. Although there is nothing wrong with the wires or cap or plugs. Other times it's a random misfire. And of course as you all know it could run really crappy for a while without setting any codes.

Those spiders I think need 50psi minimum for the poppet valve to open so I am guessing chances are fuel pressure would be good for idle at least. I would still rule out fuel pressure with a fuel pressure gauge as GM is nice and gives us a schrader valve to work with.

- 115 PSI seems a little low to me for compression.
- Plugs, Wires and Cap are good but are they new? Basically how are we saying they are good?
- The spider can cause problems if there is not enough flow through the injectors. Could do an injector balance test
- I have had to replace MAFs on these in the past. I forget what the values should be for that motor.
- Upstream O2 sensors you could try to unplug them both and see if the problem persists. Engine should stay in OPEN loop.

Checked the fuel pressure and it stays above 52....

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If this problem only shows up in damp weather I would focus on secondary ignition . Have you tried the salt water test On the wires ? I think your original post said cap and rotor were ok how were they checked?

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Ben wrote: If this problem only shows up in damp weather I would focus on secondary ignition . Have you tried the salt water test On the wires ? I think your original post said cap and rotor were ok how were they checked?

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They were checked with resistance and carefully looking at any cracks for any cross sparking marks. So yesterday I started it up and it wasn't camp at all and it still doing it. Running real rough with high short-term fuel trim numbers. I'm thinking that I should get a new cap and rotor on there anyway to rule them out completely.

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Bigfoote13 wrote:

Ben wrote: If this problem only shows up in damp weather I would focus on secondary ignition . Have you tried the salt water test On the wires ? I think your original post said cap and rotor were ok how were they checked?

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They were checked with resistance and carefully looking at any cracks for any cross sparking marks. So yesterday I started it up and it wasn't camp at all and it still doing it. Running real rough with high short-term fuel trim numbers. I'm thinking that I should get a new cap and rotor on there anyway to rule them out completely.

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Now Cheryl wrote about checking the screen in the distributor she was referring to the distributor in the Spider injector system right? I haven't had a chance to look that up yet.

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I'm not familiar with the tsb Cheryl referenced I believe it to be the distributor however I believe this is the plastic distributor and I don't believe it has a screen the spider injection usually suffer from leaking lines which would give you negative fuel trims not positive , although being a maf engine fuel trims rely a lot on maf readings

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Also I would definitely spray the wires with salt water a resistance test doesnt show leaking high voltage and a visual inspection for cracks rarely turns up the microscopic pinhole in the insulation that the high voltage arc can get out

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Ben wrote: Also I would definitely spray the wires with salt water a resistance test doesnt show leaking high voltage and a visual inspection for cracks rarely turns up the microscopic pinhole in the insulation that the high voltage arc can get out

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Thanks, will do

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Also I assume you have access to a scanner with live data ? It's always good to take a look at your maf readings with warm engine at idle you should read approx 4.1 to 4.5 grams a second and driving on a wide open throttle run you should be able to cross 100 grams a second

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Ben wrote: Also I assume you have access to a scanner with live data ? It's always good to take a look at your maf readings with warm engine at idle you should read approx 4.1 to 4.5 grams a second and driving on a wide open throttle run you should be able to cross 100 grams a second

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Well I did and it only showed a MAP reading which was 11.1 H G's at idle and 8.4 at 3500 RPMs.

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Can you verify this has a maf sensor in the intake boot? I haven't looked it up but as I recall it's right off the filter housing

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some basic inspection like water contamination of fuel or dampness in primary/secondary ignition systems...just a thought.