Misfire

[callahan347]

New to the forum have been going through a lot of older posts and haven't been able to figure out my problem. my wife has a 97 Tahoe with 140000 miles. About a month ago it started showing a random misfire code but couldn't really notice when driving it. Several months prior to this I had installed new plugs wires cap and rotor with no issues. First thing I did was pull the plugs every one looked great. Ran some fuel cleaner through it with no change. Tested the fuel pressure and it was only running at 48 to 50 psi at idle so I replaced the pump. Still had a random misfire and showed eventually showed a crank cam no correlation code. Distributor gear is good and distributor is set properly. Replaced both cam and crank sensor and the correlation code went away but still have a misfire. Pops through the exhaust at idle and idle very rough. Runs well when on the gas to accelerate but at cruising speed starts to misfire again. Does anybody have any ideas what could cause this?

 

[Doubeleive]

maybe check for a intake leak, some one else had the exact same problem just the other day, did basically that same steps you have then took it in for a diagnostic and was told it was a intake gasket leak, i wouldn't just take it for granted that that's what it is but it is something to check out, you can use starting fluid around the edges the gasket and listen for a change in idle.

 

[callahan347]

maybe check for a intake leak, some one else had the exact same problem just the other day, did basically that same steps you have then took it in for a diagnostic and was told it was a intake gasket leak, i wouldn't just take it for granted that that's what it is but it is something to check out, you can use starting fluid around the edges the gasket and listen for a change in idle.

Thanks. I checked for vacuum leaks and used some starting fluid around the throttle body looking for leaks but nothing near the intake gaskets and plenum I'll check it out and post back

 

[MeNmyHoe]

Still running the factory spider injection? If you cannot find any intake leaks it may be time to investigate...

 

[callahan347]

Any way to test the spider injection without pulling the upper intake?

 

[MeNmyHoe]

Not really. All you would do is inject excess fuel into the cylinder, possibly washing them out. In addition, you would not be able to check the spray pattern of the injectors. Look at it this way, you could do the intake gaskets and rule that out at the same time...

 

[callahan347]

Checked for intake gasket leaks after work and everything looks good their. It is sounding more and more like it's the spider injection but before I go and replace it does anyone have anything else i should check out?

 

[MeNmyHoe]

Pull the air intake off. At the rear of the spider injection there is a fuel pressure regulator. Inside is a rubber diaphram that regulates the pressure via engine vacuum. These diaphrams can rip not allowing proper control of the fuel pressure. I believe in order to access it you will have to pull the upper intake manifold. I may be misremembering this as it has been a while since I have pulled an intake on one of these. Either way, once you have access pull the vacuum line off of the nipple and check for fuel dripping out and/or a strong fuel smell. Either one is a sign of a failed regulator. If you do not see fuel or smell it then use a hand held vaccum pump to pull a vaccum on the regulator. If it leaks down at all then the regulator is at fault. If not, then time to test the injectors.
Good luck.

 

[callahan347]

Pull the air intake off. At the rear of the spider injection there is a fuel pressure regulator. Inside is a rubber diaphram that regulates the pressure via engine vacuum. These diaphrams can rip not allowing proper control of the fuel pressure. I believe in order to access it you will have to pull the upper intake manifold. I may be misremembering this as it has been a while since I have pulled an intake on one of these. Either way, once you have access pull the vacuum line off of the nipple and check for fuel dripping out and/or a strong fuel smell. Either one is a sign of a failed regulator. If you do not see fuel or smell it then use a hand held vaccum pump to pull a vaccum on the regulator. If it leaks down at all then the regulator is at fault. If not, then time to test the injectors.
Good luck.

Thank you . I have a mpfi set up on the way 2 and 4 are getting excess fuel

 

[MeNmyHoe]

That should clear up your issues. Might as well do the lower intake manifold gaskets and all that while you are there. Insurance against future issues.

 

[callahan347]

So installed the mpfi unit and new gaskets still throwing a random misfire code. Some of the plugs were fouled so I replaced them with no change. Anybody have any idea what could be causing this to still miss and smells as though it is still running rich

 

[bigfootchiro]

Did you check O2 sensors?

 

[callahan347]

Did you check O2 sensors?

Replaced them last fall and have checked them with a scan tool and they appear to be reading well no lazy signals or anything like that

 

[bigfootchiro]

Have you looked at STFT and LTFT for both banks?

 

[callahan347]

Have you looked at STFT and LTFT for both banks?

I have a scan tool to do that but dont know what I'm looking for on the fuel trims. Not sure what's good or bad or when to read them. Do I check them just sitting in the driveway idling or do I need to be out on the road

 

[bigfootchiro]

Positive fuel trim numbers (especially on LT) is leaning out the motor. Negative is rich (more fuel). Your fuel trims should be fairly equal from side to side. If you accelerate, or increase RPM up to 1500-2500 and you see your fuel trims increase, then you likely have a vacuum leak or not enough fuel (like leaky FPR, bad fuel pump, clogged injectors). If they go negative, you are getting too much fuel, which could be clogged cats, bad O2s, or stuck open injectors

 

[callahan347]

Positive fuel trim numbers (especially on LT) is leaning out the motor. Negative is rich (more fuel). You’re fuel trims should be fairly equal from side to side. If you accelerate, or increase RPM up to 1500-2500 and you see your fuel trims increase, then you likely have a vacuum leak or not enough fuel (like leaky FPR, bad fuel pump, clogged injectors). If they go negative, you are getting too much fuel, which could be clogged cats, bad O2s, or stuck open injectors

Awesome thank you I'll check them tomorrow and go from there

 

[callahan347]

Positive fuel trim numbers (especially on LT) is leaning out the motor. Negative is rich (more fuel). Your fuel trims should be fairly equal from side to side. If you accelerate, or increase RPM up to 1500-2500 and you see your fuel trims increase, then you likely have a vacuum leak or not enough fuel (like leaky FPR, bad fuel pump, clogged injectors). If they go negative, you are getting too much fuel, which could be clogged cats, bad O2s, or stuck open injectors

Stft and ltft for bank 1 stay around 0 but both for bank 2 are around positive 8 to 9 at idle. At 2500 short term comes down to about positive 3 then right back up after getting off throttle. What could cause this

 

[HiHoeSilver]

Stft and ltft for bank 1 stay around 0 but both for bank 2 are around positive 8 to 9 at idle. At 2500 short term comes down to about positive 3 then right back up after getting off throttle. What could cause this

Vacuum leak on bank 2.

 

[callahan347]

Vacuum leak on bank 2.

Cant find a vacuum leak. Pulled plugs 2 and 4 are wet with fuel but 6 and 8 look great. Did find a vacuum leak behind the cats where the flange is could that cause it?