High RPM Bogging/Backfire Between Shifts - Forums at Modded Mustangs
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post #1 of 27 Old June 19th, 2015, 12:33 PM Thread Starter
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High RPM Bogging/Backfire Between Shifts

My 98gt has been running great this season however recently I noticed some bogging and backfire issues which seem to be becoming progressively worse. I took the car out for a spin tonight and got on it down an off ramp and noticed everytime I went to shift in high rpm the car would backfire/studder and bog really bad into the next gear.

Car has a complete PI engine brand new plugs/new fuel filter, red monster cop conversion (which has worked great), Brand new upstream O2 sensors. I suspected that the fuel pump may be going out as it had seemed a bit too noisy at times. I'm sure it's still the factory one 145ks on it.

I've order a new fuel pump (walbro 255), We'll see if that clears it up; however this has me stumped as it has been running hard as hell for me tell now.


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post #2 of 27 Old June 19th, 2015, 12:36 PM
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well to me if your backfiring, fuel is entering the exhaust and igniting, I would find out why the fuel is not igniting in the cylinder like its supposed to

Edit: just noticed you mentioned something about changing you COPS, could the cops be having issues at high rpms and you not getting a good spark?

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post #3 of 27 Old June 19th, 2015, 12:49 PM Thread Starter
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well to me if your backfiring, fuel is entering the exhaust and igniting, I would find out why the fuel is not igniting in the cylinder like its supposed to

Edit: just noticed you mentioned something about changing you COPS, could the cops be having issues at high rpms and you not getting a good spark?
That could be a possibility. I have some extra coil packs I could swap out and see.


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post #4 of 27 Old June 19th, 2015, 05:26 PM
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Originally Posted by 98SnakeHorse View Post
My 98gt has been running great this season however recently I noticed some bogging and backfire issues which seem to be becoming progressively worse. I took the car out for a spin tonight and got on it down an off ramp and noticed everytime I went to shift in high rpm the car would backfire/studder and bog really bad into the next gear.

Car has a complete PI engine brand new plugs/new fuel filter, red monster cop conversion (which has worked great), Brand new upstream O2 sensors. I suspected that the fuel pump may be going out as it had seemed a bit too noisy at times. I'm sure it's still the factory one 145ks on it.

I've order a new fuel pump (walbro 255), We'll see if that clears it up; however this has me stumped as it has been running hard as hell for me tell now.
Quote:
Originally Posted by Fifth Horseman View Post
well to me if your backfiring, fuel is entering the exhaust and igniting, I would find out why the fuel is not igniting in the cylinder like its supposed to

Edit: just noticed you mentioned something about changing you COPS, could the cops be having issues at high rpms and you not getting a good spark?
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Originally Posted by 98SnakeHorse View Post
That could be a possibility. I have some extra coil packs I could swap out and see.
I'm with Horseman on this one!

Definitely swap COP's and see if that remedies the issue, if not I would definitely diagnose this further as fuel not igniting in the cylinder could be a sign of other possible major issues.

Hopefully, this is a quick fix.

Keep us updated and let me know if you have any other questions.

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post #5 of 27 Old June 19th, 2015, 06:01 PM Thread Starter
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Its got to be either an ignition or fuel issue. Shouldn't be too big of a deal just curious if anyone has had the same symptoms to remedy the situation quicker.


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post #6 of 27 Old June 19th, 2015, 06:13 PM
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After changing my clutch I was having a backfire issue with each shift. Turned out to be caused by an exhaust leak at the header.

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post #7 of 27 Old June 19th, 2015, 07:10 PM
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I doubt it is the COPs. COPs normally misfire under load when they are going out, not during a shift.

Your problem sounds like it could be your PPRV causing you issues. Search around about the PPRV causing bog at WOT shift and see if it doesn't seem to fit your symptoms pretty well.


What does your dyno sheet trap?
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post #8 of 27 Old June 19th, 2015, 07:26 PM
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Originally Posted by flyboy1294 View Post
I doubt it is the COPs. COPs normally misfire under load when they are going out, not during a shift.

Your problem sounds like it could be your PPRV causing you issues. Search around about the PPRV causing bog at WOT shift and see if it doesn't seem to fit your symptoms pretty well.
+1 to that.


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post #9 of 27 Old June 21st, 2015, 08:57 PM Thread Starter
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Quote:
Originally Posted by flyboy1294 View Post
I doubt it is the COPs. COPs normally misfire under load when they are going out, not during a shift.

Your problem sounds like it could be your PPRV causing you issues. Search around about the PPRV causing bog at WOT shift and see if it doesn't seem to fit your symptoms pretty well.
I took the negative battery terminal off to reset the ecu and replaced a couple coil packs car seems to be running good as new. But what exactly does PPRV stand for?


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post #10 of 27 Old June 21st, 2015, 09:20 PM
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Positive pressure relief valve. It's a check valve in your gas tank that keeps from fuel leaking out of the lines back into the tank when the pump is on. It's notorious for causing bogging or misfires during WOT shifts on the Cobras. I think some have experienced it on their GTs as well.

Read more here. https://www.moddedmustangs.com/forums...-what-why.html
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What does your dyno sheet trap?
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post #11 of 27 Old June 21st, 2015, 10:22 PM Thread Starter
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Quote:
Originally Posted by flyboy1294 View Post
Positive pressure relief valve. It's a check valve in your gas tank that keeps from fuel leaking out of the lines back into the tank when the pump is on. It's notorious for causing bogging or misfires during WOT shifts on the Cobras. I think some have experienced it on their GTs as well.

Read more here. https://www.moddedmustangs.com/forums...-what-why.html
Dude thanks for bringing this to my attention I actually have my tank out right now swapping to a walbro 255. Gonna delete the PPRV tomorrow. Thanks again you saved me from dropping the tank twice!



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post #12 of 27 Old June 22nd, 2015, 12:26 PM Thread Starter
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Upon further investigation I don't think the 98 fuel setup uses the PPRV like the 99/04 does. I looked at the pump and couldn't find it.


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post #13 of 27 Old June 22nd, 2015, 01:07 PM
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I also currently have my tank out, and im deleting mine as well.

This is what our tank looks like, i have a 2004. I also circled the PPRV just in case



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post #14 of 27 Old June 22nd, 2015, 03:28 PM Thread Starter
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I got a slightly different setup as I'm a return system. I don't think it coms with the PPRV on top of the pump.



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post #15 of 27 Old June 22nd, 2015, 08:02 PM
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Originally Posted by 98SnakeHorse View Post
I got a slightly different setup as I'm a return system. I don't think it coms with the PPRV on top of the pump.

You are right, if you have a factory return system there is no PPRV to my knowledge.

My bad man, I thought they switched to returnless in 96


What does your dyno sheet trap?
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post #16 of 27 Old June 23rd, 2015, 11:17 AM Thread Starter
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You are right, if you have a factory return system there is no PPRV to my knowledge.

My bad man, I thought they switched to returnless in 96
No prob dude 99 was the magic year for returnless. I have a buddy who's got a 99gt that may end up replacing his fuel pump soon so this is still relative information for me. Thanks again.

I would also like to re-describe the symptoms I was having as I think some people didn't understand what I was saying. The car would begin to break up in the upper rpm. Then when I went to shift quick to the next gear the car would do what I would describe as a fast buck and bog lol. Which at times resulted in a light pop/backfire type sound.

Like I said though I reset the ecu and swapped out some coil packs which seemed to have cleared up my issues. I'm running a PI engine with pretty much full bolt-ons and running the stock NPI tune which is probably another no no but I will eventually get a tuner ordered. Thanks again for all the help.

Nick


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post #17 of 27 Old June 23rd, 2015, 11:53 AM
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Can't this also be a tuning issue? My car bogs really bad between a aggressive 2nd to 3rd shift, basically nothing there when I get back on it, and then comes back in a second and startles you. But its fine between a 1st and 2nd. This only started happening after I deleted my pprv and put bigger pumps in so I'm pretty sure its a tuning issue. For you maybe your pump was just getting weak? Did you log your fuel pressure at all?



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post #18 of 27 Old June 23rd, 2015, 11:58 AM Thread Starter
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Can't this also be a tuning issue? My car bogs really bad between a aggressive 2nd to 3rd shift, basically nothing there when I get back on it, and then comes back in a second and startles you. But its fine between a 1st and 2nd. This only started happening after I deleted my pprv and put bigger pumps in so I'm pretty sure its a tuning issue. For you maybe your pump was just getting weak? Did you log your fuel pressure at all?
Weak fuel pump was a thought of mine since the pump was starting make some noise and has higher miles on it. I just finished getting a walbro 255 in lastnight so that shouldn't be an issue now.


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post #19 of 27 Old June 23rd, 2015, 10:06 PM
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Do you have an SCT handheld? You can start datalogging a few things so we can get a better idea what is going on.


What does your dyno sheet trap?
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post #20 of 27 Old June 23rd, 2015, 10:34 PM Thread Starter
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Do you have an SCT handheld? You can start datalogging a few things so we can get a better idea what is going on.
I do not. That's on the to buy list. Took the car out for a spin tonight after the fuel pump install. Car pulls real hard up to redline and fast shifting isn't an issue anymore. Hopefully I can get a tuner ordered within the next month.
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