Bucking when OFF THROTTLE in 4th gear

Terrorist 5.0

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Hi everyone, just got done pulling a new clutch and some other stuff in the car, and when I went to drive it for the first time, I noticed something that was new.

Typically when I drive, I don’t pull the stick out of gear until the engine is basically at idle or just above when coming to a stop. I didn’t get any bucking I’m any gear at all. After I finished putting in my new clutch, I get a bucking when my foot is off the pedal at any rpm at or under 1300 RPM in 4th only. I can’t tell if it is the engine or the driveline, but it is quite annoying.

A couple things that could provide some detail, the flywheel is slightly lighter than factory. It is just under 19 pounds. Stock is 20-24 pounds according to most sources. I understand that lighter flywheels can cause something like this, but it is barely lighter than stock so… I also found my passenger side H pipe to manifold flange to be loose (the car is significantly quieter now that I fixed that lol), but I’m wondering if that impacted what the oxygen sensor sees in a significant enough way. I could also use new U joints, which could be a culprit.

The only thing that is stumping me is why it only occurs in 4th. I get that 4th is 1 to 1 ratio, but shouldn’t this show in all gears? The only thing I can think of that can cause this is something EGR related. It should come into play at the speeds I am going during the bucking, but then again my foot is off the pedal. I checked for codes and got an all clear. If anyone else has had this before, chime in please. Thank you.
 
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I went out today and tried something. I unplugged the spout and tried driving. The bucking is gone. Obviously I shouldn’t be driving with the spout out so I put it back in and the bucking is back. This leads me to believe that whatever controls spark is a culprit. I will check the TPS for dead spots soon, but anybody care to chime in? I may throw a TFI module at it just to see what would happen. They are cheap and easy to replace. I will keep the old one.
 
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Are you still bypassing the IAC with a switch?
Yes, I drive with it off 99% of the time though, and have been doing so without issue for about a month before I changed the clutch. Works really well.
 
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95opal

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Is it also doing this with the iac active?
 
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Is it also doing this with the iac active?
That is a good point, I will go out sometime today and try this. So far it has disappeared with the spout out, which probably means it is something with ignition timing. Multiple things control that, so in addition to trying with the IAC active next drive, I will also try unplugging the EGR and capping the vacuum line (I have doubts about the last one since it has been blocked off since I got the car but doesn't hurt to try)
 
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Just tried unplugging the EGR, check engine light came on as expected, bucking still present.
 

07GtS197

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The ecu is probably being thrown off with the iac being turned off so it uses timing to help stabilize idle/rpms. Why it only happens in fourth I couldn’t say.
 
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The ecu is probably being thrown off with the iac being turned off so it uses timing to help stabilize idle/rpms. Why it only happens in fourth I couldn’t say.
I would have said the same thing, however it’s been running like this flawlessly for at least a month. I have ruled that out.
 
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Just an update, found the vents weren’t working today, so I went over how they are controlled, and turns out that a line coming from the vacuum tree controls the vents. That line was broken. I am getting a new one tomorrow, but in the meantime, I plugged it as it is a vacuum leak and went for a spin. Bucking was still present.
 

07GtS197

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I would have said the same thing, however it’s been running like this flawlessly for at least a month. I have ruled that out.
Perhaps but one thing I’ve learned from working on cars is the rules of cause and effect are distorted. I’ve had an alternator drain a battery while sitting 8 hours overnight but not while sitting for over 12 hours during the day. I had an issue with an o2 sensor pegged lean in a Mustang I owned years ago. I tried everything I knew to fix it then found the po had spliced into one of the o2 sensor returns by the ecu. I fixed the wire and it ran great but the o2 sensor was still pegged lean for some reason. Point is, don’t give up looking for other things that could be the cause but don’t be surprised if it is just the ignition timing.
 
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Perhaps but one thing I’ve learned from working on cars is the rules of cause and effect are distorted. I’ve had an alternator drain a battery while sitting 8 hours overnight but not while sitting for over 12 hours during the day. I had an issue with an o2 sensor pegged lean in a Mustang I owned years ago. I tried everything I knew to fix it then found the po had spliced into one of the o2 sensor returns by the ecu. I fixed the wire and it ran great but the o2 sensor was still pegged lean for some reason. Point is, don’t give up looking for other things that could be the cause but don’t be surprised if it is just the ignition timing.
Understood, I've seen some strange things as well at my work, but it is definitely not the IAC being rigged. I've driven without it for almost 6 months, and I've had it rigged for about a month. I am absolutely confident it is not the IAC.
 
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Smoke tested today and found no vacuum leaks (after repairing my extremely minor one by the firewall for the hvac vents). Going to try check more stuff out and will update.
 

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how old are the plug wires , plugs and coil ? Distributor still original ? These cars are 30 years old now .
 
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how old are the plug wires , plugs and coil ? Distributor still original ? These cars are 30 years old now .
Plug wires and plugs are about 9 months old, coil about 5 months old. Unsure about the distributor. That’s the one part that worries me, as the pip is in the distributor, and is a common failure point that doesn’t really show much before it fails completely. That, and the replacements out there have a pretty bad reputation.
 

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Plug wires and plugs are about 9 months old, coil about 5 months old. Unsure about the distributor. That’s the one part that worries me, as the pip is in the distributor, and is a common failure point that doesn’t really show much before it fails completely. That, and the replacements out there have a pretty bad reputation.


Yea , I’m thinking the pip might be failing . I run a parts store replacement distributor but for a fox as I’m on Holley efi. I’ll check my shop this week for my stock dizzy my car had 55k kms and when I pulled it . If I still got it I’ll give it to you next time I’m down on the coast which would probably be next week.
 
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Yea , I’m thinking the pip might be failing . I run a parts store replacement distributor but for a fox as I’m on Holley efi. I’ll check my shop this week for my stock dizzy my car had 55k kms and when I pulled it . If I still got it I’ll give it to you next time I’m down on the coast which would probably be next week.
That would be amazing thank you! I was worried about finding a suitable replacement distributor in case of failure.
 

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That would be amazing thank you! I was worried about finding a suitable replacement distributor in case of failure.


Im 99 percent sure I still have it . It’s an oem ford one never had issues with my supercharged combo. I don’t throw mustang parts away so it’ll be in my shop
 

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