Inconsistent bucking when entering 4th under full load (WOT)

Blood on Blood

rumble baby rumble
Apr 6, 2005
56,481
45,958
In the past month, the following was installled on my Mach 1

4.30 gear
aluminum drive shaft from a 2000 Mustang GT
Tune (A/F was 12.8 and 28 degrees timing)

Problem:

This has happened twice in the past couple of weeks (at night, cooler air). When shifting into 4th, under full load (WOT), the car (engine) begins to violently buck as if something sparatically pushing against the front of the car. If I take my foot off the gas, then the bucking stops. This does not happen when shifting into any other gears or when I am already in 4th and then floor it (WOT). And of course, this bucking does not occur every time I enter 4th at WOT.

I was thinking a broken motor/trans mount or wrong driveshaft; however, the inconsistency makes me think it is the tune, bad ignition coil, who knows?

Going to attempt to get the car back to the tuner prior to MSC next Saturday.

Dave
 

Dragon03

Super hated on
Mar 28, 2007
27,350
2
Blood on Blood said:
In the past month, the following was installled on my Mach 1

4.30 gear
aluminum drive shaft from a 2000 Mustang GT
Tune (A/F was 12.8 and 28 degrees timing)

Problem:

This has happened twice in the past couple of weeks (at night, cooler air). When shifting into 4th, under full load (WOT), the car (engine) begins to violently buck as if something sparatically pushing against the front of the car. If I take my foot off the gas, then the bucking stops. This does not happen when shifting into any other gears or when I am already in 4th and then floor it (WOT). And of course, this bucking does not occur every time I enter 4th at WOT.

I was thinking a broken motor/trans mount or wrong driveshaft; however, the inconsistency makes me think it is the tune, bad ignition coil, who knows?

Going to attempt to get the car back to the tuner prior to MSC next Saturday.

Dave


I agree with the fact that the tune my be suspect the only reason I say that is based that you indicate it happens at night when the air is cooler, which imo shouldnt effect driveshaft or motor mounts (would be happening all the time), and we all know our cars tend to run better whens it cool out. I hope you find the source of the problem.....I hate when these types of issues rear themselves because they happen only here and there and hard to find and fix. I feel your pain. Good Luck.
 

01GTvert

TCG Elite Member
Jan 30, 2006
1,755
2
Dragon03 said:
I agree with the fact that the tune my be suspect the only reason I say that is based that you indicate it happens at night when the air is cooler...

Bucking ussually indicates a lean condidtion. This would be made worse by cooler/denser air. I'm not sure though if it gets cool enough to throw your A/F that outa whack.
 

Blood on Blood

rumble baby rumble
Apr 6, 2005
56,481
45,958
I wanted to wait until tonight (Friday) as the tuner/I just finished working on the tune.

I was out near Elburn and we headed the car East on a country road. Two runs, from 2nd into 4th. No problems. Then we did one run heading towards the storm, where cold air from the storm was blowing in through the shaker and the car bucked after entering 4th. It began to rain almost immediately after that run.

The bucking was as if you accidently downshifted the car from 3rd to 2nd and popped the rev limitet. It was so violently that the ass end of the car was skipping.

The mass air meter read fine and the timing was at 28 degrees during WOT. We did not have time to hook up a wide band due to how quickly the storm was coming in. Only the Predator was hooked up.

We dropped the timing down to 26 degrees, turned off the knock sensor, moved the rev limiter up to 7300 rpms and made a couple other adjustments. Again, the car was acting as if you down shifted and popped the rev limiter when in actually to did properly shift to the next gear and the rpms dropped as they should.

After the roads dried and the air still cool, I did 5 separate third to fourth gear pulls and the car was fine.

Again this is an intermittent issue, so if it occurs again we are going to remove the tune and assess. If it continues, we will then at the ignition coils.

I will keep everyone posted as this does not appear to be a common issue...

Dave
 

Blood on Blood

rumble baby rumble
Apr 6, 2005
56,481
45,958
As stated in other posts, ran the car at MSC and almost every run was a train wreck. This gremlin (bucking) issue still remains as if when you shift the 4th, the car acts as if you shifted to 2nd, when in all actuality you are in 4th gear.

After the event, the ignition coils were inspected, which the look new. The spark plugs were also clean. The gapping was at 54 and now we brought it down to 35.

We performed 8 highway runs after regapping the plugs, which the bucking issue did not appear. Maybe closing the gap on the spark plugs resolved the issue, but I doubt it.

Going to wait and see if the issue returns. We are going to check the Mass Air meter to determine if it is dirty and the IAC too. Also, if the issue returns, test the ignition coils and possibly replace the plugs with a colder set.

I have only heard from one other person who had a similar issue and it turned out to be a bad coil. We shall see.


I am going to continue posting on this subject so that it is documented if someone in the future experiences similar symptoms.

Dave
 

99 Stangbanger

Phantom Fury
Nov 22, 2004
1,705
0
35 should be just fine. I ran my 99 cobra on 35 for years N/A without any issues.

You mentioned that when the temps dropped it started doing it again, I highly doubt it would be a Coil Pack issue then cuz imo it would be all the time F'ed up. Once they go bad they dont get better or stop fucking up.

Did you read your fuel pressure from your Diablo while Under load and some hits?

Edited....
I think the issue is in your tune w/your old Plug gap!
 
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