bad bucking below 40 mph during cruise.

ashford · Sep 4, 2010

i have a problem with a bad case of bucking during cruise below 40mph between 1200-1900 rpm. and during deceleration in a smaller rpm window. i know it could be a number of things but i want to know where to start.

1. clutch. i have a new zoom 10" clutch
2. driveshaft. i have changed out the dampend yoke for a non dampened one to shorten the driveshaft 1/4 inch ( went from a 7.5 to 8.8 rearend)
3. control arm bushings. i have the stock style ones in the car.

last winter i changed from a stroked windsor to my old 347(yes i know a big downgrade but i wanted a dd instead of a race car). installed a brand new t5z. and changed the gears to 3.27.

where would you guys start?

Mustang5L5 · Sep 4, 2010

Start by telling us what yup are driving and then run the check engine codes. (if the car is efi).

Your symptoms suggest running lean.

Gearbanger 101 · Sep 4, 2010

I'd think of backing off your engine timing a couple of degree's. Bucking at low RPM upon deceleration is often a sign of too far advanced ignition timing. If it also happens upon light throttle, low RPM acceleration, I might look into the age of my plug wires.

My gut feeling tells me that your driveline components don't have anyting to do with this and that its an ignition issue.

Following Mikes advice above and running it for codes should be your first step though.

ashford · Sep 4, 2010

ah sorry forgot to mention the full car setup.

85 merc capri
347, afr 165, 1.7rr, 10.5cr, e303, professional junk intake(ported) shorty headers with x-pipe.

t5z, 8.8 with 3.27.

ms3, 8 ls1 coils, sequential fuel, slc wideband.

at cruise im running between 14.5:1 and 15:1 afr. the timing is in the mid 20's below 1800rpm. ramps to 38 @2200 around 50kpa.

my problem is a mechanical oscillation between the tires and the crank. engine torque loads the driveline, car lunges forward, then the torque gets reversed and the car loads the driveline and bumps the engine faster, car slows down and it start all over again. it takes between .5 and 1 second depending on what gear im in. it dosent happen much in overdrive but driving 30mph in od get me below idle speed.

2000xp8 · Sep 4, 2010

What you describe appears to be lugging or trailer hitching.

Sounds to me like spark plug wires or the cap and rotor.
And while visual inspection works sometimes, i've had wires go bad that looked brand new.

Years ago my car did it, but under wide open throttle it was ok, it was the cruising.
What kinda like a driveline shunt. Turned out to be the wires.

jka · Sep 5, 2010

I think you should check your vacuum lines or you egr vave. I just fix the problem you are having right now on my mustang. I did a intake change and i forgot to fix one of the boken lines that goes from the intake to the vacuum tree for the egr and when i went down the highway the car had good power because of the intake swap but it was bucking hard. So i came back home and went over everything and i came across the vacuum linefor the egr on the vacuum tree so i fix it and no more problems.

bad bucking below 40 mph during cruise.

ashford

Member

Mustang5L5

That is…until I whipped out my Bissell

Gearbanger 101

Straight Outta Locash

ashford

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2000xp8

jka

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bad bucking below 40 mph during cruise.

ashford

Member

Mustang5L5

That is…until I whipped out my Bissell

Gearbanger 101

Straight Outta Locash

ashford

Member

2000xp8

jka

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