Lurching problem and I'm out of ideas.

[Ben A...]

This could be a 12 paragraph explanation, but I’ll try and keep it short. I’ve been problem solving this for months and can’t figure it out.

My question: Is it possible for a rear end to cause this? Can a torque converter be in bad shape and not leave metal shavings in the trans pan?

Vehicle: LA360 (Lumpy cam 235/249, EFI, MSD ), A999 (don’t know if it’s lockup), 8 3/4 Suregrip

Problem: When car has been warmed up for a while and driven for 1hr plus, I get a hard jerking/lurching between 1200-1300rpm when coasting down (no braking or accel). The jerking stops when rpm drops to around 1100, and disappears if I accelerate or brake.

What I've tried: I’ve taken off the intake manifold and completely redone all gaskets and checked for vacuum leaks, I’ve switched to an adjustable PCV valve to ensure it’s not a low rpm vacuum issue with tight overlap cam, I’ve tried different timing curves, CD boxes, and ignition coils. I switched from Sniper EFI to a carb and back to EFI. Fuel pressure is solid and I’ve checked every part of that system. Inspected cam and lifters too.
The engine is in very good (almost new) condition and the problem usually only starts after driving for a long time, so I don’t suspect it’s the valves. But I will do a leakdown test soon?

I’ve verified the kickdown lever activates at a 1-1 ratio with the throttle lever. I’ve taken it off, tightened it, loosened it.

I’ve gone to two transmission shops and both have said they don’t think its the trans. One shop took the pan off and said it was clean. Oil smells/looks good. I wen't to the best transmission shop in town and they said I had a "funny one" and couldn't figure it out. I kinda suspect neither shops were interested in dealing with a modified old car.

This issue did not exist for the first 5000 miles of driving, and started occurring after some hard hits.

Any ideas welcome!
Thank you for your time.
Ben

 

[Dartswinger70]

is it a clutch disc rear? can you put the car in neutral and it still does it on a roll?

 

[Bad Sport]

and started occurring after some hard hits.

I might look at the suregrip. Is is clutch or cone?

 

[Ben A...]

is it a clutch disc rear? can you put the car in neutral and it still does it on a roll?

Warning... My knowledge is weak with trans/driveline tech.
I was told it's a cone type when I got it, but I have not verified myself and the seller was a bit of a yahoo -- he could be wrong.

 

[Ben A...]

I might look at the suregrip. Is is clutch or cone?

I believe it's cone (see previous comment).

I've tried shifting to neutral and I believe the problem has gone away, but I'll have to try again because I last did that many months ago before my other fixes.

 

[Ben A...]

If this is a symptom of a busted suregrip then that helps a lot.
The guy who 'rebuilt' the rearend was always a little suspect in his worksmanship.

 

[Bad Sport]

The guy who 'rebuilt' the rearend was always a little suspect in his worksmanship.

If you get a chance do the coast thing again in neutral first and see if it does it.

How long ago was it rebuilt? Have you checked the diff fluid? Something inexpensive you could try, is to put some friction modifier in the diff. Not gear oil with modifier in it, but straight modifier. I don't know if it will help, but if it doesn't have or have enough modifier in it it might give you that lurchy feeling. Again, it's just a guess but something you can try that doesn't cost an arm and leg.

I know when I rebuilt my center section it felt like a spool when turning for a bit, catch and release catch and release.

 

[Ben A...]

If you get a chance do the coast thing again in neutral first and see if it does it.

How long ago was it rebuilt? Have you checked the diff fluid? Something inexpensive you could try, is to put some friction modifier in the diff. I don't know if it will help, but if it doesn't have or have enough modifier in it it might give you that lurchy feeling. Again, it's just a guess but something you can try that doesn't cost an arm and leg.

I know when I rebuilt my center section it felt like a spool when turning for a bit, catch and release catch and release.

Thank you for the help Bad Sport!
I'll do the coast-neutral test again today, the annoying part is I have to drive around for a long while before it starts happening.

About two years ago I purchased it from a yard that 'rebuilt it' and it immediately it was whining. So I took it back and they 'rebuilt it' again but it still whined. And they said it was "really hard to get right", which made me wonder if I was going to the right people... I've changed out the fluid and added Mopar friction modifier since. The first fluid change was very dark but I just crossed my fingers that was a break-in thing.

All that said, I haven't done an oil/lsd fluid change since the issue started occurring so I'll take another look at the oil.

Is this a potential issue for both spool and clutch type suregrips? Or just clutch?

 

[Dartswinger70]

i was going to add wouldnt the rear act up on tight turns in a parking lot? Or is that a way to isolate the issue? The neutral idea was to remove the trans etc. Process of elinitnation.

 

[Dartswinger70]

whining is more of a pinion depth/backlash ajustment etc. OR possible bearings. The actual friction /cone unit Im not so sure.i have the cone unit and its pretty durable IMO. I also have clutches in my Ford 8.8 Fox Body,and they can be "noisy" LOL!

 

[Ben A...]

whining is more of a pinion depth/backlash ajustment etc. OR possible bearings. The actual friction /cone unit Im not so sure.i have the cone unit and its pretty durable IMO. I also have clutches in my Ford 8.8 Fox Body,and they can be "noisy" LOL!

I haven't experienced any issues with tight turns at that low speed. Only noticed it when coasting down from a higher speed than I would likely do in a parking lot. That said, I'll do that neutral test today and also try in a parking lot... Maybe I just haven't noticed it.

I haven't paid too much attention to the whine because I figured it's a separate issue. It's just that they rebuilt it twice and it still whined, so I thought that was maybe telling...

 

[Bad Sport]

i was going to add wouldnt the rear act up on tight turns in a parking lot? Or is that a way to isolate the issue? The neutral idea was to remove the trans etc. Process of elimination.

One would think so. Mine did horribly until I did some figure 8s.

 

[Dartswinger70]

yeah there is a accelerate and coast side of the gears, sometimes they will whine only on '"coast" etc. I would think that is a worn gearset, or improper backlash setting. But the bearings also have their own type of noise ...and you have axle and pinion bearings.

 

[Professor Fate]

One thought other than the rear end:
You have an A999 (32RH)- Lockup convertor.
On deceleration, the convertor is not coming fully out of lockup mode; possibly locking/unlocking as you slow down causing your "lurching" feeling.
Reasons:
It only happens when hot, after an hour or so.
It happens on decel, disappears if put in neutral, if you accelerate or when brakes are applied.
Not knowing how your trans was set up/installed, I can only speculate that either the TC is failing or your line pressure is possibly low- slight chance the TC lockout solenoid isn't functioning properly, too.
Easiest thing first: verify trans fluid level. Check it hot, on level ground, in NEUTRAL. Park will give you a false reading.
Just a quick guess.

 

[Dartswinger70]

Yeah key thing is to figure out whether to focus on rear or trans...true story i had the retaining pin fall out of an open rear and the retainer was banging inside the housing, I felt it in my 4 speed shifter and I had thought something was up in the trans. I had happened to turn my ear toward the backseat area in my Dart and realized it was coming from the rear. The hammering sound was transferring up through the driveshaft to the shifter...

 

[Ben A...]

One thought other than the rear end:
You have an A999 (32RH)- Lockup convertor.
On deceleration, the convertor is not coming fully out of lockup mode; possibly locking/unlocking as you slow down causing your "lurching" feeling.
Reasons:
It only happens when hot, after an hour or so.
It happens on decel, disappears if put in neutral, if you accelerate or when brakes are applied.
Not knowing how your trans was set up/installed, I can only speculate that either the TC is failing or your line pressure is possibly low- slight chance the OD lockout solenoid isn't functioning properly, too.
Easiest thing first: verify trans fluid level. Check it hot, on level ground, in NEUTRAL. Park will give you a false reading.
Just a quick guess.

Thank you for the help.
My knowledge of driveline is weak but this makes a lot of sense to me. It also might be why the transmission shops are confused, because they don't expect a lockup is in there...

I take it to shops because I've moved to a new state with only my luggage, some tools, and the Duster... So dropping the trans in my rented driveway is intimidating. But I do need to figure out if it's a lockup.

 

[Ben A...]

Yeah key thing is to figure out whether to focus on rear or trans...true story i had the retaining pin fall out of an open rear and the retainer was banging inside the housing, I felt it in my 4 speed shifter and I had thought something was up in the trans. I had happened to turn my ear toward the backseat area in my Dart and realized it was coming from the rear. The hammering sound was transferring up through the driveshaft to the shifter...

Ever since the shop told me I have a "funny one" I drive around listening/feeling for everything like a paranoid mad man. Second guessing one sound or symptom!

 

[Dana67Dart]

I have had two cars with lockup torque converters that had problems.

1st was a Chevy that would drop out the lockup at highway speeds, felt like a misfire.

Disconnected the wire that powered the lockup and never had the problem again.


Second is a Ford and at highway speeds the rpm will I crease 100 to 200 rpm every so often. Seems to be tied to the AC. With AC off never happens, with AC on max almost never happens. With AC on normal will happen every few seconds. Some online sources suggest the lockup drops out when the AC compressor cycles

It feels like the the car also has a misfire but the rpm I creases.

I have not found or looked for the wire to disconnect yet.

My vote is the lockup TC is not releasing soon enough OR there is an issue with the overrunning parts.

I personally don't see how the diff could be doing this unless the tires are different size or the OP is turning sharp while decelerating.

Just my 2 cents

 

[AJ/FormS]

Problem: When car has been warmed up for a while and driven for 1hr plus, I get a hard jerking/lurching between 1200-1300rpm when coasting down (no braking or accel). The jerking stops when rpm drops to around 1100, and disappears if I accelerate or brake.

Assuming the trans is NOT auto downshifting during this event;
My guess is that the plugs are overheated and are pre-igniting the coast-down mixture. As soon as they cool off, or you add gas with the gas pedal or, or take away air with the brake pedal, the pre-ignition stops,and everything returns to normal. This is my guess.

If I'm right, you have three possible solutions;
1) richen up the cruise mixture
2) futz with the coast-down mixture
3) run cooler plugs
I vote for a little of each.

Retarding the timing will probably not have any effect.
That 235/249 cam wants some bypass air. The proper way to set the idle-timing is;
Begin here >
Take the carb off. drain it. Flip it over and drill one very small hole in each primary throttle blade; on the frontmost half, about half-way between the shaft and the edge and I center mine between the transfer slot and the idle-discharge port. Start with 3/32 holes.but be prepared to go up to and no more than 5/32. Chamfer the holes on both sides, just enough to get rid of burrs. Clean off the chips. Flip it back right side up.
Next; reset your curb-idle screw to achieve a transfer-slot exposure on the manifold side to appear slightly taller than wide, just enough to see it.
Next; reset your mixture screws to the mid-point of their working range; With Holley or Holley-type set it to 3/4 turn. All others are 2.25 turns. If you have a metering-rod type of carb, you may have to drop them down a hair; we'll see.
Next; make sure the WET fuel-level is correct and stays correct. Make sure none of the air-bleeds are dirty.
Next; make sure your vacuum advance is plumbed to the SPARK-port
Next; DO NOT TOUCH THE IDLE-SPEED SCREW.
To set the idle speed you will be using timing. After you start it up, and it is warmed up, it will probably be idling too fast, so RETARD the timing to no less than 12*.
But if the idlespeed is too slow, ADVANCE the timing to no more than 18*.

Your target for that cam is about 750rpm in Neutral, fully warmed up, with the timing at around 14*.

If you cannot get it there;
You may need to increase the size of the drilled Idle-air bypass holes. but do not do it yet.
If you can get the Idle speed close at 14*;
First try the mixture screws.
If it wants more fuel; increase the transfer-slot exposure by turning the speed screw 1/2 turn faster. Then put the mixture screws back.
If it still seems to want more fuel; Increase the idle-timing to 16*
STOP here>

The idle speed does not have to be 750. I have a 230/237/110 cam in my 11/1 360 and I can get her to idle just fine down to 550rpm in first gear (manual trans) and she has enough power to idle around the parking lot like that, on just 5* of idle-timing.
Your transfer slot exposure only has to be enough to prevent a tip-in sag or outright stall.
Your idle speed has to be low enough so that it doesn't
bang/clang going into gear.
You achieve these by controlling the idle-power. The three players are; throttle-opening, idle-timing, and cylinder pressure.
Don't let people bully you into trying to run more idle-timing than necessary. I can make your beast idle just fine on 5*. I'm just saying.
So now, if you don't have a tip in sag, AND the trans does not clang when engaging a gear, and the rpm drop during engagement is 100 rpm or less; you are ready for another roadtrip, but let me say it again; from this idle-speed setting, do not move that screw! If you do, and you cannot get it working, or you get lost, go back to "begin here>"

After you have found an idle timing that satisfys the engine, you will have to revisit the power-timing, to make sure it is not too much.
The very first time that your engine cares about it's Power-Timing is at stall. And with a distributor, and a stall lower than 2400rpm, it is almost impossible to give it enough timing. We can talk about that another time.
Your cruise timing will likely want to be more than 52*@2400rpm.
I for one will be waiting to see your results.
I run Champion copper cores; they last forever

Happy HotRodding

 

[Dartswinger70]

a lockup converter would have some sort of electronic sensor or switch circuit I'm thinking, in an old car there would be some sort of wiring or switch installed but i may be wrong. newer cars use the computer for example,but it is an electronic switching circuit. I guess Im saying you should be able to figure it out without pulling hte trans, look for the circuit. or computer?

 

[69FBCuda]

Put a temp gun on different areas of the rear axle ie.. differential, outer bearings, brakes/hubs, does the temp vary greatly from one side to the other? . Disk brakes? Front and/or rear? If yes check for a warped rotor or hanging shoes?

 

[Ben A...]

Assuming the trans is NOT auto downshifting during this event;
My guess is that the plugs are overheated and are pre-igniting the coast-down mixture. As soon as they cool off, or you add gas with the gas pedal or, or take away air with the brake pedal, the pre-ignition stops,and everything returns to normal. This is my guess.

If I'm right, you have three possible solutions;
1) richen up the cruise mixture
2) futz with the coast-down mixture
3) run cooler plugs
I vote for a little of each.

Retarding the timing will probably not have any effect.
That 235/249 cam wants some bypass air. The proper way to set the idle-timing is;
Begin here >
Take the carb off. drain it. Flip it over and drill one very small hole in each primary throttle blade; on the frontmost half, about half-way between the shaft and the edge and I center mine between the transfer slot and the idle-discharge port. Start with 3/32 holes.but be prepared to go up to and no more than 5/32. Chamfer the holes on both sides, just enough to get rid of burrs. Clean off the chips. Flip it back right side up.
Next; reset your curb-idle screw to achieve a transfer-slot exposure on the manifold side to appear slightly taller than wide, just enough to see it.
Next; reset your mixture screws to the mid-point of their working range; With Holley or Holley-type set it to 3/4 turn. All others are 2.25 turns. If you have a metering-rod type of carb, you may have to drop them down a hair; we'll see.
Next; make sure the WET fuel-level is correct and stays correct. Make sure none of the air-bleeds are dirty.
Next; make sure your vacuum advance is plumbed to the SPARK-port
Next; DO NOT TOUCH THE IDLE-SPEED SCREW.
To set the idle speed you will be using timing. After you start it up, and it is warmed up, it will probably be idling too fast, so RETARD the timing to no less than 12*.
But if the idlespeed is too slow, ADVANCE the timing to no more than 18*.

Your target for that cam is about 750rpm in Neutral, fully warmed up, with the timing at around 14*.

If you cannot get it there;
You may need to increase the size of the drilled Idle-air bypass holes. but do not do it yet.
If you can get the Idle speed close at 14*;
First try the mixture screws.
If it wants more fuel; increase the transfer-slot exposure by turning the speed screw 1/2 turn faster. Then put the mixture screws back.
If it still seems to want more fuel; Increase the idle-timing to 16*
STOP here>

The idle speed does not have to be 750. I have a 230/237/110 cam in my 11/1 360 and I can get her to idle just fine down to 550rpm in first gear (manual trans) and she has enough power to idle around the parking lot like that, on just 5* of idle-timing.
Your transfer slot exposure only has to be enough to prevent a tip-in sag or outright stall.
Your idle speed has to be low enough so that it doesn't
bang/clang going into gear.
You achieve these by controlling the idle-power. The three players are; throttle-opening, idle-timing, and cylinder pressure.
Don't let people bully you into trying to run more idle-timing than necessary. I can make your beast idle just fine on 5*. I'm just saying.
So now, if you don't have a tip in sag, AND the trans does not clang when engaging a gear, and the rpm drop during engagement is 100 rpm or less; you are ready for another roadtrip, but let me say it again; from this idle-speed setting, do not move that screw! If you do, and you cannot get it working, or you get lost, go back to "begin here>"

After you have found an idle timing that satisfys the engine, you will have to revisit the power-timing, to make sure it is not too much.
The very first time that your engine cares about it's Power-Timing is at stall. And with a distributor, and a stall lower than 2400rpm, it is almost impossible to give it enough timing. We can talk about that another time.
Your cruise timing will likely want to be more than 52*@2400rpm.
I for one will be waiting to see your results.
I run Champion copper cores; they last forever

Happy HotRodding

Thank you for the input-- Interesting theory here.
I've tried going from RC11YC plugs to RC9YC plugs and using high octane fuels. Ive timed from 14-20 at idle and used slow and fast timing curves. Also no vac advance.
But the problem persists.
Right now I have EFI on the engine, but I can switch back to carb pretty quickly so it's worth trying.
I had tried some of what you're saying but not in this workflow, so perhaps I'll give that a go if some of this driveline tests don't pan out.

 

[Ben A...]

a lockup converter would have some sort of electronic sensor or switch circuit I'm thinking, in an old car there would be some sort of wiring or switch installed but i may be wrong. newer cars use the computer for example,but it is an electronic switching circuit. I guess Im saying you should be able to figure it out without pulling hte trans, look for the circuit. or computer?

I believe a mid 1980s A999 is controlled hydraulically by the valve body?

 

[Ben A...]

Put a temp gun on different areas of the rear axle ie.. differential, outer bearings, brakes/hubs, does the temp vary greatly from one side to the other? . Disk brakes? Front and/or rear? If yes check for a warped rotor or hanging shoes?

Easy tests to do! I like you're thinking.

Disk front and drum rear.
I've inspected the brakes a few times in the last year but no luck finding the issue there. That said, I have to grease the backing plates on the rears so I'll take another look.

 

[Dartswinger70]

I believe a mid 1980s A999 is controlled hydraulically by the valve body?

Not sure Ive never heard of a hydraulic lockup converter though interesting. I looked into it briefly here is the link. It seems there is a hydraulic and electronic system You could pull the trans pan and see ethey hydraulic system for example. It would still avoid pulling the whole unit.

Mechanical override: Lockup torque converters

 

[69FBCuda]

Easy tests to do! I like you're thinking.

Disk front and drum rear.
I've inspected the brakes a few times in the last year but no luck finding the issue there. That said, I have to grease the backing plates on the rears so I'll take another look.

A warped rotor may not show up until it gets hot, you would also feel it in the brake pedal,

PS: We love pictures!!