brownmound

The Mound has had a worn out diff for some time now...ie it clunks and clanks when changing gears, slowing down, etc. Pretty sure it's not the u-joints because I replaced the driveshaft with a new one not too about 30,000 miles ago.

Lately, for the past few weeks or so, I will be driving, just cruising say at 40 mph or so, sometimes accelerating slightly, and it will "buck" or surge as if it is losing power. The tach shows no loss of power however, but I can definitely feel the loss. It's been doing it off and on for a few weeks.

Can a completely worn out LSD cause this symptom? Or should I start looking elsewhere...plugs and fuel filter were changed a few weeks ago.

Right on.

Paradox

iTrader: (1)

check your wheel bearings. jack up the rear of your car by the axle and see if you can wiggle the wheels with your hands, any play in the wheel could mean bearings which could also cause problems somewhat related to this, but usually it wont cause a grinding sound.

worth a shot atleast.

brownmound

I had the wheel bearings replaced not too long ago as well, in the rear. I'll look into...the diff is definitely gone though...not sure if I can even adjust it at this point. It's very sloppy.

Right on.

Kentetsu

Hmm, vacuum leak, timing, tranny or engine mounts....?

LongDuck

TPS needs to be adjusted.

Not unusual when the daytime air temps start to drop - readjust using the '2-light' rig and you'll be back to cruising smoothly.

Kentetsu

Damn, I feel like such an amateur standing next to you! I should pick up an SE just so I can get a little hands on experience with them I guess..

bliffle

Even a sound rearend can seem sorta loose and I've been fooled into thinking it was the punkin when it was UJoints. Is there someone else around there you can compare notes with?

I had a worn rearend that was howling on an old 49 ford and it finally locked up solid one day.

christaylor

The rear bearings are pretty obvious when they go, they'll make a very dubious sound for a long time and then they'll sound godawful when they get ready to go.

I'll second LD's suggestion above, even though I know nothing about -SE's. I'll be surprised if that doesn't do it, but if perchance it doesn't the next thing to check would be vacuum lines and any other computer-controlled stuff left on the car (I know the S5 I worked on was a nightmare, but I understand SE's are a little more civilized....).

brownmound

I was thinking that this morning, too...as I have had similar symptoms before, ie, the surging while cruising.

However, today, on the way to work, I was accelerating and the power dropped out for a second or two, then it comes back strong...could it still be the tps?

I'll look into it tonight....the temperature has been colder recently...but the fact that it is doing it on acceleration concerns me, and it seems more "violent" than what I remember a mal-adjusted TPS being.

Thanks and

Right on.

Wodihor

Don't know if this is something that i experanced or not... but when in 2nd or 1st trying to go though parking lots and such. if i'm off the gas i get the "jumping and jarring" car seem to bounce. no loss of power at acc. or high rpms though.. and if i push on the gas a bit and keep gears and trainy and such under a load then i don't get it.. i assume something is loose but have not and don't know where to begen... what is a TPS??

brownmound

That sounds more like the TPS (throttle position sensor) than the symptoms I have.

Right on.

ChasRX

I have the same "jumping and jarring" symptoms with my GS 12A. I know for a fact that my differential is ready for an overhaul (280,000+ miles). I have an '83 GSL disc brake and LSD complete rear end ready to go in, but still considering new front brake rotors and new brake tubing, too, as I have to bleed brakes during the swap. (may as well use that for my excuse for waiting so long.)

brownmound

At this point I can't really tell if it is the diff....I don't think it would be, really. I think it's got to be a fuel or spark type issue...right now it's very intermittent though. Hard to diagnose it at this point.

The tach stays strong though....

Eh, the Mound....

Right on.

ChasRX

Please keep us posted upon your diagnoses and how you solved it. My timing is off abit, too, as my idle is abit high as well; getting higher as the car warms up...very odd.

Sgt Fox

iTrader: (3)

i have the same problem with my 12A GS, its like that in 1 -3 gear but not at 4 or 5. the lunging as you drive down the road in 3... also the sorta grinding noise is there too in 1 and 2. i htink my tranny is fubar...

GavinJuice

brownmound i would check your fuel psi with a guage. i experienced the exact thing when i had a bad fuel pressure regulator. power would just drop and then bang kick back in. i happened to be running full pressure all the time, which was like 60psi to the injectors. so i was running about 20psi more than needed with a regulator that was stuck closed. nothing came out of my return line. your fuel pump might also be shorting out or possibly dieing with a loss of pressure when it does. i put a DMM on the plug under the driverside storage bin (it connects the pump) and monitored the voltage when power was lost etc.

or it could be the tps

LongDuck

Well, it sounds like several users here may benefit from checking or adjusting the Throttle Position Sensor (TPS), so I'll put some notes in here on how to do it. This is specific to -SE cars, since the Carb'ed 12a cars DO have a TPS as well, but it is adjusted differently.

First, the function;

The TPS is what the ECU reads to figure out where you *are* in terms of acceleration. It takes into account it's current state and load, and then compares this to what you're trying to make it do - if you're trying to speed up quickly, then the TPS is telling it 'hey, he wants to go fast!', and then the ECU can make modifications to EFI fuel mapping to help the engine accelerate. When you let off the throttle, the ECU needs to know that you're trying to slow down, and interprets this signal and makes adjustments to timing and fuel trim so that it returns to idle speed smoothly and without dieing.

Next, the symptoms;

When the TPS is set incorrectly (either way), then the computer thinks that you're telling it to accelerate or decelerate when you're actually telling it to do nothing. The ECU interprets this signal as best it can, and sometimes, the lag between accelerate and decelerate is so bad, that you get the 'surging' effects with clutch out in 1st or 2nd. This is because the car is already moving and has created a 'flywheel' effect with all the rotating parts being attached to the engine. All of this momentum tends to dampen the effects of the TPS signal, and can cause 'bucking' when all you want is steady-state throttle. If you're driving on the freeway, you might feel this in 4th or 5th gear on level ground as the car attempting to quickly accelerate briefly, then decelerate briefly, over and over. Usually, you only notice it in the lower gears because the engine has greater mechanical advantage againt the road due to gearing.

If you're really unlucky, your TPS is adjusted incorrectly exactly at the RPM range that you do most of your driving in - this will be REALLY annoying, since your throttle will be sitting right on the edge of speed-up/slow-down ALL the time that you're driving, and it will feel like the car is misbehaving nearly all the time. After you play with it, sometimes you find a 'sweetspot' for the TPS adjustment that lasts a few months. Othertimes, you may find that your best setting is right in your cruising range - oh, well. It's a passion driving these cars, remember?

What to adjust;

The TPS on the SE's is located on the right side of the TB when standing on the passenger side of the car. It is a black plastic box with a round metal tube sticking out one side that touches a lever on the primary Throttle Rod, and a 2-lead wire coming out the other end that's connected to a white connector on the harness. This TPS is adjusted through turning a flat-head machine screw mounted to the Primary Throttle Rod, and very, very, VERY slight adjustments to this screw result in massive changes in TPS 'tune'. Don't go wild on this screw, or you'll never get it set correctly until you build the ubiquitous "2 Light Rig" that SE owners covet and carry around with them...

Search on "2 Light Rig" to learn how to build one - it's not hard, but hard to explain here...

Once you get your rig installed correctly, with keys in the RUN (not START) position, slowly turn the TPS adjusting screw until both lights come on. This is the 'threshold' of where you want it. Gradually and slowly turn it the other direction until one light goes out... The closer you are in screw adjustment to the light JUST BARELY going out, maybe occasionally flashing on and off, the closer you will be to the superfine, ultraslick, level of tune that your SE can provide.

Read this thread 3 times, dance a jig, build a rig, and give it a shot. Good luck,

P.S. - it could also be your fuel pressure (!)

ChasRX

Nice explanation, but...well, phooey; I have a 12A. I'll do a search and see what threads correlate with 12A TPS adjustments and symptoms. I was thinking that the Throttle Position Sensor was the damper-plunger that only prevented the throttle linkage from slamming shut upon letting off the gas; to prevent backfiring.

GavinJuice

i always thought when tuning your tps to set it halfway in between 2 light and 1 light. are there any specific measurements as far as ohms go i could take in closed and WOT? everything i've read has always been ~1.XX (just throwing something out there) closed and WOT (~X.XX). i have always tuned with the light trick but, longduck do you have anything specific measurements you aim for when tuning your tps?

Whanrow

Verfied ! - Yup its carried in my console!

Hmmmm. When I did mine the instructions said halfway between on and off....

aussiesmg

iTrader: (1)

The usual reason for a 12a to buck is a air leak in the rats nest etc.... so if your TPS is OK try the vacuum lines.

LongDuck

'Halfway between on and off' doesn't make much sense to me - light bulbs are discrete instruments - they are either ON or OFF, not ever in-between!

If I can clarify; my original point was that the 2-Light-Rig should have both lights lit, then the screw is turned until one of them goes out. You want the TPS screw to just barely turn that one light out, so as a check, just put your thumb on the top of the screw and apply gentle pressure - that 2nd light should come back on, and when you quit pushing with your thumb, then the light will go out as the Throttle Damper releases it's pressure.

In other words, the slightest pressure on the top of the adjustment screw should light both bulbs again, but when the throttle closes completely, one of the bulbs will go out again.

Clear as mud?

SparkienSuggah

1983 12A surging can be related to the stupid *** shutter valve. block off the air bypass thingy that feeds the shutter valve inside the air cleaner, if this smoothes out your cruising, then pull off your carb, remove the little shutter plate and throw it away.

this worked wonders for me.