My car (in sig) jerks only when the torque converter is locked up. I put a vac guage on the motor, and when it's jerking, the guage is perfectly still. My question- could the problem still be in the motor if the vac guage is acting normally?
Thanks,
Jeff

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88 Trans Am 305 Auto Red/Silver, MSD coil,wires, ported plenum, gutted airbox, TB bypass, comp cams XR258HR, 206/212 @ 50 .480/.487 110 LSA everything else stock

 

?

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88 Trans Am 305 Auto Red/Silver, MSD coil,wires, ported plenum, gutted airbox, TB bypass, comp cams XR258HR, 206/212 @ 50 .480/.487 110 LSA everything else stock

 

TA-J,

"In the motor" is really pretty vague. If you meant to ask if the problem might be mechanical, it's possible, but not as likely as some other potential problems.

If you meant to ask if the problem was "Under the hood", then most likely, Yes. I would guess that you have an ignition or fuel delivery problem, not a mechanical problem with valves or compression.

At what RPM does the problem seem to be most prevalent? SBC engines are somewhat harmonic at 1,700 RPM (and multiples of that RPM). They tend to be least smooth at those speeds, since the balance of the engine components works against the engine the most at that speed. It is possible that you are experiencing that harmonic imbalance, but that is very slight, and you really have to be "in tune" with your engine to even notice it.

It is more likely that a weak spark in a cylinder is a problem, or a fuel injector with a marginal spray pattern or unbalanaced flow rate is causing the uneven operation. It is also possible that the fuel mixture is incorrect, but that typically defaults to a more rich condition, which doesn't contribute much to misfire until the situation is so bad that smoke is visible in the exhaust.

The ignition is fairly easy to check and "freshen up". You can remove and clean/inspect plugs, test plug wire resistance, inspect the distributor cap, rotor, pickup coil, reluctor, ignition coil, etc. If you perform routine maintenance, you probably already have a handle on those items.

The fuel mixture should be controlled by the ECM, and if the oxygen sensor is reliable, the mixure should be close to optimal.

Diagnosing injector problems can be a bit more difficult. Flow testing can be checked on the car to some extent, but the indications aren't that precise, only relative. Injector spray patterns are impossible to view in an assembled intake.

Once you've exhausted the ignition and mixture possibilities, you might want to consider evaluating the injectors - but only AFTER the other possibilities are exhausted.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader

[This message has been edited by Vader (edited November 02, 2001).]

 

Whoops...I read the topic and all I thought of was the 3pointer I missed the last day of the season...woulda had about 8lbs of good jerky out of that one.

 

Your season is OVER?!

Bow season is just beginning here...

Rifle/shotgun is another month off.

Never mind. I just got it - "3-pointer" and "8 lbs. of jerky". O.K.

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Later,
Vader
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If you want to beat the World, it might reach up and pull you down...
Adobe Acrobat Reader

 

What the heck are you talking about? Jerky? What the. . .

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1984 Firebird SE
1985 Camaro z28 305 engine T-5 Five Speed Tranny, 92 Camaro Leather Seats Console, carpet everything. Soon to put in 700R4 and 427CI. (Hopefully.
To view my car go to:

http://geocities.com/jmboriss/bluecar.html

It is also for sale, please make me an offer. Thanks.

 

I wish we had another season starting. We just got a little storming in the hills, woulda have brought some big ones downs. Gotta sight the danged thing in again...gotta be the scope...er the shells, er...the stocks touching the barrel...certainly not me

 

Oh, for the orignal poster, could be your TCC is locking and unlocking...ofcourse misfiring would be felt more with the direct drive...but still worth looking into.

 

Look, Jerky... (kidding!).
A little more info:
Yes the question is regarding whether the problem is in 'the motor' as opposed to the tranny.
This happens at all rpm's as long as the torque converter is locked, in 2nd, 3rd, 4th. I can accellerate gently or rapidly without a hint of jerkiness at all until the TC locks up.
A while ago I discovered the timing was wandering around so I put a new dist. gear on. Timing now steady, still JERKY.
This only started happening after my cam swap, so I checked EVERYTHING a hundred times; vac leaks, manifold leaks (re-installed intake manifold), etc, etc.
New:
coil
cap
rotor
wires
pickup coil
o2 sensor
plugs are healthy and look very normal.
I've heard that 'maybe you only notice an engine miss when the TC is locked up' stuff, but I can't believe I'd miss a miss like that without the TC locked up, especially since it also happens much past the TC stall speed. Is there something in the TC lockup that can cause it to JERK like this? The tranny otherwise acts normally.
Thanks Vader and everyone else for your replies. This is severely impacting the enjoyment of my car and I REFUSE to take it to the garage!
Thanks,
Jeff



------------------
88 Trans Am 305 Auto Red/Silver, MSD coil,wires, ported plenum, gutted airbox, TB bypass, comp cams XR258HR, 206/212 @ 50 .480/.487 110 LSA everything else stock

 

?

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88 Trans Am 305 Auto Red/Silver, MSD coil,wires, ported plenum, gutted airbox, TB bypass, comp cams XR258HR, 206/212 @ 50 .480/.487 110 LSA everything else stock

 

Jerky: seasoned and dried meat. Deer jerky is great if you like that sort of thing. I do!