missing @ highway speed

[caddy57]

I have a 96 Trans sport w/ 3.4 and a new trans. On the highway @ about 50-55 mph I get a small varience in speed that you can actually see on the tach. maybe 150-200 rpm up and down. Could this be the coil pack breaking down? Plugs and wires are new and it doesn't throw any code at all. Any input will be appreciated.
Thanks
Steve

[LMP]

I understand the RPM varies while speed stays the same...THis would look like the TCC Torque COnverter Clutch engages and disengages or engages and slips....
50-55 mph is indeed the speed at which TCC would engage.
One easy way to check that would be to keep the brake pedal very slightly depressed while you drive(first try with vehicle at rest: test to see how much you need to depress pedal just enough to light up the brake lights; it does not need much): activating the brake switch at same time disables TCC and cruise control. If the variaance in RPM disappears, then you would have a clue.
Naturally, with TCC unlocked, depressing the throttle WILL increase engine RPM because without the lock-up(TCC), you have torque converter slip that varies with torque, but at constant throttle position and constant speed, RPM would be steady.

[caddy57]

Originally Posted by LMP

I understand the RPM varies while speed stays the same...THis would look like the TCC Torque COnverter Clutch engages and disengages or engages and slips....
50-55 mph is indeed the speed at which TCC would engage.
One easy way to check that would be to keep the brake pedal very slightly depressed while you drive(first try with vehicle at rest: test to see how much you need to depress pedal just enough to light up the brake lights; it does not need much): activating the brake switch at same time disables TCC and cruise control. If the variaance in RPM disappears, then you would have a clue.
Naturally, with TCC unlocked, depressing the throttle WILL increase engine RPM because without the lock-up(TCC), you have torque converter slip that varies with torque, but at constant throttle position and constant speed, RPM would be steady.

No the speed goes up and down with the revs, and it is a new torque converter with trans.

[LMP]

OK then these symptoms are like the EGR valve that opens and closes . TO validate that option, I'd disconnect the electrical connection to the EGR valve and see what happens (you will get error codes about the EGR but they will clear within a few engine cycles after it is connected again ) . IN fact mine does something like that at certain regimes: the computer decides that the conditions are such that one more EGR valve section (3 sections) should open...then measuring the conditions after that move, it decides that it should close it back ... and does that again about every 2-3 seconds. More or less throttle sets the case. I just don't bather, kind of "gray zone" in the software.

Other option would be dirty/defective MAF sensor. If you want to overtake, will it procure instantaneous acceleration or will it accelerate only after a few seconds delay or show irregular acceleration.

[caddy57]

Originally Posted by LMP

OK then these symptoms are like the EGR valve that opens and closes . TO validate that option, I'd disconnect the electrical connection to the EGR valve and see what happens (you will get error codes about the EGR but they will clear within a few engine cycles after it is connected again ) . IN fact mine does something like that at certain regimes: the computer decides that the conditions are such that one more EGR valve section (3 sections) should open...then measuring the conditions after that move, it decides that it should close it back ... and does that again about every 2-3 seconds. More or less throttle sets the case. I just don't bather, kind of "gray zone" in the software.

Other option would be dirty/defective MAF sensor. If you want to overtake, will it procure instantaneous acceleration or will it accelerate only after a few seconds delay or show irregular acceleration.

No accelleration is fine if I step on the gas. Only happens once in a while. Also noticed that after this happens the idle gets a little rough. Then will straighten out the next time I drive it.
Thanks
Steve

[LMP]

Originally Posted by caddy57

....after this happens the idle gets a little rough.

ROugh idle also traceable to EGR valve....

[caddy57]

wouldn't this through a code?

[LMP]

It depends...was it OK before or did you buy the car recently and it was like that? With mine, I consider it part of the computer "gray zone" so the computer likes it like tnat althougnhI do not...so it will not call a code.
However, at same time,I know I have one of the sections that does not work anymore so I have a code for that.
In fact, EGR and MAF are the 2 elements best suited to cause some erratic behaviour and not necessarily call an error code.
But again, it might be something else, but disconnecting the EGR and see what happens is simple way to confirm/eliminate options.

[caddy57]

O K I'll try disconnecting the EGR. Thats in the back of the engine if I remember right?

[LMP]

...I know exactly where on 3.1 and 3.8, specially easy on 3.8, up front, near the battery. ,bout 3.4, I do not know....

[caddy57]

That was it. I had to go to Tampa this morning and no problem. Check Engine light came on but knew that it was for egr
I'll replace it tomorrow. Thanks
Steve

[LMP]

Glad this gives a clue about what causes the behaviour you described, but I'm not sure...in fact ..that a new EGR valve will stop the problem from showing again, specially since there were no CEL before so the computer considered all was OK. As told before, mine (3.8) also does that kind of trick at certain regimes (in my cse, going uphill at highway speed on a small incline) where there is some uncertainty in the computer about how many sections of EGR valve it should open. The fact that the car behaves normally, now, means that all sections close correctly otherwise you would have an erratic idle.
A CEL would have shown if one or several sections had REFUSED to open (like it shows now since it is disconnected.)
That is why I asked if the car always behaved like that to your knowledge, or if it had degraded with time.
My concern is that valve runs in the $200 ,I think, and for myself, I'd want to be sure it cures the problem before throwing that amount out of my pocket.
I'd like to know if other 3.4 owners have noticed the kind of behaviour you described in your first post.
ALso, I do not know for sure the algorithm that decides what the EGR does and when, but I'm sure the O2 analysis is part of that calculation so a shift of O2 analyser readings might have changed the calculation points about the exhaust recirculation, and possibly, some adaptive (learn) parameters in the computer may have shifted too and now place the erratic mode right in the middle of normal driving range.
Before going for expense, I try to clear the computer back to original values. With the "old" system that I have (OBD1), disconnecting the battery for several minutes (they say 30 sec. but some more does not hurt) will erase the error codes AND the adaptive values will return to default values. THis may have an impact on EGR valve operation point and I'd try that...hoping that OBD2 computer is cleared in that way too. AFter a while, it will "readapt" to actual engine parameters, but this will take several days of driving and in between, see what happens.

[LMP]

I read back through the post and noticed this that I had forgotten:
Quote:Originally Posted by caddy57
....after this happens the idle gets a little rough.
--------------------------------------------------

So....may be you have a sticky EGR section indeed....

[caddy57]

Thanks for all the help. It sure beats $95.00/hr for diognostics. I found an EGR on the web for $75 and sent for it. The price@auto parts {NAPA} was $286
Thanks again
Steve

[LMP]

I'm just waiting to hear of the result. After all, I might fix mine too some day....my wife hates that as long as I know why the car behaves wierd, I can live with it....she'd rather not..