Acceleration problem above 35 mph...

[roaddirt]

Can anybody help with this one?

Vehicle: 1999 Buick Century
Mileage: 40K
Engine: 3100 V6
Transmission: automatic 4T60E w/vacuum modulator

The car has been checked by my regular mechanic twice, a transmission shop and a Buick dealer service shop in Richmond, VA with no success.

The problem is more noticeable when car warms up.
Once I reach wanted cruising speed of 35 mph or above (and the car changes to third or fourth gear), the car has a poor response on a slowly, steady press on gas pedal.

For a second it seems that RPM sound goes up and then it drops down with dull sound ( the car has no RPM gauge on dashboard). At that point I keep pressing pedal slowly but car does not accelerate or needs eternity to speed up.
Sometimes I do not even feel that RPM goes up even for that second.

At that point,
If I release and press pedal lightly once or twice, it will eventually “pick-up” and response.
If press faster or harder, it will downshift and speed up like a champion but I do not want to race.

Air and fuel filters, transmission fluid and PCV valve has been replaced - problem persisted.
So far I tried test-swapping used TPS, MAF, MAP – problem persisted
Fuel pressure within specs (40 psi in idle) and fuel pressure regulator holds vacuum.
Spark plugs and wires are original but they should be good at 40K, (GM says the should last up to 100K).
Poured a bottle of injectors cleaner in tank - problem persisted
Replaced vacuum modulator with aftermarket part – slightly better but problem persists.
No SES light came, no malfunction codes set.

I love the car, it has low mileage and hopefully should give me years of service but I am desperate now.

Any help would be greatly appreciated.

Roaddirt

[JoeJoe231455]

I did not see..."I cleaned the throttle body with throttle body cleaner". It may be that your throttle body butterfly is having trouble getting past a certain point due to gunk buildup. 40k is actually a good time to clean it if you have not.

[roaddirt]

Thank you for trying to help JoeJoe. I could give it a try since I do not know what else to do, but I somehow do not think that throttle body is the problem.
Reason is, I do not fill hesitation in first let’s say 5 min until car warms up.Also, I do not have any hesitation in 1st and 2nd gear until the car passes 35 mph and go in higher gear.

Unfortunately I cannot figure out if this is engine or transmission related.
Warming up should have something to do with either electronic, vacuum/pressure or so.

Could this be any of these: PCM, TCC solenoid, transmission pressure control solenoid, injectors?

[kilroypr]

I would disconnect the TCC. Poor mileage but if it is the problem the car should run smoother.

[roaddirt]

Originally Posted by kilroypr

I would disconnect the TCC. Poor mileage but if it is the problem the car should run smoother.

I did try that and could not make a hundred yards. The transsmission immediately started slipping and SES light came on. Had to stop and plug TCC wiring back so I can drive.

The car 1999, is it possible that that old trick does not wotk with newer models? Cannot believe the transmission is that bad!

BTY, I removed air duct today and cleaned throttle body as JoeJoe advized. There was no much in it, like I thought, so it cannot be cause.

[kilroypr]

Well the tranny trick works on my 93 Z-34 wich has a 4t60e. Never tried it on the GTP wich has a 4t65e. So you are correct. By any chance, I know this is silly but did you used Mercom III and has the correct level on it?

[roaddirt]

Yes, I used Dexron III/Mercon and carefully filed it up to the proper level while idling hot.
I found a post from Flatrater where he said that there is GM's TBS regarding drivebility problems with 4T60E trany caused by failing Pressure Control Solenoid. I wonder if that could be my problem but it is too expensive fix to try without being sure that that is the cause.

It would be so helpful if I could distinct if the problem is engine or trany related.

[kilroypr]

Roaddirt I feel from common sense that if the engine does not hesitate or misses when you accelerate the car it is not the engine. If the engine does drown when WOTing the car I would think the problem is leaning more to the engine. But I think it leans more to the tranny since the engine iddles correctly and accelerates correctly. Depending on the load the tranny put on the engine is how it behaves so if the tranny puts an herratic load to a good engine the engine will accelerate herratically but without hesitation of misses. If the engine rough idles or misses the car will accelerate more or less ok and you can feel the engine failing directly.

[roaddirt]

I know my description of the problem can be confusing but you got me confused now. The car does hesitate when at 35-40 mph and above and it response very poor when press accelerator slowly and steadily.
If press hard it downshift, revs high and pull but it is inacceptable driving style. It is real worse on highway when cars get hot.

[kilroypr]

Roadirt, let's put it this way. The car looses power when accelerating past 40mph? If that is the case could be spark timing related. The module or something like that.
Does the engine stalls, dies, black smoke or something similar to those simptoms?

[roaddirt]

Originally Posted by kilroypr

Roadirt, let's put it this way. The car looses power when accelerating past 40mph? If that is the case could be spark timing related. The module or something like that.
Does the engine stalls, dies, black smoke or something similar to those simptoms?

Example:
Started from the stop. I press accelerator and the car goes fine through 1st, 2ng, 3rd to 4th gear. I can reach any speed, like 50 or 70 mph with no problem.
I reached wanted speed, let's say 50 mph and cruising.

Now if I press pedal to accellerate, the car hesitate, no RPM increase, no speed increase. Sometimes you can see speed gauge moves very, very slowly indicating weak speed increase.

If I release accelerator and press it again, after a couple of tries it will catch, engine revs and car speed up like it is supposed to.

A couple seconds after,as soon cruise again at steady speed, it become inresponsive again.

No stalling, no smoke, car does not die

[kilroypr]

Ok, this looks like the TCC but not is not completly gone. That happened to me once on my 93 Z-34 4t60e. I had to replace the tranny beause other issues and that stopped happening. Try adding high friction additives to the tranny to get a little more life out of it. But looks like it will die eventually(not in a short while, but in a while). What are U experiencing is a shifting problem do to a lack of enough pressure for the clutches to engage properly. Get the tranny rebuilt or look for the fricion adding additive. Ols school would add 1 cap of Wagner dot3 to the tranny oil, this will help the clutches engage better.

[roaddirt]

Originally Posted by kilroypr

Ok, this looks like the TCC but not is not completly gone. That happened to me once on my 93 Z-34 4t60e. I had to replace the tranny beause other issues and that stopped happening. Try adding high friction additives to the tranny to get a little more life out of it. But looks like it will die eventually(not in a short while, but in a while). What are U experiencing is a shifting problem do to a lack of enough pressure for the clutches to engage properly. Get the tranny rebuilt or look for the fricion adding additive. Ols school would add 1 cap of Wagner dot3 to the tranny oil, this will help the clutches engage better.

Sounds realy bad. I bought the car three months ago and I noticed the problem a couple days after I bought it.
Is there any chance that Pressure Solenoid is bad, messing with transmission pressure?

[kilroypr]

Well man, you can replace the solenoid if you want but it would be a lot better to rebuild the whole thing. Mine broke a sproket and that is why I had to replace it, after I replaced it the jerking at high speed ended. This jerking happens like the car does not decides if it has to stick on od or 3rd. Something like that. Add a capfull of Wagner DOT3 brake fluid. If the tranny shifts well after that then the clutches are gone. If not then change the tranny oil and might be oil pressure related problem. But get a reman one from Mr. Goodwrench cost you a little more thant rebuilding yours and this has 50k miles waranty

[roaddirt]

Thanks kilroypr for all your help.

Tell me what do you think about this:
I recently bought OBD-II PC adapter and software for laptop from Car Code. It is nice piece of diagnostic and can monitor some parameters like O2, TPS, FuelTerms, RPM, VSS, MAP, MAF, CTS, Spark Advance, etc.
First time when I tried it I did not know how to translate data log and get some useful info.

I gave it try yesterday again. Whenever I noticed hesitation problem, computer showed significant Spark Advance drop (retard), despite TPS was 40-50 % opened and RPM 2500-3000.

Normally, in idle and low RPM timing showed 20º which is I believe right. It would advance spark at 3000 RPM up to 32º which again should be expected but when car hesitated and had poor acceleration, it showed retard to 10º-15º at 2600-2700 RPM.
This retard spots corresponded to speeds around 35 MPH (3rd gear shift point) and 45 MPH (4th gear shift point) though I noticed such a retard at 60 MPH as well.
No such a retard at speeds from 0-35 MPH and lower gears where I do not notice the problem.
Now that was weird isn’t it?

Any thoughts anybody?