Acceleration problem - is it so tough?

[roaddirt]

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
Swapped vacuum modulator with aftermarket part – 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

[roaddirt]

I know it is tough, but there is no dumb sugggestion in my situation so please fell free to post any suggestion.
I replaced (again) trany filter and fluid with no immprovement. Reason is you can never be sure if a shop replaced filter and I wanted to eliminate plugged filter as a cause.

[michelq]

you are making thing hard to answer. try to hook up fuel pressure gauge and test drive and monitor fuel pressure.
see if you can check the exaust system.
roaddirt.i do not what to say

[roaddirt]

I myself do not know what to think. Every time I read something I start suspecting something else and try to test it but some things like checking if catalitic coverter is plugged or driving the car with fuel pressure gauge connected are beyond my knowledge.
How for God's sake I can drive with fuel pressure gauge connected in engine compartment and be able to monitor it? Hose is one foot long and Schreder port is half of an inch from running serpentine belt.
And I do not have scanner that could monitor fuel preasure readings either.

[RABarrett]

Food for thought...it is possible that the problem is normal. With the usage of overdrive and the lock up convertor, it is possible for the engine to be unable to respond well until the trans downshift or the convertor disengages. I am not sure that you are not trying to fix a normal condition. Ray

[roaddirt]

Well...I was thinking about that as well. I also have 1995 Olds Achieva with the same OD, 4T60E trany, and it does not do that. I has always driven it in OD and it runs "normal" like any other car I owned before.

I do not know if the 4T60E trany get some more electronics and more computer control since 1995, causing it to work different. I think have heard something about that though.

But again, this is something I never felt with any car before. If you do not press hard and downshift, the acceleration from cruising speeds above 35 MPH is so painfully slow that nobody would want to drive this car. I cannot believe somebody would design the car to run like this.

Did anybody heard anything about electronic control changes to 4T60E since 1995?

[Flatrater]

Every year there are updates done to the GM transmissions. Are you sure you have a 4T60E trans and not a 4T65E trans. GM has phased out the 4T60E trans in favor of the 65E since about 1996 or 97.

[roaddirt]

My User Manual says 4T60E. I also found info on Internet that Century 1994-2003 has 4T60E transmission.

"The 3100 V-6 engine is mated to a 4T60E four-speed automatic overdrive transmission, with an electronically controlled capacity clutch. It provides nearly imperceptible upshifts and downshifts and computer-controlled torque converter lockup for increased fuel economy."

[public]

Hey guys I am new here. Looking for answers to the same problem on my 98 Century. I will tell what I have so far 3 weeks with a good mechanic and he has no answer. I checked wiring to and replaced knock sensor. No help. Changed modulator valve $12, No help. My scantool showed EGR not ready or failed. I checked the EGR per instruction in Haynes and the EGR valve showes good but the harness plug did not. I left the EGR unpluged and the car is running great and the scan tool showes ERG passed???? Scantool also showes 4 EGR codes for failed circuit??? According to Haynes I need to check my harness and PCM next weekend. Hope this helps.

[roaddirt]

So you are having the same acceleration/hesitation problem and after disconnecting EGR the car runs well, right?
I would like to try the same test. I have no codes set, but I think that problem with EGR system half of times will not trigger any PCM codes.
Is it safe for engine to test run the car with unplagged EGR?

[public]

I am not aware of any problems with running the car short term with the EGR disconnected. The computer seems to make the required timming changes, so no knocking or pinging is noted. Long term it may affect the catalytic converter. Unplugging the EGR will set 4 codes and the service engine light will come on, but on my car results were instant. Autozone and some of the other auto parts stores will read and erase code for you if you need to turn the service engine light off after testing.

[roaddirt]

The other day I disconnected EGR and test drove the car for 15-20 min.
No change, car behaved like before. (This test set up SES light and threw a couple of codes that I all cleared with my OBDII laptop after reconnecting valve.)

Also, OBDII scan showed that EGR valve was opening properly under load. Commended EGR percentage was equal to actual (executed) which made me believe that my EGR valve was OK.

I wanted to check the air passage underneath of EGR valve for possible carbon blockage so I removed EGR and checked. No visible carbon deposits or blockage. EGR valve gasket has to be replaced if you remove valve.

Conclusion: EGR valve dose not seem to be cause of the problem.

[roaddirt]

Well, I have taken my Buick to another Buick dealer, again.
The car was well warmed up when Service Chancellor took it for drive with me on passenger side. As soon as we reached cruising speed around 35-40 mph, the RPM dropped down and the car exhibited famous “poor acceleration”.
He asked me if this is my problem and I confirmed. He said that was OK, the car behaved like it was supposed to. I was stunned.
He technically said “You need to keep in mind that Buick is a luxury car designed for smooth cruising, comfort and better fuel economy. Let’s say you want to cruise at 35 mph. As soon as it passes 35 MPH, first time you set up steady speed it will always switch to 4th gear and lock up torque converter, even if the current speed more suites 3rd gear. Being in 4th gear at 35-40mph will not give you any quick acceleration or passing power. What you need is to disengage “torque lock up” with any known technique like releasing and pressing pedal deeper, touching brake pedal or pressing harder to downshift. He said that PCM is not even so smart to disengage torque lock up when the car is going uphill and the car start bogging down. You need to press harder to downshift or even choose lower gear manually. WOW.

It seemed to me as a lot of leg shifting and science for an automatic trany. According to him the car is OK, it is design. My only problem is that I own a Buick Century.

My understanding was that other GM products like, let’s say Pontiac Grand Prix, will behave quite differently with the same engine and seemingly the same transmission due to sportier design targeted to younger people.

Comments are very welcome.

[bignoisey]

BAH! I think you got a bunch of double talk from someone who doesn't know what to advise.
I have a 2000 century and it doesn't do that. Maybe the lockup is occuring too early for you, but both Buicks ('86 & '00) I've had come out of lockup when you depress the accelerator slightly. Maybe you're going into O'drive too early. If you drive it in 3rd instead of OD does it work better? My transmission shop tech said the solenoud that drives the lockup will stick sometimes.

[bignoisey]

PS: My 2000 century has a 4T65E - Are you sure what you have there?