Sc400 Bucking At 40-45 Mph

[RICK323]

I'm really puzzled by a persistent problem with bucking at 40 to 45 MPH, especially when going at constant speed. If accelerating, it smooths out, or if you drop out of overdrive, the additional rpm's seem to smooth it out also. New fuel filter, fuel pump and computer installed first and a cleaning regimen performed on the injectors. Then new spark coils, caps and rotors. The mechanic says he tested the mass flow sensor and it's OK, so he thinks it's "the computer". Has anyone had this problem? The car has 145K and has run smoothly and well since we bought it over 5 years ago until just recently.

[JIBBBY]

Did you try cleaning the Throttle body and changing out your air filter?.... It almost sounds like an ECU problem maybe....

[johnsonhuang]

get a bottle of throttle body cleaner and clean that thing out. It could be because of your MAF sensors that are dirty and the ecu picks up weak signals which messes with the A/F ratio.

[john_sc400]

95 SC4. Same problem here. I even removed the throttle body to make sure I got all parts of it really clean. No difference. In addition to plugs and replacing some hoses to eliminate leaks, I also replaced the fuel filter and PCV valve.

Had it put on a computer at a dealer. They said that the O2 sensors were 'a little slow', but couldn't assure me that changing them (about $900) would fix it.

Another dealer told me that it was a typical issue with 95s. They said that 95 was the first year of drive by wire and they didn't get some things exactly right. They suggested replacing the TPS, (throttle position sensor), another expensive part. Again, they couldn't assure me that it would be a total fix.

I wonder about the EGR valve, but have not yet removed it to inspect it.

Along with the 40mph surge, I also get hesitation when accelerating normally from a dead stop. I don't get it under hard acceleration.

It makes me dislike this car.

[DILLA400]

i have the same problem as both of you.. im running a diagnostics on my car tomorrow so hopefully that will tell me something ill be sure to post if i find anything else out.

[john_sc400]

i have the same problem as both of you.. im running a diagnostics on my car tomorrow so hopefully that will tell me something ill be sure to post if i find anything else out.

Wondering how that came out...

[smooth1]

Could be a fuel supply issue, more than likely, fuel pump, or filter clog.

[DILLA400]

sorry for the slow reply, the diagnostic computer at my school hasn't been running correctly. Once thats fixed ill run a diagnostic again.