Must press accel hard to start off 97 Lumina 3.1

DTC codes P0420 (Cat below threshold) & P0751 (stuck trans solenoid) both point to possible problems for this symptom.

One muffler shop who uses infrared temp reading device says pre-cat temp is 300 deg & post-cat temp is 400 deg indicating original cat is fine.

Another muffler shop used a graphic (palm pilot) type device displaying voltage of both pre-cat & post-cat O2 sensors, neither very flat line, but both alternating up and down at idle and steady ~2200 rpms. They say the cat is bad.

Yet another technician says hard to get going from stop is more likely caused by P0751 and occassional P0753 indicators pointing to tranny solenoid/electrical tcc probs.

Once car gets going, everything thing else is fine.

Another possible indicator is as I take off from a stop, I see to always hear sirens (off from a distance), but realize there are none, i.e., a faint whine that sounds JUST LIKE an ambulance/emergency siren from a distance!

I ruled out checking the vacuum modulator right now cuz others in this forum report very different symptons, i.e., have to baby the accel pedal to re-engage gear after going into neutral, etc.

Also, the cruise control works intermittently (some days it does, others it doesn't), but vacuum lines all seems intact???

One final bit of insight, about 4 weeks ago, my son had the car out of town where he had more severe probs. He said there was a loud tappet noise for about 10-15 when car warmed up and also during that time he had trouble accelerating at all and tranny seemed to be slipping. Friend said to put in Lucas tranny fluid treatment and all seemed to clear up in a day or two after doing so.

I'm looking at about $225 to replace cat with direct fit. I haven't talked with the local, reasonable tranny shops about cost of repairing stuck solenoid, etc. in tranny--am a bit more afraid of that cost!!!

Any insight regarding these probs is greatly appreciated!

Thanks,
Scott

Wow. Too many things going on to figure it out, or the relation between them. One thing I can say with some certainty, don't spend any money on the cat. The temps quoted were fine, and if the cat is suspected of causing a performance/acceleration loss, a simple vacuum gauge test can confirm this decisively.

If you want, buy a vacuum gauge (great tool, everybody should have one), only about $20 and check it out yourself.

A good reading at idle is 18-21 in. vac. As you open the throttle, the reading will drop, as you just removed the restriction that vacuum was building against. Then, as engine speed picks up, the reading will come back up to a certain level and stabilize, provided the throttle is held at one position. Holding the throttle open to about 3000 rpm, watch the vacuum reading. It should stay steady. If it begins to drop slowly, an exhaust restriction is indicated (usually bad cat).

Before testing, verify that the intake path is clear (good filter, no bird nests or forgotten shop towels in the airbox, it happens;)) and the motor is at normal operating temp.

I don't like the sound you said you hear. A blown modulator or ruptured vacuum line will cause you to have to feather the throttle to get an upshift, but a slight vacuum leak that you may be hearing can play hell with upshift point. BUT, the noise would usually be most noticeable at idle, when vacuum is highest, and dissappear when the throttle is opened.

The alternatives aren't too good. Usually high whine like that in a tranny points to internal problems with valving, servo, accumulator, etc.

I wonder if the VSS could be at fault for the cruise issue and maybe the hard accel? I'm not real up on what kinds of problems they can cause, but it seems like something possible.

The TCC can cause an effect we used to call "trailerhitching" where it feels like you are pulling a huge load on acceleration, but the TCC won't lock up until you are over 35 mph or so, so it shouldn't be doing it from a stop. If it was, it would also stall the motor as you came to a stop, and it would be undriveable. I think for the purpose of figuring this out, you can disregard that as a cause, unless it is possible that the TCC is dragging, partially locked, and creating the whine you hear. I don't know if that is possible, but if so, the tranny wouldn't last long, being peppered with metallic debris from the dragging.

Did they clear all the codes? It might be useful to see which come back after a few days.

A good reading at idle is 18-21 in. vac. As you open the throttle, the reading will drop, as you just removed the restriction that vacuum was building against. Then, as engine speed picks up, the reading will come back up to a certain level and stabilize, provided the throttle is held at one position. Holding the throttle open to about 3000 rpm, watch the vacuum reading. It should stay steady. If it begins to drop slowly, an exhaust restriction is indicated (usually bad cat).

Thanks for you reply. Would I connect the vacuum gauge to the intake manifold to take these readings? I believe that's where the vacuum would be tested from, but wanted to double-check with you first. I know the tech who used that infrared device was also VERY confident the cat was fine.

I should have mentioned that I replaced the original pre-cat O2 sensor a year or two ago, but have never replaced the post-cat sensor because I thought all it did was tell you if the emissions coming out of cat were high or not, i.e., I didn't think it played a role in adjusting fuel/air mixture, BUT this tech said the computer DOES compare the post-cat info along with pre-cat info to determine how much fuel to tell the injectors to pass. I should probably replace that post-cat O2 sensor though, cuz tech said it does seem to running rich, again, from that infrared device. Amazing what things that tool can assist with diagnosing probs, if you know what to look for! My gas mileage has been poor, so maybe I'll replace that pricey booger, if the tranny issue can be resolved for a reasonable cost. The post-cat sensor could explain the P0420 code.

The alternatives aren't too good. Usually high whine like that in a tranny points to internal problems with valving, servo, accumulator, etc.

I wonder if the VSS could be at fault for the cruise issue and maybe the hard accel? I'm not real up on what kinds of problems they can cause, but it seems like something possible.

Did they clear all the codes? It might be useful to see which come back after a few days.

Yeah, I'm starting to think my main prob is tranny related the more I think these things through. I should have also mentioned that I still have the original tranny fluid (as recommended by GM Schedule B) and think that Lucas treatment conditioned seals, etc. but is likely only a temporary fix. And, EVERYONE says, don't change the tranny fluid/filter NOW because I'll lose the tranny within 10 miles of driving!!!

Regarding cruise control, what does VSS stand for? I can't find it on the web.

The codes were cleared (420/751/753) and 420 & 751 returned after driving about 20 miles.

Thanks again,
Scott

Vehicle Speed Sensor

Vehicle Speed Sensor

Thanks again Jeff. What about where to connect the vacuum gauge? Am I right in thinking I should connect it where it comes out of top of intake manifold?

Thanks again Jeff. What about where to connect the vacuum gauge? Am I right in thinking I should connect it where it comes out of top of intake manifold?

right. Any direct manifold vacuum source behind the throttle plate. Brake booster line is always good.