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Are you 100% positive your throttle position is the same every time on the same hills?

I'm not sure I understand the question... Do you mean is it always in the same gear when it occurs? I'm not sure; will have to pay closer attention. I do know that it happens most often when speed is between 47 and 62 mph. Makes no difference whether cruising or accelerating, although if accelerating it eventually stops on its own when it gets enough gas. Otherwise I take my foot off the gas pedal completely for a few seconds, to get it to stop. It then stops immediately.

Will try to get that list of replaced tranny parts later tonight.

Ken_W

I mean are you pressing on the gas the same amount every time it chugs? And then if you press down more, trans down shifts, and then chugging is gone?

I still think the whole chugging on these cars has to do with the torque converter in lock up mode.

Not sure. Let me drive it some more, and pay closer attention to that, and then reply back here about it. If it helps at all, I can add that I usually drive at constant speeds to save gas, so there would be little-to-no gas pedal activity to trigger gear changes when going up a gentle hill. This chugging problem has occurred when the cruise control was in use, but has also occured without it. Your theory of it being shifter-related is a possibility. Need more time for observation though.

Ken_W

Cause on a slight up hill the torque converter is supposed to unlock to raise the rpm a bit. It locks on flat ground to help with MPGs.

Depressing the brake pedal slightly will unlock the torque converter. If you can get it to "chug" consistently up a hill, you might try applying the brake, but not enough to actually slow the car down. When the TCC disengages, resume up the hill normally and note whether the chugging has gone away.

Ken_W wrote:Some new information that may aid in diagnosis:

... it was raining and the chugging problem was back with a vengeance. It most certainly gets worse in the rain. ...

I had this problem a couple times and it turned out to be bad spark plug wires.
To confirm the problem I misted the wires using a spray bottle with water while engine was idling at night and looked for a blue corona.

New discovery...

Wires are fairly new AC Delcos, and do not seem to have any issues. I'm going to withdraw my statement about humidity or moisture, because closer observation has left me unsure if there is any connection.

I've driven for several days now, trying jedwards83's suggestion, with interesting results.

Most of the time when it chugs, the car is straining around 1500 RPM. When keeping my right foot steady on the gas and tapping the brake with my left, the car IMMEDIATELY downshifts and RPM's jump up to 2500 RPM and the chugging stops. I have observed this over and over.

So; it would appear that 01BonneSC's torque converter explanation is the correct one. What does this mean? Is my transmission going bad? Should I still bother pursuing ignition-related checks & fixes?

Ken_W

Ken, I've battled this as long as I've owned the car and while I have gotten some improvement by replacing ignition components (spark plugs had the biggest effect), it's still there, but subtle.

Like you, it only happens when the car is under load at a rather low RPM. Since the SSEi has a boost/vacuum gauge, I can avoid it by not letting the boost get past about 2 PSI at that low of an RPM. If I know I'm going to need more throttle to make it up a hill, I'll downshift manually.

I don't think GM should have been so aggressive with their downshift tables. I used to have a chart, but it takes a surprising amount of throttle for the ECU to disengage the TC lockup in 3rd or 4th. Hard to tell exactly why it does it, but my gut feeling is it's something in the tranny, though I'm confused as to why replacing ignition components has such a profound effect.

So it does not appear to be damaging anything or getting worse over time? I guess that would be my most immediate concern. If it's something I can simply live with while experimenting with countermeasures, I'll sleep a lot easier.

This must be a common issue, for 2000 SSEi's if not 01-03's. I wonder how many people have brought it up here, but had difficulty describing it. The various ways of describing the issue would muddy the waters and make it seem like many different issues.

When you say "manually downshift" do you mean with the shifter itself, or by giving it gas or brake to force a shift?

Ken_W

Hasn't ever gotten worse.. I live in southern Illinois which is flat as a pancake, so I don't encounter the chugging very often.

In fact the first time I noticed it was several months after buying the car, when I took it to Maine. "Chugging" became evident once the terrain became somewhat hilly east of Ohio and beyond.

By manually down-shifting, I mean dropping it into third with the shifter. I find that using throttle to prompt a downshift is too much power once it downshifts, especially with the level of mods the car now has.

There's an article on Tripledgeperformance that comes close to describing the symptoms, though on my car it's not quite as violent as the article would suggest. Seems to imply that this is in fact a widespread problem. http://www.tripleedgeperformance.com/TC ... ation.html

I think this has been the best thread in describing what happens. We need to remember this when other people chime in about the chugging.

Its been brought up multiple times during my time here since '07. And never have I once thought it was ign related since experiencing the PCS hard shifting and finding out about the annoyances of these transmissions, 2 mos after I got my Bonneville.

To further throw a wrench into this conversation...I just bought a 2000 ssei and I had low boost issues due to a sticking BCA shaft. I drive from Northern VA to Baltimore (140 miles round trip) three times per week. The engine only has about 25k miles on it, the car has 89k. When I was driving it with the low boost issue, it was smooth all the time. Last night I got the BCA shaft lubed and it's moving freely and now boosts up to 8psi at WOT. On my trip to Baltimore today, I noticed that at highway speeds, when I accelerate ever slightly and boost goes into the positive..I get that slight chugging. No change in RPM or MPH, like the gauges don't jump but you can definitely feel it. When my boost couldn't get into the positive on slow acceleration this never happened...so I'm really starting to think that it's Air/Fuel related after reading this thread. If you think about it, under negative or neutral boost the computer doesn't have to do anything drastic to keep the air/fuel mixture in tune. But under positive boost...if there were a vac leak (and I feel like this engine/SC combo rely heavily on vac lines) then the computer would be acting very quickly to get the correct ratio sorted out and a slight leak under positive boost would probably disrupt the air fuel mixture efficacy. When new ignition components are installed, I think this effect is diminished because the margin for error is smaller with new plugs, wires, coils, etc.

All of this is just theoretical as I haven't proven any of it yet, but I think what I'm going to do is order that VAC Harness and replace the rubber vac lines w/silicone and see if the chugging goes away. Make it as air tight as possible. It just seems odd that the chugging only happens during positive boost. No boost, no issues.

I doubt it is related to the fuel mixture. The pcm relies on various sensors to determine the correct injector pulse width for the desired fuel mixture. This has nothing to do with the ignition system components. Problems with the mixture would show up in the oxygen sensor feedback and ultimately the fuel trims. I haven't seen any evidence of this.

It just seems like when boost is applied it's going to act like a suction pump to any vac leaks that may be present...pulling in more air than the computer was expecting, causing it to adjust very quickly on the fly...I don't know, it just seemed like the engine was trying to figure itself out while mine was happening...almost like a 1 injector wasn't performing normally or something.

Boost would actually want to exit the intake manifold resulting in less air, not more. Your mixture would go pig rich and set a cel if bad enough under that scenario.