Odd stumbling...

Hello everyone,

I know I don't post much, but I'm on this site every day! I lurk...a lot...

Anyway, I have an odd problem (at least to me) with my 2000 Bonneville SSEi. I posted this problem on the Bonneville forums but I wanted to see what you guys thought too. Copied from PontiacBonnevilleClub.com:

When the engine coolant gets over 100 degrees, in any gear, the car will intermittently drop 50-100 RPM and feel like it slightly stumbles. It does not, however, feel like the engine is stalling or trying to cut out. Twice, last week, on the way to the city and back (45 mile round trip) it seemed to 'cut out' for the slightest split second. Acceleration otherwise is nice and strong.

So far, I've checked fuel pressure (42 PSI with key on, 44 running) and it does not drop when the car 'stumbles'. I checked the coils and they're firing correctly and strong. I changed the spark plugs with AL104's and noticed no difference. I have not checked the plug wires yet - i have a set of 8mm AC/Delco's on the way since they're due anyhow.

My only thought is hm...torque converter? But it locks up in 3rd and 4th gear when it should, and smoothly every time. I doubt that's the problem. I really feel this is ignition related somehow, but I need a little bit of Bonneville guidance from the experts!

Thanks everyone!

-Andrew Z

Check the metal piece on poss connection for crack.

i bet you ten bucks its vacum related.Do you have a vacum tester?

Nope, but I do know that at idle my LTFT on my scanner is reading 16.4, which from what I remember, is 'lean' so the engine is adding fuel. Forgot to mention that. I remember reading in Bowers thread about vacuum issues. Problem is, I can't find any vacuum leaks.

How did you look for those vac leaks?

LTFT 16 - a vacuum leak is likely.

It is also possible you've got a bad injector somewhere that isn't doing well with small flow amounts. Will it stumble/spit at idle in park? If so you can go around to the injectors and unplug the wire one at a time, and see which one has the LEAST affect on your problem.

Sorry it took me so long to get back to this thread, guys. I appreciate the replies in the meantime!

I checked for vacuum leaks with throttle body cleaner. Didn't find a thing and I checked every seam, seal, and connection two or three times.

I have not tried the injector test yet, Derek. I will do that ASAP. That didnt even cross my mind.

Since I last posted, I had a thought. Perhaps the oxygen sensor is old? To my knowledge, it has never been changed. The reason I thought of the O2 sensor is because my old Riviera had this SAME problem. When warm, it would slightly stumble and studder a bit, occasionally accompanied by a P0171 code and lean fuel readings. Only difference is I don't have any codes set or pending.

Anyway, I changed the spark plug wires to 8mm tonight, no time to test if there was a difference. I bought a new Denso O2 sensor to install. I don't usually throw parts at cars to fix the problems, but these items are all due to be changed. I've been doing one at a time, and testing the car after each change.

I'll report back if the spark plugs do anything for the stumbling!

Take a look at the egr tube that goes from next to the thermo and up into the throttle body.Inspect carefully around the flares that tuck into the solid alu flange.

Thanks Robo...the EGR tube on the Series II is on the opposite side of the throttle body, but I definitely checked those connections too! Thanks!

did you really look closely at the tube flange? I cant tell you how many times I inspected it and never found it.Its a tiny little crack at the bend in that thin wall stainless thats allmost invisible. I guess it expands under heat and messes things up.

Ah ok thanks Robo. Didn't think of expansion. I will inspect it closely tomorrow when I change my O2 sensor.

Replaced the O2 sensor today, went pretty smoothly. I finally took off that stupid hoist bracket back there too.

The problem still exists, but I haven't had a chance to scan my LTFT's or look at the EGR tube. The car does idle WAY smoother, but it still likes to 'stumble' every once in a while. It's not as bad or frequent either, but it's still there. Might be on to something...

report back your LTFT's.

With the scanner hooked up pay special attention to your MAF readings under the stumble condition, you are looking for any "weird" dips or non-linear responses. The voltage the MAF reports is also useful. Other thing to check, since you said it was a bit lean, pull the vacuum line from your FPR and give it a smell. If you smell gas its bad and needs replacing.

...any way to get your hands on a scanner to check for intermittent failure of the crank or cam sensors?

Also, Check all ground connections while your loving her up! Mine were bad and causing mysterious cut outs as well.

I apologize I have not responded to this thread. I've been super busy. In the meantime,

The MAF readings, according to the scanner, are rock solid. The FPR is just fine, no gas smell in the vac lines there at all. I'll try to scan the LTFT tomorrow if I remember to take my scanner with me.

Albert - unfortunately I have no access to a scanner that sophisticated at this point, or I would.

Sam - the grounds all look good visually, but I haven't had time to inspect them properly.

I'll get back to you guys ASAP...thanks for the help in the mean time!

LTFT's are at 16.4 at idle still, like I expected, but immediately drop to around 0 when you accelerate, depending on how far you put your foot in the pedal.

I'm a little stumped, but I seem to remember reading around many months back and hearing this is not an uncommon problem - to be lean at idle.

just a thought on the stumbling - OT - my truck (8.1 V8) has an occasional stumble at idle and the forums I found for these engines attribute that to a wide spark plug gap. The factory spec is .060, apparently everyone who uses .060 complains about the stumbling, and re-gapping to .045 fixes it. I keep meaning to pull my plugs and re-gap them to .050 to see if it goes away.

Anyway, my point is, how are your spark plugs and what are they gapped to?