Surging/Studdering 1995
 

Here is the back ground: I have a 1995 Pontiac Bonneville SE with roughly 120,000 miles. Currently the car will studder, skip, whatever at a high load condition. Idle is less of a problem.

At the beginning of August I replaced both the upper intake manifold and the lower intake manifold. Even after those jobs it was still consuming a small amount of coolant (I now think it was/is leaking somewhere). Some time in the fall I also replaced the Crank Position Sensor and the Cam Position Sensor. I did this because the car would randomly die and not restart. Those two sensors took care of that, but it would instead of die, every once in awhile hesitate and the traction control light would light. I added a nice ground to the engine block and that was taken care of.

OK now to present day and the studdering. It started at 65 to 70 and light acceleration that didn't not allow the car to downshift. Car would kinda buck. The problem over time got worse and worse eventually it would happen at pretty much any load change. Would happen in any gear (eliminates tranny?)

I searched on here and thought that I had messed up the either the UIM or the LIM. This past weekend I re did both of those. The LIM and UIM looked fine, I don't really suspect those so much anymore. After I redid this job, car did wonderful for the first 15 miles, no sputter nothing, then at highway speed again at part throttle at 70 mph started to buck, but not so bad.

I am thinking now that something that I disconnected/jostled during the replacement of the LIM is causing the problem. So here is the list:

-Spark Plugs- I noticed on a couple of them there was rust where they clip to the coil. I am replacing them tomorrow. I'll report back.

-Fuel-Replaced the O-rings last time, not this time. What is the little cap at the front of the fuel Rail with a vacuum line on it?

-Throttle Body- It has 2 vacuum lines on it, one to fuel rail, the other to some emission looking thing. It also has 3 sensors on it, I think one is a throttle position senors, and I don't have any idea on the others.

-Mass Air Flow-?

Which of these is most likely? What test can I do to them?

Sorry this is long winded, but just trying to give you as many facts as possible (I am an engineer, can you tell?)


Thanks alot
TJ

Do you have a vacuum line running to your trans. With the engine running rev it a bit at the throttle body listen for whistles. Check out your PCV do that by shaking it if it rattles good if not, no good. Let's see what you're plugs look like and go from there. That cap you affectionly call is your fuel regulator. The 3 sensors are you MAF IAC and TPS. (reverse engineer those abbreviations) or cheat I'll have the answers at the end. The emission looking thing is your evap purge valve. so that's what you got so far.

Answers: MAF is mass air flow sensor, IAC is Idle Air Control TPS is Throttle Position Sensor.

Just checked the vacuum lines with some tubing on my ear, I did not hear any large vacuum leaks at the fuel pressure regulator or at the tranny. There might have been a faint leak at the joint where they come together and go into the throttle body. The same line that goes to the pressure regulator goes down to the tranny one. What does the tranny one do? The one on the regulator must change fuel pressure with engine load? There were definitely no vacuum leaks/whistling I could have heard without the tubing to my ear

PCV was fine when I had the UIM but I remember something wierd when I did it the first time like the new UIM looked different than the original in that area. Right now it is just a spring loaded cap on the UIM with the PCV under it. I thought maybe the orignal was different? Does the PCV cap need a vacuum line going to it? That doesn't make sense does it?

Ill change the wires tomorrow, but did plugs in August, so those seem unlikely. Maybe I will pull a couple and check them anyway though.

Those T connectors on your throttlebody tend to crack over time. Give those a good look over too.

Clean your MAF. Use Q-tips dipped in carb-choke cleaner. BE GENTLE. DO NOT BREAK THOSE LITTLE SENSORS.

Fuel filter? Didn't see it in your original post.

anyone have a source or part number for those T connectors? Or am I up to just piece it together from the "HELP" section

I tried looking on Autozone, I think the HELP section is your best bet.

Update:
I replaced both the plug wires and the vacuum hose line that goes to the tranny and the fuel pressure regulator. I did them all at the same time so I have no idea which one it was. What does the vacuum hose thing do at the transmission? My guess it was in those vacuum line because it was more prevalent at particular engine loads and speeds.

Thanks For the help
TJ

The vac line that goes to the trans deals with how the trans shifts.

could it make the trans act funny to a point where it acted like it was stuttering?

Your scenario sounds identical to mine. Incredible. I went through the same process with the sensors and everything.

Do you have a SES light sometimes at startup? I was getting a no 12x ignition pulse.

Mine ended up going away when I did my UIM and LIM, but I believe it was contributed to my ICM. I did a wire brush and scraped all of the surface underneath the coils to make sure I was getting good contact.

Plugs are a possibility too, but I didn't replace mine. I remove and cleaned mine but did not replace them.