1995 Park Avenue - missing; voltage fluctuation

Maintenance/Repairs
1

Hello all!

Got a strange one here. 1995 Buick Park Avenue, supercharged 3800, 159K miles… an older car, but an unmolested one. A little over a week ago I began to notice an occasional miss when cruising along at highway speeds, along with a slightly rougher idle. Both the fuel filter and the plugs/wires predated my ownership of the car (a little more than 6 months); so seeing no obvious culprit, I replaced them and hoped for improvement.

Unfortunately, neither had any impact. The missing continued to become more noticeable. It was never present under acceleration. Most of it was like a split-second long version of “bucking” that could be observed while cruising at lower (<2000) RPMs. It was also very noticeable at idle. Eventually a code was set: P0341, camshaft sensor issue.

If the story had ended there, it would have been just another night in the garage. But it didn’t.

As I was noticing the aforementioned symptom increase, I also noticed my voltmeter would occasionally “twitch” in the downward direction. Eventually I had an incident where I came to a stoplight… transmission in Drive, foot on brake, gauge dropped to around 10-11v. It returned to 14v momentarily, but dropped back down, held there for several seconds, returned, etc. No particular rhyme or rhythm. Of course my first guess was that the alternator was failing; a separate problem from the other.

I checked all the relevant connections (at the battery, at the alternator, etc), and cleaned them for good measure. All were tight, relatively clean, and the cables showed no resistance on the ohmmeter. It happened that I had another correct alternator on hand (used/working pull), so I swapped it in. No change. I reinstalled the original and was back where I started.

On my most recent drive, I noticed both issues had become very noticeable. The drop in voltage was happening at nearly every stop, and the missing was happening a few times per minute while coasting along. There seemed to be some correlation between the missing and voltage fluctuating, but it wasn’t absolute.

I decided to attack the voltage issue again. Re-checked all the cables; same result as before (all OK). Looked all over the car for possible bad grounds, loose connections, or other visible problems, but found none. My next thought was to run a jumper from B+ to the voltage regulator’s Sense input (have seen issues there on more than a few older vehicles) - but after reading that this car uses the PCM as the voltage regulator, I didn’t proceed.

Just to be certain I’m going to have the alternator tested by the local auto parts store. But I suspect it’ll pass.

So, basically, I’m stumped. I’m fairly sure these two problems have to be somehow connected, but just can’t see it - and I don’t want to start blindly throwing parts at it. Your thoughts?

–Keith

2

The erratic voltage is causing the misfires because the computer isn’t receiving a steady voltage supply.

When you checked the battery cables, did you peel back the red rubber cover to expose the positive terminals?

On the GM side mount terminal batteries, corrosion can form under the cover and leech down the inside of the positive battery cables causing poor connections.

Replacing the positive battery cable assembly is the best solution when corrosion is found under the red rubber cover.

Tester

3

“did you peel back the red rubber cover to expose the positive terminals?” I did (when inspecting/testing/cleaning the cables). No corrosion to be found.

Also just got back from having the alternator tested. Though I didn’t get to see any numbers, I did see PASS appear on the screen of the auto parts store’s test rig.

On the way home I stopped at the GM dealership, told one of their service guys the same story as above, and asked what he thought. His guess was pretty much the opposite of what I suspected, and what Tester said: he suggested I ignore the alternator issue for now and try to resolve the camshaft sensor issue first. The rationale was that it was most likely causing the misfire, and that the misfire was in turn causing the PCM to not regulate the alternator properly.

He also pointed to a bad PCM as a possible second step. That’s been in the back of my mind ever since reading about how voltage regulation on this car supposedly works, but I’m hoping not to reach that point.

While I had the alternator out, I decided to try a very simple test. This car has two drive belts; one runs the alternator and power steering pump, the other runs everything else. So I left the alternator/PS belt off and started the car. Sure enough, the miss was still there.

Having done that, I’m thinking it’s time to whip out the scantool and see if I can narrow down potential causes for the misfire. I think there might also be a self-serve junkyard camshaft position sensor in my future.

4

Here’s some food for thought . . . you had the alternator tested out of the car, apparently

How about reinstalling it, and having the charging system tested, with everything hooked up?

It’s possible the alternator tests fine on the bench, but the charging system still has a problem

I recommend against a junkyard cam sensor

For electrical parts like that, I recommend a new part. I don’t even think it’s that expensive.

Plus, I don’t think it’s even worth your time to drive to the junkyard, spend time removing a used sensor, driving back home, etc.

I’d go to pick a part for a used fender, but not a used cam sensor

5

Don’t buy used parts. You never know their condition. How old is the battery? I would have the charging system tested. Check to make sure there are no spots where the battery cable is bare and shorting out.

6

If you have a cam sensor code, I would probably head after that first myself. These things are notorious for the magnet coming off the cam gear which will cause cam sensor codes and also bad running.

7

Thanks for all the responses!

I know used electrical parts are a gamble (at best). But in this case, between having some “store credit” left from a previous trip and having to be in the area anyways, I figured it would be a cheap way to possibly narrow down the problem.

In 15 minutes I pulled a cam sensor, an ignition control module, and a PCM - the three easily removed parts I suspect most, in that order. No cash spent. The cam sensor was free, since not listed on the price board; and the other two items can be returned for “store credit” or exchanged if I choose to. Hard to knock a deal like that.

I’ll be spending some time tonight in hopes that I can make the miss, and the cam sensor code, go away. Will wait to see how things look if/when I reach that point.

Here’s hoping I don’t soon have to wonder “where’s that magnet?”.

8

Okay, time for closing remarks. Lucky me!

After triple-checking the wiring, I moved on to the camshaft position sensor. Removed the sensor and verified that the magnet was present (it was). Replaced the sensor with the new-to-me one; no change.

Since the cam sensor signal is supposed to run through the ignition control module, I went after that next. Removed the original, cleaned everything, and installed the new one. Again, no change.

Lastly, I popped out the PCM, swapped my original PROM into the junkyard unit, and hooked the mutt-ified computer up to the car. It idled funny while the initial relearn took place, so I couldn’t really tell if there was any improvement. After a couple minutes of semi-rough idling (but no codes or voltage swings) I took it for a drive to town and back - total 10ish miles. No traffic, one stoplight… a perfect environment to see what it would do.

It may be too early to declare total victory here, but at least I can say that the symptoms did not reappear during the course of the test drive. The replacement PCM may have solved BOTH my issues.

Seems I’m not the first one to have such results: http://rivperformance.editboard.com/t11132-pcm-ecm-ecu-computer-bad-on-1995-sc-riviera

Interestingly, the one that had been in my car was a Delphi replacement unit, while the one I got at the junkyard (from a rear-ended but otherwise immaculate '94 Bonneville SSEi) looked to be an original.

Thanks for all the responses! Always good to have more than one set of eyes on a problem.

9

Having put close to a million miles on 3800’s, I’d be having a good look at the crank sensor, then the cam sensor. While doing that, make sure the crank balancer is in good condition and not wobbling. You swapped alternators and still the same result so mathematically long odds for being the alternator.