Better Title: Rough Idle Bogging Down Relay Sound No CEL Alternator Overheating
I have reformatted this post for clarity and posted a video to Youtube which captures the symptoms. It is not possible to detect the thumping sound on the video so I would refer to it as more of a chugging. Listen to the higher pitched engine whurring to hear the off and on straining.
History: 2002 Pontiac Grand Prix GT 3800 Series II with 230,000Km (143,000Mi). No CEL. LIM gaskets, catalytic converter, fuel pressure regulator, MAF sensor, alternator, and battery where all changed which fixed various issues. IAC, TPS, MAP, ECT have been changed with little noticed impact.
Expanded History (skip if not interested): Catalytic converter was replaced and engine was noticeably relieved, regained power. Fuel pressure regulator was changed. Used to have weak starts, or will start and immediately die, or require a foot on the throttle to stay on. Fuel pressure gauge needle was vibrating in a range of 2-3psi and is now dead steady at ~45PSI IIRC. I was also having a surging idle which would oscillate between below 500rpm/dying to 1300+RPM and carry on for 10-20 seconds before settling down. Pumping the brakes over and over rapidly would reliably trigger this effect. Unable to reproduce since the fuel pressure regulator was changed. I should also note here that at one point the brake booster failed to assist but it only ever happened once. The Mass Air Flow (MAF) sensor has also been replaced fixing an issue where the car would violently buck or jerk with increasing severity as the throttle is opened further. The following components have also been changed with little noticeable effect: IAC, TPS, MAP, ECT. I have cleaned and re-tapped the battery to chassis mount point which fixed another issue I was having where I would put the key in the on position, everything looks fine, and as soon as the starter relay is triggered, the entire electrical system would black out. This happened to varying degrees with weak starts and dimming lights at best.
Current Symptoms: Idle is rough and getting bogged down approximately once every second. Sometimes the drain carries the rpm very low where the instrument panel is seen to dim and relay noises can be heard. Alternator recently shattered a bearing after idling for 20 minutes and turning on the rear defrost (it was hot, put it over the edge). It is not really possible to tell in the video, but a faint thumping, or chugging can be heard in the engine sound which corresponds exactly with the bumping/bogging down of the rpm. The engine strain can clearly be heard off and on in the higher pitched engine noises throughout most of the video.
Recent Work Performed: New Battery and Alternator. Cleaned the following with dremel and greased (dielectric) the following: Battery terminals, battery to chassis (upgraded bolt), engine block ground point, engine to chassis ground point at driver strut base, engine compartment fuse box positive link, starter motor positive link.
Video Clip Time Index:
0:00 Cold start
0:35 Rough idle dropping
1:55 Unknown Rattle/Clunking
4:15 First major bogging heard, followed by surge (engine bay)
4:40 Engine can be heard straining again, followed by surge (engine bay)
5:10 Major engine bog, lights dim, relay noises heard (tachometer view)
5:50 Another engine bog, relay noises heard (dash view)
Description (extra info): The idle likes to sit around 700 or so once warmed up. What happens is that the engine makes kind of a faint thumping or chugging noise (maybe 4/sec equal timing) intermittently and with each thump the idle is pushed lower until the thumping stops and idle returns to steady. When the rpm is being taken down, the engine is making a more strained sound and the lights may appear to dim slightly. This bogging of the engine happens for a random number of thumps between 3 and maybe 10 if it really brings down the rpm (almost down to 500rpm). It happens consistently, bogging on and off maybe 3-4 times every 10 second period (engine spends about half its time bogging and half recovering or running steady). The "thumping" sounds to me like possibly a "normal" or strained engine noise but its coinciding exactly with the idle drop. I am not sure if this is related, but I had the car running for about 20 minutes and the alternator overheated. Until this point I had no extra load on the electrical system. I turned on the rear defrost and the heater to high and the lights dimmed, rpm dropped to 500 and recovered, lights came back to normal within a second. I turned the devices off and back on once more and heard something like glass shattering. It was a bearing in the alternator, which was scorching hot and now making a rattling noise (it still puts out 14.6V after cooling off).
Other Notes: I have checked and blocked vacuum passages for a leak, carb cleaner, etc. Couldn't find anything. I don't know if this is related or even a problem, but when I pull the throttle position open, there is what sounds like a high pressure air escape, followed by a delayed engine rev. It's maybe less than half second of PSSSSSt, then VRRRRRM. I have seen videos where people pull the throttle and response seems immediate. Thought I'd mention that. One last point, I used to get ~19MPG or more and its been under 14MPG, however, the car has been off the road for a few months since the alternators/electrical started going.
I chose to keep and repair this car because I want to learn about mechanics. I have traced and fixed many problems throughout the car to keep it on the road but this is really stumping me. I have been unable to find any post which described these problems and was hoping that experience might shed some light on some next steps. There was a fusible link from the starter connection point which I believe went to the ignition switch that I didn't check yet, as well, if there is a secondary fuse box in the dash I have not checked that. I am really hoping that the engine noise description will turn on a light bulb in someone's head and they can share their experience. I will try to get a video but the sound is quite subtle to pick up. Otherwise I am stuck trying to track down a short or resistance somewhere in the electrical system which I have no experience with, but am willing to do. First car forum post, go easy. Thanks for any help. More details can be provided.