Thread: Engine makes faint intrmitnt thumping - pulls down idle (rough), alternator overheats

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.

Do you have a scanner that could watch fuel trims?

That would be the best way to see if a vacuum leak is present.

Perhaps even undo the oil cap while running, see if it sucks your hand in.

Have done a bit of research. If the plastic coolant and EGR channels in the intake manifold have been melted or breached by hot EGR gasses, the standard fix I guess would be to replace it. I found it for only $65 on rock auto which has a fix to prevent it from happening again. Another forum post about this. I know that coolant is carried to the TB, and EGR gasses are passed through, but how will this cause a vacuum leak (or the whistling sound I hear)? This could be related to the delayed throttle reaction with *PSSSSt* sound.

I have to confess that money is a constraint and I have already spent quite a bit getting it this far. Is there a way that I can test by blocking off the plenum passages? Or is there some way or telling for sure this is the issue, or perhaps a temporary repair just to see if everything runs well. Edit: I found a workaround fix that I might be able to try. If the alternator issue is totally separate, I need to know that I have a good plan in place to make all repairs with better known costs. Thanks.

I have checked over the underhood fuse box and the glove compartment fuse box and ensured that there is continuity on each fuse.

I have been operating under the assumption that the alternator is overheating because an excessive load or short is being placed on it. Is there another possible cause?

If no fuse has been blown, this means that all circuits that are being protected by these fuses are operating within standard current flow.

I believe the alternator is 110amp. It would take at least a couple of the big amp circuits in the underhood fuse box, or a majority of the large circuits in the glove compartment fuse box to consume the alternator's full capacity. Correct me if I'm wrong, just guessing here. This would point to the common areas to look at, like where multiple circuits share a ground mounting point. If one of these grounding points is damaged it may cause resistance in the circuit which acts like a load and takes a share of voltage. I think this can be tested by checking voltage drop of a live powered component, but I wouldn't know which to test, and if its even detectable outside of a severe engine bogging event.

I think a typical course of action might be to remove all unnecessary fuses, and start the car to see if a bad circuit has been eliminated. Most of the glove box fuses it look like they can go. Is there anything which is important to leave in while running the car? Many of the underhood ones seem more important. Here are some diagrams that seem to be accurate to the car.

2010-02-22_192745_cig.png

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Questions:
What are the possible causes of an overheating alternator?
Are there more ground points which I need to check? How do I find them?
What is the bare minimum set of required fuses to run the car?
Are there any other standard tests that can be done with a multimeter (10amp ammeter)?

I appreciate the views and help so far.

I have continued to pull it apart and test some things and I think I may have some new theories.

First, let me go over some of the recent tests and results:

* Pulled the upper intake off and inspected the gasket, and the coolant passages near the EGR.
The plenum gasket was replaced a couple years ago when doing the lower intake. It appears to be flattened in some of the coolant passage areas near the EGR. The mating surface to the lower intake, and to the throttle body appears to be somewhat warped, however, it doesn't appear to be bad enough to cause a gap. The actual integrity of the channels appears to be perfect, there are no holes or leaks in the coolant passages. Cleaned and polished mating surfaces and replaced plenum and throttle body gaskets. I cannot imagine a leak happening here.

* Tested amperage draw of car turned off, and turned on.
The battery has been sitting for a few days inactive and measured a 12.63Volts charge. With my 10A multimeter, I removed the negative post on the battery and bridged the gap with the meter in amp mode. Locks can be heard cycling, etc. Fluctuating around 6 amps for 10-15 secs maybe and then slowly settling down. I believe it was at 0.035 amps after about a minute where it remained. I purchased a fork type 100amp meter and used this to check the flow while the car was running. I detected 55 amp draw while the vehicle was running and this was fairly consistent, perhaps jumping around + or - 5 amps. I only ran the car for 5 minutes before the alternator was too hot to touch. Is 55 amps normal? Seems excessive since I read that charging the battery will take less than 10 amps.

* Tested coil packs (in progress)
I tested the resistance of all 3 coil packs at the main post which goes to the spark plugs. Two of them measured exactly 5.92 KOhm, and the third measured 5.81. I was unable to get the probes into the slots for the lower resistance coils and will have to figure that one out with some wire or something. I also plan to run the engine and pull and replace plugs one at a time to see if the intermittent engine bogging becomes steady. (EDIT: I read that this was a great idea. However, the hard way taught me that it wasn't so much. In the time it took me to panic, I saw maybe 10 arcs of lightning hit in all different directions with bolts of what seemed like at most 2 inches one of which used my conveniently placed finger and welding glove to find its way to ground. Didn't hurt too much but the remaining five plugs could be summarized as: NOPE. The engine took it really bad and I think would have died had I not turned the ignition off.)

I have a theory which could explain the mode of failure. Bad grounding (it was very corroded in many major points) may have caused damage to the ICM or coil packs. I am not sure how this ties into the 55 amp draw and overheating alternator as I have fixed the ground points. It is possibly just a bad alternator throwing me for a loop.

Questions:
Is 55 amps too much for a typical running vehicle? If so next step is to use the ammeter to isolate which circuit is loaded up.
Is a .11KOhm resistance typical deviation, or could indicate a problem?

Latest tests:

Current draw into main use box from alternator 35-40Amps (cant say why its not reading 55 any more).
Current on link from main fuse box daisy chain to battery 2-3Amps
Voltage at alternator output 14.4V
Voltage at main fuse box 14.25V
Battery at ignition off 12.63V

Coil packs. Main posts 6.09kohm, 6.2kohm, and 6.0kohm (spec 5k-8k). Other coils 0.4ohm, 0.4ohm and 0.4ohm. (spec 0.5 – 0.9) IS THIS BAD? Every time I try measure it starts at 0.0 ohm and counts up 0.1, 0.2, 0.3, and 0.4 over course of a few seconds. Sometimes I dont think I get a good connection and it blasts up to 1.9ohm and back down kind of going all over and settling down below 1 ohm)

Putting the ammeter above the IGN relay, I am showing about 15 amps, but it is not the proper way to use the meter and I don't know if its accurate. Shows the fuel pump at about 5-6 Amps, and the rest don't seem to be drawing much at all but it's difficult to get a proper reading.

I unplugged the MAF sensor and this causes the rpm to exaggerate the bogging cycle. Instead of bumping up and down within a range of less than 50 rpm (hardly noticeable), it drops and surges in a range of 200-300rpm consistently.

The voltage remains steady according to my digital multimeter. If it truly is steady, and not fluctuating too fast to be picked up, then I am assuming that the electrical system is supplying a clean signal and a power drain is not the direct cause of the bogging. Perhaps someone can shed some light on what the next steps might be. I am almost starting to think that live data will be 10x easier than scouring every last part for a problem... suggestions appreciated.

I'd be very interested as to why the alt is pushing so much power. Not sure where to go beyond that.. but hey.. someone had to post in your thread besides you.

Originally Posted by BillBoost37

I'd be very interested as to why the alt is pushing so much power. Not sure where to go beyond that.. but hey.. someone had to post in your thread besides you.

Lol, thanks! It is rated for 110 amps I believe, and according to my touchless ammeter has a load of only 30-40 amps on average (I don't know if this is a normal range). Would a bad ground increase the load on the alt? I am sure a short would, but a single circuit with a bad ground should cause issues on the circuit device itself, not the alt right? I am willing to accept that it's a faulty alternator, but my last one also fried I believe in a similar manner. I wonder if a damaged wire in the bundle that clips to the alt could cause this effect.

I could also accept that the O2 sensor is defective, but with this new tool in my arsenal (Torque), I hope to be able to narrow it down. I will see if I can monitor actual fuel trim values when I figure out how.

Your O2 sounds like it's functioning normally..

Sometimes.. it'll sit for a second, then bounce. All depends on the speed of the scan tool etc

I think you guys are going to love this...



I hope I am using the tool correctly. LTFT seemed to be showing positive up to 15% for the furst half of the video and then it was generally negative for the second half. Tried to show some throttle response here and there. When I went to Scan Data the values looked negative when LTFT graph was showing positive values. I don't know what this means... Let me know if I should be doing this test differently.

Quick note: I charged the battery to full and this seemed to cause other behavior. The Relay noises were happening more often, the idle seemed much higher for the first minute than before, possibly sounding worse overall.

Tested all fuel injectors at 13ohm exactly. Also I agree about the O2 sensor, its just reporting what it sees (crazy ****). I also believe that my exhaust headers may have a small but audible leak if this could be contributing.