3.1GM Engine goes "Chug...Chug...Chug" at idle &makes engine leap up and down a bit

I had described this problem in another post concerning removing the gas tank to check out the evap cannister and solenoid valves due to this chugging issue. But it really needs to be in a post directly concerned with my chugging issue itself. It only chugs at idle, accelerates and cruises ok. But at steady slow speeds like 25 mph for example, sometimes it jerks a bit as if I had a tranny issue. No constant trouble codes, although not long ago I got the vague code 0300. Evap system was ok. I replaced idle air control valve.Some plugs & wires had conductiviy issues so I replaced and siliconed them all. A 100 dollar diagnostic at a shop showed only the random misfire. Compression in cylinders showed 150-158 with normal pump up speed. No vacuum leaks. Shop told me to use fuel system cleaner which I did 3 times. I also added 5 oz. of Seafoam to the crankcase. EGR 7 PCV systems OK. I cleaned the MAF sensor with alcohol. It runs so good at high speeds that I didn’t suspect a clogged filter or exhaust component. This chugging is described with that cursed word intermittent. Sometime when not chugging, it has a rough idle.I had replaced the fuel pressure regulator a few years ago as it had leaked out from somewhere. I sucked on the current one’s vacuum nipple but it held vacuum against my tongue, although I was told that it could still leak at a higher amount of vacuum suction. I just made a trip in the car over the road for 4 hours 200 miles with the Seafoam in the oil with no improvement. Nothing I’ve done to it this summer has helped one bit, I still can’t have this malfunction identified. If it were the crankshaft position sensor, wouldn’t it also happen at speeds other than idle? Might the MAP sensor be bad without a trouble code? How would it be checked out? Again, no vacuum leaks found anywhere. No plugs off location. Of course it could be a bad wiring connection. Or trouble in the PCM.I welcome all insight, tips, suggestions, similiar experience comments, etc.

That’s where I would start. Sounds like the shop didn’t get very far with tracing down the misfire. P0300 a code for random cylinder misfire. I would want to see which cylinder is misfiring and how often. GM cars of that era often won’t set a code for a specific cylinder misfiring, that information would be found in the data section. You will find current and history misfires for each cylinder there. I also would want to see fuel control data, specifically short term and long term fuel trim when the idle is irregular.

MAP sensor reading out of range could cause this issue, but that’s unlikely. I would like to see the MAP reading (in volts) and EGR position sensor while the issue is happening.

Your car has 3 coil packs, it’s possible one of them is weak, I would also have started with basic ignition tests. If the car accelerates well and has plenty of power on wide open throttle, we can eliminate fuel supply issues like regulator or fuel pump.

I understand how difficult it is to get a proper engine performance diagnostic done these days. Too many places just scan for fault codes and don’t have an understanding of engine management and don’t have professional level scan tools or understand how to use the tools they have. On the other hand I wouldn’t diagnose an engine performance issue for $100. Last year the shop I ran charged $149 for a basic check engine light diag, with approval to go up to $250 if needed. And prices have gone up since then.

Ask your friends and coworkers for a referral to a good independent shop, and when you find one ask if they have an engine performance diag specialist on staff.

Maybe that random misfire is caused by the crank sensor connector? I seem to remember some complaints along that line and with the 300 code just maybe. It could be that the misfire occurs at elevated RPMs and is being covered up due to the subtlety. The compression is decent for the age and miles? but as compression drops a miss at elevated RPMs is more easily covered up.

Just curious, but how are you checking for vacuum leaks? Eyeballing and hose wiggling is only good up to a point as a leak can exist in hidden places including inside the dashboard and which might vary according to the position of the climate controls which operate various vacuum pods.
My opinion is that a vacuum gauge (15 or so? at Harbor Freight) is a lifesaver for issues like this and will work fine.

A vacuum leak below the throttle plate generally means a rough idle and an EGR sticking open to some degree is also a vacuum leak or sorts. Hope that random guessing helps… :slight_smile:

I seem to recall years ago on my 3800 when they replaced the crank sensor said oil in the connector was a common problem. But yeah getting a guy with the time,experience, and equipment for a good diagnosis is not cheap.

Thanks for these replies, gentlemen. Yet my chugging & rough idle only occurs with low rpm rather than when rpm’s increase. I plan to look into a CKP sensor issue. But would it only have it’s oil fouling ground fault issue at idle, wouldn’t it also occur under acceleration?

MAP sensor is pretty easy to check as long as you can get at it. It should register a voltage corresponding to about 15 psi with engine off. That’s the normal atmospheric pressure. Engine off, it acts like a barometer. Engine on, intake manifold vacuum is around -20 in Hg, or about -10 psi, so it should register a voltage corresponding to about 5 psi at idle. MAP is pretty robust, unlikely to be the cause. Fuel pressure problem, misfires, or unmetered air entering engine much more likely.

I have attached an audio file of what my engine sounds like when it is chugging normally. But sometimes the chugging is more strained and has stalled a few times when it was under load hauling my boat in town in stop & go traffic. That has been rare though. A lot of times when starting cold, it will not chug, but idle rough at first. Infrequently it will settle down to almost acceptable idle but that’s not often. I haven’t caught it in the act of any other degree of chugging, when recording, though. Yet this should give people the sound of the chugging even though at times it is more intense, & strained.

No experience you car’s GM 3.1 engine. How is it configured? v6? Does it use variable valve timing? Does it use a cylinder shut-off mode for better mpg? As I recall, the compression tests “ok” in all cylinders, right? Any important engine function parts been replaced recently? Any repair work done just prior to noticing this problem? The chug chug symptom is often related to a serious engine problem of some sort. Not saying “serious” necessarily means “expensive to repair”, but something inside the engine is definitely amiss, so should be fairly easy for a shop to discover the cause.



This is a pretty basic V-6 :smile:

Does your intake gasket look like this?

1 Like

I don’t think my intake gasket looks like that. A shop replaced it a couple years ago due to the chronic issue of coolant leaking out under the left side of the intake manifold

Leaking intake gaskets can make the motor run poorly if coolant is being sucked into intake too. So, we can assume your newer gaskets are good. Pulling the intake is a pain. Did you change plugs with intake off? How old are they? And plug wires?

I think it is finally fixed! I had checked the EGR valve as best as I could, all seemed ok and no EGR trouble codes. Then I gave up and turned myself in to my local repair shop, where it had been inconclusively diagnosed for $100. He did some vacuum checking and guessing, tried a new MAF sensor which made no improvement. He didn’t charge me for that. He found a vacuum leak in the intake manifold and it seemed to him to be a leaking EGR valve diaphragm. He put in an OEM new one and it is now running normal- “so far”. I had replaced the EGR valve 2-1/2 years ago with an off brand online, it cost about $14. The OEM one was $266.80. Total parts & labor was $584.73. But at least it seems fixed, finally. Of course, professor hindsight says that if I would have done the guessing that I needed a new EGR valve and replaced it myself, even with an OEM from the dealer, it would have cost much much less…

I had changed the plugs and wires about a month ago.

I’m glad to hear that things are looking good!

I’ve had the experience that sometimes it’s best to avoid the cheapest parts . . . either because they simply don’t work at all, or they don’t last very long before failing

Suggest to include in your shop’s routine tune-up procedures to verify EGR valve actuation will stall an idling engine. That tests the EGR diaphragm integrity. I’ve done that for many years, whenever doing a tune-up of my vehicles, Rabbit, Corolla, truck.