I got one for you.... 1994 3100 engine stumble

[DTMAce]

Ok.

Here is the deal.

The past week, I have been noticing an increasing amount of engine stumble, particularly at idle. So bad in fact that I have killed it a couple of times while doing a stop-turn or even coming to a stop at say a light or sign. Also it is very hard to start when its doing this, I even have to hit the gas a bit to get it to start and go. I have had it die so hard once that it even reset the clock and settings on my radio. That was another weird item of note.

But the funny part is, it doesn't do it all the time. It can be rough idle at one point, and smooth and normal later. Heat doesn't seem to make a difference, so it does this cold or warm.

Next, there have been NO engine codes at all. No light, nothing.

WOT runs feel perfectly normal, all the power is there, its smooth, but as soon as I drive normal, use the cruise even you can feel it stumbling a bit. Almost like the SFI mode is having trouble. Exhaust stinks strong while its stumbling, heavy rotten egg smell, more so than normal. It literally acts like its misfiring. No wires seem loose anywhere, and if I manually hit the throttle at the TB, it smooths right out, till I hold it at an RPM, then it may not always stumble there, unless I throttle it more. Just seems that under acceleration its fine, but other times it stumbles. Then I can stop someplace and its fine, starts normal, then later on its back again.

My thoughts at this point:

It acts like it has a loose connection of some kind.
Spark wires possibly? They have been on there a good while, probably 100k, but wouldn't that still show up, even under WOT?
Issue with the coils maybe? I could reseat the bolts that hold it down, maybe there is a grounding issue?
Bad plug, but they were just installed at 112k, so nearly 20k on these now. Stock OEM replacements.

Any suggestions from the peanut gallery?

[Barry]

Coil packs. I had the same issue with my 95. Drive on the highway at around 2000 rpms and wait for the converter to lock up. You will probably feel it misfiring for sure at that point.

[Barry]

Also you can spray different components with water and see if anything changes. Look in your engine bay at night and look for sparks (literally). I had 2 plug wires arcing between each other on a 3.1. Only at idle for some reason.

[DTMAce]

2000 RPM means I have to run 70... LOL

But no, even at around 1500 at 55 with the cruise I can feel it stumble. So confident its the same symptom.

Next question, does anyone have a spare 3100 series coil pack laying around that's good? lol

[heavywoody]

I've got some I can send your way. PM me your address.

[DTMAce]

Let me check a local source first Woody, and see if the guy still has one. If not, I will get back with you. I forgot about him earlier. Same guy I bought the base from.

[heavywoody]

Ok, I've got 3 of them I believe, might even have an ICM.

[Cliff8928]

Same guy I bought the base from.

The coils on the base are the same as well.. Just that there's only 2 of them, not 3.

[Barry]

Yeah I was gonna say that I think the 4 banger dis is the same.

[DTMAce]

Woody, go ahead and send me that coil pack, and maybe for giggles the ICM too. Let me know what you need for them or at least the shipping costs, we can work something out. Will send you a PM with the address shortly. And I don't want to chance with the ones off the 4 banger. Especially since someone had played with the spark plugs and wires on it already. lol

[Money pit Beretta]

What kind of plugs? The old style stock replacements were only good for 5K.

[3X00-Modified]

I'm gonna say fuel pump lol

[SuzukiGhostRider]

I seem to recall you have alot of charging issues with this car. I'd look into alternator connections, ground wires, etc.

The clock and radio resetting kind of also makes this a dead giveaway.

[DTMAce]

What kind of plugs? The old style stock replacements were only good for 5K.

Actually they were good for 30 something k. Where did you hear 5k? That sounds a bit off. But I usually got near 100k out of the stock OEMs that it originally came with, that is what I put in. I installed the last set at 212,500. The previous set was installed at around 140k and was the same set that I put the last set of cables in. The previous set to that was done at around 30 something k, only because that was their supposed interval. And at that mileage they looked new. Hell the last set I pulled out still looked nice, other than the obvious discoloring from normal use.

So I highly doubt its plugs, for two reasons:

One, they have low mileage.
Two, the misfire (or appearance of) does not happen at WOT (for example, getting on it hard up to like 80+).

Same goes for me doubting its the spark wiring, I would be getting random misfires any time, not just at idle or set cruise speeds if there was an issue with the wiring. I have had bad wires before, and it acted differently than this.

I'm gonna say fuel pump lol

You would. LOL

But if the fuel pressure was weak, then WOT would actually run worse, as it would need more fuel to run WOT, vs idling. At least that's my thinking.

I seem to recall you have alot of charging issues with this car. I'd look into alternator connections, ground wires, etc.

The clock and radio resetting kind of also makes this a dead giveaway.

I tend to agree with you. But I have gone over all those connections now twice. They seem to be solid and tight. I can't find any real problems with the main connections, even reseated them, reseated the connections on the coils, loosened and re-tightened the coil mounting bolts, no change.

Its acting as if SPFI mode is causing the stumble, whereas when you give it a lot of throttle and make the computer switch to MPFI mode it works fine. That doesn't make sense, but that's what it acts like. Runs smooth as ever, no issues with performance or power as long as I'm hard in it. But when its idling or cruising, it can stumble.

The other odd thing, is that it will tend to go away at times and be just fine. But when its acting up, it can be hard to start even. This also tells me that it sounds like a loose connection someplace. I hope to have a bit of time tomorrow, I will pull the coil pack and ohm them, see what I get..

All I know is, it has never done this before, even with some of the other things I have had happen over the past couple years. The alt thing has been going on forever. This has just been in the past week.

Thanks for the suggestions.

[DTMAce]

A further thought on the radio getting reset. What if whatever is causing this, is somehow causing a short in the electrical system at the same time. Could a bad coil cause a really hard drain on your electrical system? One other thing I have noticed for many years, is the harder I get on it, the more it drops the voltage. I figure that to be normal, but what would be the expected amount of draw the coil packs should have on your electrical system? Obviously at WOT its going to draw a lot more than say idle or cruise speed. Anyway something isn't making sense with this, as I have a hard time finding a consistency, other than in how it acts. Ah well.