Intermittent miss at hot idle

[99Saturn]

Short version: 1995 Ford Aspire ~103K miles. Seems to have a miss from time to time at hot idle. It does not occur consistently, but when it is happening it is at a 6 to 10 second interval.

Further details: My girlfriend took possession of the car at 56,000 miles. Before that, it was her aunt's who basically decided she didn't want a car anymore (secretly I think she was fed up with this car but that's another story). We have the full service history since the car's been new. Since 56,000 I've done the basic stuff here and there that is within my (limited) skill set, and everything else has been done by a local (fairly reliable) shop.

The car has never been as smooth or quiet at idle as other vehicles but makes it from point a to point b. A few months/3,000 miles ago I had some lack of pickup from a stop, sputtering when I was getting going and had been noticing that the rough idle from time to time was more a consistent miss/sputter after about 30 minutes of driving, but only when at a full stop. I installed new cap and rotor but the miss/sputter didn't go away (the lack of pickup did). I probably should have continued and just done the plugs and wires but I assumed they were a little too new to be causing the problem (25K to 30K on the plugs and wires) so I decided to ask the shop I use to take a look. They regapped the plugs and asked me to drive it for a while and let them know, for about 200 miles this worked, but the miss/sputter reappeared after that. Back to shop, installed new plugs this was about 1,500 - 1,800 miles ago, and I am noticing that the miss seems to be returning - no where near as bad as before, but I'm thinking it may continue to get worse with time.

Compression was checked and was told it was correct (within range) for the vehicle - unfortunately I did not get the exact reading.

My thought it I should check the plug gaps and go from there, maybe replace what the shop put in with the same brand plugs I had before (NGKs), only because I know those went 25K, but I feel like doing this I might be masking the problem for a few thousand miles rather than getting to the cause of it. Any advice on how to proceed/what could be causing this?

 

[JDenyer232]

Sounds like a weak ignition coil.

 

[SatinSilver]

How about a big bottle of Chevron Techron, Redline S-1 etc....all are fuel system cleaners. I'd start with that.

 

[ThirdeYe]

Oxygen sensor, maybe? On my car it seemed to be a combination of that and the spark plugs, when it used to misfire and stall on me.

 

[Phishin]

1.) Get the car good and hot
2.) In your driveway, put it in drive, and hold the brake down and get the car to sputter. Once you have accomplished this...
3.) Put the car in neutral and let it idle. Does the car still sputter?

If it does NOT sputter at idle in neutral, then this indicates that you have a vacuum leak. Pretty common. This is how I diagnoised this same phenomenon on my truck. It would miss at a stop light with the brake depressed, but NOT idling in park or in neutral.

If the car does sputter in neutral or park, I think one of your plugs is fouling. Does the car burn oil? How much? Have you ever inspected the plugs? Maybe when your mechanic re-gapped them, he wiped them dry/clean them off, giving you a good idle, but after a bit of usage, a plug was fouling again. Pull the plugs and see if one of them is soaked. Replace/clean just that one and see if it fixes your miss.

 

[meep]

you may be able to rule out the weak ign coil. a weak coil will be ok at idle and higher rpms--- will cause the most trouble under load when the engine is in the lower rpm range.

is there a t-belt---? they can start to go this way if it's starting to stretch.

2nd the vacuum leak

2nd the possibility of a clogging injector

 

[doitmyself]

I had a hard to diagnose intermittent miss on my old Caravan. No codes were thrown.

It turned out to be a bad plug wire that would cause an occasional misfire. Maybe when it heated up or was jostled it would trigger a weak point in the "wire"??

 

[99Saturn]

Thanks for the responses so far guys.

Originally Posted By: Phishin
1.) Get the car good and hot
2.) In your driveway, put it in drive, and hold the brake down and get the car to sputter. Once you have accomplished this...
3.) Put the car in neutral and let it idle. Does the car still sputter?

If it does NOT sputter at idle in neutral, then this indicates that you have a vacuum leak. Pretty common. This is how I diagnoised this same phenomenon on my truck. It would miss at a stop light with the brake depressed, but NOT idling in park or in neutral.

I will try this - I have not been able to get it do miss/sputter in park, but I have not tried in neutral. I'm fairly certain that it was not doing it in park a few thousand miles ago as I went out of my way to leave it running when I pulled in to have it looked at and was pretty surprised when I put it in park and idled relatively well.

Originally Posted By: Phishin

If the car does sputter in neutral or park, I think one of your plugs is fouling. Does the car burn oil? How much? Have you ever inspected the plugs? Maybe when your mechanic re-gapped them, he wiped them dry/clean them off, giving you a good idle, but after a bit of usage, a plug was fouling again. Pull the plugs and see if one of them is soaked. Replace/clean just that one and see if it fixes your miss.


Oil consumption is under 1 quart every 5000 miles, probably about 3/4 a quart. Having a Saturn has made oil checking a weekly ritual so I've never thought much of consumption and never measured actual consumption.

I installed plugs and wires now about 27,000 miles ago (maybe it was a bit longer. I did pull them about 15,000 miles ago and didn't see anything too out of the ordinary. I haven't inspected the new one's (installed 2,000 miles ago) yet, but I do plan to pull them (I underestimated how cold it was going to be here Sunday and have a project that is monopolizing the garage at the moment).

----
I have run a bottle of Techron in the tank as well as a few doses of MMO. All of this pre- spark plug change, but made no difference. I had thought perhaps clogged injector - would that disappear for a few thousand miles (masked by new plugs)?

Yes to the t-belt, 60K interval with 45K on it.

On the coil - it is internal to the distributor so that would be unfortunate, though it has about 45,000 miles on it.

On the plug wires - they were installed 30,000 miles ago with the plugs I put in. I went with NGKs as well - I would think/hope they would last longer but I'll look for arcing in the next few days - with daylight savings time and "falling back" an hour, it should be dark enough by the time I get home now and a good opportunity as I usually don't see symptoms until the last 10 minutes of the commute.

 

[Phishin]

One easy way to check spark plug wires is at night when it's dark.

Use a misting/spray bottle with water in it, and spray all around the cap, and following each wire back to the plug. Spraying each wire liberally as you move down each wire from the cap to the plug.

If there is a short, you should see a "spark" in the wire.

 

[99Saturn]

Update - I'd gotten busy with a few other things, but made my way back to the car (one long morning stuck in traffic with the sputter was a good reminder). Pulled the plugs, and replaced the wires.

Plugs were light gray - keep in mind these have about 2,300 only miles on them. Gap was in the acceptable range for the car.

But I had two boots with clear cracks. I looked for a minute before I started for arcing (water on the wires from spray bottle) and saw nothing, but honestly wasn't investigating too hard, already bought the wires and they are lifetime warranty from defect.

I'm guessing the plug wires may have been at fault? Plan to continue monitoring as I went ~1,500 miles on last set of plugs before the sputter returned. Thoughts?