Unusual rough running problem

[lee]

Last Friday filled up (w/93 octane) at a Chevron station where I frequently buy my gasoline. Drove home (about 8 miles) and parked the car overnight. Didn't drive it again until Sunday morning to make the run for the newspaper. Car stumbled/shuddered pretty bad. Thought maybe I got a load of bad gas, so since then I have put in a bottle of Techron and 2 qts of toluene. Running better but not right. Here's what it's like today....

Once up to speed, and on flat ground (like there's any other kind around here), the car runs fine, don't feel any engine misfires. When at about 1/3 to 2/3 throttle there is some stumbling. When at WOT, there is sometimes a surging sensation - where it changes from full-strength acceleration to something less- still relatively smooth, but down on power.

Does this during "first run of the morning" cold to fully warmed up - but cold is a bit worse.

Just filled up on my way home today with Shell V-Power. Only drove about a mile to the house, and it's parked until I go to work tomorrow.

Interestingly, I normally get about 19.8 mpg. I drive a pretty standard route to/from work up & down Hiway A1A. The mpg for this tank while suffering the performance issue - 24.2 mpg!? It wasn't babied either, I accelerated harder than normal to avoid running in the middle throttle position and getting all that misfire.

No check engine light, no hard starting. Haven't yet checked anything except battery ground.

So,

[SVXRide]

Originally Posted by lee

Last Friday filled up (w/93 octane) at a Chevron station where I frequently buy my gasoline. Drove home (about 8 miles) and parked the car overnight. Didn't drive it again until Sunday morning to make the run for the newspaper. Car stumbled/shuddered pretty bad. Thought maybe I got a load of bad gas, so since then I have put in a bottle of Techron and 2 qts of toluene. Running better but not right. Here's what it's like today....

Once up to speed, and on flat ground (like there's any other kind around here), the car runs fine, don't feel any engine misfires. When at about 1/3 to 2/3 throttle there is some stumbling. When at WOT, there is sometimes a surging sensation - where it changes from full-strength acceleration to something less- still relatively smooth, but down on power.

Does this during "first run of the morning" cold to fully warmed up - but cold is a bit worse.

Just filled up on my way home today with Shell V-Power. Only drove about a mile to the house, and it's parked until I go to work tomorrow.

Interestingly, I normally get about 19.8 mpg. I drive a pretty standard route to/from work up & down Hiway A1A. The mpg for this tank while suffering the performance issue - 24.2 mpg!? It wasn't babied either, I accelerated harder than normal to avoid running in the middle throttle position and getting all that misfire.

No check engine light, no hard starting. Haven't yet checked anything except battery ground.

So,

Lee,
given that when you put in the techron and toluene it ran better and you're experiencing surging, I'd take a look at the injectors and/or the fuel filter.
-Bill

[lhopp77]

Are you sure it is hitting on all six cyl? Mine was somewhat like that and one of the coils was bad.

Lee

[lee]

Assuming my wife gives me some time this weekend I'll start checking.

I have been assuming it was fuel related since it did get better with the cleaner additions. The fuel filter is only about 10K miles old (like that means anything) - but since it actually runs better under WOT, which would mean highest flow rate, I'm doubting the filter.

I wonder about a bad sparkplugs/coil, again since it gets better under WOT - but then I always learn new things - can they behave in this way? Plugs are Bosch-4, about 25K miles old.

Idle is fine, more or less OK at WOT, poor at mid-throttle. Note, this is throttle induced and is not speed sensitive, I can get the shuddering/misfire if at 15 or 50 mph. Also noted today that if I give enough gas to cause it to start, then back off a little, then give a little more (past where it would have been an issue) it seems to work OK. Using this self-induced kind of surge - press, slight release, press some more, repeat - I can almost totally avoid the issue.

[Beav]

I'd begin by checking the TPS. If it's o.k. try disconnecting the EGR valve and drive to see if the problem remains. If it goes away let me know.

[Amazonparrot]

I have also been having this problem off and on for just about a year now. I have been watching over that great period of time for any number of consistant circumstances, but have found almost none. It may go away for weeks, and then come back full force, sputtering popping and shuddering that it takes forever to attain 40mph. When I took it to a mechanic to just get analyzed, he said it was the number 3 or 5 cylinder (second and third from you on the passenger side whilst facing the car). I guess it made total sense, being that if the rubber "insulation" around the coil pack was bad that it would ground somewhere and cause that cylinder to misfire. But if this was the case...then wouldnt it be persistant? And also, (and maybe its like a placebo effect) it seems like sometimes when i fill up after being empty that it gets worse for a little while, but there is really no telling b/c its so random (also I just changed the fuel filter less than 2k ago, or at least Chike did it for me...haha)

-Andrew R.

Pitch us some help here!

[lee]

Originally Posted by Beav

I'd begin by checking the TPS. If it's o.k. try disconnecting the EGR valve and drive to see if the problem remains. If it goes away let me know.

I finally got around to trying today on my way to work. It's not that I've been lazy, we ripped out all the interior walls and ceiling over the weekend. I got the ceilling back and some of the walls...lots of work left over from the hurricane season still yet to go.

Anyway, I pulled off the line from the EGR vac controller to the EGR valve. Seemed no different with the vac line hanging loose.

[SilverSpear]

Originally Posted by SVXRide

Lee,
given that when you put in the techron and toluene it ran better and you're experiencing surging, I'd take a look at the injectors and/or the fuel filter.
-Bill

A+++++++++++