johnlocke

I'm looking for culprits. Here's the issue I'm calling "rough idle":

A normal idle is around 700 rpms, I think. When it gets "rough", it will dip first to about 500, then come back up, but dip back down ... slowly it seems like it bogs even further and then will oscillate between 300 and 500 (while the battery charge indicator drains) and then it'll just bounce to zero eventually and die. This whole sequence can happen within 15 seconds. Sometimes it will bounce all the way to dead, sometimes it will just grind down to about 300rpms and bounce back up and down between that for a while.

About half the time I start the jeep, it goes immediately into a rough idle. The other half, it idles fine and has no issue. I haven't found a reason why it does one or the other (temperature, or anything else).

When coming to a stop, this is most frequent. It drives fine, and even seems to get pretty decent mpg's in general. It's got a slight "tick" sound at higher rpm's, but that could be normal or completely unrelated (the sound was there well before I ever noticed this problem and the sound hasn't changed).

Another odd thing I've noticed is that if I shift to neutral while coming to a stop, I don't see this problem /nearly/ as frequently. If the problem is occurring in general, then every time I come to a stop in drive, I will have this problem. Shifting to neutral then doesn't stop it. If I shift to neutral when this problem is occurring, then about 95% of the time, I don't see the issue happen at a stop until I shift back into drive.

That all said, and not knowing nearly as much about automatic transmissions as I should, it almost sounds and feels like the engine is bogging down a little more than it should ... like the "clutch" isn't fully releasing, but like I said, it's an automatic, so I'm not sure how that works.

Any ideas of tests I can run to narrow down what the issue is? Or ideas about what it is in general?

I've run a few tanks of sea foam, and even done some in the crank case. No improvement. I've cleaned the throttle body with a spray cleaner and not seen any problems. I haven't tried spraying starter fluid around the vacuum lines in the manifold, but they look good. The plugs and wires appear to be in good external shape (but again, I haven't pulled them to check). I've got a K&N that I've just pulled and cleaned with no change (it even had the problem with /no/ air filter at a stop). The jeep isn't throwing any check engine codes, though I'm not sure if it's throwing codes that wouldn't light up the dash.

(it's a 2001 XJ w/ 150k on it if my profile doesn't say it)

DEVILSTOY

id try to give it a tune up and clean your throttle body , even if it doesnt fully fix it , its probly a good idea to do it anyway, but for the most part it sounds like it needs a good tune up

johnlocke

Well, as mentioned, I did clean the air filter and clean the throttle body. I'm not 100% sure when the plugs and wires were last changed (just bought the car this year), but the previous owner said he'd changed them before selling it to me (don't think I mentioned that part) and they look pretty new. The oil has been changed in the last 1000 miles, and the rest of the fluids look clean.

I guess one other thing I need to check is the fuel filter.

johnlocke

Update: I still haven't figured this out, but it's presenting less and less. I just switched to midgrade gas from the lower grade and am not seeing this as much. The idle still drops down to about 600, and looks like it's /threatening/ to bounce down to nothing, but it hasn't happened in the last 200 miles.

Could I have been unlucky enough to get /two/ bad loads of gas, one right after another? Does the jeep actually /need/ midgrade at this point in it's life? Alternately, I know Colorado ups the ethanol content for the winter, and higher grades of gas use less ethanol, but I don't know any of my other jeep buddies that use midgrade here.

DEVILSTOY

right after i originaly read this thread my DD started doing this (chevy avalanche) yet i didnt go to midgrade gas , i simply just bought gas from a differant gas station and my problem nearly fixed itself

johnlocke

Obviously I can't swear to it, but I don't /think/ it's this gas station. It's the same place I get gas for my motorcycle (a cruiser that uses the low grade gas as well) and there haven't been any problems there that I can see.

But gas certainly /may/ have been part of the problem. I know there's got to be something else to it still since the idle still isn't quite stable.

DEVILSTOY

its possable your fuel filter could need replacement

cltdba

Possible throttle position sensor? O-reilly auto parts (and possibly others) can test it for you.

johnlocke

I'm hoping not because it appears the fuel filter and the fuel pump (and the sender) are all part of one unit that's in the gas tank and cost $200. I've got a friend with a fuel pressure gauge, so I'll report back once I've looked at that.

If I'm right in thinking O-reily is Checker's, then nope, they won't check codes or parts anymore (according to the guy behind the counter). New policy this year he said.

Took it to auto zone and there's no codes being thrown (I didn't realize that /any/ code will set off a check engine light ... thought there were "non critical" codes that could hang out behind the scenes), and to have them test the TPS, I have to remove it. No time for that today, but once I get an option to do so, they can check it if I bring it to them.

johnlocke

So ... found the problem and I'm posting here in case someone else has it. Found out the throttle body was really dirty on the inside (along with the air intake sensor). So the throttle couldn't close completely (because of the carbon build up) and the air sensor couldn't tell what was going on, so it was "hunting for idle". Once I got those good and cleaned up, zero problem.