any ideas where to begin to troubleshoot for rough idle

 

Dirty throttle body , bad maf sensor , irregular air flow . Do you read check engine light on the gauge ?

 

Dirty fuel injectors......?


Cheers
DD

 

I ran my 95 XJ6 to near to the empty mark, and it is idling rough and sometimes it stalls out. I took it autozone and they checked it and said I had a problem with cylinder 1 ingition. I replaced all the plugs and while we were in there we saw that the valve cover gasket needed to be replaced. So we replaced that too. Still idling rought. I replaced the ingition coil for cylinder one, no change. I bought a new fuel filter and I have a new fuel injector on order. I am hoping this fixes it. If it doesn't, I don't know what we will do next. Any suggestions would be accepted and appreciated.
Thanks

 

Rough idle problems can be very hard to sort out. Sometimes the problem can be isolated to a single cylinder and traced to a single fault. This is where a "check engine light" and fault code, when present, can be helpful.

Lacking that, a scan tool with "live data" capability might reveal an anomoly that can be traced out.

Other times the roughness is actually a "stack up" of small faults....none of which are individually severe enough to trigger a fault code or be easy to recognize by any other means.

A DIYer can check for vacuum leaks, clean the MAF, replace consumable items (spark plugs, filters, etc), clean the throttle body... and that's about it unless he wants to go on a parts replacing spree :-)

Having the fuel injectors *professionally* cleaned will often solve the problem. Take the car to a good shop that has a "Motor Vac" or similar method of cleaning/flushing the injectors or remove the injectors and send them to a injector specialist.

You can try pour-in-the-tank injectors cleaners. Some of them are pretty good. If the idle improves, great. If there's no improvement, though, you haven't proven that there *isn't* a problem with the injectors. You may have some that are too dirty to be cleaned with pour-in-addtives.

Speaking of injectors.....

On X300s there is a known glitch regarding fuel injectors: if the idle is very rough *only* when the gearshift is in "P", it's almost certainly an injector problem. At idle in "P" the engine load is minimal and the ECU commands a very short "injector pulse width". It's been discovered that some injectors to not respond to the command. My own car has this problem, in fact. One day I'll try to figure out which injector is giving trouble. I may have to buy one new one and try it in 6 differeent locations :-)

Enough rambling.

Cheers
DD

 

"I ran my 95 XJ6 to near to the empty mark". Here's the clue -- almost certainly fuel-associated, most likely clogged injector. They clean them ultasonically now apparently. No need to buy new.
Leedsman.

 

Thanks to everybody for your recomendations. Looks like I needed to replace spark plugs. I also cleaned the throtle body as suggested and so far the rough idle I had before is gone.

 

Good news indeed Tony.

@Doug can't you just hook up one of those Noid Light jobbies, damn sight cheaper than an injector just for testing purposes.

 

No, I don't think so, not in the situation I was describing above.

The noid light would identify a "dead" nor "not dead" injector but probably not be sensitive enough to tell us if the injector was responding poorly to low pulse width commands.

But.....I might just try it. I've got a set of noid lights.

Cheers
DD

 

I replaced the spark plugs and cleaned the throttle body and it ran good for a while but problem came back. I have no check engine light. I checked for vacuum leaks and found none. I guess I will just have to take it in for a diagnostics check and spend the money on the fix instead of guessing at it.

 

Bummer! Just a thought, but you cleaned the throttle body, have a good check of all the rubber bits that are around there.

The bellows for seating and any perforations.

Also check all the breather pipes for cracks and or blockages. The one from the oil filler tube to the bellows can become full of crud.

Any openings that should not be there can easily affect idle on a closed system.

How did you check for vacuum leaks? A quick method is to mix up some washing up liquid and squirt it onto the pipes/lines any leaks tend to show up as frothing/bubbles.

Even check your dipstick seating and oil filler cap.

Costs nothing to try, and today is Sunday.

@Doug, OK twas just a thought.

 

I've had mine tested time and time again to determine the reason for the rough idle i have been having in my Jag, but due to a "fluke" investigation, i found as i was moving a few things around i noticed the boot between the throttle body intake and the air intake had a split in the rubber between the ribs..... I cleand the split and applied... get this...super glue to the split!..it sealed up, and my rough idle was gone. this was my solution..so i suggest snooping around where you wouldnt expect it to be, you may just find the hidden problem...

Cheers
Kyle

 

Quite so, Kyle-Smith, quite so... The empirical method of fault finding is a favorite method among real diagnosticians; this posting by Kyle-Smith should be read and absorbed by everyone attempting to track down problems. Didn't Sherlock Holmes say famously, "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth"?
Leedsman.