Idle

Hi all,

Tired of my 400rpm idle. It idles fine while in cold loop, right around 800 but after the engine warms up it drops off to 300-500 range. Where should I start hunting for a problem?

Thanks

Seems to me if it knows when to come out of cold loop, then the O2 sensor isn't suspect. You may want to carefully check for vacuum leaks, including loose intake manifold bolts. And of course, clean the throttle body. How old is your TPS?
Perhaps others have more suggestions?...

Vacuum leaks will usually cause it to idle high and I've repeatedly convinced myself that the TPS doesn't really do anything to idle speed. Is this your '88 XJ or one of your other vehicles. If it's the '88, I'd pull off the throttle body and clean the IAC port. If someone has fooled with the idle air screw on the street side of the TB, that might need to be adjusted properly. It starts life hidden under a soft metal plug and if the plug is gone, then someone has probably diddled with it. With the IAC port blocked off, you should idle around 400-500 RPM. Adjust the idle air screw if that's not the case. The MAP could be a culprit. If it is not seeing enough vacuum, idle speed could be low. A lazy or dying MAP sensor might do the same thing.

Thanks for the suggestions. Here are some of the things that have been addressed:

TPS is brand new
MAP is new
Idle air screw still happily hidden behind metal plug

O2 sensor is busted (rebuilt tranny and the shop broke it putting tranny back in). No idea if it still works but I'm running SUPER rich so I doubt it. It's had this problem forever anyway, I'm just now getting around to messing with it.

I was wondering if it was something sensor related since it idles fine in cold loop. Once it hits normal temp range it starts to idle low and rough.

I'll take a look at the IAC. Do I understand that if that stuff all checks out then it's time to unplug the screw and start tuning?

Thanks!

why would you not fix the o2 sensor. fuel injection requires all sensors to be working properly, if not they run like shit.

fix the o2 sensor and try some top end cleaner and see what happens.

also could be a bad egr valve

It has already been ordered... it was suposed to have been here a couple days ago but they had some kind of problem so now I have to wait a couple more days.

EGR is also new... Like I said this has been an ongoing problem since I bought the XJ 3 years ago. It was in REALLY tough shape due to the po's neglect of just about everything. This is my last problem and it's as good as a brand new 15 yo XJ can be.

WOW. :shocked:

What a differance a $2.48 can of throttle body cleaner can make. My XJ is now running PERFECTLY. Sprayed it down with cleaner and after the clouds of gray smoke cleared and the engine quit sputtering I was left with a perfect idle. Why, oh why did I wait so long to do this? :doh: Fixed all my problems after hours of vacuum diagnostics and tinkering with all kinds of stuff.

Now that it runs perfectly, why does the instrument lighting suddenly fail to work? Sigh, the Chief refuses to be completely operational. Hopefully I'll get it's last problem solved in the morning with a new part...

Thanks for the tip Martin, it made the Chief happy again. :looney: