89 laredo idle

my 89 4.0(226,xxx)idles comfortably at 300 rpm and never dies but obviously this is wrong.it runs fine,gets good gas mileage but because of the low idle i get an occasional knock in the bottom end taking off from a light.oil pressure is 65 at startup cold,25-30 at a warm idle and 40 while driving.i don't want to adjust the idle screw cuz i saw on the "idles too high"thread that you should'nt touch it.help my laredo and she will appreciate it:eye:

a7273chargerguy said:

my 89 4.0(226,xxx)idles comfortably at 300 rpm but obviously this is wrong.it runs fine,gets good gas mileage but because of the low idle i get an occasional knock in the bottom end taking off from a light.oil pressure is 65 at startup cold,25-30 at a warm idle and 40 while driving.i don't want to adjust the idle screw cuz i saw on the "idles too high"thread that you should'nt touch it.help my laredo and she will appreciate it:eye:

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Behind a plug (soft metal, easy to remove) on the drivers side of the TB, there is a screw to adjust a small air pypass, I'd guess for fine idle adjustments. It's a torx type screw in mine. The orifice for the air bypass, could be plugged on yours, maybe squirting a little cleaner down there might help. This adjuster isn't a cure all, you add air and the O2 sensor is going to try and compensate (especially when the motor is hot). But a small adjustment may work out for you. I use a little dab of mild Loc Tite in the threads, to keep the screw from backing out on it's own. Just a tiny drop, it deosn't take much and you may want to adjust it again in the future (some people use a drop of RTV).
When I adjusted mine, I'd either have a good cold idle or a good hot idle but never both, I split the difference. I found the orifice to be completely plugged up on mine and it took removing the screw adjuster and some fairly serious cleaning to get it open again.
Poor idle can also be a sign of a weak or failing O2 sensor. O2 sensor heater relay or the wiring for the O2 sensor that goes down the front of the motor and very near the exhaust manifold (it can get cooked).

My 90' also idles at 300-400 rpm, adjusting the air bypass doesn't affect my idle either way. Since every sensor and output device is new and my TPS is adjusted correctly, I think its more of a computer issue/calibration type of thing. Mine also does not die or stall but I'm not having any knock/ping like you are having. Here is a list of whats new on mine.

block/head-newly rebuilt
Intake and TB cleaned with ultrasonic cleaner
injectors
distributer
TPS
CPS
MAT
CTS
MAP
IAC
O2
new cat
EGR and solonoid
fuel pressure reg.
fuel pump/pickup screen/fuel filter
knock sensor
cap/rotor/NGK plugs/OEM wires
NSS has been cleaned
all relays
All connectors under the hood and dash have been pulled apart and cleaned with die-electric grease added.

Cleaned all grounds, and added a battery- to chassis ground from the side post on my OPTIMA.

No change in my idle, it sounds like a old John Deere tractor. The idle kicks up when the AC is on to maybe 500 rpm. I'm getting great mileage and the power is unreal. It runs much better/stronger than my other 3 HO XJ's.

I've decided to live with what ever the low idle issue is.

xjtrailrider said:

My 90' also idles at 300-400 rpm, adjusting the air bypass doesn't affect my idle either way. Since every sensor and output device is new and my TPS is adjusted correctly, I think its more of a computer issue/calibration type of thing. Mine also does not die or stall but I'm not having any knock/ping like you are having. Here is a list of whats new on mine.

block/head-newly rebuilt
Intake and TB cleaned with ultrasonic cleaner
injectors
distributer
TPS
CPS
MAT
CTS
MAP
IAC
O2
new cat
EGR and solonoid
fuel pressure reg.
fuel pump/pickup screen/fuel filter
knock sensor
cap/rotor/NGK plugs/OEM wires
NSS has been cleaned
all relays
All connectors under the hood and dash have been pulled apart and cleaned with die-electric grease added.

Cleaned all grounds, and added a battery- to chassis ground from the side post on my OPTIMA.

No change in my idle, it sounds like a old John Deere tractor. The idle kicks up when the AC is on to maybe 500 rpm. I'm getting great mileage and the power is unreal. It runs much better/stronger than my other 3 HO XJ's.

I've decided to live with what ever the low idle issue is.

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my 89 runs better than my other one as well.i guess i am just going to adjust the idle screw and see if that helps.thanx for the ideas!

I recently solved my low idle speed (combined with an idle speed cycling problem between 300 and 700 rpm on a 20 to 30 second cycle). I had lots of new parts too. Turned out the wiring harness connectors were old and missing the locking clips and I had attached them to the O2 sensor and the TPS sensor backwards. I did not realise at the time that they could be attached backwards so I was not paying enough attention to the orientation, wire colors and markings (A-B-C) on the connectors. It still ran, but had a 300 rpm idle problem.

Mike McGinness
South Houston, TX

Ecomike, You may be onto something there, I'm going to check that first thing a.m., will repost with results on mine.