XJ 1991 limited. Experiencing high but constant and steady idle at 1250. Runs good but the idle bothers me.

I’ve replaced

TPS
IAC
MAP
02 sensor
thermostat

Vacuum lines seem fine and I havent found any leaks either.

I also have really bad fuel economy too - around 9-11 mpg, a bit better on highway drives. Not sure if related.


What do you suggest to do next?

Notes: my spark plugs seems fine by colour( light brown, not dark), so it doesnt seem that it would run rich either.

EDIT: solved with an oem TPS - make sure the tps sensor is proper and working even if it is a new one.

 

Check the exhaust manifold for leaks/cracks at the collector (where the individual pipes are welded together), or the gasket-donut where the manifold connects to the exhaust. Exhaust manifold gasket could be blown out too, but that wasn’t the problem on the 3 that I have fixed

Really only one way to do check this. A visual inspection first. Some cracks at the welds will be very obvious.

Nothing obvious…? Make sure you are not wearing anything that will get caught in a moving belt or fan - hoodie strings, baggy sleeves, open jacket, long hair. Start it up, stand to the drivers side and reach bare hand into engine bay alongside the exhaust - a couple of inches away. If it’s leaking you will feel the blow by. Just be real careful.

Changing the manifold is another story. Not really that hard, but you will need a good assortment of hand tools. Ratcheting box-end wrench is the bees-knees for the nuts between the manifold and the block! You have to unbolt all of the accessories on the drivers side of the engine, but shouldn’t have to disconnect them. Just pull to the side (power steering pump and hoses). Fuel rail can be left in place on the intake manifold.

 

Did you wake up one day and out of the blue the idle jumped to 1250 or did you do some sort of work and then the idle went high. High idle is normally associated with an intake leak..( vacuum) ..the engine can't run faster without more air and the extra fuel added because of the map signal sent sensing higher manifold pressure ( lower manifold vacuum) due to suspected leak. Pull the intake tube off the throttle body, is it fully closed? Does the rpm stay the same if you put it in gear with brakes set as in P or N. Do you have cruise control...disconnect the cable from the TB linkage. When you changed the IAC, did you clean the mating surface on the TB where the iac cone seats. The engine runs (idles) a little faster when it senses the vehicle is moving as opposed to a complete stop in D. Pull the small black hose off the fuel rail regulator...it should be dry even with engine running. A mechanical vacuum gauge is a few bucks.get one and attach to any intake port..what does it read at idle. Pull the connector off the temp sensor in the thermostat...any change? A cold signal increases rpm, you will not see this on your temp gauge. With engine hot pull the same connector off and read the resistance of the sensor should be between 600 and 800 ballpark

 

Thanks for the input and sorry for not responding earlier. However the problem persists.

The idle has been quite high as long as I remember. When I started looking into the bad gas mileage I realised that the idle shouldnt be over 1100.

Funnily there are/has been times, where the idle comes down to 800 and stays there nicely. It could be couple days working fine and then the next day it jumps to 1400 on a cold start and sets to 1100-1200 after warming up. That has been the situation now for few weeks and it hasnt calmed down anymore.

It has led me to believe that this should not be vacuum leak related problem - if there was a leak, it would probably be there all the time, meaning the idle wouldnt sometimes settle at 800 where its supposed to be.

Another notable thing is that sometimes if I rev the engine on neutral up to 3500rpm, the idle then suddenly sets down to 1000, or at least way lower than what it was before that.

I know everyone always says that ECU is rarely at fault, but I really cannot find any more solutions other than that. All the sensors have been replaced too. I have also resetted the ECU with no effect.

If you still have any more ideas, I appreciate your help alot.

 

Thanks for the reply. The problem seems to be complicated. I have been able to get the Jeep running with 800 idle - totally randomly after highway driving or just city driving for couple hours! It feels like then the jeep has better power output too, the rig moves quite easily even on idle and with little bit of gas, but when the idle is at 1200, the jeep is moving slower than it is on 800 idle. All this makes no sense.

Very frustrating to say the least. I’m getting no codes or check engine light either, so I’m not suspecting any misfires (although the engine does have quite strong vibrations that occur quite constantly). If there was a misfire, woulndt the idle be low too, or at least engine would run rough?

I have also replaced the injectors to newer 4hole ones, this didnt seem to make any kind of difference.

Wondering if I should take it to a shop next.

 

Well, as it turns out, the new TPS I had installed before was malfunctioning. I changed the oem tps from my brothers same year wrangler to the xj and the rig runs great now. I’m not sure yet if the mpg will be affected too, but the idle is working perfectly. I found out the problem when I decided to try to drive without the IAC attached. the idle slowly came down to 700 and I figured the tps was giving it faulty data. Changed the tps and the idle is steady at 700 now and responds well to gas. Thanks all for help and answers.

If someone is having similar problems, make sure your TPS is working even if you have a new one.